6 - Cell Division

Front 1

A pair of chromosomes that contain genes for the same characteristics

Back 1

Homologous

Front 2

A group of organs working together to perform an essential function

Back 2

System

Front 3

A type of cell division that produces genetic variation

Back 3

Meiosis

Front 4

The mitotic cell cycle is divided into a number of stages. In which of the stages will the chromosomes lineup at the equator of the cell.

Back 4

Metaphase

Front 5

In meiosis what can lead to variation within a species

Back 5

Independent assortment of homologous chromosomes

Front 6

Discuss the ways in which genetic variation is produced including the role of nuclear division

Back 6

Independent assortment of homologus chromosomes in metaphase 1 of chromatin in metaphase 2 so homologous chromosomes have different alleles and produce a large number of allele combinations.

crossing over of chromosomes in prophase 1 so chromatin will have new combination of alleles, amount of variation depends on distance between crossover points.

mutation changes the nucleotide sequence, DNA checks during duplicaton did not recognise any damage and there will be a change in the amino acid sequence changing the structure of protein.

non-disjunction, homologous chromosomes don't seperate n metaphase 1, one more chromosomes present.

random fertilisation, gametes are not genetically identical, produces large number of allele combinations.

Front 7

cells can be counted and analysed using a technique called for cytometry. DNA in the cells is stained with a fluorescent dye before analysis. the degree of fluorescence is dependant on the amount of DNA present. Name the stages of interphase that match the description: the stage in which a cell produces the lest fluorescence?

Back 7

G1, first growth phase

Front 8

cells can be counted and analysed using a technique called for cytometry. DNA in the cells is stained with a fluorescent dye before analysis. the degree of fluorescence is dependant on the amount of DNA present. Name the stages of interphase that match the description: the stage in which a produces the most fluorescence

Back 8

G2 the second growth

Front 9

stage 1 - prophase - of mitosis

Back 9

1. Chromatin fibres begin to coil and condense to form chromosomes that will take up stain to become visible under light microscope. the nucleolus disappears and the nuclear membrane begins to breakdown.

2. Protein microtubules form spindle-shaped structures linking the poles of the cell. the fibres forming the spindle are necessary to move the chromosomes into the correct positions before division.

3. In animal cells and some plant cells, two centrioles migrate to opposite poles of the cell. the centrioles are cylinderical bundles of proteins that help in the formation of the spindle.

4. The spindle fibres attach to specific areas on the centromeres and start to move the chromosomes to the centre of the cell.

5. By the end of prophase the nuclear envelope has disappeared.

Front 10

stage 2 of mitosis - metaphase

Back 10

chromosomes are moved by the spindle fibres to form a plane in the centre of the cell, called the metaphase plate and then held in position.

Front 11

stage 3 of mitosis - anaphase

Back 11

the centromeres holding together the pairs of chromatids in each chromosome divide during anaphase. the chromatids are seperated - pulled to opposite poles of the cell by shortening the spindle fibres. The 'V' characteristics of the chromatids moving towards the poles is a result og them being dragged by their centromeres through the liquid cytosol.

Front 12

stage 4 of mitosis - telophase

Back 12

the chromatids have reached the poles and are now called chromosomes. the two new sets of chromosomes assemble at each pole and the nuclear envelope reforms around them. the chromosomes start to uncoil and the nucleolus is formed. cytokinesis occurs.

Front 13

cytokinesis in animals

Back 13

a cleavage furrow forms around the middle of the cell. the cell-surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle forming two cells.

Front 14

cytokinesis in plants

Back 14

plant cells have cell walls so it is not possible for a cleavage furrow to be formed. Vesicles from the golgi apparatus begin to assemble in the same place as where the metaphase plate was formed. the vesicles fuse with each other and the cell surface membrane, dividing the cell into 2. New sections of the cell wall then form along the new sections of membrane.

Front 15

meiosis 1: prophase 1

Back 15

chromosomes condense, the nuclear envelope disintegrates, the nucleolus disappears and spindle formation begins, as in prophase in mitosis. homologous chromosomes pair up forming bivalents. chromosomes are large molecules of DNA and moving them through the liquid cytoplasm as they are bough together results in the chromatids entangling. this is called crossing over.

Front 16

meiosis 1: metaphase 1

Back 16

the same as metaphase in mitosis except that the homologous pares of chromosomes assemble along the metaphase plate instead of the individual chromosomes. the orientation of each homologous pair on the metaphase plate is random and independent of any other homologous pair. The maternal or paternal chromosomes can end up facing either pole. this is called independent assortment, and can result in many different combinations of alleles facing the poles. Independent assortment of chromosomes results in genetic variation.

Front 17

meiosis 1: anaphase 1

Back 17

Homologous chromosomes are pulled to the opposite poles and the chromatids stay joined to each other. Sections of DNA on 'sister' chromatids, which became entangled during crossing over now break off and re-join sometimes resulting in the exchange of DNA. the points at which the chromatids break and re-join are called chiasmata. When exchange occurs, this forms recombinant chromatids, with genes being exchanged between chromatids. the genes being exchanged may be different alleles of the same gene, meaning the combination of alleles on the recombinant chromatids will be different from the allele combination on either the original chromatids. Genetic variation arises from this new combinations of alleles - the sister chromatids are no longer identical.

Front 18

meiosis 1: telophase 1

Back 18

The chromosomes assemble at each pole and the nuclear membrane reforms. Chromosomes uncoil. Cell undergoes cytokinesis and divides into two cells. the reduction of chromosome number from diploid to haploid is complete.

Front 19

meiosis 2 - prophase 2

Back 19

the chromosomes, which still consist of two chromatids, condense and become visible again. the nuclear envelope breaks down and spindle formation begins.

Front 20

meiosis 2: metaphase 2

Back 20

the individual chromosomes assemble on the metaphase plate, as in metaphase in mitosis. Due to the crossing over, the chromatids are no longer identical so there is independent assortment again and more genetic variation produced in metaphase 2.

Front 21

meiosis 2: anaphase 2

Back 21

Unlike anaphase 1, anaphase 2 results in the chromatids of the individual chromosomes being pulled to opposite poles after division of the centromeres - the same as in anaphase of mitosis.

Front 22

meiosis 2: telophase 2

Back 22

the chromatids assemble at the poles at telophase 2 as in telophase of mitosis. the chromosomes uncoil and form chromatin again. the nuclear envelope reforms and the nucleolus becomes visible. cytokinesis results in division of the cells forming four daughter cells in total. the cells will be haploid due to the reduction division. they will also be genetically different from each other, and from the parent cell, due to the processes of crossing over and independent assortment.

Front 23

erythrocytes

Back 23

red blood cells have a flattened biconcave shape, which increases their SA:V ratio. this is essential to their role of transpoting oxygen around the body. in mammals these cells do not have nuclei or many other organelles which increases the space available for haemoglobin. they are also flexible to be able to squeeze through narrow capillaries.

Front 24

neutrophilis

Back 24

a type of white blood cell which play an essential role in the immune system. they have a characteristic multi-lobed nucleus, which amkes it easier for them to squeeze though small gaps to get to the site of infections. the granular cytoplasm contains many lysosomes that contain enzymes used to attack pathogens.

Front 25

sperm cell

Back 25

male gametes. their function is to deliver genetic information to the female gamete. sperm have a tail or flagellum so they are capable of movement and contain many mitochondris to supply thr energy needed to swim. the acrosome on the head of their sperm contains digestive enzymes which are released to digest the protective layers around the ovum nd allow the sperm to penetrate, leading to fertilisation.

Front 26

palisade cells

Back 26

present in the mesophyll contain chloroplaststo absorb large amounts of light for photosynthesis. the cells are rectangular box shapes, which can be closely packed to form a continuous layer. they have thin cell walls, increasing the rate of diffusuion of CO2. they have a large vaculoe to maintain turgor pressure. chloroplasts can move within the cytoplasm in order to absorb more light.

Front 27

root hair cells

Back 27

present at eh surfaces of roots near the growing tips, have long extensions called root hairs, which increase the surafce area of the cell. this maximises the uptake of water and minerals from the soil.

Front 28

phloem tissue

Back 28

another type of vascular tissue in plants, responsible for the transport of organic nutrients, particularly in sucrose, from leaves and stems where it is made by photosynthesis to all parts of the plant where it is needed. it is composed og columns of sieve tube cells separated by perforated walls called sieve plates.

Front 29

xylem tissue

Back 29

type of vascular tissue responsible for transport of water and minerals thoughout plants. the tissue is composed of vessel elements, which are elongated dead cells. the walls of these cells are strengthened with a waterproof material called lignin, which provide structural support for plants.

Front 30

cartilage

Back 30

a connective tissue found in the outer ear, nose and at the ends of, and between bones. it contains fibres of the proteins elastin and collagen. cartilage is a firm, flexible connective tissue composed of chondrocyte cells embedded in an extracellular matrix. cartilage among other things, prevents the ends of bones from rubbing together and causing damage. many fish have whole skeletons made of cartilage, not bone.

Front 31

squamous epithelium

Back 31

made up of specialised squamous epithelial cell, is sometimes knows as pavement epithelium due to its flat appearance. it is very thin due to the squat or falt cells that make it up and also becaus it is only one cell thick. it is present when rapid diffusion across a surface is essential. it forms the lining of the lungs and allows rapid diffusion of oxygen into the blood.

Front 32

cells can be counted and analysed using a technique called for cytometry. DNA in the cells is stained with a fluorescent dye before analysis. the degree of fluorescence is dependant on the amount of DNA present. Name the stages of interphase that match the description: the stage in which a produces the most fluorescence

Back 32

G2 the second growth

Front 33

cells can be counted and analysed using a technique called for cytometry. DNA in the cells is stained with a fluorescent dye before analysis. the degree of fluorescence is dependant on the amount of DNA present. Name the stages of interphase that match the description: the stage in which the highest number of cells is recorded

Back 33

G1, the first growth

Front 34

suggest why fluorescent dyes in flow cytometry is inappropriate when analysing red blood cells

Back 34

they do not contain DNA

Front 35

explain why a plant leaf is described as an organ

Back 35

organ is a collection of tissues which are adapted to perform a certain function. leaves have many tissues including the xylem, phloem, spongy mesophyll and epidermis to help carry out photosynthesis and gaseous exchange

Front 36

explain the role of embryonic stem cells in the development of the embryo

Back 36

embryonic cells are undifferentiated (not specialised) and they are a renewing source of cells (have ability to divide continuously. they can differentiate into any cell type.

Front 37

explain why the cells of the inner cell mass are not totipotent stem cells

Back 37

they cannot form a whole organism or form tissues such as placenta. they cannot give rise to extra-embryonic tissues.

Front 38

name stages of mitosis in order

Back 38

prophase, metaphase, anaphase, telophase

Front 39

Cell cycle

Back 39

The cell cycle is a highly ordered sequence of events that takes place in a cell, resulting in division of the cell, and the formation of two genetically identical daughter cells.

Front 40

Plant stem cells and medicines

Back 40

Plant stem cells have a huge potential role to play in medicine. Many drugs used in medicines are derived from plants. Plant stem cells can be cultured, leading to an unlimited, and cheap, supply of plant-based drugs. Paclitaxel is a common drug used in the treatment of breast and lung cancer. It cannot be chemically synthesised and must be obtained from the bark of yew trees. The trees have to be mature, which means the supply is limited and the extraction process difficult and expensive. An alternative way of producing the drug was developed using a related plant but it is still a difficult and expensive process. Recently stem cells from the yew tree have been used to produce paclitaxel cheaply and in sustainable quantities.

Front 41

Experimental gene therapy for SCID

Back 41

More recently experimental gene therapy has been used to treat SCID. The aim is that stem cells from the patient’s own bone marrow are removed and genetically modified so that they function normally to produce the white blood cells needed. These are then put back into the patient and the condition should be corrected. This treatment was initially successful in a small number of children, but in some of the children another gene was damaged in the process, and they went on to develop leukaemia. However, gene therapy is still seen as having the most potential for treating SCID in the future.

Front 42

Gene therapy using stem cells for SCID

Back 42

Children born with the rare genetic condition Severe Combined Immunodeficiency (SCID) are extremelyvolnerable to all infections and without treatment are unlikely to live for more than a year. They produce no T cells, and without T cells the B cells do not function either (T cells and B cells are types of white blood cell). Normally SCID is treated with a bone marrow transplant, which depends on finding a matching donor. The transplanted stem cells divide and differentiate into the different types of white blood cells needed for a healthy immune system.

Front 43

How ethics are holding back progress of stem cells in medicine

Back 43

This controversy is holding back progress that could lead to the successful treatment of many incurable diseases. The use of umbilical cord stem cells overcomes these issues to a large extent, but these cells are merely multipotent, not pluripoent like embryonic stem cells, thus restricting their usefulness. Adult tissue stem cells can also be used but they do not divide as well as umbilical stem cells and are more likely to have aquired mutations. Developments are being made towards artificially transforming tissue stem cells into pluripotent cells. Induced pluripotent stem cells (iPSC) are adult stem cells that have been genetically modified to act like embryonic stem cells and so are pluripotent. The use of plant stem cells does not raise the same ethical issues as animal cells.

Front 44

Ethics of stem cells – removal of stem cells from embryos

Back 44

Stem cells have been used in medicine for many yeats in the form of bone marrow transplants. More recently, the use of embryonic stem cells in therapies and research ahs led to controversy and debated regarding the ethics of such use. The removal of stem cells from embryos normally results in the destruction of the embryos, although techniques are being developed that will allow stem cells to be removed without damage to embryos. There are not only religious objections to the use of embryos in this way but moral objections too – many people believe that life begins at conception and the destruction of embryos is, therefore, murder. There is a lack of consensus as to when the embryo itself has rights, and also who owns the genetic material that is being used for research.

Front 45

Stem cells are already medicine for - Developmental biology

Back 45

with their ability to divide indefinitely and differentiate into almost any cell within an organism, stem cells have become an important area of study in developmental biology. This is the study of the changes that occur as multicellular organisms grow and develop from a single cell, such as a fertilised egg – and why things sometimes go wrong.

Front 46

Stem cells are already medicine for - Drug trials

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potential new drugs can be tested on cultures of stem cells before being tested on animals and humans

Front 47

Stem cells are already medicine for - The treatment of burns

Back 47

stem cells grown on biodegradable meshes can produce new skin for burn patients, this is quicker than the normal process of taking a graft from another part of the body.

Front 48

Uses of stem cells for medical treatment - Spinal injuries

Back 48

scientists have restored some movement to the hind limbs of rate with damaged spinal cords using stem cell implants.

Front 49

Uses of stem cells for medical treatment - Birth defects

Back 49

scientists have already successfully reversed previously untreatable birth defects in model organisms such as mice.

Front 50

Uses of stem cells for medical treatment - Macular degeneration

Back 50

this condition is responsible for causing blindness in the elderly and diabetics; scientists are currently researching the use of stem cells in its treatment and early results are very encouraging.

Front 51

Uses of stem cells for medical treatment - Alzheimer’s disease

Back 51

brain cells are destroyed as a result of the build up of abnormal proteins; drugs currently only alleviate the symptoms.

Front 52

Uses of stem cells for medical treatment - Parkinson’s disease

Back 52

the symptoms (shaking and rigidity) are caused by the death of dopamine – producing cells in the brain: drugs currently only delay the progress of the disease.

Front 53

Uses of stem cells for medical treatment - Type 1 diabetes

Back 53

with insulin-dependent diabetes the body’s own immune system destroys the insulin-producing cells in the pancreas; patients have to inject insulin for life – this has been tried experimentally with some success already.

Front 54

Uses of stem cells for medical treatment - Heart disease

Back 54

muscle tissue in the heart is damaged as a result of a heart attack, normally irreparably – this has been tried experimentally with some success already

Front 55

Sources of plant stem cells

Back 55

Stem cells are present in meristematic tissue (meristems) in plants. This tissue is found wherever growth is occurring in plants, for example at the tips of roots and shoots (termed apical meristems). Meristematic tissue is also located sandwiched between the phloem and xylem tissues, and this is called the vascular cambium. Cells originating from this region differentiate into the different cells present in xylem and phloem tissues. In this way the vascular tissue grows as the plant grows. The pluripotent nature of stem cells in the meristems continues throughout the life of the plant.

Front 56

Sources of animal stem cells - Tissue (adult) stem cells

Back 56

these cells are present throughout life from birth. They are found in specific areas such as bone marrow. They are multipotent, although there is growing energy that they can be artificially triggered to become pluripotent. Stem cells can also be harvested from the umbilical cords of new-born babies. The advantages of this source are the plentiful supply of umbilical cords, and that invasive surgery is not needed. These stem cells can be stored in case they are ever needed by the individual in the future, and tissues cultured from such stem cells would not be rejected in a transplant to the umbilicus’ owner.

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Sources of animal stem cells - Embryonic stem cells

Back 57

these cells are present at a very early stage of embryo development and are totipotent. After about seven days a mass of cells, called a blastocyst, has formed and the cells are now in a pluripotent state. They remain in this state in the foetus until birth.

Front 58

Replacement of red and white blood cells

Back 58

Mammalian erythrocytes are essential for the transport of oxygen around the body. They are adapted to maximise their oxygen-carrying capacity by having only a few organelles so there is more room for haemoglobin. Due to the lack of nucleus and organelles they only have a short lifespan of around 120 days. They therefore need to be replaced constantly. The stem cell colonies in the bone marrow produce approximately three billion erythrocytes per kilogram of body mass per day to keep up with the demand. Neutrophils have an essential role in the immune system. They live for only about 6 hours and the colonies of stem cells in bone marrow produce in the region of 1.6 billion per kg per hour. This figure will increase during infection.

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Differentiation

Back 59

Erythrocytes (red blood cells) and neutrophils (white blood cells) are both present in blood. They look very different because they have different functions. When cells differentiate they become adapted to their specific role. What form this adaption takes is dependent on the function of the tissue, organ, and organ system to which the cell belongs. All blood cells are derived from stem cells in the bone marrow.

Front 60

Need for differentiation in multicellular organisms

Back 60

Multicellular organisms like animals and plants have evolved from unicellular (single-celled) organisms because groups of cells with different functions working together as one unit can make use of resourced more efficiently than single cells operating on their own. In multicellular organisms cells have to specialise to take on different roles in tissues and organs. They may be required to form barriers such as skin to be motile such as sperm cells. Cells have adapted to different roles in an organism and so have many shapes (and sizes) and often contain different organelles.

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Multipotent

Back 61

these stem cells can only form a range of cells within a certain type of tissue. Haematopoietic stem cells in bone marrow are multipotent because this gives rise to the various types of blood cell

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Pluripotent

Back 62

these stem cells can form all tissue types but not whole organisms. They are present in early embryos and are the origin of the different types of tissue within an organism.

Front 63

Totipotent

Back 63

these stem cells can differentiate into any type of cell. A fertilised egg, or zygote and the 8 or 16 cells from its first few mitotic divisions are totipotent cells, which are destined eventually to produce a whole organism. They can also differentiate into extra-embryonic tissues like the amnion and umbilicus.

Front 64

Stem cell potency

Back 64

A stem cell’s ability to differentiate into different cell types is called potency. The greater the number of cell types it can differentiate into, the greater its potency. Stem cells differ depending on the type of cell they can turn into totipotent, multipotent or pluripotent.

Front 65

Stem cells

Back 65

All cells in plants and animals begin as undifferentiated cells and originate from mitosis or meiosis. They are not adapted to any particular function (they are unspecialised) and they have the potential to differentiate to become any one of the range of specialised cell types in the organism. These undifferentiated cells are called stem cells. Stem cells are able to undergo cell division again and again and are the source of new cells necessary for growth, development, and tissue repair. Once stem cells have become specialised they lose the ability to divide, entering the G0 phase of the cell type. The activity of stem cells has to be strictly controlled. If they do not divide fast enough then tissues are not efficiently replaced, leading to ageing. However, if there is uncontrolled division then they form masses of cells called tumours, which can lead to the development of cancer.

Front 66

Differentiation

Back 66

Different cells in a multicellular organism are specialised for different functions. The process of a cell becoming specialised is called differentiation. Despite being differentiated in structure and function, all body cells within an organism have the same DNA (except those like erythrocytes and sieve tube elements which don’t have a nucleus). Differentiation involves the expression of some genes but no others in the cells genome.

Front 67

Animal examples include of organ systems

Back 67

The digestive system, which takes in food, breaks down the large insoluble molecules into small soluble ones, absorbs the nutrients into the blood, retains water needed by the body and removes any undigested material from the body.

The cardiovascular system, which moves blood around the body to provide an effective transport system for the substances it carries

The gaseous exchange system, which brings air into the body so oxygen can be extracted for respiration, and carbon dioxide can be expelled.

Front 68

Organ systems

Back 68

Large multicellular organisms have coordinated organ systems. Each organ system is composed of a number of organs working together to carry out a major function in the body.

Front 69

Organs

Back 69

An organ is a collection of tissues that are adapted to perform a particular function in an organism. For example, the mammalian heart is an organ that is adapted for pumping blood around the body. It is made up of muscle tissue and connective tissue. The leaf is a plant organ that is adapted for photosynthesis. It contains epidermis tissues and vascular tissues.

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Specialised plant tissues - Phloem tissue

Back 70

another type of vascular tissue in plants, responsible for the transport of organic nutrients, particularly sucrose, from leaves and stems where it is made by photosynthesis to all parts of the plant where it is needed. It is composed of columns of sieve tube cells separated by perforated walls called sieve plates.

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Specialised plant tissues - Xylem tissue

Back 71

a type of vascular tissue responsible for transport of water and minerals throughout plants. The tissue is composed of vessel elements, which are elongated dead cells. The walls of these cells are strengthened with a waterproof material called lignin, which provides structural support for plants.

Front 72

Specialised plant tissues - The epidermis

Back 72

a single layer of closely packed cells covering the surfaces of plants. It is usually covered by a waxy, waterproof cuticle to reduce the loss of water. Stomata, formed by a pair of guard cells that can open and close are present in the epidermis, they allow carbon dioxide in and out, and water vapour and oxygen in and out.

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Tissues in plants

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There are a number of different tissues in plants, including: Epidermis tissue, adapted to cover plant surfaces Vascular tissue, adapted for transport of water and nutrients.

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Specialised animal tissues - Muscle

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a tissue that needs to be able to shorten in length (contract) in order to move bones, which in turn move the different parts of the body. There are different types of muscle fibres. Skeletal muscle fibres (muscles which are attached to bone) contain myofibrils (dark pink bands on the micrograph) which contain contractile proteins. The skeletal muscle micrograph shown here has several individual muscle fibres (pink) separated by connective tissue (thin white strips)

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Specialised animal tissues - Cartilage

Back 75

a connective tissue found in the outer ear, nose and at the ends (and between) bones. It contains fibres of the proteins elastin and collagen. Cartilage in a firm, flexible connective tissue composed of chondrocyte cells embedded in an extracellular matrix. Cartilage, among other things, prevents the ends o bones from rubbing together and causing damage. Many fish have whole skeletons made of cartilage, not bone.

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Specialised animal tissues - Ciliated epithelium

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made up of ciliated epithelial cells. The cells have their ‘hair-like structures called cilia on one surface that move in a rhythmic manner. Ciliated epithelium lines the trachea, for example, causing mucus to be swept away from the lungs. Goblet cells are also present, releasing mucus to trap any unwanted particles present in the air. This prevents the particles, which may be bacteria, from reaching the alveoli once inside the lungs.

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Specialised animal tissues - Squamous epithelium

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made up of specialised squamous epithelial cells, is sometimes known as pavement epithelium due to its flat appearance. It is very thin due to the squat or flat cells that make it up and also because it is only one cell thick. It is present when rapid diffusion across a surface is essential. It forms the lining of the lungs and allows rapid diffusion of oxygen into the blood.

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There are four main categories of tissues in animals

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Nervous tissue, adapted to support the transmission of electrical impulses. Epithelial tissue, adapted to cover body surfaces, internal and external Muscle tissue, adapted to contract Connective tissue adapted either to hold other tissues together or as a transport medium.

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Tissue

Back 79

A tissue is made up of a collection of differentiated cells that have a specialised function or functions. As a result, each tissue is adapted for a particular function within the organism.

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Specialised plant cells - Guard cells

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pairs of guard cells on the surfaces of leaves form small openings called stomata. These are necessary for carbon dioxide to enter plants for photosynthesis. When guard cells lose water and become less swollen as a result of osmotic forces, they change shape, and the stoma closes to prevent further water loss from the plant. The cell wall of a guard cell is thicker on one side so the cell does not change shape symmetrically as its volume changes.

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Specialised plant cells - Root hair cells

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present at the surfaces of roots near the growing tips, have long extensions called root hairs, which increase the surface area of the cell. This maximised the uptake of water and minerals from the soil.

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Specialised plant cells - Palisade cells

Back 82

present in the mesophyll contain chloroplasts to absorb large amounts of light for photosynthesis. The cells are rectangular box shapes, which can be closely packed to form a continuous layer. They have thin cell walls, increasing the rate of diffusion of carbon dioxide. They have a large vacuole to maintain turgor pressure. Chloroplasts can move within the cytoplasm in order to absorb more light.

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Sperm cells

Back 83

male gametes. Their function is to deliver genetic information to the female gamete, the ovum (or egg). Sperm have a tail or flagellum, so they are capable of movement and contain many mitochondria to supply the energy needed to swim. The acrosome on the head of the sperm contains digestive enzymes, which are released to digest the protective layers around the ovum and allow the sperm to penetrate, leading to fertilisation.

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Neutrophils

Back 84

a type of white blood cell. Play an essential role in the immune system. The have a characteristic multi-lobed nucleus, which makes it easier for them to squeeze through small gaps to get to the site of infections. The granular cytoplasm contains many lysosomes that contain enzymes used to attack pathogens.

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Erythrocytes

Back 85

or red blood cells have a flattened biconcave shape, which increases their surface area to volume ratio. This is essential to their role of transporting oxygen around the body. In mammals these cells do not have nuclei or many other organelles, which increases the space available for haemoglobin, the molecule that carries oxygen. They are also flexible so that they are able to squeeze through narrow capillaries.

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Specialised cells

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The cells within a multicellular organism are differentiated, meaning they are specialised to carry out very specific functions.

Front 87

Levels of organisation in multicellular organisms

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Specialised cells --> tissues à organs à organ systems à whole organism

Front 88

Meiosis cytokinesis

Back 88

Cytokinesis results in division of the cells forming four daughter cells in total. The cells will be haploid due to the reduction division. They will also be genetically different from each other, and from the parent cell, due to the processes of crossing over and independent assortment.

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Meiosis 2 - Telophase 2

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the chromatids assemble at the poles at telophase 2 as in telophase of mitosis. The chromosomes uncoil and form chromatin again. The nuclear envelope reforms and the nucleolus becomes visible.

Front 90

Meiosis 2 - Anaphase 2

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unlike anaphase 1, anaphase 2 results in the chromatids of the individual chromosomes being pulled to opposite poles after division of the centromeres – the same as in anaphase of mitosis.

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Meiosis 2 - Metaphase 2

Back 91

metaphase 2 differs from metaphase 1, as the individual chromosomes assemble on the metaphase plate, as in metaphase in mitosis. Due to crossing over, the chromatids are no longer identical so there is independent assortment again and more genetic variation produced in metaphase 2.

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Meiosis 2 - Prophase 2

Back 92

in prophase 2 the chromosomes, which still consist of two chromatids, condense, and become visible again. The nuclear envelope breaks down and spindle formation begins.

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Meiosis 1 - Telophase 1

Back 93

telophase 1 is essentially the same as telophase in mitosis. The chromosomes assemble at each pole and the nuclear membrane reforms. Chromosomes uncoil. The cell undergoes cytokinesis and divides into two cells. The reduction of chromosome number from diploid to haploid is complete.

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Meiosis 1 - Anaphase 1

Back 94

anaphase 1 is different from anaphase of mitosis as the homologous chromosomes are pulled to the opposite poles and the chromatids stay joined to each other. sections of DNA on ‘sister’ chromatids, which became entangled during crossing over, now break off and re-join – sometimes resulting in an exchange of DNA. The points at which the chromatids break and re-join are called chiasmata. When exchange occurs this forms recombinant chromatids, with genes being exchanged between chromatids. The genes being exchanged may be different alleles of the same gene, meaning the combination of alleles on the recombinant chromatids will be different from the allele combination on either of the original chromatids. Genetic variation arises from this new combinations of alleles – the sister chromatids are no longer identical.

Front 95

Meiosis 1 - Metaphase 1

Back 95

metaphase 1 is the same as metaphase in mitosis except that the homologous pairs of chromosomes assemble along the metaphase plate instead of the individual chromosomes. The orientation of each homologous pair on the metaphase plate is random and independent of any other homologous pair. The maternal or paternal chromosomes can end up facing either pole. This is called independent assortment and can result in many different combinations of alleles facing the poles. Independent assortment of chromosomes in metaphase 1 results in genetic variation.

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Meiosis 1 - Prophase 1

Back 96

during prophase 1, chromosomes condense, the nuclear envelope disintegrates, the nucleolus disappears, and spindle formation begins, as in prophase of mitosis. The difference in prophase 1 is that the homologous chromosomes pair up, forming bivalents. Chromosomes are large molecules of DNA and moving them through the liquid cytoplasm as they are brought together results in the chromatids entangling. This is called crossing over.

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Meiosis 2

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the second division is similar to mitosis, and the pairs of chromatids present in each daughter cell are separated, forming two more cells. Four haploid daughter cells are produced in total.

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Meiosis 1

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the first division is the reduction division when the pairs of homologous chromosomes are separated into two cells. Each intermediate cell will only contain one full set of genes instead of two, so the cells are haploid.

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Alleles

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Genes for a particular characteristic may vary, leading to differences in the characteristic, for example blue eyes and brown eyes. The genes are still the same type as they both code for eye colour, but the colour is different, meaning they are different versions of the same gene. Different versions of the same gene are called alleles (also known as gene variants). The different alleles of a gene will all have the same locus (position on a particular chromosome).

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Homologous chromosomes

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Each characteristic of an organism is coded for by two copies of each gene, one from each parent. Each nucleus of the organism’s cells contains two full sets of genes, a pair of genes for each characteristic. Therefore, each nucleus contains matching sets of chromosomes, called homologous chromosomes, and is termed diploid. Each chromosome in a homologous pair has the same genes at the same loci. As homologous chromosomes have the same genes in the same positions, they will be the same length and size when they are visible in prophase. The centromeres will also be in the same positions.

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Chromosomes in meiosis

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In sexual reproduction two sex cells (gametes), one from each parent, fuse to produce a fertilised egg. the fertilised egg (zygote) is the origin of all cells that the organism develops. gametes must therefore only contain half of the standard (diploid) number of chromosomes in a cell, or the chromosome number of an organism would double with every round of reproduction. Gametes are formed by another form of cell division known as meiosis. Unlike in mitosis, the nucleus divides twice to produce four daughter cells – like gametes. Each gamete contains half of the chromosome number of the parent cell – it is haploid. Meiosis is therefore known as reduction division.

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Chromosomes in mitosis

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Normal cells have two chromosomes of each type (termed diploid) – one inherited from each parent. During mitosis, the nucleus divides once following DNA replication. This results in two genetically identical diploid daughter cells.

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Cytokinesis in plant cells

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Plant cells have cell walls, so it is not possible for a cleavage furrow to be formed. Vesicles from the Golgi apparatus begin to assemble in the same place as where the metaphase plate was formed. The vesicles fuse with each other and the cell surface membrane, dividing the cell into two. New sections of cell wall then form along the new sections pf membrane (if the dividing cell wall were formed before the daughter cells separated they would immediately undergo osmotic lysis from the surrounding water).

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Cytokinesis in animal cells

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In animal cells a cleavage furrow forms around the middle of the cell. The cell-surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle, forming two cells.

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Cytokinesis

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The actual division of the cell into two separate cells, begins during telophase.

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Telophase

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In telophase the chromatids have reached the poles and are now called chromosomes. The two new sets of chromosomes assemble at each pole and the nuclear envelope reforms around them. The chromosomes start to uncoil, and the nucleolus is formed. Cell division – or cytokinesis begins.

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Anaphase

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The centromeres holding together the pairs of chromatids in each chromosome divide during anaphase. The chromatids are separated – pulled to opposite poles of the cell by the shortening spindle fibres. The characteristic ‘V’ shape of the chromatids moving towards the poles is a result of them being dragged by their centromeres through the liquid cytosol.

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Metaphase

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During metaphase, the chromosomes are moved by the spindle fibres to form a plane in the centre of the cell, called the metaphase plate, and then held in position.

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Stages of prophase

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During prophase, chromatin fibres (complex made up of various proteins, RNA and DNA) begin to coil and condense to form chromosomes that will take up stain to become visible under the light microscope. The nucleolus, a distinct are of the nucleus responsible for RNA synthesis, disappears. The nuclear membrane begins to break down.

Protein microtubules form spindle-shaped structures linking the poles of the cell. The fibres forming the spindle are necessary to move the chromosomes into the correct positions before division.

In animal cells and some plant cells, two centrioles migrate to opposite poles of the cell. The centrioles are cylindrical bundles of protein that help in the formation of the spindle.

The spindle fibres attach to specific area on the centromeres and start to move the chromosomes to the centre of the cell.

By the end of prophase, the nuclear envelope has disappeared.

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The stages of mitosis - There are four stages of mitosis

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prophase, metaphase, anaphase, and telophase. We describe them separately but in fact they flow seamlessly from one to another. Each of these phases can be viewed and identified using a light microscope. Dividing cells can be easily obtained from growing toot tips of plants. The root tips can be treated with a chemical to allow the cells to be separated – then they can be squashed to form a single layer of cells on a microscope slide. Stains that bind DNA are used to make the chromosomes clearly visible.

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Chromosomes

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Before mitosis can occur, all of the DNA in the nucleus is replicated during interphase. Each DNA molecule (chromosome) is converted into two identical DNA molecules, called chromatids. The two chromatids are joined together at a region called the centromere. It is necessary to keep the chromatids together during mitosis so that they can be precisely manoeuvred and segregated equally, one each into the two new daughter cells. During interphase DNA combines with proteins called histones to form a dense complex called chromatin

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The importance of mitosis

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Mitosis is necessary when all the daughter cells have to be identical. This is the case during growth, replacement, and repair of tissues in multicellular organisms such as animals, plants, and fungi. Mitosis is also necessary for asexual reproduction, which is the production of genetically identical offspring from one parent in multicellular organisms including plants, fungi, and some animals, and also in eukaryotic single-celled organisms such as Amoeba species. Prokaryotic organisms, including bacteria, do not have a nucleus and they reproduce asexually by a different process known as binary fission.

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What is mitosis

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Mitosis is the term usually used to describe the entire process of cell division in eukaryotic cells. It actually refers to nuclear division (division of the nucleus), an essential stage in cell division. Mitosis ensures that both daughter cells produced when a parent cell divides are genetically identical (except in the rare events where mutations occur). Each new cell will have an exact copy of the DNA present in the parent cell and the same number of chromosomes.

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Cell-cycle regulation

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If overexpression of a cyclin gene results from mutation, the abnormally large quantity of cyclins produced would disrupt the regulation of the cell cycle, resulting in uncontrolled cell division, tumour formation, and possible leading to cancer. Cyclin-dependant kinases can be used as a possible target for chemical inhibitors in the treatment of cancer. If the activity of CDKs can be reduced it may reduce or stop cell division and therefore cancer formation.

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Cancer

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Cancer is a group of many different diseases caused by the uncontrolled division of cells. An abnormal mass of cells is called a tumour. Tumours can be benign, meaning that they stop growing and do not travel to other locations in the body. If a tumour continues to grow unchecked and uncontrolled, it is termed malignant. A malignant tumour is the basis of cancer. Tumours are often the results of damage or spontaneous mutation of the genes that encode the proteins that are involved in regulating the cell cycle, including the checkpoint proteins.

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Checkpoints occur at various stages of the cell cycle

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Spindle assembly checkpoint (also called metaphase checkpoint) - this checkpoint is at the point in mitosis where all the chromosomes should be attached to spindles and have aligned. Mitosis cannot proceed until this checkpoint is passed.

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Checkpoints occur at various stages of the cell cycle - G2 checkpoint

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this checkpoint is at the end of G2 phase, before the start of the mitotic phase. In order for this checkpoint to be passed, the cell has to check a number of factors, including whether the DNA has been replicated without error. If this checkpoint is passed, the cell initiates the molecular processes that signal the beginning of mitosis.

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Checkpoints occur at various stages of the cell cycle - G1 checkpoint

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this checkpoint is at the end of the G1 phase before entry into S phase. If the cell satisfies the requirements of this checkpoint it is triggered to begin DNA replication. If not, it enters a resting state (G0).

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Control of the cell cycle

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It is vital to ensure a cell only divides when it has grown to the right size, the replicated DNA is error-free (or is repaired) and the chromosomes are in their correct positions during mitosis. This is to ensure the fidelity of cell division – that two identical daughter cells are created from the parent cell. Checkpoints are the control mechanisms of the cell cycle. They monitor and verify whether the process at each phase of the cell cycle have been accurately completed before the cell is allowed to progress into the next phase.

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The two main phases of the cell cycle

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In eukaryotic cells the cell cycle has two main phases – interphase and mitotic (division) phase.

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Interphase

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Cells do not divide continuously – long periods of growth and normal working separate divisions. These periods are called interphase and a cell spends the majority of its time in this phase. Interphase is sometimes referred to as the resting phase as cells are not actively dividing. However, this is not an accurate description – interphase is actually a very active phase of the cell cycle, when the cell is carrying out all of its major functions such as producing enzymes or hormones, while also actively preparing for cell division.

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What happens during interphase

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DNA is replicated and checked for errors in the nucleus Protein synthesis occurs in the cytoplasm Mitochondria grow and divide, increasing in number in the cytoplasm Chloroplasts grow and divide in plant and algal cell cytoplasm, increasing in number The normal metabolic processes of cells occur (some, including cell respiration, also occur throughout cell division).

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The three stages of interphase: G1

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the first growth phase: proteins from which organelles are synthesised are produced and organelles replicate. The cell increases in size. S – synthesis phase: DNA is replicated in the nucleus. G2 – the second growth phase: the cell continues to increase in size, energy stores are increased, and the duplicated DNA is checked for errors.

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Mitotic phase

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The mitotic phase is the period of cell division. Cell division involves two stages: Mitosis – the nucleus divides Cytokinesis – the cytoplasm divides and two cells are produced

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Differentiation and reasons for G0

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a cell that becomes specialised to carry out a particular function (differentiated) is no longer able to divide. It will carry out this function indefinitely and not enter the cell cycle again.

The DNA of a cell may be damaged, in which case it is no longer viable. A damaged cell can no longer divide and enters a period of permanent cell arrest (G0). The majority of normal cells only divide a limited number of times and eventually become senescent.

As you age, the number of these cells in your body increases. Growing numbers of senescent cells have been linked with many age-related diseases, such as cancer and arthritis.

A few types of cells that enter G0 can be stimulated to go back into the cell cycle and start dividing again, for example lymphocytes (white blood cells) in an immune response.

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G01

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G0 is the name given to the phase when the cell leaves the cycle, either temporarily or permanently.