Cellular Basis Of Reproduction And Inheritance

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Life Cycle

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"Like Begets Like"
Maple trees make other maple trees, people make other people, bears make other bears....

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Cell Division

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--The 2 "Daughter" cells that result are genetically identical to each other and to the original "parent" cell
--Before the "parent" cell splits into 2 it duplicates its chromosomes and the sets are distributed to the daughter cells

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Chromosomes

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The structures that contain most of the cell's DNA
--Made of Chromatin in Eukaryotic Chromosomes
--Eukaryotic Chromosomes are much more complex and they have many more protein molecules-- which helps maintain the structure and control the activity of its genes
Humans-- 46 Chromosomes
Dogs-- 78 Chromosomes
--Early in the division process, chromosomes duplicate
--Each chromosome appears as two sister chromatids, containing identical DNA molecules
Sister chromatids are joined at the centromere, a narrow region

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Asexual Reproduction

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--Offspring are identical to the original cell or organism
--Involves inheritance of all genes from one parent
--Singal Parent--No participation from a sperm and an egg
--Examples: Single Celled Prokaryotes, Eukaryotic Yeast Cells, Sea Stars, Some Plants
--Mitosis

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Sexual Reproduction

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--Offspring are similar to parents, but show variations in traits
--Involves inheritance of unique sets of genes from two parents
--Fertilization of an Egg by a Sperm
--Meiosis

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Rudolf Virchow’s Principle

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Every Cell from a Cell

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Roles of Cell Division

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--Asexual reproduction
-Reproduction of an entire single-celled organism
-Growth of a multicellular organism
-Growth from a fertilized egg or zygote into an adult
-Repair and replacement of cells in an adult
--Sexual reproduction
-Sperm and egg production

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Binary Fission

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--Means “dividing in half”
--Occurs in prokaryotic cells (Bacteria and Archaea)
--Two identical cells arise from one cell
--Steps in the process
-A single circular chromosome duplicates, and the copies begin to separate from each other moving to opposite sides of the cell
-The cell elongates, and the chromosomal copies separate further
-When chromosome duplication is complete the cell has reached about 2x its size
-The plasma membrane grows inward and more cell wall is made-- dividing the parent cell into 2 daughter cells

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Chromatin

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--Eukaryotic chromosomes are composed of chromatin
--Chromatin = DNA + proteins
--To prepare for division, the chromatin becomes highly compact, coils up, and the chromosomes are visible with a microscope
--Chromatin is too thin to be seen through a light microscope
--DNA in its "loose state" composed of roughly equal amounts of DNA and protein molecules

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Sister Chromatids

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--Each chromosome right before dividing consists of 2 copies which contain identical copies of the DNA molecule
--Joined together REALLY tightly at a narrow waist called the Centromere
--Once seperated from each other each is called a Chromosome each goes to one of the daughter cells

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Centromere

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The narrow "waist" where the 2 Sister Chromatids are joined together tightly

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Cell Cycle

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--An ordered sequence of events for cell division
--Extends from the time a cell is first formed fromed a dividing parent cell until its own division into 2 cells
--It consists of two stages: Interphase and Mitotic Phase
--Interphase: duplication of cell contents
-G1—growth, increase in cytoplasm
-S—duplication of chromosomes
-G2—growth, preparation for division
--Mitotic phase: division
-Mitosis—division of the nucleus
-Cytokinesis—division of cytoplasm

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Interphase

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--Most of the cell cycle is spent in this phase-- Usually 90% of total time
--Cell's metabolic activity is very high and the cell performs its various functions within the organism
--Cell makes more cytoplasm, Increases its supply of proteins, Creates more cytoplasmic organelles (Mitochondria and Ribosomes), and Grows in size
--Chromosomes duplicate during this period--S Phase

--Three Subphases--During these stages the cell grows
-G1--First Gap—growth, increase in cytoplasm
-S—duplication of chromosomes--Synthesis of DNA
-G2--Second Gap—growth, preparation for division

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S Phase

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--Synthesis of DNA
--Chromosomes are duplicated only during this phase
-At the beginning of this phase each chromosome is single
-At the end of this phase - after DNA replication, the chromosomes are double - each consisting of 2 Sister Chromatids

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Mitotic Phase

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--The part of the cell cycle when the cell actually divides
--Accounts for 10% of the total time required for the cell cycle
--Divided into 2 overlapping Stages
-Mitosis—division of the nucleus
-Cytokinesis—division of cytoplasm

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Mitosis

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--The nucleus and its contents-- most importantly the chromosomes divide and are evenly distributed- forming 2 daughter nuclei
-Unique to eukaryotes
-Remarkably Accurate Mechanism -- only an error once in about 100,000 cell divisions
--Continuum but biologist distinguish 5 main stages
--Prophase
--Prometaphase
--Metaphase
--Anaphase
--Telophase

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Cytokinesis

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--Usually begins before Mitosis ends
--Cytoplasm is divided into seperate cells
In animal cells -- occurs by the process called Cleavage -- a Cleavage Furrow developes and a contracting ring of microfilaments (when the actin interact with the myosin the ring contracts) --the contraction is much like pulling the string on a hooded sweatshirt -- pulling the drawstring contracts the hood inward eventually pinching it shut-- similarly the cleavage furrow deepens and eventually pinches the parent cell into 2 completely seperate daughter cells each with its own nucleus and cytoplasm
In Plant Cells--Membranous vesicles containing cell wall material collect at the middle of the parent cell -- the vesicles fuse forming a membranous disk called the Cell Plate -- the Cell Plate grows outward accumulating more cell wall materials as more vesicles fuse with it -- Eventually the membrane of the Cell Plate fuses with the plasma membrane and the cell plate's contents join the parental cell wall -- results in 2 daughter cells each bounded by its own plasma membrane and cell wall

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Mitotic Spindle

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A football shaped structure of microtubules that guides the seperation of the 2 sets of daughter chromosomes
--Emerge from 2 Centrosomes

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Centrosomes

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--Clouds of cytoplasmic material that in animal cells contain pairs of centrioles
--AKA Microtubule Organizing Centers
--The role of centrioles in cell division is unclear

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Interphase

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--In the cytoplasm
-Cytoplasmic contents double
-Two centrosomes form
--In the nucleus
-Chromosomes duplicate during the S phase
-Nucleoli, sites of ribosome assembly, are visible
--The period of cell growth when the cell synthesizes new molecules and organelles

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Prophase

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--In the cytoplasm
-Microtubules begin to emerge from centrosomes, forming the spindle
--In the nucleus
-Chromosomes coil and become compact
-Each duplicated chromosome appears as 2 identical sister chromatids joined together at the centromere
-Nucleoli disappear

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Prometaphase

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--Spindle microtubules reach chromosomes, where they
-Attach at kinetochores on the centromeres of sister chromatids
-Move chromosomes to the center of the cell through associated protein “motors”
--Other microtubules meet those from the opposite poles
--The nuclear envelope disappears

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Metaphase

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--Spindle is fully formed
--Chromosomes align at the cell equator -- the Metaphase Plate
--Kinetochores of sister chromatids are facing the opposite poles of the spindle

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Anaphase

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--Sister chromatids separate at the centromeres
--Daughter chromosomes are moved to opposite poles of the cell
--Motor proteins move the chromosomes along the spindle microtubules
--Kinetochore microtubules shorten

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Telophase

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--The cell continues to elongate
--The nuclear envelope forms around chromosomes at each pole, establishing daughter nuclei
--Chromatin uncoils
--Nucleoli reappear
--The spindle disappears

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By the end of Metaphase
1)How many chromosomes are present in one human cell?
2)How many chromatids are present in one human cell?

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1) 46 Chromosomes
2) 92 Chromatids

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By the end of Interphase
1) How many chromosomes are present in one cell?
2) How many chromatids are present in one cell?

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1) 46 Chromosomes
2) 92 Chromatids

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By the end of Anaphase
1) How many chromosomes are present in one human cell?
2) How many chromatids are present in one human cell?

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1) 92 Chromosomes
2) 0 Chromatids

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By the end of Telophase
1) How many chromosomes are present in one nucleus within the human cell?
2) Are the nuclei identical or different?

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1) 46 Chromosomes
2) Identical

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Factors that Control Cell Division

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--Presence of essential nutrients
--Growth factors, proteins that stimulate division
--Presence of other cells causes density-dependent inhibition
--Contact with a solid surface; most cells show anchorage dependence

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Growth Factor

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A protein secreted by certain body cells that stimulates other cells to divide
--Atleast 50 different types that can trigger cell division
--Different cell types respond specifically to different certain GFs or a combination of GFs

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Density Dependent Inhibition

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A phenomenon in which crowded cells stop dividing until vacant space opens up
--Cancer Cells do not have this phenomenon... they keep dividing and just pile up

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Anchorage Dependence

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Cells must be in contact with a solid surface (like a culture dish or extracellular matrix) in order to divide

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Cell Cycle Control System

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A set of molecules, including growth factors, that triggers and coordinates events of the cell cycle

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Checkpoint

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--Control points where signals regulate the cell cycle
--G1 checkpoint allows entry into the S phase or causes the cell to leave the cycle, entering a nondividing G0 phase
--G2 checkpoint
--M checkpoint

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Effects of a Growth Factor at the G1 Checkpoint

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--A growth factor binds to a receptor in the plasma membrane
--Within the cell, a signal transduction pathway propagates the signal through a series of relay molecules
--The signal reaches the cell cycle control system to trigger entry into the S phase

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Tumor

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An abnormally growing mass of body cells

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Benign Tumor

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--Remain at the original site
--Can cause problems if they grow in and disrupt certain organs like the brain but often they can be removed by surgery

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Malignant Tumor

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--Can spread to other locations by metastasis
--Cancerous Tumors

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Metastasis

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The spread of cancer cells beyond their original site

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Carcinomas

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Arise in external or internal body coverings such as the skin or the lining of the intestines

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Sarcomas

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Arise in supportive and connective tissue such as bone or muscle

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Leukemias and Lymphomas

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Arise from blood-forming tissues such as bone marrow, spleen, and lymph nodes

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Cancer Cells

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"Immortal"--They can go on dividing indefinitely as long as they have a supply of nutrients
--They do no heed the normal signals that regulate the cell cycle-- they proceed past checkpoints even in the absence of growth factors

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Somatic Cells

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A typical body cell has 46 chromosomes

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Homologous Chromosomes

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2 Chromosomes which match and they both carry genes controlling the same inherited characteristics
Matched in:
--Length
--Centromere position
--Gene locations
-A locus (plural, loci) is the position of a gene
-Different versions of a gene may be found at the same locus on maternal and paternal chromosomes

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Locus

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The position of a gene

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Sex Chromosomes

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X and Y Chromosomes
Male - An X and a Y
Female - 2 X Chromosomes

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Autosomes

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The remaining 22 pairs of chromosomes which are the same size and have the same genetic composition

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Diploid Cell

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Have two homologous sets of chromosomes
--2n-- Diploid Number
Humans-2n=46

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Gametes

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The egg and the sperm cells
--Each has a single set of chromosomes-- 22 Autosomes and a Sex Chromosome either X or Y

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Haploid Cell

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Have one set of chromosomes
Haploid Number = n
Humans-- n=23

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Zygote

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Fertilized Egg
--Has 2 sets of homologous chromosomes so its Diploid

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Meiosis

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A type of cell division that produces haploid gametes in diploid organisms
--Were it not for meiosis then each generation would have twice as much genetic information as the generation before

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Meiosis and Mitosis
Similarities and Differences

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--Like mitosis, meiosis is preceded by interphase
-Chromosomes duplicate during the S phase
--Unlike mitosis, meiosis has two divisions
-During meiosis I, homologous chromosomes separate
-The chromosome number is reduced by half
-During meiosis II, sister chromatids separate
-The chromosome number remains the same

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Prophase I

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--Chromosomes coil and become compact
--Homologous chromosomes come together as pairs by Synapsis
--Each pair, with four chromatids, is called a tetrad
--Nonsister chromatids exchange genetic material by crossing over

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Metaphase I

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--Tetrads align at the cell equator (the metaphase plate)

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Anaphase I

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--Homologous pairs separate and move toward opposite poles of the cell
--Unlike Mitosis --The sister chromatids remain attached at their centromeres

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Telophase I
and
Cytokinesis

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--Duplicated chromosomes have reached the poles
--A nuclear envelope forms around chromosomes in some species
--Each nucleus has the haploid number of chromosomes
C-- 2 Haploid daughter cells are formed

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Prophase II

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--Chromosomes coil and become compact
--A spindle forms and moves the chromosomes toward the middle of the cell

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Metaphase II

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--Duplicated chromosomes align at the cell equator (the metaphase plate) with the kinetochores of sister chromatids of each chromosome pointing toward opposite poles

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Anaphase II

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--Sister chromatids separate and chromosomes move toward opposite poles

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Telophase II

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--Chromosomes have reached the poles of the cell
--A nuclear envelope forms around each set of chromosomes
--With cytokinesis, four haploid cells are produced

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Similar Characteristics of Mitosis and Meiosis

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--One duplication of chromosomes

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Characteristics Unique to Meiosis

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--Two divisions of chromosomes
--Pairing of homologous chromosomes
--Exchange of genetic material by crossing over

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Meiosis I

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Homologues Seperate

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Meiosis II

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--Starts with a Haploid Cell
--Sister Chromatids Seperate

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Tetrads

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--Sets of four chromatids
-With each pair of sister chromatids joined at the centromeres

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Mitosis Conserves Chromosome Number while Meiosis Reduces the Number from Diploid to Haploid

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--In Mitosis, the duplication of chromosomes is followed by one division of the cell
--In Meiosis, homologous chromosomes seperate in the first of 2 cell divisions- after the second division- each new cell ends up with just a single haploid set

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Meiosis Outcome

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-- Four genetically different cells, with half the chromosome number of the original cell

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Mitosis Outcome

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--Two genetically identical cells, with the same chromosome number as the original cell

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Independent Orientation of Chromosomes in Meiosis and Random Fertilization lead to Varied Offspring

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--Independent orientation at metaphase I
-Each pair of chromosomes independently aligns at the cell equator
-There is an equal probability of the maternal or paternal chromosome facing a given pole
-The number of combinations for chromosomes packaged into gametes is 2n where n = haploid number of chromosomes
--Random fertilization
-The combination of each unique sperm with each unique egg increases genetic variability
-64 Trillion Different Possibilities

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Crossing Over

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An exchange of corresponding segments between nonsister chromatids of homologous chromosomes
--Occurs very early in Prophase I of Meiosis
--

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Chiasma

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--The sites of crossing over
--Appear as X Shaped Regions
--Place where 2 homologous chromatids are attached to each other

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Genetic Recombination

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The production of gene combinations different from those carried by the original parental chromosomes

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Recombinant

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They result from Genetic Recombination

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Sources of Variability

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--Mutations
--Crossing Over
--Independent Orientation
--Random Fertilization

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Maternal

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Came from Mother

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Paternal

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Came from Father

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Karyotype

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--Shows stained and magnified versions of chromosomes
--Karyotypes are produced from dividing white blood cells, stopped at metaphase
--Karyotypes allow observation of:
-Homologous chromosome pairs
-Chromosome number
-Chromosome structure

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Trisomy 21

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The basis of Down Syndrome
--There are 3 #21 chromosomes making 47 chromosomes in total
--It can be detected by Karyotypes

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Nondisjunction

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--The failure of chromosomes or chromatids to separate during meiosis
--During Meiosis I
-Both members of a homologous pair go to one pole
--During Meiosis II
-Both sister chromatids go to one pole

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Kleinfelter Syndrome

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XXY Men with this disorder have male sex organs and normal intelligence, but the testes are abnormally small and they are sterile
He often has enlarged breasts and other female body characteristics
XXYY, XXXY, XXXXXY if more than XXY they are more likely to have developmental disabilities

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Turner Syndrome

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Women who lack an X chromosome are marked XO
Normally have a short stature and a web of skin extending between the neck and the shoulders
Sex organs do not fully mature so they are sterile
If left untreated girls with this disorder will have poor development of breasts
Normal Intelligence
Only 45 chromosomes-- only known case with less chromosomes

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Polyploid Organism

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One with more than 2 sets of homologous chromosomes in its body cells

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Deletion

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If a fragment of a chromosome is lost

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Duplication

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If a fragment from one chromosome joins to a sister chromatid or homologous chromosome

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Inversion

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If a fragment reattaches to the original chromosome but in reverse orientation

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Cri Du Chat

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A child born with this syndrome is mentally retarded, has a small head with unusual facial features and has a cry that sounds like the meowing of a distressed cat
They usually die in infancy or early childhood

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Translocation

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The attachment of a chromosomal fragment to a nonhomologous chromosome
May or may not be harmful!