Chapter 12/13 The Cell Cycle And Cell Division

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The life of an Organism is linked to

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cell division

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Four events must occur for cell division

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Reproductive signal, replication, segregation and cytokinesis

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Reproductive signal

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To initiate cell division

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Replication

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Of DNA

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Segregation

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Distribution of the DNA into the two new cells

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Cytokinesis

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Separation of the two new cells

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In prokaryotes, _________ results in two new cells.

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binary fission

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External environmental factors initiate

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cell division

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Most prokaryotes have one _________, a single molecule of DNA—usually circular.

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chromosome

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Two important regions in reproduction

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ori—where replication starts
ter—where replication ends

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Replication begins near the

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center of the cell.

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In eukaryotes, signals for cell division are related to the

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needs of the entire organism.

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Eukaryotes usually have many chromosomes

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the processes is more complex

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DNA replication usually occurs between cell divisions.

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sister chromatids and mitosis

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Sister chromatids

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newly replicated chromosomes are closely associated

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Mitosis

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separates them into two new nuclei, identical to the parent cell

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Meiosis

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is nuclear division in cells involved in sexual reproduction.

The cells resulting from meiosis are not identical to the parent cells.

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The cell cycle

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The period between cell divisions, divided into mitosis/cytokinesis and interphase

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Interphase

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The cell nucleus is visible and cell functions including replication occur.

begins after cytokinesis and ends when mitosis starts.

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Interphase has three subphases

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G1, S, and G2

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G1: Gap 1

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between end of cytokinesis and onset of S phase; chromosomes are single, unreplicated structures

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Restriction point

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at the G1-to-S transition a commitment is made to DNA replication and cell division

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S phase

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DNA replicates; one chromosome becomes two sister chromatids

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G2

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Gap 2—end of S phase, cell prepares for mitosis

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M phase

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Mitosis and cytokinesis occur during this phase

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Growth factors

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External chemical signals that stimulate these cells to divide

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Platelet-derived growth factor

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From platelets that initiate blood clotting, stimulates skin cells to divide and heal wounds

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Some white blood cells produce________ that promote cell division in other white cells

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interleukins

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Erythropoietin

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produced in the kidneys stimulates division of bone marrow cells and production of red blood cells

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After DNA replicates, its segregation occurs during

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mitosis

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The DNA molecule is complexed with proteins to form dense

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chromatin

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The sister chromatids are held together by cohesin, which is removed during mitosis except at the

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centromere

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DNA molecules are extensively “packed” even during

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interphase

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Packing is achieved by ______________—proteins with positive charges that attract negative phosphate groups of DNA.

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histones

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Interactions result in the formation of beadlike units, or

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nucleosomes

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Mitosis can be divided into 5 phases:

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Prophase
Prometaphase
Metaphase
Anaphase
Telophase

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During __________, chromatids become visible

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prophase

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______________________, for movement, develop in the centromere regions.

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Kinetochores

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Centrosomes serve as mitotic centers or poles; microtubules form between the poles to make the _________.

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spindle

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During ___________, the nuclear envelope breaks down.

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prometaphase

Chromosomes consisting of two chromatids attach to the kinetochore mictotubules.

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__________: Chromosomes line up at the midline of the cell.

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Metaphase

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___________: The separation of sister chromatids is controlled by M phase cyclin-Cdk; cohesin is hydrolyzed by separase.

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Anaphase

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After separation they move to opposite ends of the spindle and are referred to as ________________

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daughter chromosomes.

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A protein at the kinetochores—cytoplasmic dynein—hydrolyzes ATP for energy to move chromosomes along the microtubules towards the poles.

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kinetochores—cytoplasmic dynein—hydrolyzes ATP

and Microtubules also shorten, drawing chromosomes toward poles.

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Telophase occurs after chromosomes have separated:

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-Spindle breaks down

-Chromosomes uncoil

-Nuclear envelope and nucleoli appear

-Two daughter nuclei are formed with identical genetic information

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____________________ is based on mitotic division of the nucleus.

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Asexual reproduction

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A unicellular organism may reproduce itself

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reproduce itself

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Cells of multicellular organisms

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break off to form a new individual

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The offspring are ___________—genetically identical to the parent.

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clones

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__________________: The offspring are not identical to the parents.

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Sexual reproduction

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It requires gametes created by ________; two parents each contribute one gamete to an offspring.

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meiosis

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_________________differ genetically from each other and from the parents.

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Gametes—and offspring—

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_____________—body cells not specialized for reproduction.

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Somatic cells

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Each somatic cell contains _________________ of chromosomes with corresponding genes. Each parent contributes one ______________.

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homologous pairs

homolog

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Gametes contain only

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one set of chromosomes.

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Haploid

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: Number of chromosomes = n

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_____________: Two haploid gametes (female egg and male sperm) fuse to form a _____________; chromosome number = 2n

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Fertilization

diploid zygote

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Sexual reproduction generates

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diversity among individual organisms

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_____________: The number, shapes, and sizes of the metaphase chromosomes in a cell.

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Karyotype

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Individual chromosomes can be recognized by

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length, position of centromere, and banding patterns

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Meiosis consists of two nuclear divisions but DNA is replicated only once. The function of meiosis is to:

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-Reduce the chromosome number from diploid to haploid

-Ensure that each haploid has a complete set of chromosomes

-Generate diversity among the products

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In meiosis I, homologous pairs of chromosomes

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come together and pair along their entire lengths

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After metaphase I, the homologous pairs separate, but

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individual chromosomes made up of sister chromatids remain together

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Each chromosome then consists of two sister chromatids, held together by

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cohesin proteins.

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At the end of meiosis I

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two nuclei form, each with half the original chromosomes —still composed of sister chromatids.

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The homologs seem to repel each other but

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are held together at chiasmata that form between non-sister chromatids.

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Exchange of genetic material occurs at the chiasmata—called

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crossing over

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Crossing over results in _____________ chromatids and increases genetic variability of the products.

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recombinant

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Prophase I may last a long time.

Human males: Prophase I lasts about

Human females: Prophase I begins

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1 week, and 1 month for entire meiotic cycle

before birth, and ends up to decades later during the monthly ovarian cycle

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Crossing over is one reason for genetic diversity—___________________ during anaphase I also allows for chance combinations.

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independent assortment

(This does not occur during mitosis.)

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Metaphase I: Chromosomes are at the

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equatorial plate; homologous pairs are held together by chiasmata.

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Anaphase I: Homologous chromosomes separate; daughter nuclei contain only

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one set of chromosomes. Each chromosome consists of two chromatids.

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Telophase I: Occurs in some organisms
Nuclear envelope ____________________

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reaggregates, followed by an interphase called interkinesis.

In other organisms, meiosis II begins immediately.

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Differences between meiosis II and mitosis:

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-DNA does not replicate before meiosis II

-In meiosis II the sister chromatids may not be identical because of crossing over

-The number of chromosomes at the equatorial plate in meiosis II is half the number of those in mitosis

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Meiotic errors:
____________: Homologous pairs fail to separate at anaphase I; sister chromatids fail to separate, or homologous chromosomes may not remain together

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Nondisjunction

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Either results in ______________—chromosomes lacking or present in excess

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aneuploidy

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In humans, if both chromosome 21 go to the same pole it will be ____________ for chromosome 21. (Down Syndrome.)

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trisomic

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Lacking chromosome 21 will be _______________, which is lethal.

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monosomic

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____________________ A piece of chromosome may break away and attach to another chromosome.
Trisomies and monosomies for chromosomes other than 21 are lethal—many miscarriages are due to this.

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Translocation:

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Cell death occurs in two ways:

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Necrosis and Apoptosis

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Necrosis

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cell is damaged or starved for oxygen or nutrients. The cell swells and bursts

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Apoptosis

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is genetically programmed cell death.

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Cell death cycle is controlled by signals:
Lack of a mitotic signal (growth factor)
Recognition of damaged DNA
External signals cause membrane proteins to change shape and activate enzymes called____________—hydrolyze proteins of membranes.

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caspases