cell division

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The Cell Cycle andCell Division

Jackquero DatinguinooDe La Salle Araneta University

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Phases of the Cell Cycle

The cell cycleconsists of Interphase

normal cellactivity

The mitoticphase celldivision

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Phases of the Cell Cycle

Interphase G1 - primary growth S - genome

replicated G2 - secondary

growth M - mitosis C - cytokinesis

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Interphase

G1 - Cells undergo majority of growth S - Each chromosome replicates (Synthesizes) to

produce sister chromatids Attached at centromere Contains attachment site (kinetochore)

G2 - Chromosomes condense - Assemble machineryfor division such as centrioles

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G2 of Interphase

A nuclear envelope bounds the nucleus. The nucleus contains one or more nucleoli(singular, nucleolus). Two centrosomes have formed by replication of a

single centrosome. In animal cells, each centrosome features two

centrioles. Chromosomes, duplicated during S phase,cannot be seen individually because they have notyet condensed.

The light micrographs show dividing lung cells froma newt, which has 22 chromosomes in its somaticcells (chromosomes appear blue, microtubulesgreen, intermediate filaments red). For simplicity,the drawings show only four chromosomes.

G2 OF INTERPHASECentrosomes(with centriole pairs) Chromatin

(duplicated)

Nucleolus Nuclearenvelope

Plasmamembrane

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Functions of Cell Division

20 m100 m 200 m

(a) Reproduction. An amoeba,a single-celled eukaryote, is

dividing into two cells. Eachnew cell will be an individualorganism (LM).

(b) Growth and development. Thismicrograph shows a sand dollarembryo shortly after the fertilized eggdivided, forming two cells (LM).

(c) Tissue renewal. These dividingbone marrow cells (arrow) willgive rise to new blood cells (LM).

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Cell Division

An integral part of the cell cycle Results in genetically identical daughter

cells Cells duplicate their genetic material

Before they divide, ensuring that eachdaughter cell receives an exact copy ofthe genetic material, DNA

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Mitosis

Some haploid & diploid cells divide by mitosis.Each new cell receives one copy of every chromosome

that was present in the original cell.Produces 2 new cells that are both genetically identical

to the original cell.

DNAduplication

duringinterphase Mitosis

Diploid Cell

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Mitotic Division of an Animal Cell

G2 OF INTERPHASE PROPHASE PROMETAPHASECentrosomes(with centriole pairs) Chromatin

(duplicated)Early mitoticspindle

AsterCentromere

Fragmentsof nuclearenvelope

Kinetochore

Nucleolus Nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Kinetochoremicrotubule

Nonkinetochoremicrotubules

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Prophase The chromatin fibers become more tightlycoiled, condensing into discretechromosomes observable with a lightmicroscope The nucleoli disappear. Each duplicated chromosome appears astwo identical sister chromatids joinedtogether. The mitotic spindle begins to form. It iscomposed of the centrosomes and themicrotubules that extend from them. Theradial arrays of shorter microtubules thatextend from the centrosomes are calledasters (stars). The centrosomes move away from eachother, apparently propelled by thelengthening microtubules between them.

PROPHASE

Early mitoticspindle

AsterCentromere

Chromosome, consistingof two sister chromatids

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Metaphase

Metaphase is the longest stage of mitosis,lasting about 20 minutes. The centrosomes are now at opposite ends ofthe cell.The chromosomes convene on the metaphaseplate, an imaginary plane that is equidistantbetween the spindles two poles. Thechromosomes centromeres lie on the metaphaseplate. For each chromosome, the kinetochores of thesister chromatids are attached to kinetochoremicrotubules coming from opposite poles. The entire apparatus of microtubules is calledthe spindle because of its shape.

METAPHASE

Spindle

Metaphaseplate

Centrosome atone spindle pole

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METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphaseplate Nucleolus

formingCleavagefurrow

NuclearenvelopeformingCentrosome at

one spindle poleDaughterchromosomes

Mitotic Division of an Animal Cell

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The Mitotic Spindle The spindle includes the centrosomes, the spindle

microtubules, and the asters The apparatus of microtubules controls chromosome

movement during mitosis The centrosome replicates, forming two centrosomes that

migrate to opposite ends of the cell Assembly of spindle microtubules begins in the centrosome,

the microtubule organizing center An aster (a radial array of short microtubules) extends from

each centrosome

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The Mitotic Spindle

Some spindle microtubules attach to the kinetochores ofchromosomes and move the chromosomes to the metaphaseplate

In anaphase, sister chromatids separate and move along thekinetochore microtubules toward opposite ends of the cell

Microtubules ChromosomesSisterchromatids

AsterCentrosome

Metaphaseplate

Kineto-chores

Kinetochoremicrotubules

0.5 m

Overlappingnonkinetochoremicrotubules

1 mCentrosome

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Anaphase Anaphase is the shortest stage ofmitosis, lasting only a few minutes.

Anaphase begins when the two sisterchromatids of each pair suddenly part.Each chromatid thus becomes a full-fledged chromosome.

The two liberated chromosomes beginmoving toward opposite ends of the cell,as their kinetochore microtubulesshorten. Because these microtubules areattached at the centromere region, thechromosomes move centromere first (atabout 1 m/min).

The cell elongates as thenonkinetochore microtubules lengthen.

By the end of anaphase, the two ends ofthe cell have equivalentandcompletecollections of chromosomes.

ANAPHASE

Daughterchromosomes

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Telophase Two daughter nuclei begin toform in the cell.

Nuclear envelopes arise from thefragments of the parent cells nuclearenvelope and other portions of theendomembrane system. The chromosomes become lesscondensed. Mitosis, the division of one nucleusinto two genetically identical nuclei, isnow complete.

TELOPHASE AND CYTOKINESIS

Nucleolusforming

Cleavagefurrow

Nuclearenvelopeforming

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Mitosis in a plant cell

1Prophase.The chromatinis condensing.The nucleolus isbeginning todisappear.Although notyet visiblein the micrograph,the mitotic spindle isstaring to from.

Prometaphase.We now see discretechromosomes; eachconsists of twoidentical sisterchromatids. Laterin prometaphase, thenuclear envelop willfragment.

Metaphase. Thespindle is complete,and the chromosomes,attached to microtubulesat their kinetochores,are all at the metaphaseplate.

Anaphase. Thechromatids of eachchromosome haveseparated, and thedaughter chromosomesare moving to the endsof cell as theirkinetochoremicrotubles shorten.

Telophase. Daughternuclei are forming.Meanwhile, cytokinesishas started: The cellplate, which willdivided the cytoplasmin two, is growingtoward the perimeterof the parent cell.

2 3 4 5

NucleusNucleolus

ChromosomeChromatinecondensing

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Cytokinesis

Cleavage of cell into two halves Animal cells

Constriction belt of actinfilaments = cleavagefurrow

Plant cells Cell plate

Fungi and protists Mitosis occurs within the

nucleus

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Cytokinesis In Animal And Plant Cells

Cleavage furrow

Contractile ringofmicrofilaments

Daughter cells

100 m1 mVesicles

formingcell plate

Wall ofpatent cell Cell plate

New cell wall

(a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM)20

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Meiosis andSexual Life Cycles

Living organisms are distinguished by their ability to reproduce their ownkind

Heredity Is the transmission of traits from one generation to the next

Variation Shows that offspring differ somewhat in appearance from parents and

siblings

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Inheritance of Genes

Genes are segments of DNA, units of heredity Offspring acquire genes from parents by

inheriting chromosomes Genetics is the scientific study of heredity and

hereditary variation

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Each gene in an organisms DNA has a specific locuson a certain chromosome

We inherit one set of chromosomes from our motherand one set from our father

Two parents give rise to offspring that have uniquecombinations of genes inherited from the two parents- sexual reproduction

Inheritance of Genes24

Asexual Reproduction

In asexual reproduction, oneparent produces geneticallyidentical offspring by mitosis ParentBud

0.5 mm

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SexualReproduction

Fertilization and meiosisalternate in sexual life cycles

A life cycle is thegeneration-to-generationsequence of stages in thereproductive history of anorganism

Gametes

Diploidmulticellular

organism

Key

MEIOSIS FERTILIZATION

n

n

n

2n2nZygote

HaploidDiploid

Mitosis

(a) Animals

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Sex Cells - Gametes

Unlike somatic cells, sperm and egg cells are haploidcells, containing only one set of chromosomes

At sexual maturity the ovaries and testes producehaploid gametes by meiosis

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Sexual Reproduction The Human Life Cycle

During fertilization,sperm and ovum fuseforming a diploid zygote

The zygote develops intoan adult organism

Haploid (n)Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

SpermCell (n)

MEIOSIS FERTILIZATION

Ovary Testis Diploidzygote(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

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MEIOSIS Reduces the chromosome number such that each

daughter Consists of 2 successive nuclear divisions Cell has a haploid set of chromosomes Ensures that the next generation will have:

Diploid number of chromosome Exchange of genetic information (combination of

traits that differs from that of either parent)

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Only diploid cells candivide by meiosis.

Prior to meiosis I, DNAreplication occurs.

During meiosis, there willbe two nuclear divisions,and the result will be fourhaploid nuclei.

No replication of DNAoccurs between meiosis Iand meiosis II.

MEIOSIS

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Meiosis reduces the numberof chromosome sets fromdiploid to haploid

Meiosis takes place in twosets of divisions Meiosis I reduces the

number of chromosomesfrom diploid to haploid

Meiosis II produces fourhaploid daughter cells

InterphaseHomologous pairof chromosomesin diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids Diploid cell with

replicatedchromosomes

1

2

Homologouschromosomesseparate

Haploid cells withreplicated chromosomes

Sister chromatidsseparate

Haploid cells with unreplicated chromosomes

Meiosis I

Meiosis II

MEIOSIS

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Phases of Meiosis Meiosis involves the same four phases seen in mitosis

prophase metaphase anaphase telophase

They are repeated during both meiosis I and meiosis II. The period of time between meiosis I and meiosis II is called

interkinesis. No replication of DNA occurs during interkinesis because the

DNA is already duplicated.

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Prophase I

Prophase I occupies more than 90% of the time required for meiosis Chromosomes begin to condense In synapsis, the 2 members of each homologous pair of chromosomes

line up side-by-side, aligned gene by gene, to form a tetrad consistingof 4 chromatids

During synapsis, sometimes there is an exchange of homologous partsbetween non-sister chromatids. This exchange is called crossing over

Each tetrad usually has one or more chiasmata, X-shaped regionswhere crossing over occurred

Prophase Iof meiosis

Tetrad

Nonsisterchromatids

Chiasma,site ofcrossingover

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Metaphase I

At metaphase I, tetrads line up at themetaphase plate, with one chromosome facingeach pole

Microtubules from one pole are attached to thekinetochore of one chromosome of each tetrad

Microtubules from the other pole are attachedto the kinetochore of the other chromosome

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Sisterchromatids

Chiasmata

Spindle

Centromere(with kinetochore)

Metaphaseplate


Sister chromatidsremain attached

Microtubuleattached tokinetochore

Tetrad

PROPHASE I METAPHASE I ANAPHASE I

Homologous chromosomes(red and blue) pair andexchange segments; 2n = 6in this example

Pairs of homologouschromosomes split up

Tetrads line up

Metaphase I

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Anaphase I

In anaphase I, pairs of homologous chromosomesseparate

One chromosome moves toward each pole, guidedby the spindle apparatus

Sister chromatids remain attached at thecentromere and move as one unit toward the pole

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Sisterchromatids

Chiasmata

Spindle

Centromere(with kinetochore)

Metaphaseplate


Sister chromatidsremain attached

Microtubuleattached tokinetochore

Tetrad

PROPHASE I METAPHASE I ANAPHASE I

Homologous chromosomes(red and blue) pair andexchange segments; 2n = 6in this example

Pairs of homologouschromosomes split up

Tetrads line up

Anaphase I

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Telophase I and Cytokinesis

In the beginning of telophase I, each half of the cell has ahaploid set of chromosomes; each chromosome still consistsof two sister chromatids

Cytokinesis usually occurs simultaneously, forming twohaploid daughter cells

In animal cells, a cleavage furrow forms; in plant cells, a cellplate forms

No chromosome replication occurs between the end ofmeiosis I and the beginning of meiosis II because thechromosomes are already replicated

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Prophase II

Meiosis II is very similar to mitosis In prophase II, a spindle apparatus forms In late prophase II, chromosomes (each still composed of two

chromatids) move toward the metaphase plate

Cleavagefurrow

PROPHASE II METAPHASE II ANAPHASE IITELOPHASE I ANDCYTOKINESIS

TELOPHASE II ANDCYTOKINESIS


Haploid daughter cellsforming

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Metaphase II

At metaphase II, the sister chromatids are at the metaphase plate Because of crossing over in meiosis I, the two sister chromatids of each

chromosome are no longer genetically identical The kinetochores of sister chromatids attach to microtubules extending from

opposite poles

Cleavagefurrow





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Anaphase II

At anaphase II, the sister chromatids separate The sister chromatids of each chromosome now move

as two newly individual chromosomes toward oppositepoles

Cleavagefurrow





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Telophase II and Cytokinesis

In telophase II, the chromosomes arrive at opposite poles Nuclei form, and the chromosomes begin decondensing Cytokinesis separates the cytoplasm At the end of meiosis, there are four daughter cells, each with a haploid set of

unreplicated chromosomes Each daughter cell is genetically distinct from the others and from the parent cell

Cleavagefurrow





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A Comparison of Mitosis and Meiosis

Mitosis conserves the number of chromosome sets,producing cells that are genetically identical to theparent cell

Meiosis reduces the number of chromosomes setsfrom two (diploid) to one (haploid), producing cells thatdiffer genetically from each other and from the parentcell

The mechanism for separating sister chromatids isvirtually identical in meiosis II and mitosis

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Three events are unique to meiosis, and all three occur inmeiosis l: Synapsis and crossing over in prophase I: Homologous

chromosomes physically connect and exchange geneticinformation

At the metaphase plate, there are paired homologouschromosomes (tetrads), instead of individual replicatedchromosomes

At anaphase I of meiosis, homologous pairs move towardopposite poles of the cell. In anaphase II of meiosis, thesister chromatids separate

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MEIOSIS DNA duplication

followed by 2 celldivisions

Sysnapsis Crossing-over One diploid cell

produces 4 haploidcells

Each new cell hasa uniquecombination ofgenes

MITOSIS Homologous

chromosomes do notpair up

No genetic exchangebetween homologouschromosomes

One diploid cellproduces 2 diploid cellsor one haploid cellproduces 2 haploid cells

New cells are geneticallyidentical to original cell(except for mutation)

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COMPARISON BETWEEN MITOSIS AND MEIOSIS

Basis ofComparison Mitosis Meiosis

Cell type Somatic cell andgerm cell

Sex cell(gametes)

Number of Karyokinesis 1 2

Daughter cellformed

2 (2N) 4 (N)

Duplication of homologues YES YES

Pairing of chromosomes Absent present

END

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cell division

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