Cell DivisionCell DivisionChapter 5Chapter 5

Why does a cell divide?Why does a cell divide?

-As a cell absorbs nutrients and gets larger, the volume of the cell increases faster than the surface area.

-Therefore, the demands of the cell (the volume) exceed the ability of the cell to bring in nutrients and export wastes. Solution?

Divide into two smaller cells

► Cell division: Mitosis (division of nucleus or

karyokinesis) Cytokinesis (division of cytoplasm)

► Apoptosis (cell death) decreases the number of cells.

► Both cell increase and apoptosis occur during normal development and growth.

Cell Increase and DecreaseCell Increase and Decrease

Cell Division, Cell DeathCell Division, Cell Death

► Functions of Cell DivisionFunctions of Cell Division MitosisMitosis

► Increase number of somatic cellsIncrease number of somatic cells► Wound healingWound healing► Asexual reproductionAsexual reproduction

MeiosisMeiosis► Production of gametes (germ cells, sperm and eggs)Production of gametes (germ cells, sperm and eggs)

Apoptosis—Programmed cell deathApoptosis—Programmed cell death

Amoeba

Asexual reproduction 2

Hydra

The cell cycleThe cell cycle

Interphase—3 StagesInterphase—3 Stages►GG11 stage stage – –

cell growthcell growth cell doubles its organellescell doubles its organelles accumulates materials for DNA synthesisaccumulates materials for DNA synthesis

►S stageS stage – – DNA synthesis occursDNA synthesis occurs DNA replication results in duplicated DNA replication results in duplicated

chromosomeschromosomes

►GG22 stage stage – – cell synthesizes proteins needed for cell cell synthesizes proteins needed for cell

divisiondivision

Chromosome – Condensed chromatin

ChromosomesChromosomes

Same sizeSame sizeSame genes in Same genes in the same orderthe same order

Homologous Chromosomes

Chromosome pairs, one from mom, one from dad

a. Diploid number (2n) - total number of chromosomes

in a cell (somatic cells) b. Haploid number (n) - only one homologue of each chromosome is present (gametes)

The Big PictureThe Big Picture

The Cell CycleThe Cell Cycle

► Interphase Interphase (90% of (90% of

cycle)cycle) • G1 phase~ • G1 phase~ growth growth • S phase~ • S phase~ synthesis of DNA • G2 synthesis of DNA • G2 phase~ preparation for phase~ preparation for

cell divisioncell division

► Mitotic phaseMitotic phase • Mitosis~ nuclear • Mitosis~ nuclear division • Cytokinesis~ division • Cytokinesis~ cytoplasm cytoplasm divisiondivision

The Mitotic StageThe Mitotic Stage

►Following interphase is the Following interphase is the M stageM stage, , including mitosis and cytokinesis.including mitosis and cytokinesis.

►During mitosis, During mitosis, sister sister chromatidschromatids of of each chromosome separate.each chromosome separate.

►The cell cycle ends when The cell cycle ends when cytokinesis, the cleaving of the cytokinesis, the cleaving of the cytoplasm, is complete.cytoplasm, is complete.

Figure 5.30 The DNA double helix and its replicationFigure 5.30 The DNA double helix and its replication

In mitosis each chromosome is replicated and then divides

Sister chromatids attached at centromere

Sister chromatids separate during cell division

Duplicated ChromosomeDuplicated Chromosome

► PloidyPloidy - number of copies of each chromosome - also number of genomes

► HaploidHaploid (N) (N)= one set of unpaired chromosomes= one set of unpaired chromosomes► DiploidDiploid (2N or 2n)= one set of paired (2N or 2n)= one set of paired

chromosomeschromosomes► TriploidTriploid = three copies of each chromosome = three copies of each chromosome► TetraploidTetraploid = four copies of each chromosome = four copies of each chromosome► HexaploidHexaploid = six copies of each chromosome = six copies of each chromosome► Etc, etc, etcEtc, etc, etc

PLOIDY

Chloroplasts have their own DNA, separate fromplant nuclear DNA, derived from their cyanobacterial origin

Most eukaryotes also have a mitochondrial genome, derived from a eubacterial endosymbiont ancestor

Human mtDNA 37 genes, 16 kilobasepairs

ApoptosisApoptosis►ApoptosisApoptosis - - programmed cell programmed cell death.death.

►Function?Function?

Mitosis OverviewMitosis Overview

►1 diploid (2N) cell>>>>2 diploid 1 diploid (2N) cell>>>>2 diploid (2N)cells(2N)cells

►Genetically identical daughter cellsGenetically identical daughter cells►Function---Maintain the somatic cell Function---Maintain the somatic cell

chromosome numberchromosome number

Mitosis OverviewMitosis Overview

Chromosome DistributionChromosome Distribution

Spindle and FibersSpindle and Fibers► Spindle made of?Spindle made of?

microtubules that that disassemble and disassemble and assemble.assemble.

► Centrosomes (MTOC)——function? function?

► Types of Fibers Types of Fibers Polar Spindle Fibers Centomeric (kinetochore)

fibers Asters

Phases of MitosisPhases of Mitosis

►ProphaseProphase►MetaphaseMetaphase►AnaphaseAnaphase►TelophaseTelophase►PPretty retty MMen en AAre re TToughough►OverviewOverview

As cell enters mitosis from interphase it has 2 complete sets of chromosomes because of replication in the S phase. Each set must be re-arranged and distributed into the 2 new daughter nuclei. This is mitosis.

Late InterphaseLate Interphase

Early ProphaseEarly Prophase

Late ProphaseLate Prophase

MetaphaseMetaphase

AnaphaseAnaphase

TelophaseTelophase

Review

Results of MitosisResults of Mitosis

►Two daughter nuclei Two daughter nuclei

►Each with same Each with same chromosome number chromosome number as parent cellas parent cell

►Genetically identical Genetically identical to each other and to each other and the parent cellthe parent cell

-Animal cells: Cleavage furrow forms from outside in.

-

Plant cells: Division plate forms from inside out.

 

CYTOKINESIS – Cytoplasm splits into 2 cells.

Cytokinesis in Animal Cytokinesis in Animal CellsCells

Cytokinesis in plant Cytokinesis in plant cellscells

REVIEW OF MITOSISREVIEW OF MITOSIS

Parent Cell

DNA Synthesis (“S” Phase)

Mitosis & Cytokinesis

Two daughter cells

Remember:Mitosis Is Part Of The Cell Cycle!!

Parent cell and daughter cells contain the same complement of chromosomes.

Cell Division in ProkaryotesCell Division in Prokaryotes

►Binary fission---process of asexual ---process of asexual reproduction in prokaryotes.reproduction in prokaryotes.

►Two daughter cells Two daughter cells ► Identical to the original parent cell and Identical to the original parent cell and

each other each other► All contain a single chromosome.All contain a single chromosome.

The Big PictureThe Big Picture

Overview of MeiosisOverview of Meiosis

►1 diploid (2N) cell>>>>4 haploid 1 diploid (2N) cell>>>>4 haploid (1N) cells(1N) cells

►Genetically unique daughter cellsGenetically unique daughter cells►Function---reduce the chromosome Function---reduce the chromosome

number by ½number by ½►The main source of genetic The main source of genetic

diversity in sexually reproducing diversity in sexually reproducing organismsorganisms

MEIOSISMEIOSISTwo Parts:Two Parts:

► Meiosis I:Meiosis I: Separation of Separation of homologous homologous

pairs pairs Ploidy Level?Ploidy Level?

► Meiosis II:Meiosis II: Separation of Separation of sister sister

chromatidschromatids Ploidy Level?Ploidy Level?

COMPARISON-- MITOSIS and MEIOSIS

Mitosis:

Occurs in somatic (body) cells

Growth, development, replacement & repair

Produces 2 daughter cells► Clones of parent and

each other

Daughter cells are diploid (2N)

Meiosis:

Occurs in germ cells (immature reproductive cells)

Production of gametes (sex cells)

Produce 4 daughter cells► Distinct from parent and

each other

Daughter cells are haploid (1N)

Genetic RecombinationGenetic Recombination

• Meiosis increases genetic diversity. Two points of genetic recombination.

• Crossing-over of nonsister of nonsister chromatids—Prophase 1chromatids—Prophase 1

• Independent assortment of of homologous chromosomes homologous chromosomes during Metaphase 1during Metaphase 1

Synapsis and crossing-overSynapsis and crossing-over

Defs

More TerminologyMore TerminologyCrossing OverCrossing Over

►Segments of paternal/maternal chromatids Segments of paternal/maternal chromatids exchange during Meiosis Iexchange during Meiosis I

Synapsis Synapsis ►Process of homologs coming together Process of homologs coming together

physically during Meiosis Iphysically during Meiosis I

TetradTetrad ►Consists of 4 sister chromatids from 2 Consists of 4 sister chromatids from 2

homologous chromosomeshomologous chromosomes

Independent assortmentIndependent assortment

Meiosis in DetailMeiosis in Detail

►PhasesPhases ProphaseProphase MetaphaseMetaphase AnaphaseAnaphase telophasetelophase

► Interkinesis.Interkinesis.►Replication of DNA??Replication of DNA??

Meiosis I in an animal cellMeiosis I in an animal cell

Meiosis IIMeiosis II

Sources of Genetic VariationSources of Genetic Variation

► Three sources of genetic Three sources of genetic recombination:recombination:

1)1) Independent alignment – Independent alignment – metaphase Imetaphase I

2)2) Crossing-over---prophase ICrossing-over---prophase I3)3) FertilizationFertilization

SUMMARY-- MITOSIS and MEIOSIS

Mitosis:

Occurs in somatic (body) cells

Growth, development, replacement & repair

Produces 2 daughter cells► Clones of parent and

each other

Daughter cells are diploid (2N)

►Meiosis: Occurs in germ cells

(immature reproductive cells)

Production of gametes (sex cells)

Produce 4 daughter cells► Distinct from parent and

each other

Daughter cells are haploid (1N)

comparison

Meiosis compared to mitosisMeiosis compared to mitosis

Life cycle of humansLife cycle of humans

SpermatogenesisSpermatogenesis

OogenesisOogenesis

FERTILIZATION RESTORES FERTILIZATION RESTORES FULL SET OF FULL SET OF

CHROMOSOMESCHROMOSOMESFemale gamete Male gamete

fertilization

Diploid zygote contains homologous pairs of chromosomes

Sources of Genetic VariationSources of Genetic Variation

► Three sources of genetic Three sources of genetic recombination:recombination:

1)1) Independent assortment – Independent assortment – metaphase Imetaphase I

2)2) Crossing-over---prophase ICrossing-over---prophase I3)3) FertilizationFertilization

MEIOSIS & GENETIC MEIOSIS & GENETIC VARIATIONVARIATION

Independent Independent AssortmentAssortment

► Diploid organisms Diploid organisms can produce 2can produce 2n n diff. diff. gametesgametes

► Ea. homologous pair can Ea. homologous pair can orient in two different orient in two different waysways

► Humans: 2Humans: 22323 = = 8,388,608 8,388,608

((est. 8.4 millionest. 8.4 million))

Each homologous pair can orient in either of two ways

HYPOTHETICAL EXAMPLE OF INDEPENDENT ASSORTMENT

Gene for

browneyes

Eye color

Gene for

blueeyes

Gene for

black hair

r

Gene for red hair

Hair color

INDEPENDENT INDEPENDENT ASSORTMENTASSORTMENT

OR

During meiosis I, tetrads can line up 2n different ways.

Brown eyesBlack hair

Blue eyesRed hair

Brown eyesRed hair

Blue eyesBlack hair

Meiosis I & II

MEIOSIS & GENETIC MEIOSIS & GENETIC VARIATIONVARIATION

Crossing Over:Crossing Over:► Physical exchange of Physical exchange of

sections of chromosomessections of chromosomes► Results in shuffling of Results in shuffling of

genome genome ► New combo of genes on New combo of genes on

chromosomes (chromosomes (recombinationrecombination))

A single crossover in each A single crossover in each homologous pair = 5.0 x 10 homologous pair = 5.0 x 10 27 27

different zygotes possible different zygotes possible (4951,760,200,000,000,000,000,0(4951,760,200,000,000,000,000,000,000)!00,000)!

MEIOSIS & GENETIC MEIOSIS & GENETIC VARIATIONVARIATION

Random Fertilization:Random Fertilization: ► Each human can produce Each human can produce

an nearly 8.4 million an nearly 8.4 million different kinds of gametesdifferent kinds of gametes

► Crossing over not taken into Crossing over not taken into account!!!account!!!

► Sex yields:Sex yields:► 8.4 mill x 8.4 mill = 70.6 x 10 8.4 mill x 8.4 mill = 70.6 x 10 1212

genetically distinct offspring genetically distinct offspring can be producedcan be produced

SUMMARY-- MITOSIS and MEIOSIS

Mitosis:

Occurs in somatic (body) cells

Growth, development, replacement & repair

Produces 2 daughter cells► Clones of parent and

each other

Daughter cells are diploid (2N)

►Meiosis: Occurs in germ cells

(immature reproductive cells)

Production of gametes (sex cells)

Produce 4 daughter cells► Distinct from parent and

each other

Daughter cells are haploid (1N)

comparison