Chapter 11

Chapter 11Chapter 11

Cell DivisionCell Division

TerminologyTerminology

• Nucleic AcidNucleic Acid• GeneGene• ChromatinChromatin

– HistonesHistones– NucleosomesNucleosomes– Linker DNALinker DNA

• ChromosomeChromosome

TerminologyTerminology• PlasmidPlasmid

• Condensed Condensed ChromosomeChromosome

• CentromereCentromere

• TelomereTelomere

• Sister ChromatidsSister Chromatids

Cell DivisionCell Division

The life cycle of an organism is linked to The life cycle of an organism is linked to cell division.cell division.

Unicellular organisms use cell division Unicellular organisms use cell division primarily for reproduction.primarily for reproduction.

In multicellular organisms, cell division is In multicellular organisms, cell division is also important in growth and repair of also important in growth and repair of tissues.tissues.

Cell DivisionCell Division

Reproductive signalReproductive signal :To initiate cell :To initiate cell divisiondivision

ReplicationReplication

SegregationSegregation :Distribution of the DNA into :Distribution of the DNA into the two new cellsthe two new cells

CytokinesisCytokinesis: Separation of the two new : Separation of the two new cellscells

Prokaryotic Cell DivisionProkaryotic Cell Division

Binary Fission Binary Fission

External factors such as nutrient External factors such as nutrient concentration and environmental concentration and environmental conditions are the reproductive signals conditions are the reproductive signals that initiate cell division.that initiate cell division.

For many bacteria, abundant food For many bacteria, abundant food supplies speed up the division cycle.supplies speed up the division cycle.

Binary FissionBinary Fission

Plasmid

Eukaryotic Cell DivisionEukaryotic Cell Division

Highly Controlled/ RegulatedHighly Controlled/ Regulated

Two TypesTwo Types

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

MeiosisMeiosis is nuclear division in cells involved in sexual is nuclear division in cells involved in sexual reproduction.reproduction.

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

Cell CycleCell Cycle

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

InterphaseInterphase

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

Begins after cytokinesis and ends when Begins after cytokinesis and ends when mitosis starts.mitosis starts.

Has three subphases: G1, S, and G2Has three subphases: G1, S, and G2

Interphase SubphasesInterphase Subphases

G1: Gap 1G1: Gap 1

Between end of cytokinesis and onset of S phase; Between end of cytokinesis and onset of S phase; chromosomes are single, unreplicated structures (Normal chromosomes are single, unreplicated structures (Normal Cell Function)Cell Function)

S PhaseS PhaseDNA replicates; one chromosome becomes two sister DNA replicates; one chromosome becomes two sister chromatidschromatids

G2: Gap 2G2: Gap 2End of S phase, cell prepares for mitosisEnd of S phase, cell prepares for mitosis

M phaseM phaseMitosis and cytokinesis occur during this phaseMitosis and cytokinesis occur during this phase

CentrosomesCentrosomes• Determines the plane of cell divisionDetermines the plane of cell division

• It doubles during S phase and will determine the It doubles during S phase and will determine the spindle orientationspindle orientation

• Each Each centrosomecentrosome can consist of can consist of two centriolestwo centrioles

• The centrosomes move to opposite ends of the The centrosomes move to opposite ends of the nuclear envelope during G2-to-M transition.nuclear envelope during G2-to-M transition.

• Orientation determines the plane at which the cell will Orientation determines the plane at which the cell will divide and the spatial relationship of the two new cells.divide and the spatial relationship of the two new cells.

• Plants don’t have centrosomes (but have something Plants don’t have centrosomes (but have something similar)similar)

MitosisMitosis

Can be divided into phases:Can be divided into phases:

• ProphaseProphase

• PrometaphasePrometaphase

• MetaphaseMetaphase

• AnaphaseAnaphase

• TelophaseTelophase

ProphaseProphase

Cohesin disappears except at Cohesin disappears except at the centromere; chromatids the centromere; chromatids become visible.become visible.

Kinetochores, for movement, Kinetochores, for movement, develop in the centromere develop in the centromere regions. regions.

Centrosomes serve as Centrosomes serve as mitotic mitotic centerscenters or or polespoles;;

Microtubules form between the Microtubules form between the poles to make the spindle.poles to make the spindle.

PrometaphasePrometaphase

The nuclear envelope breaks down.The nuclear envelope breaks down.

Chromosomes consisting of two chromatids attach to Chromosomes consisting of two chromatids attach to the kinetochore mictotubules.the kinetochore mictotubules.

MetaphaseMetaphase

• Chromosomes line Chromosomes line up at the midline of up at the midline of the cell.the cell.

AnaphaseAnaphase

• The separation of The separation of sister chromatids sister chromatids

• Cohesin is Cohesin is hydrolyzed by hydrolyzed by separase.separase.

• Critical check pointCritical check point

TelophaseTelophase

Occurs after Occurs after chromosomes have chromosomes have separated:separated:

•Spindle breaks downSpindle breaks down

•Chromosomes uncoilChromosomes uncoil

•Nuclear envelope and Nuclear envelope and nucleoli appearnucleoli appear

•Two daughter nuclei are Two daughter nuclei are formed with identical formed with identical genetic informationgenetic information

CytokinesisCytokinesis

Division of the Division of the cytoplasm differs in cytoplasm differs in plant and animals.plant and animals.

Animals: Animals:

Contractile RingContractile Ring

CytokinesisCytokinesis

Plants:Plants:Cell PlateCell Plate

Types of ReproductionTypes of Reproduction

AsexualAsexual

- Mitosis- Mitosis

The offspring are The offspring are clones (clones (genetically genetically identicalidentical to the to the parent).parent).

Types of ReproductionTypes of Reproduction

SexualSexual

The offspring are The offspring are not not identical to the parents.identical to the parents.

It requires It requires gametesgametes created by meiosis; two parents created by meiosis; two parents each contribute one gamete to an offspring. each contribute one gamete to an offspring.

Gametes (and offspring) differ genetically from each Gametes (and offspring) differ genetically from each other and from the parents.other and from the parents.

More TerminologyMore Terminology

Somatic cellsSomatic cells: body cells not specialized for : body cells not specialized for reproduction.reproduction.

Homologous pairs of chromosomesHomologous pairs of chromosomes: Each parent : Each parent contributes one homolog.contributes one homolog.

GenomeGenome: all the genetic information in a cell: all the genetic information in a cell

Diploid (2n)Diploid (2n): Contains : Contains twotwo sets of each chromosome sets of each chromosome

Haploid (n)Haploid (n): Contains : Contains oneone set of each chromosome set of each chromosome

FertilizationFertilization: Two haploid gametes (female egg and : Two haploid gametes (female egg and male sperm) fuse to form a diploid male sperm) fuse to form a diploid zygotezygote

KaryotypeKaryotype

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

Individual chromosomes Individual chromosomes can be recognized by can be recognized by length, position of length, position of centromere, and centromere, and banding patterns.banding patterns.

CytogeneticsCytogenetics uses uses karyotypes to aid in the karyotypes to aid in the diagnosis of certain diagnosis of certain diseases.diseases.

MeiosisMeiosis

Consists of Consists of twotwo nuclear divisions (1 and nuclear divisions (1 and 2) but 2) but DNA is replicated only onceDNA is replicated only once. .

The function of meiosis is to: The function of meiosis is to:

• Reduce the chromosome number from Reduce the chromosome number from diploid to haploiddiploid to haploid

• Ensure that each haploid has a Ensure that each haploid has a complete set of chromosomescomplete set of chromosomes

• Generate diversity among the productsGenerate diversity among the products

InterphaseInterphase

Prior to Meiosis the cell must commit to Prior to Meiosis the cell must commit to sell division moving from the G1 phase sell division moving from the G1 phase to the S phaseto the S phase

During S phase the DNA is ReplicatedDuring S phase the DNA is Replicated

During G2 phase the cell prepares for During G2 phase the cell prepares for Meiosis Meiosis

Prophase 1Prophase 1

Very Long PhaseVery Long Phase

The homologous The homologous chromosomes pair: chromosomes pair: synapsissynapsis..

The four chromatids of The four chromatids of each homologous pair each homologous pair form a form a tetradtetrad..

ChiasmataChiasmata form between form between non-sister chromatids.non-sister chromatids.

Crossing OverCrossing Over can occur can occur

Crossing OverCrossing Over

Source of Genetic Source of Genetic DiversityDiversity

Form Form RecombinantRecombinant ChromatidsChromatids

Prophase/Prometaphase 1Prophase/Prometaphase 1

• Nuclear Envelope Breaks DownNuclear Envelope Breaks Down

• Spindle FormsSpindle Forms

• Centrosomes move to the polesCentrosomes move to the poles

• Cohesin is limited to the centromeresCohesin is limited to the centromeres

Metaphase 1Metaphase 1Homologous Paris line up Homologous Paris line up

down the Metaphase down the Metaphase PlatePlate

Independent assortment Independent assortment (how the homologous (how the homologous pairs line up) during pairs line up) during Metaphase/Anaphase I Metaphase/Anaphase I also allows for chance also allows for chance combinations.combinations.

““More Genetic Diversity”More Genetic Diversity”

Anaphase 1Anaphase 1

Homologous Homologous chromosomes chromosomes separateseparate

Telophase 1Telophase 1

Occurs in some Occurs in some organismsorganisms

Nuclear envelope Nuclear envelope reaggregates, reaggregates, followed by an followed by an interphase called interphase called interkinesisinterkinesis..

In other organisms, In other organisms, meiosis II begins meiosis II begins immediately.immediately.

Meiosis IIMeiosis IISimilar to MitosisSimilar to Mitosis

Differences:Differences:

1) 1) DNA does not replicate before DNA does not replicate before meiosis IImeiosis II

2) In meiosis II the sister 2) In meiosis II the sister chromatids may not be chromatids may not be identical because of crossing identical because of crossing overover

3) The number of chromosomes 3) The number of chromosomes at the equatorial plate in at the equatorial plate in meiosis II is half the number meiosis II is half the number of those in mitosisof those in mitosis

Mitosis vs. Meiosis

Meiotic Errors

Nondisjunction:

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

Cell Death

Necrosis

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

Apoptosis

is genetically programmed cell death.