Chapter 11Chapter 11

Continuity of Life:Continuity of Life:

Cellular ReproductionCellular Reproduction

Chapter 11 2

Cellular ReproductionCellular Reproduction

Intracellular activity between one cell division Intracellular activity between one cell division to the next is the to the next is the cell cyclecell cycle

• Some activities involve growth (enlargement) Some activities involve growth (enlargement) of the cellof the cell

• Some activities involve duplication of genetic Some activities involve duplication of genetic material and cellular division (reproduction)material and cellular division (reproduction)

Binary Fission (prokaryotes)Binary Fission (prokaryotes)Mitosis (new individual cells)Mitosis (new individual cells)

Budding (eukaryotes)Budding (eukaryotes)Meiosis (new gametes)Meiosis (new gametes)

Chapter 11 3

The Prokaryotic Cell CycleThe Prokaryotic Cell Cycle

1.1. Long growth phaseLong growth phase– Replication of circular DNA chromosome occursReplication of circular DNA chromosome occurs– Duplicate chromosomes anchored to membraneDuplicate chromosomes anchored to membrane

2.2. Cell increases in size, pulling duplicated Cell increases in size, pulling duplicated chromosomes apart…chromosomes apart…

3.3. Plasma membrane grows inward between Plasma membrane grows inward between chromosome copieschromosome copies

4.4. Fusion of membrane along cell equator Fusion of membrane along cell equator completes separation (completes separation (binary fissionbinary fission or “splitting or “splitting in two”)…in two”)…

5.5. Daughter cells are genetically identicalDaughter cells are genetically identical• Under ideal conditions Under ideal conditions Escherichia coliEscherichia coli bacteria bacteria

complete a cell cycle every 20 minutescomplete a cell cycle every 20 minutes

Chapter 11 4

Binary FissionBinary Fission

DNA replicated

Membrane added

Chapter 11 5

Binary Fission 2Binary Fission 2

constriction

fission

Chapter 11 6

False-Color EMFalse-Color EMof Dividing Bacteriumof Dividing Bacterium

Division planeDivision plane

Cell wallCell wall

CytoplasmCytoplasm

NuclearNuclearmaterialmaterial

Chapter 11 7

Mitotic Mitotic

cell cell divisiondivision

Mitotic Mitotic cell cell division & division &

differ-differ-entiationentiation

Functions of MitosisFunctions of Mitosis

TissuesTissues

OrgansOrgansFertilized eggFertilized egg(zygote)(zygote) Multicell stageMulticell stage

Chapter 11 8

Protozoa:Protozoa:Asexual Reproduction by MitosisAsexual Reproduction by Mitosis

New individualsNew individuals

Chapter 11 9

Yeasts:Yeasts:Asexual Reproduction by MitosisAsexual Reproduction by Mitosis

BuddingBuddingNucleus divides by Nucleus divides by

mitosismitosisBud forms on cellBud forms on cellNucleus moves into Nucleus moves into

budbudBud separatesBud separates

Chapter 11 10

Hydra:Hydra:Asexual Reproduction by MitosisAsexual Reproduction by Mitosis

Chapter 11 11

GeneticallyGeneticallyIdentical Aspen GrovesIdentical Aspen Groves

Three separate aspen Three separate aspen grovesgroves

Each produced Each produced asexually from single asexually from single ancestorancestor

Variable between Variable between grovesgroves

Identical within grovesIdentical within groves

Trees Trees synchronously synchronously

lost leaveslost leaves

Trees Trees synchronously synchronously

lost leaveslost leaves

Trees Trees synchronously synchronously turned yellowturned yellow

Trees Trees synchronously synchronously turned yellowturned yellow

Trees still Trees still greengreen

Trees still Trees still greengreen

Chapter 11 12

11 single-stranded chromosomesingle-stranded chromosome22 double helixdouble helix

uncondenseduncondensed

DNA replicationDNA replication

11 double-stranded chromosomedouble-stranded chromosome22 double helicesdouble helices

still uncondensedstill uncondensed

Chromosome Chromosome condensationcondensation

11 double-stranded chromosomedouble-stranded chromosome22 double helicesdouble helices

now condensednow condensed

centromere centromere aachromatidchromatid

cellcell basebasepairspairs

closer lookstill closer lookeven closer look

Chromosome CondensationChromosome Condensation

Chapter 11 13

HumanHumanChromosomes during MitosisChromosomes during Mitosis

Chapter 11 14

Human Karyotype, MaleHuman Karyotype, Male

These are These are chromosomes from chromosomes from mitosismitosis

Stained to show Stained to show regionsregions

Numbered by lengthNumbered by lengthOccur in pairsOccur in pairs

Chapter 11 15

telophase

metaphase

anaphase

cell

divi

sion

GG00: nondividing: nondividing

interphaseinterphase

The EukaryoticThe EukaryoticCell CycleCell Cycle

S: Synthesis S: Synthesis of DNA; of DNA; chromosomes chromosomes duplicatedduplicated

GG11: Growth: Growth

GG22: Growth: Growth

prophase

cytokinesis

Mito

sis

Chapter 11 16

Interphase : The chromosomes (blue) are in the thin, extended state and appear as a mass in the center of the cell. The microtubules (red) extend outward from the nucleus to all parts of the cell.

Metaphase: The chromosomes have moved along the spindle microtubules to the equator of the cell.

Late prophase: Chromosomes (blue) have condensed and attached to microtubules of spindle fibers (red). Microtubules have reorganized to form the spindle; chromosomes, now condensed, are clearly visible.

Phases of Mitosis, 1Phases of Mitosis, 1

Chapter 11 17

Separation of Sister ChromatidsSeparation of Sister Chromatids

In metaphase, sister chromatids In metaphase, sister chromatids are held together at centromereare held together at centromere

At end of metaphase, centromere At end of metaphase, centromere releases sister chromatidsreleases sister chromatids

In anaphase, they move to opposite In anaphase, they move to opposite polespoles

In metaphase, sister chromatids In metaphase, sister chromatids are held together at centromereare held together at centromere

At end of metaphase, centromere At end of metaphase, centromere releases sister chromatidsreleases sister chromatids

In anaphase, they move to opposite In anaphase, they move to opposite polespoles

Chapter 11 18

Anaphase: Sister chromatids have separated, and one set of chromosomes moves along the spindle microtubule to each pole of the cell.

Telophase: The chromosomes have gathered into two clusters, one at the site of each future nucleus.

Next interphase: Chromosomes are relaxing again into their extended state. Spindle fibers are disappearing, and the microtubules of the 2 daughter cells rearrange into the interphase pattern.

Phases of Mitosis, 2Phases of Mitosis, 2

Chapter 11 19

Mitosis:Mitosis:Prophase - MetaphaseProphase - Metaphase

Kinetochores align at cell’s

equator

Nucleolus disappears;

Nuclear envelope breaks down

Microtubules attach to

kinetochores

Chromosomes condense and

shorten

Centrioles begin to move apart;Spindle forms

Duplicated chromosomes

remain elongated

Centrioles have also been duplicated

LateLateInterphaseInterphase

EarlyEarlyProphaseProphase

LateLateProphaseProphase MetaphaseMetaphase

Chapter 11 20

MitosisMitosisAnaphase - CytokinesisAnaphase - Cytokinesis

Free spindle fibers push poles apart

Chromatids become

independent chromosomes

One set of chromosomes;

Begin unwinding

Nuclear envelope re-

forms

Cytoplasm divided along

equator

Each daughter gets 1 nucleus &

half of cytoplasm

Spindle disappears; Nucleolus reappears

AnaphaseAnaphase TelophaseTelophase CytokinesisCytokinesisNextNext

InterphaseInterphase

Chapter 11 21

Cytokinesis of a Ciliated CellCytokinesis of a Ciliated Cell

Cleavage FurrowCleavage Furrow

Daughter CellsDaughter Cells

Chapter 11 22

Cytokinesis in PlantsCytokinesis in Plants

Vesicles fuse to form cell wall

and membranesComplete

separation of daughter cells

Chapter 11 23

Control of Cell CycleControl of Cell Cycle

The cells of some tissues divide frequently The cells of some tissues divide frequently throughout lifespanthroughout lifespan

– e.g. skin, intestine e.g. skin, intestine

Cell division occurs rarely or not at all in other Cell division occurs rarely or not at all in other tissuestissues

– e.g. brain, heart, skeletal musclese.g. brain, heart, skeletal muscles

Cell division in eukaryotes is driven by Cell division in eukaryotes is driven by enzymes and controlled at specific enzymes and controlled at specific checkpointscheckpoints

Chapter 11 24

Enzymes Drive the Cell CycleEnzymes Drive the Cell Cycle

• The cell cycle is driven by proteins The cell cycle is driven by proteins called called CCyclin-yclin-ddependent ependent kkinases, or inases, or Cdk’s Cdk’s

• Kinases are enzymes that phosphorylate Kinases are enzymes that phosphorylate (add a phosphate group to) other (add a phosphate group to) other proteins, stimulating or inhibiting their proteins, stimulating or inhibiting their activity activity

• Cdk’s are active only when they bind to Cdk’s are active only when they bind to other proteins called cyclinsother proteins called cyclins

Chapter 11 25

Enzymes Drive the Cell CycleEnzymes Drive the Cell Cycle

• Cell division occurs when Cell division occurs when growth growth factorsfactors bind to cell surface receptors, bind to cell surface receptors, which leads to cyclin synthesis which leads to cyclin synthesis

• Cyclins then bind to and activate Cyclins then bind to and activate specific Cdk’sspecific Cdk’s

Chapter 11 26

Enzymes Drive the Cell CycleEnzymes Drive the Cell Cycle

• Activated Cdk’s promote a variety of cell Activated Cdk’s promote a variety of cell cycle eventscycle events– Synthesis and activation of proteins required Synthesis and activation of proteins required

for DNA synthesisfor DNA synthesis– Chromosome condensationChromosome condensation– Nuclear membrane breakdownNuclear membrane breakdown– Spindle formationSpindle formation– Attachment of chromosomes to spindleAttachment of chromosomes to spindle– Sister chromatid separation and movementSister chromatid separation and movement

Chapter 11 27

Chapter 11 28

Checkpoints Control Cell CycleCheckpoints Control Cell Cycle• Although Cdk’s drive the cell cycle, multiple Although Cdk’s drive the cell cycle, multiple

checkpoints ensure that…checkpoints ensure that…– The cell successfully completes DNA synthesis The cell successfully completes DNA synthesis

during interphase during interphase – Proper chromosome movements occur during Proper chromosome movements occur during

mitotic cell divisionmitotic cell division • There are three major There are three major checkpointscheckpoints in the in the

eukaryotic cell cycle, each regulated by eukaryotic cell cycle, each regulated by protein complexesprotein complexes

1.1. G1 to S: G1 to S: 2.2. G2 to mitosisG2 to mitosis3.3. Metaphase to anaphaseMetaphase to anaphase

Chapter 11 29

Chapter 11 30

Checkpoints Control Cell CycleCheckpoints Control Cell Cycle

• G1 to S: G1 to S: Ensures that the cell’s DNA is Ensures that the cell’s DNA is suitable for replicationsuitable for replication– p53 protein expressed when DNA is p53 protein expressed when DNA is

damageddamagedInhibits replicationInhibits replicationStimulates synthesis of DNA repair Stimulates synthesis of DNA repair

enzymesenzymesTriggers cell death (apoptosis) if Triggers cell death (apoptosis) if

damage can’t be repaireddamage can’t be repaired

Chapter 11 31

Chapter 11 32

Checkpoints Control Cell CycleCheckpoints Control Cell Cycle

• G2 to mitosis:G2 to mitosis: Ensures that DNA has Ensures that DNA has been completely and accurately been completely and accurately replicatedreplicated– p53 protein expression leads to decrease p53 protein expression leads to decrease

in synthesis and activity of an enzyme in synthesis and activity of an enzyme that facilitates chromosome condensation that facilitates chromosome condensation

– chromosomes remain extended and chromosomes remain extended and accessible to DNA repair enzymes, which accessible to DNA repair enzymes, which fix DNA before cell enters mitosisfix DNA before cell enters mitosis

Chapter 11 33

Checkpoints Control Cell CycleCheckpoints Control Cell Cycle

• Metaphase to anaphase:Metaphase to anaphase: Ensures that Ensures that the chromosomes are aligned properly the chromosomes are aligned properly at the metaphase plateat the metaphase plate– a variety of proteins prevent separation of a variety of proteins prevent separation of

the sister chromatids if there are defects the sister chromatids if there are defects in chromosome alignment or spindle in chromosome alignment or spindle functionfunction

Chapter 11 34

Prevalence of Sexual ReproductionPrevalence of Sexual Reproduction

Asexual reproduction by mitosis produces Asexual reproduction by mitosis produces genetically identical offspringgenetically identical offspring

Sexual reproduction by meiosis shuffles the Sexual reproduction by meiosis shuffles the genes to produce genetically unique offspringgenes to produce genetically unique offspring

• Wide use of sexual reproduction suggests that Wide use of sexual reproduction suggests that DNA reshuffling is advantageousDNA reshuffling is advantageous

• Variation in offspring provided by sexual Variation in offspring provided by sexual reproduction confers a large evolutionary reproduction confers a large evolutionary advantageadvantage

Chapter 11 35

Genetic Variability from MutationGenetic Variability from Mutation

Mutations are the ultimate source of genetic Mutations are the ultimate source of genetic variabilityvariability

Most mutations are harmful or lethal; a few are Most mutations are harmful or lethal; a few are neutral or even beneficialneutral or even beneficial

Mutation gives rise to new Mutation gives rise to new allelesalleles Alleles are alternate gene forms that may produce Alleles are alternate gene forms that may produce

differences in structure or functiondifferences in structure or functionHomologous chromosomes carry alleles for the Homologous chromosomes carry alleles for the

same genes or characteristicssame genes or characteristics• Each chromosome may carry a different allele of a Each chromosome may carry a different allele of a

gene (e.g. for eye color)gene (e.g. for eye color)

Chapter 11 36

Chapter 11 37

Combination of Parental AllelesCombination of Parental Alleles

Combining the parental chromosomes Combining the parental chromosomes through sexual reproduction can produce through sexual reproduction can produce offspring with allelic combinations that offspring with allelic combinations that may be advantageousmay be advantageous

Sexual reproduction causes variability Sexual reproduction causes variability 1.1. Combinations of gene alleles on one Combinations of gene alleles on one

homologous chromosome are homologous chromosome are combined with combinations of gene combined with combinations of gene alleles on the other homologous alleles on the other homologous chromosomechromosome

Chapter 11 38

Combination of Parental AllelesCombination of Parental Alleles

Sexual reproduction causes variabilitySexual reproduction causes variability

2. Different homologous chromosomes with 2. Different homologous chromosomes with certain alleles are combined with other certain alleles are combined with other homologous chromosomes in a random homologous chromosomes in a random manner… manner…

3. Two gametes produced by meiosis each 3. Two gametes produced by meiosis each contribute their unique allelic combinations contribute their unique allelic combinations to produce a new offspringto produce a new offspring

Chapter 11 39

Meiosis IMeiosis IHomologous

chromosomes pair and cross over

Homologous chromosomes

exchange DNA & align on equator

Homologous chromosomes move to

opposite poles

Prophase IProphase I Metaphase IMetaphase I Anaphase IAnaphase I Telophase ITelophase I

Chapter 11 40

Meiosis IIMeiosis II

Prophase IIProphase II Metaphase IIMetaphase II Anaphase IIAnaphase II Telophase IITelophase IIFourFour

HaploidHaploidCellsCells

Similar to MitosisSimilar to Mitosis

Chapter 11 41

Crossing OverCrossing Over

Homologues Homologues pair uppair up

Protein strands Protein strands zip togetherzip together

Recombination Recombination enzymes snip enzymes snip and rejoin and rejoin DNADNA

Homologs Homologs separate with separate with new gene new gene combinationscombinations

Chapter 11 42 Meiosis vs. Mitosis:Meiosis vs. Mitosis:

Comparison of SpindlesComparison of SpindlesMeiosis: Duplicated chromosomes Meiosis: Duplicated chromosomes with one kinetochore; Paired with one kinetochore; Paired homologues go to opposite poles.homologues go to opposite poles.

Meiosis: Duplicated chromosomes Meiosis: Duplicated chromosomes with one kinetochore; Paired with one kinetochore; Paired homologues go to opposite poles.homologues go to opposite poles.

Mitosis: Duplicated chromosomes Mitosis: Duplicated chromosomes with two kinetochores; Unpaired with two kinetochores; Unpaired homologs split between sister homologs split between sister chromatids, which go to opposite chromatids, which go to opposite poles.poles.

Mitosis: Duplicated chromosomes Mitosis: Duplicated chromosomes with two kinetochores; Unpaired with two kinetochores; Unpaired homologs split between sister homologs split between sister chromatids, which go to opposite chromatids, which go to opposite poles.poles.

Chapter 11 43 Meiosis vs. Mitosis:Meiosis vs. Mitosis:

Comparison of StagesComparison of Stages

Chapter 11 44

Novel Chromosome CombinationsNovel Chromosome Combinations

Genetic variability among organisms is Genetic variability among organisms is essential in a changing environmentessential in a changing environment

Mutations produce new variation but are Mutations produce new variation but are relatively rare occurrencesrelatively rare occurrences

Randomized line up and separation of Randomized line up and separation of homologous chromosomes in Meiotic homologous chromosomes in Meiotic Metaphase I and Anaphase I increase Metaphase I and Anaphase I increase variationvariation

• The number of possible combinations is 2The number of possible combinations is 2nn, , where n = number of homologous pairswhere n = number of homologous pairs

Chapter 11 45

Crossing OverCrossing Over

Variation also enhanced by genetic Variation also enhanced by genetic recombinationrecombination

Crossing over in Meiotic Prophase I creates Crossing over in Meiotic Prophase I creates chromosomes with new allele chromosomes with new allele combinationscombinations

Combined with homologue shuffling in Combined with homologue shuffling in Metaphase/Anaphase I, each gamete Metaphase/Anaphase I, each gamete produced in meiosis is virtually uniqueproduced in meiosis is virtually unique

Chapter 11 46

Metaphase Alignment ScenariosMetaphase Alignment Scenarios

Chapter 11 47

Fusion of GametesFusion of Gametes

Fusion of games from two individuals further Fusion of games from two individuals further increases possible 2n combinationsincreases possible 2n combinations

Gametes from two humans could produce Gametes from two humans could produce about 64 trillion different 2n combinationsabout 64 trillion different 2n combinations

Taken together with crossing over, each Taken together with crossing over, each human individual is absolutely genetically human individual is absolutely genetically uniqueunique

Chapter 11Chapter 11

The endThe end