CH 8

Reproduction and Inheritance

Reproduction• Asexual Reproduction• Sexual Reproduction

LM 3

40

Prokaryoticchromosome

Plasmamembrane

Cell wall

Duplication of chromosomeand separation of copies

1

Continued elongation of thecell and movement of copies2

Division intotwo daughter cells

3

Prokaryotes are asexual• Via binary fission

– Recall that Prokaryotes have circular DNA

Figure 8.3A

Prokaryotic chromosomes

Eukaryotes

• Complex cell division

• Chromosomes occur as chromatin unless dividing

• Individual chromosomes visible when cell is dividing

LM 6

00

Chromosomes• DNA organizes into chromosomes

– Chromosomes duplicate as cell prepares to divide– After duplication, each chromosome consists of 2

sister chromatids• Constricted at centromere

• 23 pair of homologous chromosomes in humans

Fig. 8-4bc

Sister chromatids

Centromere

Chromosomeduplication

Sister chromatids

Chromosomedistribution

todaughter

cells

Mitosis & Meiosis• Mitosis- nuclear division that maintains

chromosome numbers• Meiosis- nuclear division that halves the

chromosome number

Cell cycle

• Ordered sequence of events from time a cell is first formed until its own division– Growth phase

• Interphase

– Division phase (mitotic phase)• Mitosis • Cytokinesis

Fig. 8-5

S(DNA synthesis)G1

G2

Cytokinesis

Mito

sis

INTERPHASE

MITOTICPHASE (M)

Interphase

• G1– Cell growth before DNA replication– Contains nucleoli indicating cell is making proteins

• S– DNA replication

• G2– Second stage of growth before division– Make proteins to drive mitosis

Cell cycle control system

• Set of molecules that triggers and coordinates key events in cell cycle– Checkpoints

• Cell is set to STOP until told to GO• Some cells stuck in “stop”, i.e. nerve cells always in G1

Mitosis

• Prophase• Metaphase• Anaphase• Telophase• Cytokinesis

Prophase• Chromatin fibers more tightly coiled and folded

– Form discrete chromosomes– Nucleoli disappear– Duplicated chromosomes appear and joined at

centromere– Nuclear envelope beings to dissolve

• Centromsomes duplicate and move to opposite ends of nucleus– Mitotic spindle forms in cytoplasm

Prophase

Mitotic Spindle• Centrosome

– Region near the nucleus that organizes microtubules– Two barrel-shaped centrioles (not found in plant cells)– Microtubules grow from centrosome to form a spindle

• The spindle attaches to and moves chromosomes during nuclear division– Attach to chromosome at kinetochore– Attach to cell wall– Add or loose subunits to push and pull chromosomes apart

Prometaphase/Metaphase

• Nuclear envelope fragments and disappears• Kinetochore visible• Mitotic spindle formed and microtubules

attach to sister chromatids• Chromosomes line up at metaphase plate• Centromeres of chromosomes line up

Fig. 8-6ad

PROMETAPHASE METAPHASE

Anaphase

• Two centromeres of each chromosome come apart– Motor proteins on spindle drag chromatids apart

• Sister chromatids separate• Poles move farther apart, elongating cell• Complete collection of chromosomes at each

pole

ANAPHASE

Telophase

• Telophase– Nuclear envelope reforms– Chromosomes uncoil into chromatin– Nucleoli reappear

• Cytokinesis– Cell divides in two

TELOPHASE

Fig. 8-6a

Centrosomes(with centriole pairs) Kinetochore

Early mitoticspindle

Chromatin

INTERPHASE PROMETAPHASEPROPHASE

Centrosome Fragmentsof nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Nuclearenvelope

Spindlemicrotubules

Nucleolus

Centromere

Fig. 8-6b

Metaphaseplate

Nucleolusforming

METAPHASE TELOPHASE AND CYTOKINESISANAPHASE

Cleavagefurrow

Daughterchromosomes

NuclearenvelopeformingSpindle

Fig. 8-5b (6), p. 141

Stepped Art

6) TelophaseThe chromosomes reach the spindle poles and decondense. A nuclear envelope forms around each cluster. Mitosis is over.

centrosome 1) Early ProphaseMitosis begins. In the nucleus, the DNA begins to appear grainy as it organizes and condenses. The centrosome is duplicated.

2) Prophase The chromosomes become visible as distinct structures as they condense further. Microtubules assemble and move one of the two centrosomes to the opposite side of the nucleus, and the nuclear envelope breaks up.

pole pole 3) Transition to MetaphaseThe nuclear envelope is gone, and the chromosomes are at their most condensed. Spindle microtubules assemble and attach sister chromatids to opposite spindle poles.

microtubule of spindle

4) Metaphase All of the chromosomes are aligned midway between the spindle poles. Microtubules attach each chromatid to one of the spindle poles, and its sister to the opposite pole.

5) Anaphase Motor proteins moving along spindle microtubules drag the chromatids toward the spindle poles, and the sister chromatids separate. Each sister chromatid is now a separate chromosome.

Cytokinesis• Cleavage- Animal cells

– Starts in telophase or late anaphase– Cleavage furrow

• Shallow groove on cell formed via contractile ring• Microfilaments draw together and split cell in two

• Cell wall– Vesicles containing cell wall material form cell

plate• Form cell plate that grows out to fuse with existing wall

Cytokinesis

Cytokenesis

Cleavagefurrow

Cleavage furrow

Contracting ring ofmicrofilaments

Daughter cellsCell plate

Cell wall

Vesicles containingcell wall material

Wall ofparent cell

New cell wall

Daughter cells

Cell plateforming

Cell Division Control

• Growth factors– Proteins that stimulate cell to divide

• Density-Dependent inhibition– Stop cells from dividing under crowded conditions

• Anchorage dependence– Need surface on which to divide

G1 checkpoint

Controlsystem

M

S

G2

G1

M checkpoint

G2 checkpoint

G0

Cell cycle control system

Set of molecules that triggers and coordinates key events in cell cycle

Control

Fig. 8-9b

G1 checkpoint

Controlsystem

M

S

G2

G1

Receptorprotein

Signaltransductionpathway

Relayproteins

Plasma membrane

Growth factor

Out of control

• Cancer– Do not respond to cell control system– No density-dependent inhibition– Divide indefinitely– No anchorage dependence

Meiosis terms• Somatic cells- non-reproductive cells• Gametes- reproductive cells (sex cells)• Homologous chromosomes- chromosomes with

same genes at same loci• Sex chromosomes- determine sex• Autosomes- non-sex chromosomes• Diploid- 2 sets of chromosomes• Haploid- single set of chromosomes• Alleles- different forms of the same gene

Fig. 8-13

Haploid gametes (n = 23)

nnEgg cell

Sperm cellFertilizationMeiosis

Multicellulardiploid adults

(2n = 46)

Mitosis anddevelopment

nn

22nn

Diploidzygote

(2n = 46)

Meiosis

• Produces haploid gametes in diploid organisms

• Duplication of chromosomes– Two cell divisions– Form a tetrad

Crossing over• A chromosome and its homologous partner

exchange a corresponding piece of DNA

crossover

Fig. 8-10a, p. 146

Fig. 8-10b, p. 147

Fig. 8-15

Prophase

Metaphase IMetaphase

2n = 4

Tetradsalign at themetaphase plate

Duplicatedchromosome(two sisterchromatids)

Parent cell(before chromosome duplication)

Chromosomeduplication

Chromosomesalign at themetaphase plate

AnaphaseTelophase Sister chromatids

separate duringanaphase

Daughter cellsof mitosis

2n 2n

n

Chromosomeduplication

Site ofcrossing over

Tetrad formedby synapsis ofhomologouschromosomes

MEIOSIS

Prophase I

Anaphase ITelophase I

MITOSIS

MEIOSIS I

Haploidn = 2

Daughtercells of

meiosis I

MEIOSIS II

n n n

Daughter cells of meiosis II

Homologouschromosomesseparate(anaphase I);sister chroma-tids remaintogether

No furtherchromosomalduplication;sisterchromatidsseparate(anaphase II)

Diversity

• Random arrangement of homologous chromosomes

• Different gene versions• Genetic recombination

– “Crossing over”

Nondisjunctionin meiosis I

Normalmeiosis II

n + 1

Gametes

Number of chromosomes

n + 1 n – 1 n – 1

Error

•Abnormal sex chromosomes

•Trisomy 21

Error

• Down syndrome– Extra copy of

chromosome 21

Videos• Overview

– http://www.youtube.com/watch?v=3kpR5RSJ7SA&feature=related

– http://www.youtube.com/watch?v=s4PaOz7eWS8&feature=related

• Cytokinesis– http://www.youtube.com/watch?v=mzeowbIxgwI

• Meiosis– http://www.youtube.com/watch?v=D1_-mQS_FZ0&featur

e=related– http://www.youtube.com/watch?

v=3xtD8uUZBhM&feature=related

Videos

• Mutations and DNA• http://www.youtube.com/user/

greatpacificmedia#p/u/43/hkK5hDbxJ0M