ch 8 reproduction and inheritance. reproduction asexual reproduction sexual reproduction lm 340
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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
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
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
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
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
Telophase
• Telophase– Nuclear envelope reforms– Chromosomes uncoil into chromatin– Nucleoli reappear
• Cytokinesis– Cell divides in two
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
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. 8-15
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
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