1、CELL DIVISIONMITOSIS&MEIOSISInterphaseMitoticCell Cycle2 distinct phasesWhats the most important event of interphase?Chromosome duplicationSG2All chromosomes are duplicatedDo they contain identical genes?What is significant about DNA in the S and G2 phases?That means:two copies of each chromosomeWha
2、t are the copies called?Sister chromatidsYes,butWhat about meiosis?Some terminology Double-chromatid chromosomes Single-chromatid chromosomes Chromatin Homologous chromosomes ChromatidWhen do each of these occur?Interphase(actually,this is not part of mitosis itself)Prophase Metaphase Anaphase Telop
3、haseTHE STEPS OF MITOSISOnion root tipWhitefish blastulaInterphase Nuclear envelop intact Chromosomes duplicated but not apparent 2 nucleoli visible in onion root tip Nucleoli not visible in whitefish blastulaMitosis DNA loose,uncondensed,called chromatinOnion root tipWhitefish blastulaProphaseMitos
4、is Double-chromatid chromosomes evident Chromatin becomes super-coiled&compact Nuclear envelop breaks down Nucleoli disappear in onion root tip Centrioles migrate toward poles of cell forming the spindle Double-chromatid chromosomes Onion root tipWhitefish blastulaMetaphase Mitosis Double-chromatid
5、chromosomes line up on equatorial plate of cell The spindle fibers attach to the chromosomes at the centromeresOnion root tipWhitefish blastulaAnaphase Mitosis Centromeres are pulled apart Groups of single-chromatid chromosomes move opposite poles of the cell Spindle fibers pull chromatids apartOnio
6、n root tipWhitefish blastulaTelophase Mitosis Cytokinesis begins with appearance of cleavage furrow Cytokinesis begins with appearance of cell plate Groups of single-chromatid chromosomes reach poles of cell Nuclear envelop begins to reform Nucleoli reform 2 new daughter cells formedOnion root tipWh
7、itefish blastulaReturning to Interphase Mitosis Cytokinesis completes Chromosomes disappear as interphase chromatin reforms 2 new daughter cells enter G1 phase of the cell cycleA SUMMARY OF MITOSISMitosis Questions1.What does“diploid”mean?2.We have _ total chromosomes.3.In the term 2n,what does“n”st
8、and for in us?In a gypsy moth?4.Why does mitosis occur?Major functions?5.In what cells(general term)does mitosis occur?Mitosis Questions6.What are chromatids?7.What happens in Anaphase to result in each new cell receiving duplicate parental DNA?8.In a species whose diploid number is 224,what would i
9、ts sperm/eggs contain?Why is this duplication of parental DNA necessary?Something to do with passing on genetic information?Why undergo mitosis at all?Something to do with cells getting damaged,old,lost?Something to do with the organism growing,infant to adult?SOMATIC CELLS What is the process of so
10、matic cell duplication called?What are typical body cells called?These cells divide continuouslyThe new cells receive an exact copy of all the parent cells:Mitosis DNAWhat are these somatic cells?Diploid or 2nWhat does this mean?They contain the full number of chromosomesin pairsHow many in humans?4
11、623 pairsMitosis occurs only in somatic cells Gametes are not diploid(2n)Instead,they are haploid(n)What about sex cells?Called gametes Eggs and spermProduced in ovaries or testesOur haploid(n)number is 23So our eggs and sperm have how many chromosomes?Half the numberWhy?Fertilization is the union o
12、f an egg and a spermIf the egg and sperm were both diploid,what would the fertilized egg(zygote)be?It would be a genetic mess!At fertilization,n+n=23;23+23=46!Instead,gametes are haploid(n).Egg and sperm both have exactly half the number of chromosomes of somatic cellsNote how mitosis and meiosis di
13、ffer:Number of divisions?Number of chromosomes?Number of products?Meiosis!Meiosis is the cell division process that enables the transformation from 2n to nSomehow somatic cells(2n)in our ovaries or testes must produce gametes(n)How is meiosis more complicated than mitosis?Each chromosome has a partn
14、er Gametes must contain precisely half the diploid number of chromosomesThey must contain one of each homologous pair of chromosomesRemember karyotypes?They come in pairsOne from momOne from dadHuman KaryotypeWhich pair of chromosomes in us in not homologous?All our other pairs of chromosomes are ho
15、mologousIts the 23rd pair in males,the XY pairRemember what homologous means?What are the divisions called?How many divisions does meiosis have?12341.One pair of homologues to start2.DNA is replicated3.Meiosis 1=2n-n4.Meiosis 2=chromatids separate into 4 productsIn males,how many progeny are produce
16、d?Typically 4 viable sperm are produced following each Meiosis 2In females,how many progeny are produced?Just one viable ovum(egg)is produced,plus 3 small polar bodiesCrossing overWhen does it occur?Only during Prophase of Meiosis 1Homologous chromosomes get together in temporary tetradsOverlap(cros
17、s over)and trade their DNAWhy is this a good thing to do,generally?Meiosis Summary1.Meiosis 1a.DNA replication takes placeb.A parent cell produces two daughter cells each with one member of each original pair of homologous chromosomes(to create haploid daughter cells)c.Crossing over may occurMeiosis
18、 Summary2.Meiosis 2a.There is no more DNA replicationb.The chromatids of each chromosome separate and each daughter cell dividesc.At the end of Meiosis 2,there are 4 daughter cells from each parent cell.Each daughter cell has half the number of chromosomes as the parent cell1.The cells produced in meiosis are _(haploid or diploid?)2.Sex cells are called _._ are produced by males,_ by females.3.Whats crossing over and why is it important?Meiosis Questions4.In what meiotic stage does crossing over occur?5.Why are sex cells n,not 2n?6.If a species diploid number is 50,what is n?Meiosis Questions
