2 Sexual life cycle Made up of meiosis and fertilization Diploid cells Somatic cells of adults have 2 sets of chromosomesHaploid cellsGametes have only 1 set of chromosomesAllows offspring to inherit genetic material from 2 parents
4 In most animals, diploid state dominates Life cycles of sexually reproducing organisms involve the alternation of haploid and diploid stagesSome life cycles include longer diploid phases, some include longer haploid phasesIn most animals, diploid state dominatesZygote first undergoes mitosis to produce more diploid cellsmulticellularityLater in the life cycle, some of these diploid cells undergo meiosis to produce haploid gametes(sperm & eggs)
6 Meiosis DNA replicates (chromosome # doubled) Meiosis I Stages: Prophase I – synapsis occursMetaphase I – homologous chromosome pairs line up (4 chromosomes)Anaphase I – homologous chromosomes seperateTelophase IResults in 2 cells, but still diploid numberMeiosis IIStagesProphase IIMetaphase IIAnaphase II – sister chromosomes seperateTelophase IIResults in 4 cells, haploid number, one of each homologous pair in every cell
7 Features of Meiosis Meiosis includes 2 rounds of division Meiosis I and meiosis IIEach has prophase, metaphase, anaphase, and telophase stagesSynapsis (crossing-over)During early prophase IHomologous chromosomes become closely associated (eventually swapping DNA)Includes formation of synaptonemal complexesFormation also called tetrad or bivalents
8 First meiotic division is termed the “reduction division” Results in daughter cells that contain one homologue from each chromosome pairNo DNA replication between meiotic divisionsSecond meiotic division does not further reduce the number of chromosomesSeparates the sister chromatids for each homologueMeiosis II is very much like mitosis (but half the DNA)
10 Prophase IChromosomes coil tighter and become visible, nuclear envelope disappears, spindle formsEach chromosome composed of 2 sister chromatidsSynapsis (crossing-over)Homologues become closely associatedCrossing over occurs between nonsister chromatidsNonsister chromatids remain attached at chiasmataChiasmata move to the end of the chromosome arm before metaphase I
12 Crossing over Occurs between nonsister chromatids Allows the maternal and paternal homologues to exchange chromosomal material (recombination)Alleles of genes that were formerly on separate homologues can now be found on the same homologueChiasmata – site of crossing overContact maintained until anaphase I
13 Metaphase ITerminal chiasmata hold homologues together following crossing overMicrotubules from opposite poles attach to each homologue (not each sister chromatid)Homologues are aligned at the metaphase plate side-by-sideOrientation of each pair of homologues on the spindle is random(This sets up independent assortment)
15 Anaphase I Microtubules of the spindle shorten Chiasmata break Homologues are separated from each other and move to opposite polesSister chromatids remain attached to each other at their centromeresEach pole has a complete haploid set of chromosomes consisting of one member of each homologous pairIndependent assortment of maternal and paternal chromosomes occurs
16 Telophase I Nuclear envelope re-forms around each daughter nucleus Sister chromatids are no longer identical because of crossing over (prophase I)Cytokinesis may or may not occur after telophase IMeiosis II occurs after an interval of variable length
17 Meiosis II Resembles a mitotic division But with a haploid rather than diploid cellProphase II: nuclear envelopes dissolve and new spindle apparatus formsMetaphase II: chromosomes align on metaphase plateAnaphase II: sister chromatids are separated from each otherTelophase II: nuclear envelope re-forms around 4 sets of daughter chromosomes; cytokinesis follows
18 Prophase II Following a brief interphase, with no S phase, meiosis II begins.During prophase II,a new spindle apparatus formsin each cell, and the nuclearenvelope breaks down.In some species the nuclearEnvelope does not re-form intelophase I, removing the need forNuclear envelope breakdown.
19 Metaphase II In metaphase II, chromosomes consisting of sister chromatidsjoined at the centromere alignalong the metaphase plate ineach cell.Now, kinetochore microtubules from opposite poles attach to kinetochores of sister chromatids, as in mitosis.
20 Anaphase II When microtubules shorten In anaphase II, sister chromatidsare pulled to opposite polesof the cells, as in mitosis.
21 Telophase II The nuclear membranes re-form around four different clusters ofchromosomes.After cytokinesis, four haploidcells result.No two cells are alikedue to the random alignment ofhomologous pairs atmetaphase I and crossing overduring prophase I.
22 Final result of Meiosis Four cells containing haploid sets of chromosomesIn animals, these cells develop directly into gametesIn Spermtogenesis: 4 spermIn Oogenesis: 1 egg, 3 polar bodiesIn plants, fungi, and many protists, haploid cells then divide mitoticallyProduce greater number of gametes
23 Errors in MeiosisNondisjunction – failure of chromosomes to move to opposite poles during either meiotic divisionAneuploid gametes – gametes with missing or extra chromosomesMost common cause of spontaneous abortion in humans
24 Meiosis vs. Mitosis Meiosis is characterized by 4 features: Synapsis and crossing overSister chromatids remain joined at their centromeres throughout meiosis IKinetochores of sister chromatids attach to the same pole in meiosis IDNA replication is suppressed between meiosis I and meiosis II