2Chromosome Review Let’s review… What is a chromosome? Condensed chromatin (DNA + proteins)only visible during cell division!!How many chromosomes do you have in each of your cells?46 (23 pairs)But wait..do any of your cells NOT have 46?SEX CELLS!
3Chromosome Number What does it mean to have 23 pairs of chromosomes? 23 chromosomes: one copy of each from mom & one from dad!Cells that have a chromosome from each parent are DIPLOID (2N)!Somatic cells (body cells)“two sets”
4contain one set of genes! Chromosome NumberSome cells don’t have a pair of each chromosome!Gametes (sex cells) of sexually reproducing organismsThese cells are HAPLOID (N)!“one set”Single set of genes!Why do they only have half thenumber of chromosomes?Each gamete can onlycontain one set of genes!They COMBINE toBecome DIPLOID!
5Chromosome Structure Review ChromatidOne strand of a duplicated chromosomeJoined by a centromere to its sister chromatidSister chromatidsTwo chromatids joined by a common centromereEach carries identical genetic informationTogether called a DYAD
6MeiosisThe genetic information that we (and other sexually reproducing organisms) inherit comes from two cells: sperm and eggMeiosis – cell division in which the chromosome number is cut in halfGametes (sperm and egg) divide this way!Germ cells in the testis and ovaryundergo meiosis and producegametes.
7Meiosis: Reduces the number of chromosomes/cell by half Occurs in two steps:Meiosis IMeiosis IIMeiosis I and II each have prophase, metaphase, anaphase, and telophase stages2N = 4N = 2
8Meiosis2 dyadsDNA replication occurs during interphase before the beginning of meiosis I but not before meiosis IIChromosomes will associate with the other member of its pair during MeiosisHomologous chromosomes (also called a TETRAD)1 tetrad
9MeiosisMeiosis I is a reductional division: the number of centromeres is reduced by half after this divisionMeiosis II is an equational division: the number of centromeres remains equal after this division
10Meiosis 1: Prophase 1 Prophase I Chromosomes become visible Homologous chromosomes pair up to form tetradsconsumes 90% of the time for meiosisCrossing Over occursResults in Genetic VariationNew allele combinations!
11Two major sources of genetic variation in Meiosis I Crossing OverCreates new combinations of mom and dad’s allelesThink about chromosomal mutations!Independent Assortment
12Metaphase 1 As prophase I ends, tetrads attach to spindle fibers Tetrads line up at center of cell
13Anaphase 1 Dyads pulled toward opposite poles by spindle fibers Disjunction: separation of chromosomesNondisjunction leads to polyploidy! (extra chromosomes)Note the exchange of information between paternal and maternal chromosomes
14Telophase 1 Separated chromosomes cluster at opposite ends of cell Nuclear membrane forms around each clusterCytokinesis follows and forms 2 new cells
16This was a reductional division: number of centromeres reduced per cell. Prophase I: 4 centromeres, therefore 4 chromosomesProphase II: 2 centromeres, therefore 2 chromosomes/cell
17Results of Meiosis 1 Two daughter cells Neither with two complete sets of chromosomes (haploid)Sets have been shuffled and independently assortedChromosomes differ between each other and the original cell
18Meiosis II Two cells enter second meiotic division Neither cell goes through DNA replication prior to this division!
19Prophase II Chromosomes (dyads) become visible Do not form tetrads because they are already separated from homologous pair!
20Metaphase II & Anaphase II Chromosomes (dyads) attach to spindle fibers and line up in center of cellRemember…they aren’t paired with another chromosome!Anaphase II – chromatids (monads) separate from each other at the centromeremove to opposite poles!
21Telophase II & Cytokinesis Four genetically different haploid cells produced (N)Each monad may be an entirely new combination of maternal and paternal genetic information
23Meiosis Review Meiosis II Meiosis I Prophase II: Dyads reappear, no tetrads!Metaphase II: Dyads line up in middleAnaphase II: Monads pulled apart (separated at centromere)Telophase II: four new genetically different haploid daughter cells with monadsMeiosis IProphase I: tetrads form (homologous chromosomes pair), crossing over occursMetaphase I: tetrads line up in middleAnaphase I: Dyads pulled apartTelophase I: two new genetically different haploid daughter cells with dyads
24Meiosis: Formation of Gametes Meiosis results in two kinds of haploid, sexual gametesMales produce spermFemales produce eggs (usually only one of the four egg cells is used!)Sperm fertilizesegg to produce2N zygote!Goes through mitosis& cell specializationto form a neworganism!
25Mitosis vs. Meiosis: a Comparison Both preceded by DNA replicationBoth are methods of cell divisionBoth include Prophase, Metaphase, Anaphase, and TelophaseBoth are followed by cytokinesisMitosis vs. Meiosis
26Contrasting Meiosis & Mitosis Each daughter cell receives a complete set of chromosomesLess genetic diversityDoesn’t change the chromosome number of the original cellSingle cell divisionTwo genetically identical diploid daughter cellsAsexual reproductionMakes somatic cellsMeiosisTwo alleles for each gene segregated and end up in different cellsGreater variety of possible gene combinationsReduces the chromosome number by halfTwo rounds of cell divisionFour genetically different haploid daughter cellsSexual reproductionMakes gametes
28Gene Linkage Genes on different chromosomes assort independently What about genes on the same chromosome?Tend to be linked!Chromosomes assort independently, but typically genes on the samechromosome are inherited togetherEspecially when close together!Crossing over causes some genes onthe same chromosome to assortindependently
29Gene MapsFrequency of crossing-over between genes during meiosis is used to determine genes’ locationsFarther apart, more likely that crossing over occurs between themClose together, crossovers rareUse frequency of crossing over to determine distances from each other and map genes’ locations on chromosomes!