Metaphase I • Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) • Homologous pairs of chromosomes align along the equator of the cell.
Anaphase I • Spindle fibres contract and split the bivalent • Homologs separate and move to opposite poles. • Sister chromatids remain • attached at their centromeres .
Telophase I • Chromosomes decondense. • Nuclear envelopes may reassemble. • Spindle disappears. • Cytokinesis divides cell into two haploid daughter cells.
Prophase II: ● No interphase ● No crossing over ● Spindle forms Metaphase II: ● Chromosomes line up Anaphase II: ● Sister chromatids separate Telophase II: ● 4 haploid cells ● Nuclei reappear ● Each daughter cell genetically unique Meiosis II (2nd division) = create gametes - 4 phases
Meiosis II Only one homolog of each chromosome is present in the cell . Meiosis II produces gametes with one copy of each chromosome and thus one copy of each Sister chromatids carry identical genetic information . Gene X
Meiosis II: Reducing Chromosome Number Prophase II Metaphas e II Anaphas e II Telophas e II 4 Identical haploid cells
Prophase II • Chromosomes condense, nuclear membrane dissolves, centrioles move to opposite poles (perpendicular to before) • Nuclear envelope fragments.
Metaphase II • Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) • Chromosomes align along equator of cell .
Anaphase II • Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles • Sister chromatids separate and move to opposite poles . Equator Pole
Telophase II Nuclear envelope reassembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two to form four haploid daughter cells .
Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome
3 Sources of Genetic Variation: 1. Crossing Over ● Exchange genetic material ● Recombinant chromosomes ● Offspring with recombinant chromosomes will have unique gene combinations that are not present in either parent
1. Independent Assortment of Chromosomes ● Random orientation of homologous pairs in Metaphase I ● When they line up, their orientation towards the opposing poles is random , as is the subsequent assortment of chromosomes into gametes ● The orientation of each bivalent occurs independently , meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in Anaphase I ● The final gametes will differ depending on whether they got the maternal or paternal copy of a chromosome after Anaphase I.
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