Diploid eukaryotic germ cells produce haploid sex-cell gametes {meiosis, biology}|, such as sperm and eggs. One germ cell makes four haploid cells. First stage is prophase. Next stage is segregation. Then cell divides, making two diploid cells {doublet, cell}, with doubled, paired chromosomes. In both cells, chromosomes separate to cell sides on spindles. Then nucleus splits. Cytokinesis makes each diploid cell into two cells.
First meiosis phase {prophase, meiosis} has synapsis, replication, and tetrad formation. First, homologous chromosomes have synapsis. Then all chromosomes replicate, by semiconservative replication, except at chromosome centromere, where chromosome pairs remain attached. Then homologous, doubled chromosomes synapse to make tetrads.
In meiosis prophase, diploid germ cell pairs homologous chromosomes {synapsis}.
After synapsis, paired nucleic acids replicate to make two new nucleic acids {semiconservative replication, meiosis}.
Homologous, doubled chromosomes synapse to make four-chromosome groups {tetrad}, attached at centromeres.
One chromosome pair from each tetrad goes to one centriole pole, and the other chromosome pair goes to other centriole pole {segregation, chromosome}| {chromosome segregation}. Because any two chromosomes can segregate, meiosis increases variation.
At meiosis prophase first synapsis makes chromosome pairs. Pairs can interchange chromosome segments {recombination, DNA}| {homologous recombination} {crossing-over}.
process
Both double helices unwind. Enzyme splits homologous strands at same positions. Ends can reattach to other homologous strand or repair themselves. Both double helices rewind.
crossing over: one cut
Enzymes can split homologous nucleic acids at the same position. Ends can reattach so halves exchange. Left end is from one nucleic acid, and right end is from other nucleic acid. Left end is from other nucleic acid, and right end is from one nucleic acid.
crossing over: two cuts
Enzymes can split homologous nucleic acids at same two positions. Ends can reattach so middle section exchanges. Left end is from one nucleic acid, middle is from other nucleic acid, and right end is from one nucleic acid. Left end is from other nucleic acid, middle is from one nucleic acid, and right end is from other nucleic acid.
recombination
Recombination makes strands with different allele sequences. Because recombination mixes alleles, meiosis increases variation. Only homologous chromosomes have recombination, because only homologous chromosomes pair and because enzymes can split them at same place. Yeast has high recombination.
Gene-middle recombination inactivates genes. Gene-end recombination allows recombined genes to replace original genes {transplacement}. Transplacement can replace normal genes with inactive genes, so cell loses gene function or product {gene knockout}. For experiments, knockout mice can have gene deactivation.
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Date Modified: 2022.0225