All eukaryotic cells have the same cell-division sequence {cell cycle}| and cell-cycle controls.
After mitosis, cells pinch membrane in at middle until both sides meet, and then cell splits to form two cells {cytokinesis}. Mitosis and cytokinesis are independent processes.
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.
Cell-nucleus DNA can replicate to make two DNA sets {mitosis}|. Growing cells spend 5% of time in mitosis.
cell cycle
All eukaryotic cells have the same cell-division sequence and cell-cycle controls. First, cell differentiates and grows but does not divide, in gap 1 stage. Then cell begins cell division. DNA synthesizes, and chromosomes replicate, in S stage. Then cell grows with no DNA replication, in gap 2 stage. In M stage, mitosis has chromosome doubling prophase, chromosome pairing metaphase, chromosome separation on spindles anaphase, and cell-nucleus splitting telophase. The long interphase between mitoses includes gap-1, S, and gap-2 stages.
mitosis start
Nuclear-to-cytoplasmic volume ratio controls mitosis. Cell division starts after cell grows enough. Perhaps, hormones control mitosis.
proteins
More than 60 cell-division-cycle (CDC) proteins control cell cycle. They regulate metabolism in non-growing cells, trigger mitosis, and coordinate growth and division in growing cells.
cytokinesis
After mitosis, cell splits, as membrane pinches in, to make two cells. Mitosis and cytokinesis are independent processes.
yeast
In Saccharomyces cerevisiae budding yeast, bud is start of DNA synthesis. Bud grows throughout other stages. Cell nucleus differs in gap 2 and mitosis. In Schizosaccharomyces pombe fission yeast, cell length shows cell-cycle stage. Cell nucleus differs in gap 2 and mitosis. Budding yeast CDC28 and fission yeast cdc2 make serine, threonine kinase, which binds to different cyclins to make proteins {maturation promoting factor} (MPF) that start cell division and act from gap 2 to mitosis. Factor complex breakup ends mitosis.
temperature
Mutant organisms {temperature-sensitive organism} can have temperature-sensitive proteins and die or stop growing at higher temperature.
metaphase stoppage
Colcemid, trypsin, and Giemsa stain stop cells at metaphase. Typically, bromodeoxyuridine precedes chemical stoppage.
First mitosis phase {prophase, mitosis} doubles chromosomes. Chromosomes condense, and human chromosomes have 350 to 550 bands. RNA-containing spindle-pole body centrosomes separate to cell sides. Microtubules form between centrosomes. Chromosome kinetochores attach to microtubules.
Second mitosis phase {metaphase} pairs chromosomes. Chromosomes align halfway between centrosomes.
Third mitosis phase {anaphase} separates chromosomes on spindles. Chromosomes move toward spindle poles.
Last mitosis phase {telophase} splits cell nucleus. Poles move apart.
Between mitoses, long phase {interphase} includes gap-1, S, and gap-2 stages.
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Date Modified: 2022.0225