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📢 ｜ Discover a common mechanism for sex chromosome evolution using the unique sex chromosomes of Drosophila



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Meiotic recombination creates diversity in children with different combinations of genomes than their parents.

1. Overview The sex chromosome is one of the typical sex-determining mechanisms that exist in many organisms.The sex chromosomes were originally stained normally ...　→ Continue reading

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Meiosis (Gensubunretsu,alone: meiosis,British: meiosis) IsEukaryote OfCell divisionOne of the styles.In animalsGameteIt was performed and occurred during the formation of (spores in moss, fern, etc.)Daughter cellsThen.ChromosomeNumbers before division細胞Will be half of.On the other hand, the form that cells normally take when they proliferate isMitosisOrMitosisIs called.The difference in mode from somatic cell division is that homologous chromosomes pair after chromosome replication, and cell division (decreased first division, second division) occurs twice in a row without replicating DNA in the middle. That is.Meiosis in English Meiosis Means "decrease" in Greek.

Meiosis is a phenomenon that was foreseen in the late 19th century.In fertilization, a pair of chromosomes is supplied from the egg and sperm,DiploidWhen it was revealed that cells had one set of chromosomes derived from maternal and paternal lines, it was considered that the number of chromosomes needed to be reduced prior to fertilization.The actual observation isWalter SuttonByLocust OfGerm cellsReported in.From this, it is assumed that the gene is on the chromosome.Chromosome theoryWas proposed.

Biological significance of meiosis

Meiosis is in gametogenesisHeredityIt is thought that it has caused diversification and contributed to the response and evolution of environmental changes.For example, 2 sets of 4ChromosomeIn organisms with, 22= 4 gametes are created with 4 combinations, and the next generation obtained from this is XNUMX2= 16 ways.Human capitalIn the case of, there are 23 sets of homologous chromosomes, a total of 46 chromosomes, so 223= 8,388,608 (a little less than 840 million) gametes, 8,388,6082= There is a possibility that 70,368,744,177,664 (about 70 trillion) next generations will occur.In addition, parts of the chromosomes are replaced in the mechanism that creates genetic diversity.Transfer(Homologous recombination).Transfers are made in the first division of meiosis.The molecular mechanism is that DNA double-strand breaks occur, which isDNA repairOccurs when reconnecting by.

The phenomenon that chromosomes are not distributed normally in meiosis is nondisjunction (chromosome nondisjunction)non-disjunction).Nondisjunction produces gametes with different numbers of chromosomes,Down's syndromeな どChromosomal abnormalitiesCause of.

Chromosome behavior in meiosis

In diploid cells, one chromosome has another similar chromosome (upper left in the figure).These are very similar to each other (the sameIt is a chromosome, and two chromosomes that have such a relationshipHomologous chromosome(homologous chromosomes).One of the homologous chromosomes is inherited from the maternal side and the other from the paternal side. (note:AutosomalWell, that's right, for example, humansSex chromosomesThe two pairs are different in length from each other and the gene contents are not very similar).

Prior to meiosis, the cellsDNA replicationDoDNADouble the amount of.As a result, two chromosomes having the same sequence as the original chromosome are formed (middle left in the figure).These two chromosome pairs with the same genetic informationSister chromatids(Two of the same color in the left of the figure).In mitosis, the doubled chromosomes are inherited by each daughter cell and become the same as the mother cell (upper right).

First division of meiThen, homologous chromosomes consisting of two chromatids are paired with each other and consist of four chromatids.Divalent chromosome(bivalent chromosomes) Is formed (lower left of the figure).After that, each homologous chromosome (two chromatids) divides in different directions, and the first division ends (lower center of the figure; this division is also called reduction division).Continuing, without going through new DNA synthesisSecond division of meiStarts.Two in the second divisionSister chromatidsDivides in another direction (this division is also called equal division).Each of the four daughter cells thus created contains half the amount of DNA as the original cell (bottom right).

In prophase meiosis, between homologous chromosomesTransfer(Also called crossing or crossing;Crossovers) Occurs and replaces some sequences (Recombinant).The structure formed at the site where the homologous chromosome is transferred isChiasmaIs called.Thus, the importance of meiosis lies in the formation of gametes with different combinations of chromosomes that produce various gene combinations by recombination.That is,Meiosis plays a major role in creating the genetic diversity of offspring..Recent studies have also revealed that the crossover process itself is an essential event for normal chromosome segregation.

Cell cycle of meiosis

The meiotic cycle of vertebrates is

  1. Stopping division in the first meiotic prophase and releasing the stop by hormone stimulation (activation of MPF [Cdc2 / Cyclin complex])
  2. Omission of S phase at the transition of the first and second divisions (partial inactivation / reactivation of MPF by partial decomposition / resynthesis of Cyclin)
  3. Division arrest in metaphase of the second meiosis (maintenance of MPF activity by CSF), deactivation by fertilization stimulation (MPF inactivation by complete decomposition of Cyclin, etc.)

It has a very important feature that is different from the mitotic cycle in three points.


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