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Unlike mitosis, meiosis aims for genetic diversity in its creation of haploid gametes. Dividing germ cells first begin this process in prophase I, where each chromosome—replicated in S phase—is now composed of two sister chromatids (identical copies) joined centrally.
The homologous pairs of sister chromosomes—one from the maternal and one from the paternal genome—then begin to align alongside each other lengthwise, matching corresponding DNA positions in a process...
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Crossing over is the exchange of genetic information between homologous chromosomes during prophase I of meiosis I. Genetic recombination gives rise to allelic diversity in the newly formed daughter cells. In humans, crossing over produces genetically distinct haploid egg and sperm cells that undergo fertilization to produce unique offspring. Before cell division starts, the germ cell’s chromosome(s) undergo duplication in the S phase of the cell cycle. As the cells enter prophase I,...
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Other than maintaining genome stability via DNA repair, homologous recombination plays an important role in diversifying the genome. In fact, the recombination of sequences forms the molecular basis of genomic evolution. Random and non-random permutations of genomic sequences create a library of new amalgamated sequences. These newly formed genomes can determine the fitness and survival of cells. In bacteria, homologous and non-homologous types of recombination lead to the evolution of new...
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Meiosis is a carefully orchestrated set of cell divisions, the goal of which—in humans—is to produce haploid sperm or eggs, each containing half the number of chromosomes present in somatic cells elsewhere in the body. Meiosis I is the first such division, and involves several key steps, among them: condensation of replicated chromosomes in diploid cells; the pairing of homologous chromosomes and their exchange of information; and finally, the separation of homologous chromosomes by...
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Meiosis is the division of a diploid cell into haploid cells forming sperm and eggs in animals through differentiation. Meiosis I is the first stage of meiosis, where the genetic recombination of homologous chromosomes and the reduction of the ploidy level by half occurs.
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Meiosis II02:02

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Meiosis II entails cell division and segregation of the sister chromatids, resulting in the production of four unique haploid gametes. The steps for meiosis II are similar to mitosis, except that meiosis II occurs in haploid cells, whereas mitosis occurs in diploid cells.
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Preparing Maize Synaptonemal Complex Spreads and Sequential Immunofluorescence and Fluorescence In Situ Hybridization.

Methods in molecular biology (Clifton, N.J.)·2019
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Sequential Immunofluorescent Light Microscopy and Electron Microscopy of Recombination Nodules During Meiotic Prophase I.

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Frequency and Distribution of Crossovers in Caenorhabditis elegans Meiosis by SNP Genotyping using Real-time PCR
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Meiotic Crossing Over in Maize Knob Heterochromatin.

Stephen M Stack1, Lindsay A Shearer2, Leslie Lohmiller2

  • 1Department of Biology, Colorado State University, Fort Collins, Colorado 80523 stephen.stack@colostate.edu.

Genetics
|January 22, 2017
PubMed
Summary

Crossing over occurs in maize knob heterochromatin, challenging previous assumptions. While not suppressed by SC length, it is reduced by DNA density within these regions.

Keywords:
crossing overheterochromatinknobsmaizesynaptonemal complex

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Area of Science:

  • Genetics
  • Molecular Biology
  • Cytogenetics

Background:

  • Recombination (crossing over) is known to be suppressed in centromeric and NOR-associated heterochromatin.
  • This suppression has been broadly generalized to all heterochromatin, but this may not be accurate.

Purpose of the Study:

  • To investigate the relationship between crossing over and heterochromatin not associated with centromeres or NORs.
  • Specifically, to examine recombination frequencies within maize interstitial knobs.

Main Methods:

  • Utilized fluorescence in situ hybridization (FISH) to locate maize 180-bp knob repeat heterochromatin.
  • Employed fluorescent immunolocalization of MLH1 and AFD1 proteins to identify recombination nodules (MLH1 foci) on synaptonemal complexes (SCs).

Main Results:

  • MLH1 foci were observed at similar frequencies per unit SC length in interstitial knobs and adjacent euchromatin.
  • Crossing over occurs within knob heterochromatin and is not suppressed relative to SC length.
  • However, crossing over is suppressed relative to DNA length in knobs due to higher DNA density.

Conclusions:

  • Heterochromatin, specifically maize knobs, can accommodate crossing over.
  • The relationship between heterochromatin and recombination is complex and depends on the context (SC length vs. DNA length).
  • Generalizing recombination suppression to all heterochromatin is not fully justified.