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Related Concept Videos

Nondisjunction01:29

Nondisjunction

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During meiosis, chromosomes occasionally separate improperly. This occurs due to failure of homologous chromosome separation during meiosis I or failed sister chromatid separation during meiosis II. In some species, notably plants, nondisjunction can result in an organism with an entire additional set of chromosomes, which is called polyploidy. In humans, nondisjunction can occur during male or female gametogenesis and the resulting gametes possess one too many or one too few chromosomes.
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Nondisjunction01:21

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Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate correctly and move to the opposite poles of the cells. This produces daughter cells with abnormal chromosome numbers.  Nondisjunction is common during anaphase I or anaphase II of meiosis.  Mutations in synaptonemal complex proteins that attach homologous chromosomes increase the chances of nondisjunction in anaphase I of meiosis I. In contrast, mutations in topoisomerases and condensins that hold...
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Speciation describes the formation of one or more new species from one or sometimes multiple original species. The resulting species are discrete from the parent species, and barriers to reproduction will typically exist. There are two primary mechanisms, speciation with and without geographic isolation—allopatric and sympatric speciation, respectively.
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Polytene chromosomes are giant interphase chromosomes with several DNA strands placed side by side. They were discovered in the year 1881 by Balbiani in salivary glands, intestine, muscles, malpighian tubules, and hypoderm of larvae Chironomus plumosus. Hence, these are also called "Salivary gland chromosomes." These are found in insects of the order Diptera and Collembola; in certain organs of mammals; and synergids, antipodes of flowering plants. Polytene chromosomes are also...
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The seminal work of Ohno in 1970 popularized the idea of gene duplication and divergence. DNA sequence comparison studies reveal that a large portion of the genes in bacteria, archaebacteria, and eukaryotes was  generated by gene duplication and divergence, indicating its critical role in evolution.
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Manipulation of Ploidy in Caenorhabditis elegans
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Genome downsizing after polyploidy: mechanisms, rates and selection pressures.

Xiaotong Wang1,2, Joseph A Morton1,2, Jaume Pellicer1,3

  • 1Royal Botanic Gardens, Kew, Surrey, TW9 3AB, UK.

The Plant Journal : for Cell and Molecular Biology
|June 2, 2021
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Plant genome downsizing, driven by polyploidy, suggests selection against large genomes. DNA repair pathways and nutrient costs may explain this evolutionary trend, impacting photosynthesis and water use efficiency.

Keywords:
DNA loss rateDNA repairgenome downsizingselection pressureswhole-genome duplications

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Area of Science:

  • Plant evolutionary biology
  • Genomics
  • Molecular genetics

Background:

  • Polyploidy is common in plant evolution, yet most species have smaller genomes than expected.
  • A slow rate of DNA loss following polyploidy (4-70 Mb/million years) challenges explanations for genome downsizing.
  • The mechanisms by which small DNA losses are selected for and overcome genetic drift remain unclear.

Purpose of the Study:

  • To investigate the role of double-strand break (DSB) repair pathways in plant genome downsizing.
  • To explore hypotheses explaining selection for genome downsizing, including reduced nutrient costs and improved cell size regulation.
  • To examine whether DNA loss is rapid post-polyploidy or a continuous byproduct of other selection pressures.

Main Methods:

  • Analysis of over 10,000 plant genome sizes (GSs).
  • Comparative analysis of ancestral GS and polyploidy incidence.
  • Exploration of double-strand break (DSB) repair pathways (non-homologous end joining and homologous recombination).
  • Evaluation of hypotheses related to nitrogen (N) and phosphate (P) costs and cell size scaling effects.

Main Results:

  • Most plant species exhibit smaller genomes than predicted by polyploidy incidence, indicating selection for downsizing.
  • The rate of DNA loss post-polyploidy appears very low, posing a challenge for selection-driven downsizing.
  • DSB repair pathways and potential benefits like reduced nutrient costs and optimized cell size are explored as drivers.

Conclusions:

  • Genome downsizing in plants, despite slow DNA loss rates, may be driven by DSB repair mechanisms.
  • Selection for reduced nucleic acid synthesis costs (N and P) and improved cell size regulation (CO2 uptake, water loss) could favor smaller genomes.
  • Genome downsizing might be a byproduct of selection against damaging repetitive DNA, with impacts on photosynthesis and nutrient use being emergent properties.