Gas Exchange 5. Homeostasis Higher Level 7: Nucleic Acids 1. DNA Structure 2. Transcription 3. Translation 8: Metabolism 1. Metabolism 2. Cell Respiration 3. Photosynthesis 9: Plant Biology 1. Xylem Transport 2. Phloem Transport 3. Plant Growth 4. Plant Reproduction Genetics 1.
Meiosis 2. Inheritance 3. Speciation Animal Physiology 1. Antibody Production 2. Movement 3. Genetic Mechanisms of Sex Determination. Sex Chromosomes and Sex Determination. Sex Chromosomes in Mammals: X Inactivation. Sex Determination in Honeybees. Citation: O'Connor, C. Nature Education 1 1 How is the same process responsible for genetic recombination and diversity also the cause of aneuploidy? Understanding the steps of meiosis is essential to learning how errors occur.
Aa Aa Aa. Figure 1. Figure Detail. Meiosis Is a Highly Regulated Process. Figure 2. Meiosis I. Figure 3. Meiosis II. Figure 4. Figure 5. Figure 6: Visualization of chromosomal bridges in Allium fistulosum and Allium cepa plant meiocytes. The sites of double-stranded break DSB dependent homologue interaction can be seen as approximately nm bridges between chromosome axes.
These bridges, which probably contain a DSB that is already engaged in a nascent interaction with its partner DNA, occur in large numbers. Their formation depends on the RecA recombination protein homologues that are expressed in this species. In the next phase of homologue interaction, these nascent interactions are converted to stable strand-invasion events.
This nucleates the formation of the synaptonemal complex SC. Homologous chromosome interactions in meiosis: diversity amidst conservation. Nature Reviews Genetics 6, All rights reserved. References and Recommended Reading Gerton, J. Science , — Petes, T. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel. Email your Friend. Submit Cancel. This content is currently under construction.
Explore This Subject. Chromosome Analysis. Chromosome Structure. Mutations and Alterations in Chromosomes. Chromosome Number. Chromosome Theory and Cell Division. Sex Chromosomes. Topic rooms within Chromosomes and Cytogenetics Close. No topic rooms are there. Or Browse Visually. Other Topic Rooms Genetics. Student Voices. Creature Cast. Simply Science. Green Screen. Green Science. Bio 2. The Success Code. Why Science Matters.
The Beyond. Plant ChemCast. Postcards from the Universe. Brain Metrics. Each maternal chromosome has a corresponding paternal chromosome of the same gene sequence, gene loci, chromosomal length, and centromere location.
The pair comes close to each other during meiosis so that they could exchange genes between sister and non-sister chromatids. Although both members of the homologous pair have similar genes and loci, they may differ in the alleles.
For example, both of them carry genes coding for the eye color trait. One has alleles for the brown-eye trait whereas the other, for the blue-eye trait. In this case, the alleles are different and the homologous chromosomes are described as heterozygous. On the contrary, when the alleles are the same e. The significance of alleles was demonstrated by Gregor Mendel through his pioneering works in genetics using garden peas.
He was able to show that one of the alleles may be dominant over the other. In this case, only one allele will be expressed.
A trait demonstrating this pattern is referred to as Mendelian inheritance. Nevertheless, in humans, genetic expressions are not straightforward. Many of the human traits do not conform to the Mendelian pattern of inheritance. In this regard, they are referred to as non-Mendelian. Meiosis is a form of cell division wherein diploid organisms halve their number of chromosomes and homologous chromosomes pair up. In contrast, mitosis another form of cell division results in two daughter cells, each containing the same chromosomal number as the parent cell.
In meiosis, a diploid 2n cell will give rise to four haploid n cells. The cells that undergo meiosis are the gametes producing haploid sperm cell and egg cell. Haploidy is essential so that at fertilization the chromosomal number remains the same throughout generations. In order to achieve haploidy, the cell undergoes two consecutive nuclear divisions. They are referred to as meiosis I and meiosis II.
To prepare the cell to meiosis, one of the major preparatory steps is DNA replication. The chromosomes duplicate their DNA, particularly in the S phase of interphase.
At this point, each of the chromosomes will consist of two strands sister chromatids joined at the centromere. The pairing synapse of homologous chromosomes will occur at prophase I. DNA exchanges occur between homologous chromosomes via homologous recombination and crossover at chiasmata between non-sister chromatids. Then, the homologous pairs line up at the metaphase plate. Next, the homologous chromosomes separate during anaphase I and move to the opposite poles of the cell.
Then, the cell divides for the first time during telophase I resulting in two genetically non-identical daughter cells but with sister chromatids still intact. Each cell will undergo meiosis II so that the resulting daughter cells will each have a chromosomal number reduced by half. In humans, the nucleus typically contains 46 chromosomes. Thus, there are 22 pairs of autosomes with approximately the same length, staining pattern, and genes with the same loci. As for the sex chromosomes, the two X chromosomes are considered as homologous whereas the X and Y chromosomes are not.
Thus, females have 23 homologous chromosomes i. They also bear the genetic information that determines the trait of an organism. Homologous chromosomes, therefore, are vital in the same way. They carry genetic information that has been passed down from one generation to the next.
And since alleles may possibly be different in the same gene, the result is varying phenotypes. Thus, the distinctiveness of an individual of the same species is established. Apart from this, the organism is capable of reproducing offspring that is genetically different from itself as well as from the rest of its descendants. This event is crucial to promote genetic variation. The homologous pair exchanges genes via genetic recombination so that genetic diversity may be promoted.
This is regarded as one of the advantages of having been able to reproduce sexually. Those that reproduce asexually create a clone of themselves.
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