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Homologous chromosome

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Homologous chromosomes are pairs of chromosomes which contain a paternal and maternal copy. These copies have the same genes in the same locations, or loci, as one another. This allows them to pair correctly with one another before separating during mitosis or meiosis.[1] The homologous chromosomes randomly segregate, experience genetic recombination, pair up, and separate into two different daughter cells.[2] This is the basis for Gregor Mendel’s laws of genetics.

Karyogram showing homologous chromosome pairs of a human female

Overview

Homologous chromosomes are made up of chromosome pairs of approximately the same length, centromere position, and staining pattern, with genes for the same corresponding loci. One homologous chromosome is inherited from the organism's mother; the other from the organism's father. After mitosis occurs the daughter cells will have the correct number of genes that are most likely a mix of the two parents' genes. In diploid (2n) organisms, the genome is composed of pairs of homologous chromosomes with one coming from the father and the other from the mother. The alleles on the homologous chromosomes may be different, resulting in different phenotypes of the same genes. This mixing of maternal and paternal traits is enhanced crossing over during meiosis.[3]

History

Early in the 1900s William Bateson and Reginald Punnett were studying genetic inheritance and they noted that some combinations of alleles appeared more frequently than others. That data and information was further explored by Thomas Morgan. He discovered using test crosses, which focus and reveal the alleles of a single parent that genes near to one another move together. Using this logic he concluded that the two genes he was studying were located on homologous chromosomes. Later on during the 1930s Harriet Creighton and Barbara McClintock were studying meiosis and discovered that the new allele combinations present in the offspring and the event of crossing over were directly correlated with each other.[4] Creighton and McClintock proved this intrachromosomal genetic recombination by examining gene loci on corn chromosomes.[4]

Structure

Homologous chromosomes are chromosomes which contain the same genes in the same order on their chromosomal arms. There are two main properties of which homologous chromosomes are concerned with: the length of chromosomal arms and the placement of the centromere [5] This is the basis for Gregor Mendel’s laws of genetics.. The actual length of the arm, in accordance with the gene locations, is critically important for proper alignment. Centromere placement can be characterized by four main arrangements, consisting of either being metacentric, submetacentric, telocentric, or acrocentric. Both of these properties are the main factors for creating the homology of the structure between chromosomes. Therefore when two chromosomes of the exact structure exist, they are able to pair together to form homologous chromosomes [5]. Homologous chromosomes differ from sister chromatids in that sister chromatids are identical, duplicates of each other that are made during DNA replication.[2]

In humans

Humans have 46 chromosomes, but there are only 22 pairs of true homologous chromosomes. The additional 23rd pair is comprised of the sex chromosomes, X and Y. If this pair is made up of an X and Y chromosome, then the pair is not truly homologous because their size and types of genes differ slightly. The 22 pairs of homologous chromosomes contain the same genes but code for different traits in their allelic forms due to the fact that one was inherited from the mother and one from the father.[6] So humans have two homologous chromosome sets in each of our cells, meaning we are diploid organisms.[4]

Functions

Homologous chromosomes are important in the processes of meiosis and mitosis. They allow for the recombination and random segregation of genetic material from the mother and father into new cells.[7]

In meiosis

Depiction of chromosome 1 after undergoing homologous recombination in meiosis
During the process of meiosis, homologous chromosomes can recombine and produce new combinations of genes in the daughter cells.
Sorting of homologous chromosomes during meiosis
Sorting of homologous chromosomes during meiosis.

Meiosis reduces the chromosome number by half by separating the homologous chromosomes in a germ cell. In prophase, the homologous chromosomes pair up with one another. The process of meiosis I is generally longer than in meiosis II because time elapses during it for homologous chromosomes to be properly oriented and segregated through the processes of pairing, synapsis, and recombination.[2] The implications of genetic recombination by random segregation and crossing over between chromosome are that the daughter cells all contain different combinations of maternally and paternally coded genes. This creates genetic variation which helps make a population more stable by providing a wider range of genetic traits for natural selection to act on.[4] Creighton and McClintock proved this intrachromosomal genetic recombination by examining gene loci on corn chromosomes.[4]

Prophase I

In prophase I of meiosis I, each chromosome becomes aligned with its homologous partner and will pair completely. This occurs by a synapsis process where the synaptonemal complex - a protein scaffold - is assembled and joins the chromosome along their lengths.[2] Cohesion crosslinking occurs between the homologous chromosomes and helps them resist being pulled apart until anaphase, when they are cleaved by the enzyme separase to release the homologous chromosome arms from each other.[6] The synaptonemal complex disassembles before anaphase which also allows homologous chromosomes to separate (while the sister chromatids stay associated).[2] Genetic crossing over occurs during genetic recombination in prophase of meiosis I. In this process, genes are exchanged by the breakage and union of a portion of the chromosomes’ lengths.[2]Structures called chiasmata are the site of the exchange. They physically link the homologous chromosomes once crossing over occurs and throughout the process of chromosomal segregation during meiosis.[2] This genetic recombination allows for the introduction of new gene pairings and genetic variation.[4] Creighton and McClintock proved this intrachromosomal genetic recombination by examining gene loci on corn chromosomes.[4]

Metaphase I

In metaphase I of meiosis I, the pairs of homologous chromosomes, also known as bivalents, line up randomly along the metaphase plate. The random orientation is another way for cells to introduce genetic variation. Meiotic spindles emanating from opposite spindle poles attach to each of the homologs (each pair of sister chromatids) at the kinetochore.[6]

Anaphase I

In anaphase I of meiosis I the homologs are pulled apart, which releases the cohesion that held the chromosome arms together, thus allowing the chiasmata to release and the homologs to move to opposite poles of the cell.[6] The homologous chromosomes have now been randomly segregated into two daughter cells that will then undergo meiosis II to create four haploid daughter germ cells.[4] Creighton and McClintock proved this intrachromosomal genetic recombination by examining gene loci on corn chromosomes.[4]

Meiosis II

The two diploid daughter cells resulting from meiosis I then undergo another cell division in meiosis II, but without another round of chromosomal replication. The sister chromatids of the two daughter cells are pulled apart in anaphase II by nuclear spindle fibers, resulting in four haploid daughter cells.[4]

In mitosis

Homologous chromosomes function similarly in mitosis as in meiosis, but there are some differences. Prior to the start of mitosis, the chromosomes in the cell replicate themselves so that each daughter cell will have the same number of chromosomes as the parent cell. These replicants, or sister chromatids, will then separate in the same way as meiosis I by being pulled apart by nuclear mitotic spindles.[8] However, there is no crossing over between sister chromatids in mitotis.[4]

Problems

1. Meiosis I 2. Meiosis II 3. Fertilization 4. Zygote Nondisjunction is when chromosomes fail to separate normally resulting in a gain or loss of chromosomes. In the left image the blue arrow indicates nondisjunction taking place during meiosis II. In the right image the green arrow is indicating nondisjunction taking place during meiosis I.

There are severe repercussions when chromosomes do not segregate properly. It can lead to fertility problems, embryo death, birth defects, and cancer.[9] Though the mechanisms for pairing and adhering homologous chromosomes vary among organisms, proper functioning of those mechanisms is imperative in order for the final genetic material to be sorted correctly.[9]

Nondisjunction

Proper homologous chromosome separation in meiosis I is crucial for sister chromatid separation in meiosis II.[9] Failure to separate properly is known as nondisjunction. There are two main types of nondisjunction that occur: trisomy and monosomy. Trisomy is caused by the presence of three chromosomes in the zygote, and monosomy is characterized by the presence of a single chromosome in the zygote. If this uneven division occurs in meiosis I, then none of the daughter cells will have proper chromosomal distribution and severe effects can ensue, including Down’s syndrome.[10]. Unequal division can also occur during the second meiotic division. Nondisjunction which occurs here can result in normal daughter cells and deformed cells. [5]

Embryonic death

If unequal genetic crossover occurs within the gametes involved in meiosis, then the resulting zygote may suffer unviability. [5] This is caused by events such as trisomy and monosomy because the cells are not able to develop normally with these characteristics. In other terms, one cell will gain the proper amount of genetic material while the other cell lacks that material.[10] Consequently, these deformed cells may cause a spontaneous abortion because the zygote will never develop correctly.[10]

Other uses

While the main function of homologous chromosomes is their use in nuclear division, they are also used in repairing double-strand breaks of DNA.[11] These double-stranded breaks typically occur in DNA that serve as template strands for DNA replication, and they are the result of mutation, replication errors, or any type of malfunctioning DNA.[12] Homologous chromosomes can repair this damage by aligning themselves with chromosomes of the same sequence.[11] Once they are oriented correctly, the homologous chromosomes perform a process that is very similar to recombination, or crossing over, as seen in meiosis. Part of the intact DNA sequence overlaps with that of the damaged chromosome. Replication proteins and complexes are then recruited, allowing replication to occur correctly.[12] Through this functioning, double-strand breakscan be repaired and DNA can function normally.[11]

See also

References

  1. ^ "Homologous chromosomes". Genetics Home Reference - Glossary Entry. U.S. National Library of Medicine.
  2. ^ a b c d e f g Pollard TD, Earnshaw WC, Lippincott-Schwartz J (2008). Cell Biology (2 ed.). Philadelphia: Saunders/Elsevier. pp. 815, 821–822. ISBN 1-4160-2255-4.{{cite book}}: CS1 maint: multiple names: authors list (link)
  3. ^ Campbell NA, Reece JB (2002). Biology. San Francisco: Benjamin Cummings. ISBN 0-8053-6624-5.
  4. ^ a b c d e f g h i j k Griffiths JF, Gelbart WM, Lewontin RC, Wessler SR, Suzuki DT, Miller JH (2005). Introduction to Genetic Analysis. New York: W.H. Freeman and Co. pp. 34–40, 473–476, 626–629. ISBN 0-7167-4939-4.{{cite book}}: CS1 maint: multiple names: authors list (link)
  5. ^ a b c d Klug, William S. (2012). Concepts of Genetics. Boston: Pearson. pp. 21–22.
  6. ^ a b c d Lodish HF (2013). Molecular cell biolog. New York: W.H. Freeman and Co. pp. 355, 891. ISBN 1-4292-3413-X.
  7. ^ Gregory MJ. "The Biology Web". Clinton Community College – State University of New York.
  8. ^ "The Cell Cycle & Mitosis Tutorial". The Biology Project. University of Arizona. Oct. 2004. {{cite web}}: Check date values in: |date= (help)
  9. ^ a b c Gerton JL, Hawley RS (2005). "Homologous chromosome interactions in meiosis: diversity amidst conservation". Nat. Rev. Genet. 6 (6): 477–87. doi:10.1038/nrg1614. PMID 15931171. {{cite journal}}: Unknown parameter |month= ignored (help)
  10. ^ a b c Tissot, Robert and Kaufman, Elliot. "Chromosomal Inheritance". Human Genetics. University of Illinois at Chicago.{{cite web}}: CS1 maint: multiple names: authors list (link)
  11. ^ a b c Sargent RG, Brenneman MA, Wilson JH (1997). "Repair of site-specific double-strand breaks in a mammalian chromosome by homologous and illegitimate recombination" (PDF). Mol. Cell. Biol. 17 (1): 267–77. PMC 231751. PMID 8972207. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  12. ^ a b Kuzminov A (2001). "DNA replication meets genetic exchange: chromosomal damage and its repair by homologous recombination". Proc. Natl. Acad. Sci. U.S.A. 98 (15): 8461–8. doi:10.1073/pnas.151260698. PMC 37458. PMID 11459990. {{cite journal}}: Unknown parameter |month= ignored (help)

Further reading

Wikimedia Commons has media related to homologous chromosomes.
  • Gilbert SF (2003). Developmental biolog. Sunderland, Mass.: Sinauer Associates. ISBN 0-87893-258-5.
  • OpenStaxCollege (25 Apr. 2013). "Meiosis". Rice University. {{cite web}}: Check date values in: |date= (help)
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