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Homologous chromosomes are a pair of chromosomes which contain a paternal and maternal copy. These copies have the same genes in the same location, or Locus, as one another. This allows them to pair correctly with one another before separating in Mitosis or Meiosis (U.S. National Library of Medicine, 2013). The homologous chromosomes are randomly segregated, experience Genetic recombination, pair up, and separate into two different daughter cells (Pollard, Earnshaw 2008). This is the basis for Gregor Mendel’s laws of genetics.

Overview

See also Homologous recombination

Homologous chromosomes are made up of Chromosome pairs of approximately the same length, Centromere position, and staining pattern, with genes for the same characteristics at 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 while the genes are the same. This trait is do to crossing over during Meiosis (Reese et al 2002)

History and the discovery of Homologous Chromosomes

Early in the 1900’s William Bateson and Reginald Punnett were studying 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 that logic he concluded that the two genes he was studying were both located on homologous chromosomes. Later on during the 1930’s 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. (Griffiths et al. 2000).

Structure of Homologous Chromosomes

Homologous chromosomes are chromosomes that have the same order of genes at the same location. The rest of their structure is identical to that of a regular chromosome.

Homologous Chromosomes in Humans

Humans have 46 chromosomes, which make up 23 pairs of homologous chromosomes. They 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 (Lodish et al. 2013). So humans have two homologous chromosome sets in each of our cells, meaning we are diploid organisms (Griffiths et al. 2000).

Functions of Homologous Chromosomes

Homologous chromosomes are very 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 (Gregory, The Biology Web). Homologous chromosomes are different from Sister chromatids in that sister chromatids are identical, duplicates of each other that are made during DNA replication (Pollard, Earnshaw 2008).

Homologous Chromosomes in 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 (Pollard, Earnshaw 2008). 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 genetic traits for Natural selection to act on.

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 (Pollard, Earnshaw 2008). 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 (Lodish et al. 2013). The Synaptonemal complex disassembles before anaphase which also allows homologous chromosomes to separate (while the Sister chromatids stay associated) (Pollard, Earnshaw 2008). 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 (Pollard, Earnshaw 2008). 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 (Pollard, Earnshaw 2008). This Genetic recombination allows for the introduction of new gene pairings and Genetic variation.

Harriet Creighton and Barbara McClintock proved this intrachromosomal Genetic recombination by examining gene loci on corn chromosomes (Griffiths et al. 2000).

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 homologs (each pair of Sister chromatids) at the Kinetochore (Lodish et al. 2013) (“Meiosis”, 2013).

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 (Lodish et al. 2013). 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.

==== Problems with Homologous Chromosomes in Meiosis ==== There are severe repercussions when chromosomes do not segregate properly. It can lead to fertility problems, [[Embryo#Miscarriage|embryo death}}, birth defects, and Cancer (Gerton, Hawley 2005). Proper homologous chromosome separation in meiosis I is crucial for Sister chromatids separation in meiosis II. 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 (Gerton, Hawley 2005).

Homologous Chromosomes 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 itself so that each [cell division|daughter cell]] will have just as many chromosomes as the parental cell. These replicants represent the homologous chromosomes in Mitosis, which will then separate in the same way as meiosis I (“The Cell Cycle and Mitosis Tutorial, 2004).

Other Uses of Homologous Chromosomes

While the main function of homologous chromosomes is their use in [Mitosis|nuclear division]], they are also used in repairing double-strand breaks of DNA (Sargent et al., 1996). 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 (Kuzminov, 2001). Homologous chromosomes can repair this damage by aligning themselves with chromosomes of the same sequence (Sargent et al., 1996). 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 proteinsand complexes are then recruited, allowing replication to occur correctly (Kuzminov, 2001). Through this functioning, double-strand breakscan be repaired and DNA can function normally.

Bibliography

Alberts, Bruce, and A. Lewis. Molecular Biology of the Cell. 4th ed. New York: Garland Science, 2002. Print.

Gerton, Jennifer L., and R. S. Hawley. "Homologous Chromosome Interactions in Meiosis: Diversity amidst Conservation." Nature Reviews Genetics (2005): 477-87. Web. <http://www.nature.com/nrg/journal/v6/n6/abs/nrg1614.html>.

Gilbert, Scott F. Developmental Biology. 6th ed. Sunderland, MA: Sinauer Associates, 2000. Print. Gregory, Michael J., Ph.D. The Biology Web. SUNY, n.d. Web. 31 Oct. 2013. <http://faculty.clintoncc.suny.edu/faculty/michael.gregory/default.htm>.

Griffiths, A. J. F., J. H. Miller, and D. T. Suzuki. An Introduction to Genetic Analysis. 7th ed. New York: W.H. Freeman, 2000. Print.

Klug, William S. Concepts of Genetics. Boston, Mass.: Pearson, 2012. Print.

Kuzminov, Andrei. "DNA Replication Meets Genetic Exchange: Chromosomal Damage and Its Repair by Homologous Recombination." PNAS 98.15 (2001): 8461-468. Web. 31 Oct. 2013. <http://www.pnas.org/content/98/15/8461.full.pdf+html>.

Lodish, Harvey F. Molecular Cell Biology. 5th ed. New York: W. H. Freeman and, 2013. Print.

Moulton, Glen E. The Complete Idiot's Guide to Biology. Indianapolis, IN: Alpha, 2004. Print.

Pollard, Thomas D., and William C. Earnshaw. Cell Biology. 2nd ed. Philadelphia: Saunders/Elsevier, 2008. Print.

Reece, Jane; Campbell, Neil (2002). Biology. San Francisco: Benjamin Cummings. ISBN 0-8053-6624-5.

Sargent, Geoffrey, Mark Brenneman, and John Wilson. "Repair of Site-Specific Double-Strand Breaks in a Mammalian Chromosome by Homologous and Illegitimate Recombination." Molecular and Cell Biology 17.1 (1996): 267-77. American Society for Microbiology. Web. 31 Oct. 2013. <http://mcb.asm.org/content/17/1/267.full.pdf+html>.

Tissot, Robert, and Elliot Kaufman. "Chromosomal Inheritance." Human Genetics. University of Illinois at Chicago, n.d. Web. 14 Oct. 2013. <http://www.uic.edu/classes/bms/bms655/lesson9.html>.

Zickler D. and Kleckner N (1999) Meiotic Chromosomes: Integrating Structure and Function. Annual Review of Genetics, 33: 603-754.

Austin, TX: Landes Bioscience, 2000. Madame Curie Bioscience Database. Web. 8 Oct. 2013. Meiosis." Connexions. OpenStax College, 25 Apr. 2013. Web. 31 Oct. 2013. <http://cnx.org/content/m45466/latest/?collection=col11487/latest>.

"Homologous Chromosomes." - Glossary Entry. U.S. National Library of Medicine, 23 Sept. 2013. Web. 14 Oct. 2013. <http://ghr.nlm.nih.gov/glossary%3Dhomologouschromosomes>.

"The Cell Cycle and Mitosis Tutorial." The Biology Project. University of Arizona, Oct. 2004. Web. 31 Oct. 2013. <http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/cells3.html>.