thumb|260px|As this [[karyotype displays a diploid human cell contains 22 pairs of homologous chromosomes and 2 sex chromosomes. The cell has two sets of each chromosome; one of the pair is derived from the mother and the other from the father. The maternal and paternal chromosomes in a homologous pair have the same genes at the same locus, but possibly different alleles. ]]
Homologous chromosomes or homologs are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during meiosis. Homologs have the same genes in the same loci, where they provide points along each chromosome that enable a pair of chromosomes to align correctly with each other before separating during meiosis. This is the basis for Mendelian inheritance, which characterizes inheritance patterns of genetic material from an organism to its offspring parent developmental cell at the given time and area.
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. Using test cross experiments, he revealed that, for a single parent, the alleles of genes near to one another along the length of the chromosome 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 in corn cells and examining gene loci on corn chromosomes. This proved interchromosomal genetic recombination. There are two main properties of homologous chromosomes: 1) the length of chromosomal arms and 2) the placement of the centromere.
The actual length of the arm, in accordance with the gene locations, is critically important for proper alignment. Centromere placement on the chromosome can be characterized by four main arrangements, either metacentric, submetacentric, acrocentric, or telocentric. Both of these properties (i.e., the length of chromosomal arms, and the placement of the chromosomal centromere) are the main factors for creating structural homology between chromosomes. Therefore, when two chromosomes containing the relatively same structure exist (e.g., maternal chromosome 15 and paternal chromosome 15), they are able to pair together via the process of synapsis to form homologous chromosomes.
Since homologous chromosomes are not identical and do not originate from the same organism, they are different from sister chromatids. Sister chromatids result after DNA replication has occurred, and thus are identical, side-by-side duplicates of each other.
In humans
Humans have a total of 46 chromosomes, but there are only 22 pairs of homologous autosomal chromosomes. The additional 23rd pair is the sex chromosomes, X and Y.
In humans, the 22 pairs of homologous autosomal chromosomes contain the same genes but may code for different traits in their allelic forms, as one was inherited from the mother and one from the father.
So, humans have two sets of 23 chromosomes in each cell that contains a nucleus. One set of 23 chromosomes (n) is from the mother (22 autosomes, 1 sex chromosome (X only)) and one set of 23 chromosomes (n) is from the father (22 autosomes, 1 sex chromosome (X or Y)). Ultimately, this means that humans are diploid (2n) organisms.
In meiosis
thumb|275px | alt=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.
thumb|289px | alt= Sorting of homologous chromosomes during meiosis | Sorting of homologous chromosomes during meiosis.
Meiosis is a round of two cell divisions that results in four haploid daughter cells that each contain half the number of chromosomes as the parent cell. It reduces the chromosome number in a germ cell by half by first separating the homologous chromosomes in meiosis I and then the sister chromatids in meiosis II. The process of meiosis I is generally longer than meiosis II because it takes more time for the chromatin to replicate and for the homologous chromosomes to be properly oriented and segregated by the processes of pairing and synapsis in meiosis I. If any crossing over does occur between sister chromatids during mitosis, it does not produce any new recombinant genotypes. One notable function of this is the sexually dimorphic regulation of X-linked genes.
Problems
[[File:Nondisjunction Diagrams.svg|300px|thumb|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. Faulty segregation can lead to fertility problems, embryo death, birth defects, and cancer. 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. Unequal division can also occur during the second meiotic division. Nondisjunction which occurs at this stage can result in normal daughter cells and deformed cells. These double-stranded breaks may occur in replicating DNA and are most often the result of interaction of DNA with naturally occurring damaging molecules such as reactive oxygen species. Homologous chromosomes can repair this damage by aligning themselves with chromosomes of the same genetic sequence. The deletion of HOP2 in mice has large repercussions in meiosis. Other current studies focus on specific proteins involved in homologous recombination as well.
There is ongoing research concerning the ability of homologous chromosomes to repair double-strand DNA breaks. Researchers are investigating the possibility of exploiting this capability for regenerative medicine. This medicine could be very prevalent in relation to cancer, as DNA damage is thought to be contributor to carcinogenesis. Manipulating the repair function of homologous chromosomes might allow for bettering a cell's damage response system. While research has not yet confirmed the effectiveness of such treatment, it may become a useful therapy for cancer.
See also
- Homologous recombination
- Mendelian inheritance
- Developmental biology
- Synapsis
- Non-disjunction
- Heredity