They are a set of maternal and paternal chromosomes that pair within a cell during meiosis.
Homologs have the same genes at the same loci where they provide points along each chromosome that allow a pair of chromosomes to line up correctly with each other before separating during meiosis.
This is the basis of Mendelian inheritance that characterizes the inherited patterns of genetic material from an organism to its descendant parental developmental cell in the given time and area.
Chromosomes are linear arrangements of condensed deoxyribonucleic acid (DNA) and histone proteins, which form a complex called chromatin.
Homologous chromosomes are composed of pairs of chromosomes of approximately the same length, centromere position, and staining pattern for genes with the same corresponding loci.
A homologous chromosome is inherited from the organism’s mother; the other is inherited from the organism’s father. After mitosis occurs within the daughter cells, they have the correct number of genes that are a mixture of the genes of the two parents.
In diploid (2n) organisms, the genome comprises one set of each pair of homologous chromosomes, compared to tetraploid microorganisms that can have two sets of each pair of homologous chromosomes.
Alleles on homologous chromosomes can be different, resulting in different phenotypes of the same genes.
This mix of maternal and paternal traits is enhanced by crossover during meiosis. The lengths of the chromosome arms and the DNA they contain within a pair of homologous chromosomes are swapped.
In the early 1900s, William Bateson and Reginald Punnett studied genetic inheritance and noted that some combinations of alleles appeared more frequently than others. That information and data were explored by Thomas Morgan.
Using cross-trial experiments, he revealed that, for a single parent, alleles of genes close to each other along the chromosome move together. Using this logic, he concluded that the two genes he studied were on homologous chromosomes.
Later, during the 1930s, Harriet Creighton and Barbara McClintock studied meiosis in maize cells and examined the genetic loci on maize chromosomes.
Creighton and McClintock found that the new allele combinations present in the offspring and the crossing event were directly related. This demonstrated interchromosomal genetic recombination.
Structure of homologous chromosomes
Homologous chromosomes contain the same genes in the same order along their chromosomal arms. There are two main properties of homologous chromosomes: the length of the chromosome arms and the location of the centromere.
According to the gene locations, the actual arm length is critically essential for proper alignment. The placement of the centromere can be characterized by four primary arrangements consisting of being metacentric, submetacentric, telocentric, or acrocentric.
Both properties are the main factors in creating a structural homology between chromosomes. Therefore, when there are two chromosomes of the exact structure, they can pair up 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 are produced after DNA replication and are identical; they duplicate side by side.
Humans have 46 chromosomes, but there are only 22 homologous autosomal chromosome pairs. The extra 23 pairs are the sex chromosomes, X and Y.
If this pair is made up of an X and Y chromosome, then the chromosome pair is not homologous because their size and genetic content differ significantly.
The 22 homologous chromosomes contain the same genes but encode different traits in their allelic forms since one was inherited from the mother and the other from the father.
So humans have two sets of homologous chromosomes in each cell, which means that humans are diploid organisms.
Homologous chromosomes are essential in the processes of meiosis and mitosis. They allow the recombination and random segregation of genetic material into new cells from the mother and father.
Meiosis is a round of two cell divisions that results in four haploid daughter cells that contain half the number of chromosomes as the parent cell.
It halves the number of chromosomes in a germ cell 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 longer for chromatin to replicate and for homologous chromosomes to properly orient and secret through the synchronization and synapse processes in meiosis I.
During meiosis, genetic recombination (by random segregation) and crossing produce daughter cells that contain different combinations of genes encoded by the mother and father.
This recombination of genes allows the introduction of new pairs of alleles and genetic variation. Genetic variation between organisms helps make the population more stable by providing a more comprehensive range of genetic traits for natural selection to operate.
Each chromosome lines up with its homologous pair and pairs completely in prophase I of meiosis I. DNA has already been replicated in prophase I, so each chromosome consists of two identical chromatids connected by a common centromere.
During the zygotene stage of prophase I, homologous chromosomes pair with each other. This pairing occurs by a synapse process where the synaptonemal complex – a protein scaffold – assembles and binds to homologous chromosomes along its length.
Cohesin crosslinking occurs between homologous chromosomes and helps them resist separation to anaphase. Genetic crossing, a type of recombination, occurs during the pachytene stage of prophase I.
In addition, another type of recombination frequently occurs, called synthesis-dependent chain annealing (SDSA).
Synthetic-dependent chain-pairing recombination involves information exchange between paired homologous chromatids but is not a physical business. Synthesis-dependent chain-pairing recombination does not cause crossover.
In crossing over, genes are exchanged by breaking and joining homologous parts of chromosome lengths. Structures called chiasmata are the site of the exchange.
The chiasmata physically connect the homologous chromosomes once the crossover occurs and throughout the chromosome segregation process during meiosis.
Both non-crossover and crossover types of recombination function as processes to repair DNA damage, particularly double-stranded breaks.
In the diplotene stage of prophase I, the synaptonemal complex disarms before the homologous chromosomes can separate, while the sister chromatids remain associated at their centromeres.
In metaphase I of meiosis I, homologous chromosome pairs, also known as bivalent or tetrads, line up in a random order along with the metaphase plate. Random orientation is another way for cells to introduce genetic variation.
The meiotic spindles emanate from opposite poles of the spindle and bind to each of the homologs (each pair of sister chromatids) in the kinetochore.
In anaphase I of meiosis, homologous chromosomes separate from each other. The enzyme separase cleaves the homologs to release the cohesin that holds the homologous chromosomal arms together.
This allows the chiasmata to break free and the homologues to move to opposite poles of the cell. Homologous chromosomes are now randomly secreted into two daughter cells that will undergo meiosis II to produce four haploid daughter germ cells.
After the tetrads of the homologous chromosomes separate in meiosis I, the sister chromatids of each pair separate. Both haploids (because the chromosome # has been halved).
Previously, there were two sets of chromosomes. Still, now each group exists in two different daughter cells that have arisen from the single diploid parent cell by meiosis I) daughter cells resulting from meiosis undergo another cell division in meiosis II but without another round of replication chromosome.
The sister chromatids in the two daughter cells are separated by nuclear spindle fibers during anaphase II, resulting in four haploid daughter cells.
Homologous chromosomes do not function the same in mitosis as in meiosis. Before each mitotic division that a cell undergoes, the original cell’s chromosomes replicate.
Homologous chromosomes within the cell will ordinarily not pair up and will undergo genetic recombination with each other.
Instead, the replicants, or sister chromatids, will line up along the metaphase plate and then separate in the same way as meiosis II, by separating at their centromeres by nuclear mitotic spindles.
If a cross between sister chromatids occurs during mitosis, it does not produce new recombinant genotypes.
In somatic cells
Homologous pairing in most contexts will refer to germline cells. However, it also takes place in somatic cells.
For example, in humans, somatic cells have very tightly regulated homologous pairs (separated into chromosomal territories and pairing at specific loci under the control of developmental signaling).
However, other species (especially Drosophila) frequently exhibit homologous pairing. Various functions of homologous pairing in somatic cells have been elucidated through high-throughput screens in the early 21st century.
Problems with homologous chromosomes
There are severe repercussions when chromosomes do not segregate properly. Faulty segregation can lead to fertility problems, embryonic death, congenital disabilities, and cancer.
Although the mechanisms for matching and attaching homologous chromosomes vary between organisms, the correct functioning is essential for the final genetic material to be correctly classified.
Adequate homologous chromosome separation in meiosis I is crucial for separating sister chromatids in meiosis II. A failure to separate correctly is known as nondisjunction.
Two main types of nondisjunction occur trisomy and monosomy. An extra chromosome causes trisomy in the zygote compared to the average number, and monosomy is characterized by one less chromosome in the zygote than the average number.
If this uneven division occurs in meiosis I, none of the daughter cells will have a proper chromosomal distribution, and serious effects, including Down syndrome, can occur.
Unequal division can also occur during the second meiotic division. The nondisjunction that arises at this stage can result in normal daughter cells and deformed cells.
While the primary function of homologous chromosomes is their use in the nuclear division, they are also used to repair double-stranded DNA breaks.
These double-stranded breaks can occur in DNA replication and, most often, result from DNA’s interaction with harmful naturally-occurring molecules, such as reactive oxygen species.
Homologous chromosomes can repair this damage by lining up with chromosomes of the same genetic sequence.
Once the base pairs have been correctly paired and oriented between the two strands, the homologous chromosomes go through a process that is very similar to recombination, or they cross over, as seen in meiosis.
Part of the entire DNA sequence overlaps with the damaged chromosome sequence. Replication proteins and complexes are recruited at the site of damage, allowing for proper replication and repair.
This operation can repair double-stranded breaks, and DNA can function normally.
Current and future research on homologous chromosomes is highly focused on the roles of various proteins during recombination or during DNA repair.
In an article recently published by Pezza et al., the protein known as HOP2 is responsible for both homologous chromosomal synapse and double-strand break repair by homologous recombination.
The elimination of HOP2 in mice has a significant impact on meiosis. Other current studies are also focusing on specific proteins involved in homologous recombination.
There is ongoing research on the ability of homologous chromosomes to repair double-stranded DNA breaks. Researchers are investigating the possibility of exploiting this ability for regenerative medicine.
This drug could be prevalent in cancer, as DNA damage is believed to contribute to carcinogenesis.