A pair of homologous chromosomes , or homologous , is a set of one maternal and one paternal chromosome that join with each other inside a cell during fertilization . Homologs have identical genes at the same locus where they provide points along each chromosome that enable a pair of chromosomes to align correctly with each other before they separate during meiosis. [1] It is the basis of Mendelian inheritance , which refers to the pattern of inheritance of genetic material from an organism to the developmental cell of its descendant parent over a given period of time and region.
Observation
Chromosomes are linear arrangements of condensed deoxyribonucleic acid (DNA) and histone proteins, which form a complex called chromatin . [2] Homologous chromosomes are composed of chromosomal pairs of approximately equal length, centromere position, and staining pattern, for genes with identical related loci . A homologous chromosome is inherited from the mother of the organism; The second is inherited from the father of the organism. After mitosis occurs within the daughter cells, they have the correct number of genes which are a mixture of the genes of the two parents. In diploid (2n) organisms, the genome is composed of one set of each homologous chromosome pair, compared to tetraploid organisms which may have two sets of homologous chromosomes each.Alleles may separate on homologous chromosomes, resulting in different phenotypes of the same gene. This mix of maternal and paternal traits is enhanced by crossing over during meiosis, which includes the length of the chromosome arms and the exchange of their DNA with each other within a homologous chromosome pair.
History
In the early 1900s William Bateson and Reginald Punett were studying genetic inheritance and found that some combinations of alleles appeared more frequently than others. That data and information was further explored by Thomas Morgan . Using test cross experiments, they revealed that, for a single parent, alleles of genes that are close to each other run together along the length of the chromosome. Using this logic, they concluded that the two genes they were studying were located on homologous chromosomes. Harriet Creighton and Barbara McClintock during the later 1930sWas studying meiosis in corn cells and examining gene loci on corn chromosomes. [2] Creighton and McClintock found that new allele combinations present in the offspring and the incidence of crossing over were directly related. [2] This proved to be interchromosomal genetic recombination.
Structure
Homologous chromosomes are those chromosomes that contain the same genes along their chromosome arms in the same order. Homologous chromosomes have two main properties: the length of the chromosome arms and the location of the centromere. [4]
Actual arm length, according to gene locations, is critically important for proper alignment. Centromere placement can be characterized by four main arrangements, which include either metacentric , submetacentric , acrocentric , or telocentric . Both{{clarification needed|reason=both, but four were listed, so what do both mean?|date=June 2021} These properties are the main factors for creating structural homology between chromosomes. Therefore, when two chromosomes of the exact structure [ clarification needed ] are present, they are able to join together to form homologous chromosomes. [5]
Since homologous chromosomes are not identical and do not originate from the same organism, they differ 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 only 22 pairs of homologous autosomal chromosomes. The additional 23rd pair is the sex chromosomes, X and Y. The 22 pairs of homologous chromosomes contain the same genes, but their allelic forms code for different traits because one was inherited from the mother and one from the father. [7] So humans have two homologous chromosome sets in each cell, which means that humans are diploid organisms. [2]
Tasks
Homologous chromosomes are important in the processes of meiosis and mitosis. They allow the recombination and random separation of genetic material from the mother and father into new cells.
In meiosis
Meiosis is a round of two cell divisions resulting in four haploid daughter cells each containing half the number of chromosomes as the parent cell. [9] It halves the number of chromosomes in a germ cell by first separating homologous chromosomes in meiosis I and then sister chromatids in meiosis II. The process of meiosis I is generally longer than that of meiosis II because it takes longer for chromatin to replicate and homologous chromosomes to be properly oriented and separated by the processes of pairing and synapsis in meiosis I. [6]During meiosis, genetic recombination (by random segregation) and crossing over generate daughter cells that each contain different combinations of maternally and paternally coded genes. [9] This recombination of genes allows the introduction of new allele pairs and genetic variations. [2] Genetic variation among organisms helps make populations more stable by providing a wider range of genetic traits for natural selection to act on. [2]
Phase I
In the prophase of meiosis I, each chromosome is fully aligned with its homologous partner and pair. In prophase I, the DNA has already undergone replication, so each chromosome consists of two identical chromatids attached to a common centromere. [9] During the zygotene stage of prophase I, homologous chromosomes associate with each other. [9] This pairing occurs by a synapsis process where the synaptonemal complex – a protein scaffold – is assembled and links homologous chromosomes along their length. [6] Cohesin crosslinking occurs between homologous chromosomes and helps prevent them from separating until anaphase. [7]Genetic crossing-over, a type of recombination, occurs during the pectin phase of prophase I. [9] In addition, another type of recombination called synthesis-dependent strand annealing (SDSA) often occurs. SDSA recombination involves the exchange of information between paired homologous chromatids, but not physical exchange. SDSA recombination does not cause crossing-over.
In the process of crossing-over, genes are exchanged by the breaking and union of homologous parts of the length of the chromosomes. [6] Structures called chiasmata are sites of exchange. Chiasmata physically assemble homologous chromosomes once crossing over occurs and during the process of chromosomal segregation during meiosis. [6] Both non-crossover and crossover recombination function as processes for DNA damage repair, especially double-strand breaks. The diplotene stage of prophase I disassembles the synaptonemal complex, allowing the first homologous chromosomes to separate, while sister chromatids remain attached to their centromeres. [6]
Metaphase I
In metaphase I of meiosis I, pairs of homologous chromosomes, also known as bivalents or tetrads, line up in a random order along the metaphase plate. [9] 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) in the kinetochore. [7]
Anaphase I
Homologous chromosomes separate from each other in anaphase I of meiosis I. The homologs are cleaved by the enzyme separase to release the cohesin that holds the homologous chromosome arms together. [7] This allows the chiasmata to be released and homologs to travel to opposite poles of the cell. [7] The homologous chromosomes are now randomly divided into two daughter cells that will undergo meiosis II to form four haploid daughter germ cells. [2]
Meiosis II
After the tetrad separation of homologous chromosomes in meiosis I, sister chromatids from each pair separate. Two haploid (because the chromosome number is reduced to half. Earlier two sets of chromosomes were present, but now each set is present in two separate daughter cells that have been produced by meiosis I from a single diploid parent cell) daughter produced by meiosis Cells I undergo another cell division in meiosis II but without another round of chromosome replication. The sister chromatids in the two daughter cells are separated by nuclear spindle fibers during anaphase II, resulting in four haploid daughter cells. [2]
In mitosis
The mitotic chromosomes do not perform the same function in mitosis as they do in meiosis. Before each mitotic division a cell undergoes, the chromosomes in the parent cell repeat themselves. Homologous chromosomes within the cell will usually not pair up and undergo genetic recombination with each other. [9] Instead, the replicons, or sister chromatids, will line up along the metaphase plate and then separate in the same way as in meiosis II – pulled apart at their centromere by the nuclear mitotic spindle. [10] If a crossing over occurs between sister chromatids during mitosis, it does not create a new recombinant genotype. [2]
In somatic cells
Homologous coupling in most contexts would refer to germline cells, although it also occurs in somatic cells. For example, in humans, somatic cells have very tightly controlled homologous pairing (separating into chromosomal regions, and coupling at a specific locus under the control of developmental signalling). However other species (notably Drosophila ) exhibit homologous pairings more frequently. In Drosophila homologous pairing supports a gene regulatory event called transvection in which an allele on one chromosome affects the expression of homologous alleles on homologous chromosomes. [11] One of its notable functions is the sexually dimorphic regulation of X-linked genes. [12]
Problem
The blue arrow in the left image indicates non-disjunction occurring during meiosis II.
The green arrow in the right image is indicating non-disjunction occurring during meiosis I.
Serious consequences occur when chromosomes do not separate properly. Faulty isolation can lead to reproductive problems, fetal death, birth defects and cancer. [13] Although the mechanisms for linking and adherence to homologous chromosomes differ among organisms, proper functioning of those mechanisms is essential for the final genetic material to be sequenced correctly. [13]
Nondisjunction
Proper homologous chromosome separation in meiosis I is critical for sister chromatid separation in meiosis II. [13] Failure to separate properly is known as nondisjunction. There are two main types of nondisjunctions: trisomy and monosomy. Trisomy is caused by the presence of an extra chromosome in the zygote than the normal number, and monosomy is characterized by the presence of one less chromosome in the zygote than the normal number. If this unequal division occurs in meiosis I, none of the daughter cells will have proper chromosome distribution and nonspecific effects may occur, including Down syndrome. [14] Uneven division may also occur during the second meiosis. Nondisjunction that occurs at this stage can result in normal daughter cells and malformed cells.[४]
Other uses
While the main function of homologous chromosomes is their use in nuclear division, they are also used in the repair of double-strand breaks of DNA. [15] These double-stranded breaks can occur in replication of DNA and are often the result of interactions of DNA with naturally occurring harmful molecules such as reactive oxygen species. Homologous chromosomes can repair this damage by aligning themselves with chromosomes of the same genetic sequence. [15]Once the base pairs are correctly matched and oriented between the two strands, the homologous chromosomes undergo a process that is similar to recombination, or seen in meiosis. The part of the intact DNA sequence overlaps with the damaged chromosome sequence. Replication proteins and complexes are then recruited to the site of damage, allowing repair and proper replication to occur. Through this mechanism, double-strand breaks can be repaired and DNA can function normally. [15]
Relevant search
Current and future research on the topic of homologous chromosomes is heavily focused on the roles of different proteins during recombination or during DNA repair. In a recently published article by Pezza et al. [ Which one? ] A protein known as HOP2 is responsible for both homologous chromosome synapsis as well as double-strand break repair through homologous recombination. Deletion of HOP2 in mice has major effects in meiosis. [16] Other current studies also focus on specific proteins involved in homologous recombination.
Current and future research on the topic of homologous chromosomes is heavily focused on the roles of different proteins during recombination or during DNA repair. In a recently published article by Pezza et al. [ Which one? ] A protein known as HOP2 is responsible for both homologous chromosome synapsis as well as double-strand break repair through homologous recombination. Deletion of HOP2 in mice has major effects in meiosis. [16] Other current studies also focus on specific proteins involved in homologous recombination.
There is ongoing research concerning the ability of homologous chromosomes to repair double-strand DNA breaks. Researchers are investigating the possibility of harnessing this potential for regenerative medicine. [17] This drug may be very popular in relation to cancer, as DNA damage is known to be a contributor to carcinogenesis. Manipulating the repair function of homologous chromosomes may allow the cell’s damage response system to be improved. Although research has not yet confirmed the effectiveness of this type of treatment, it could become a useful therapy for cancer.
