Which is a Homologous Chromosome Pair Chromatid Zygote Gamete Tetrad

Chromosomes that pair in fertilization

A couple of
homologous chromosomes, or
homologs, are a gear up of one maternal and ane paternal chromosome that pair upwardly with each other inside a cell during fertilization. Homologs have the same genes in the same loci where they provide points along each chromosome which enable a pair of chromosomes to align correctly with each other before separating during meiosis.^[i]
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 fourth dimension and area.^[2]

Overview

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Chromosomes are linear arrangements of condensed deoxyribonucleic acrid (Dna) and histone proteins, which form a complex called chromatin.^[two]
Homologous chromosomes are made up of chromosome pairs of approximately the same length, centromere position, and staining blueprint, for genes with the same respective loci. Ane homologous chromosome is inherited from the organism’s mother; the other is inherited from the organism’s begetter. Subsequently mitosis occurs within the daughter cells, they have the correct number of genes which are a mix of the 2 parents’ genes. In diploid (2n) organisms, the genome is composed of ane set of each homologous chromosome pair, as compared to tetraploid organisms which may accept two sets of each homologous chromosome pair. The alleles on the homologous chromosomes may be unlike, resulting in different phenotypes of the same genes. This mixing of maternal and paternal traits is enhanced by crossing over during meiosis, wherein lengths of chromosomal artillery and the Deoxyribonucleic acid they contain within a homologous chromosome pair are exchanged with one another.^[three]

History

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Early in the 1900s William Bateson and Reginald Punnett were studying genetic inheritance and they noted that some combinations of alleles appeared more oftentimes 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 well-nigh to one another along the length of the chromosome movement together. Using this logic he ended 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.^[2]
Creighton and McClintock discovered that the new allele combinations present in the offspring and the effect of crossing over were directly related.^[2]
This proved interchromosomal genetic recombination.^[two]

Structure

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Homologous chromosomes are chromosomes which comprise the aforementioned genes in the same order along their chromosomal artillery. There are ii chief properties of homologous chromosomes: 1) the length of chromosomal artillery and 2) the placement of the centromere.^[4]

The bodily length of the arm, in accordance with the gene locations, is critically of import for proper alignment. Centromere placement on the chromosome tin be characterized by 4 master arrangements, either metacentric, submetacentric, acrocentric, or telocentric. Both of these properties (i.e., the length of chromosomal artillery, 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 (eastward.grand., maternal chromosome 15 and paternal chromosome 15), they are able to pair together via the process of synapsis to class homologous chromosomes.^[five]

Since homologous chromosomes are not identical and do non originate from the aforementioned organism, they are unlike from sister chromatids. Sister chromatids result afterward DNA replication has occurred, and thus are identical, side-by-side duplicates of each other.^[6]

In humans

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Humans have a total of 46 chromosomes, but there are but 22 pairs of homologous autosomal chromosomes. The boosted 23rd pair is the sex chromosomes, Ten and Y.

Note that the pair of
sex chromosomes
may or may non be homologous, depending on the sex of the individual. For example, females comprise Xx, thus have a homologous pair of sex chromosomes. This ways that females accept 23 pairs of homologous chromosomes in total (i.due east., 22 pairs of not-sex activity chromosomes (autosomes), one pair of sex chromosomes). Conversely, males contain XY, which means that they have a non-homologous pair of sex chromosomes as their 23rd pair of chromosomes.

In humans, the 22 pairs of homologous autosomal chromosomes contain the aforementioned genes only code for different traits in their allelic forms, as one was inherited from the mother and one from the father.^[7]

Then, humans have ii sets of 23 chromosomes in each prison cell that contains a nucleus. Ane fix of 23 chromosomes (n) is from the mother (22 autosomes, 1 sex activity chromosome (X just)) and ane set of 23 chromosomes (north) is from the male parent (22 autosomes, 1 sexual activity chromosome (X or Y)). Ultimately, this ways that humans are diploid (2n) organisms.
^[two]

Functions

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

In meiosis

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Meiosis is a round of two cell divisions that results in 4 haploid girl cells that each contain half the number of chromosomes every bit the parent prison cell.^[9]
Information technology reduces the chromosome number in a germ cell by half by offset separating the homologous chromosomes in meiosis I and then the sister chromatids in meiosis 2. The process of meiosis I is by and large longer than meiosis II because it takes more time for the chromatin to replicate and for the homologous chromosomes to exist properly oriented and segregated past the processes of pairing and synapsis in meiosis I.^[6]
During meiosis, genetic recombination (by random segregation) and crossing over produces daughter cells that each contain dissimilar combinations of maternally and paternally coded genes.^[nine]
This recombination of genes allows for the introduction of new allele pairings and genetic variation.^[2]
Genetic variation among organisms helps make a population more stable by providing a wider range of genetic traits for natural selection to deed on.^[two]

Prophase I

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In prophase I of meiosis I, each chromosome is aligned with its homologous partner and pairs completely. In prophase I, the DNA has already undergone replication so each chromosome consists of two identical chromatids connected past a common centromere.^[9]
During the zygotene phase of prophase I, the homologous chromosomes pair up with each other.^[ix]
This pairing occurs past a synapsis process where the synaptonemal complex – a poly peptide scaffold – is assembled and joins the homologous chromosomes along their lengths.^[6]
Cohesin crosslinking occurs betwixt the homologous chromosomes and helps them resist existence pulled autonomously until anaphase.^[vii]
Genetic crossing-over, a type of recombination, occurs during the pachytene stage of prophase I.^[9]
In addition, another type of recombination referred to equally synthesis-dependent strand annealing (SDSA) frequently occurs. SDSA recombination involves information exchange between paired homologous chromatids, but not concrete substitution. SDSA recombination does non cause crossing-over.

In the process of crossing-over, genes are exchanged by the breaking and union of homologous portions of the chromosomes’ lengths.^[six]
Structures chosen chiasmata are the site of the substitution. Chiasmata physically link the homologous chromosomes one time crossing over occurs and throughout the procedure of chromosomal segregation during meiosis.^[6]
Both the non-crossover and crossover types of recombination function as processes for repairing Deoxyribonucleic acid impairment, peculiarly double-strand breaks. At the diplotene phase of prophase I the synaptonemal circuitous disassembles before which will allow the homologous chromosomes to separate, while the sister chromatids stay associated past their centromeres.^[six]

Metaphase I

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In metaphase I of meiosis I, the pairs of homologous chromosomes, as well known equally bivalents or tetrads, line upward in a random order along the metaphase plate.^[ix]
The random orientation is another manner for cells to introduce genetic variation. Meiotic spindles emanating from contrary spindle poles adhere to each of the homologs (each pair of sis chromatids) at the kinetochore.^[vii]

Anaphase I

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In anaphase I of meiosis I the homologous chromosomes are pulled apart from each other. The homologs are broken by the enzyme separase to release the cohesin that held the homologous chromosome artillery together.^[7]
This allows the chiasmata to release and the homologs to motility to reverse poles of the jail cell.^[7]
The homologous chromosomes are now randomly segregated into two girl cells that will undergo meiosis II to produce four haploid daughter germ cells.^[2]

Meiosis Two

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Afterwards the tetrads of homologous chromosomes are separated in meiosis I, the sis chromatids from each pair are separated. The 2 haploid daughter cells (the number of chromosomes has been reduced to half: earlier two sets of chromosomes were present, but now each set exists in ii different daughter cells that have arisen from the single diploid parent cell past meiosis I) resulting from meiosis I undergo another prison cell partitioning in meiosis Ii but without another round of chromosomal replication. The sis chromatids in the two daughter cells are pulled apart during anaphase Ii by nuclear spindle fibers, resulting in 4 haploid daughter cells.^[2]

In mitosis

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Homologous chromosomes practice not office the same in mitosis as they do in meiosis. Prior to every single mitotic sectionalisation a cell undergoes, the chromosomes in the parent cell replicate themselves. The homologous chromosomes within the cell will unremarkably not pair upward and undergo genetic recombination with each other.^[nine]
Instead, the replicants, or sister chromatids, volition line upwards forth the metaphase plate and and then separate in the same mode every bit meiosis II – by being pulled apart at their centromeres by nuclear mitotic spindles.^[10]
If whatsoever crossing over does occur betwixt sister chromatids during mitosis, information technology does not produce whatsoever new recombinant genotypes.^[2]

In somatic cells

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Homologous pairing in most contexts will refer to germline cells, notwithstanding also takes identify in somatic cells. For instance, in humans, somatic cells have very tightly regulated homologous pairing (separated into chromosomal territories, and pairing at specific loci nether control of developmental signalling). Other species notwithstanding (notably
Drosophila) showroom homologous pairing much more than ofttimes. In
Drosophila
the homologous pairing supports a gene regulatory phenomenon called transvection in which an allele on one chromosome affects the expression of the homologous allele on the homologous chromosome.^[11]
Ane notable function of this is the sexually dimorphic regulation of Ten-linked genes.^[12]

Bug

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There are severe repercussions when chromosomes exercise non segregate properly. Faulty segregation can lead to fertility problems, embryo death, birth defects, and cancer.^[13]
Though the mechanisms for pairing and adhering homologous chromosomes vary amongst organisms, proper operation of those mechanisms is imperative in social club for the final genetic material to be sorted correctly.^[13]

Nondisjunction

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Proper homologous chromosome separation in meiosis I is crucial for sister chromatid separation in meiosis II.^[13]
A failure to separate properly is known as nondisjunction. In that location are two chief types of nondisjunction that occur: trisomy and monosomy. Trisomy is acquired by the presence of 1 additional chromosome in the zygote equally compared to the normal number, and monosomy is characterized by the presence of ane fewer chromosome in the zygote as compared to the normal number. If this uneven division occurs in meiosis I, then none of the daughter cells will have proper chromosomal distribution and non-typical furnishings tin can ensue, including Downwardly’south syndrome.^[14]
Diff segmentation tin besides occur during the second meiotic division. Nondisjunction which occurs at this phase can effect in normal daughter cells and deformed cells.^[4]

Other uses

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While the main function of homologous chromosomes is their utilize in nuclear division, they are also used in repairing double-strand breaks of Dna.^[15]
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 every bit reactive oxygen species. Homologous chromosomes can repair this harm by adjustment themselves with chromosomes of the same genetic sequence.^[15]
Once the base of operations pairs have been matched and oriented correctly between the two strands, the homologous chromosomes perform a process that is very similar to recombination, or crossing over as seen in meiosis. Role of the intact Dna sequence overlaps with that of the damaged chromosome’southward sequence. Replication proteins and complexes are then recruited to the site of impairment, allowing for repair and proper replication to occur. Through this functioning, double-strand breaks can be repaired and Dna can function normally.^[15]

Relevant research

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Electric current and time to come research on the subject area of homologous chromosome is heavily focused on the roles of various proteins during recombination or during DNA repair. In a recently published article by Pezza et al.^[
which?
]
the protein known as HOP2 is responsible for both homologous chromosome synapsis as well as double-strand interruption repair via homologous recombination. The deletion of HOP2 in mice has large repercussions in meiosis.^[16]
Other current studies focus on specific proteins involved in homologous recombination every bit well.

At that place 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.^[17]
This medicine could be very prevalent in relation to cancer, every bit DNA damage is idea to exist correspondent to carcinogenesis. Manipulating the repair office of homologous chromosomes might allow for bettering a jail cell’s impairment response system. While research has not even so confirmed the effectiveness of such treatment, it may become a useful therapy for cancer.^[eighteen]

See also

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• Homologous recombination
• Mendelian inheritance
• Developmental biology
• Synapsis
• Non-disjunction
• Heredity

References

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Farther reading

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