What is homologous chromosome?
The genetic code is a vital element for all biological life forms, as it provides the instructions necessary for the process of protein synthesis. Genetic material is found in all living cells in the form of the nucleic acid deoxyribonucleic acid (DNA) or ribonucleic acid (RNA).
DNA and RNA are polymers of small molecules called nucleotides in DNA and ribonucleotides in NA. DNA and RNA have various structural differences, but the most striking one is that DNA is double stranded whereas RNA is single stranded.
DNA is a very long molecule, reaching up to 3m in length in one cell. DNA can be divided into sections called genes, where each gene is a basic unit of inheritance that codes for a protein or a specific trait.
In order to fit in the nucleus of a eukaryotic cell, DNA is wrapped around special proteins called histones to form a structure commonly known as the nucleosome. Upon cell division, nucleosomes condense even further to form chromatin.
Moreover, cell division requires the occurrence of DNA replication before it proceeds in order to ensure equal inheritance between daughter cells.
During cell division, chromatin condenses further to form chromosomes, which are composed of two identical sister chromatids as a result of DNA replication.
The term chromosome is of Latin origin, where the term “chromo” means colored and “soma” means bodies. This refers to the observation that chromosomes appear as colored bodies upon visual staining under the microscope.
Chromosomes exist in pairs called homologous chromosomes. Each chromosome has a similar copy to it, where one set of chromosomes originate from the father and the other from the mother.
In other words, chromosomes are numbered from 1-22 in humans, where each cell contains two copies of each chromosome. For example, there are two copies of chromosome 1 in each cell, one copy from the mother and one copy from the father.
Human cells contain 23 pairs of homologous chromosomes. A homologous chromosome pair contains the same genes as each other. This term is not to be confused with heterologous chromosomes, where the pair carries different genes from each other.
Characteristics of Homologous Chromosomes
The chromosomes of a eukaryotic cell are located in the nucleus. The chromosomes in the nucleus are of two sets. One set would come from the male gamete (sperm cell) and the other set, from the female gamete (egg cell).
Prior to fertilization, both gametes are haploid. When the sperm cell successfully fertilizes the egg cell the resulting zygote is a diploid, which means the union of the two haploid cells results in a single cell with two sets of chromosomes: one set that is maternally derived and another set that is paternally derived.
Each maternal chromosome has a corresponding paternal chromosome of the same gene sequence, gene loci, chromosomal length, and centromere location. The pair comes close to each other during meiosis so that they could exchange genes between sister and non-sister chromatids.
Although both members of the homologous pair have similar genes and loci, they may differ in the alleles. For example, both of them carry genes coding for the eye color trait. One has alleles for the brown-eye trait whereas the other, for the blue-eye trait.
In this case, the alleles are different and the homologous chromosomes are described as heterozygous. On the contrary, when the alleles are the same (e.g. both alleles for the same gene code for the blue-eye trait) then, they are said to be homozygous.
The significance of alleles was demonstrated by Gregor Mendel through his pioneering works in genetics using garden peas. He was able to show that one of the alleles may be dominant over the other. In this case, only one allele will be expressed.
A trait demonstrating this pattern is referred to as Mendelian inheritance. Nevertheless, in humans, genetic expressions are not straightforward.
Many of the human traits do not conform to the Mendelian pattern of inheritance. In this regard, they are referred to as non-Mendelian.
Structure of Homologous Chromosomes
Homologous chromosomes are pairs of chromosomes in a diploid organism that have similar genes, although not necessarily identical.
There are two main properties of homologous chromosomes:
- the length of chromosomal arms and
- 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.
Note that the pair of sex chromosomes may or may not be homologous, depending on the sex of the individual. For instance, females contain XX, thus have a homologous pair of sex chromosomes.
This means that females have 23 pairs of homologous chromosomes in total (i.e., 22 pairs of non-sex chromosomes (autosomes), 1 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 same genes but 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.
Function of Homologous Chromosome
During mitosis, the homologous chromosomes duplicate and fuse to form sister chromatids. These sister chromatids get segregated to opposite poles and get divided between the daughter cells.
During meiosis, the homologous chromosomes exchange genetic information in the process of recombination. Here, they form four daughter cells, with each having half the number of parental chromosomes.
The segregation of homologous chromosomes happens during meiosis Ⅰ, and the segregation of sister chromatids occurs in meiosis Ⅱ.
The failure in the segregation of homologous chromosomes during meiosis can result in nondisjunction.
Thus homologous chromosomes play a vital role in nuclear division. They also play a role in fixing damaged DNA. Any double-stranded damage can often be fixed by matching up the broken ends to their homologous partners.
Importance of Homologous Chromosome
Homologous chromosomes are 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.
The chromosomes are essential to all living things because they carry the genetic instructions for all of the cell’s activities. They also bear the genetic information that determines the traits of an organism. Homologous chromosomes, therefore, are vital in the same way.
They carry genetic information that has been passed down from one generation to the next. And since alleles may possibly be different in the same gene, the result is varying phenotypes. Thus, the distinctiveness of an individual of the same species is established.
Apart from this, the organism is capable of reproducing offspring that is genetically different from itself as well as from the rest of its descendants. The homologous chromosomes in the nucleus of the organism’s gamete pair up during meiosis. This event is crucial to promote genetic variation.
The homologous pair exchanges genes via genetic recombination so that genetic diversity may be promoted. This is regarded as one of the advantages of having been able to reproduce sexually. Those that reproduce asexually could create a clone of themselves. Thus, this could reduce diversity in the gene pool.
A small gene pool means low genetic diversity. It could be unfavorable because it means there is less opportunity to acquire genes essential for adapting to an environment prone to inexorable physicochemical changes.
In contrast, greater genetic variability means a higher propensity to acquire better genes. High genetic diversity also means a large gene pool. This, in turn, implicates increased chances of acquiring genes that could enhance biological fitness and survival.
Use in clinical context
The chromosomes in a homologous pair each have the same genes in the same order, but there may be variation between them, resulting in different alleles. Some of the variants in these alleles might have functional implications while others will not.
During meiosis, homologous chromosomes line up and recombination occurs between them. This results in gametes with unique combinations of alleles on each chromosome and therefore unique individuals.