Table of Contents
What are Homologous Chromosome?
A pair of chromosome having same gene sequence, loci, chromosomal length, and centromere location is called homologous chromosome. The pair consists of one paternal and one maternal chromosome.
The human genome has total of 46 chromosomes. The offspring inherit half of the chromosome by its father and other half from the mother. During meiosis, the maternal chromosomes pair up with paternal chromosome. In humans, the female have all the 23 pairs of homologous chromosomes whereas male have 22 pairs of homologous and 23rd chromosome is not homologous.
At meiosis, the genetic recombination occurs between homologous pairs of chromosomes that is crucial for genetic variation. Due to this genetic recombination or crossing over, the offspring produced are genetically different from each other.
Homologous Chromosome Definition
Paired chromosome with same gene sequence, length, and centromere position are called homologous chromosomes. Homologous chromosomes may have different alleles that is composed of paternal and maternal chromosomes.
Homologous Chromosome Etymology
The word homologous is composed of two words, homos, meaning “same” and logos meaning “relation”. A scientist named as Wilhelm von Waldeyer – Hartz coined the term chromosome in 1889.
Homologous Chromosome vs Heterologous Chromosomes
The chromosomes can be of two types, homologous and heterologous chromosomes. The chromosomes that have similar gene sequences, chromosomal length, and centromere position are termed as homologous chromosomes whereas different chromosomes are termed as heterologous chromosomes.
During meiosis, exchange of genetic material occur in between homologous chromosomes whereas it is absent in heterologous chromosomes. Sometimes, due to any mutation, like translocation, heterologous chromosomes also exchange chromosomal parts.
Let’s compare the homologous chromosomes and heterologous chromosomes with this table-
|Homologous Chromosomes||Heterologous Chromosomes|
|Pairing of chromosomes occur during meiosis||Pairing absent|
|Exchange of genetic material may occurs at synapse||Exchange of genetic material may occur during translocation|
|Homologous chromosome is composed of – Centromere Long arm Short arm||Heterologous chromosomes are composed of- Centromere Long arm Short arm|
|Examples- Autosomal chromosomes, sex chromosomes in female(XX)||Examples- sex chromosome (XY)|
Homologous Chromosome Karyotype
Human nucleus consist 46 chromosomes. There are 22 pairs of autosomes having same length, same position of centromere that are considered as homologous chromosomes. The sex chromosomes have two XX chromosomes are considered as homologous chromosomes whereas X and Y chromosomes are considered as heterologous chromosomes. According to this, the female have all homologous chromosomes and male consists 22 pairs of homologous and one pair of heterologous chromosome.
Homologous Chromosomes vs Sister Chromatids
The centromere join two identical chromatids of homologous chromosomes. These identical chromatids are called sister chromatids. Chromatins are thread like structures that condense into chromosomes at time of cell division.
At interphase of cell division, the genetic material present in chromosome replicates by the process of DNA replication. These two copies of DNA are called chromatids that are joined by a centromere. The chromatids are commonly known as sister chromatids.
Homologous Chromosome Characteristics
In eukaryotic organisms, the genetic material or chromosome in located in the nucleus of the cell. There are two sets of the chromosomes present in the nucleus- male gamete called sperm and female gamete known as egg cell.
After fertilization, the egg cell develop into a diploid zygote by fusion of both male and female haploid gametes. All the organisms consist two sets of chromosomes, one set from each parent that can be similar or may be differ in gene sequence, chromosomal length, and position of centromere.
The pair of chromosomes exchange their genetic material during meiosis. Homologous chromosomes may have different alleles that carries genes for a specific trait. When both the alleles carry different genes for a particular trait, they are described as heterozygous and when they have same genes they are termed homozygous. The concept of dominance of alleles was described by Gregor John Mendel.
Homologous Chromosome Pairing Process
A cell can divide by two methods- mitosis and meiosis. The mitosis results in formation of two daughter cells, each having same no. of chromosomes as the parent cell whereas meiosis is described as reduction division where four daughter cells are formed with half of the chromosomes present in parent cell.
The gametes also undergo meiosis to produce haploid sperm cell and egg cell. These haploid gametes fuse in the process of fertilization. Meiosis completes in two stages- meiosis I and meiosis II. DNA replicates in S phase of interphase and sister chromatids are joined by centromere that exchange their genetic material at prophase I.
This process is called synapse or pairing. During anaphase I the homologous chromosomes separate and moves to the opposite poles of the cell. Telophase I results in formation of two daughter cells that undergo meiosis II Meiosis II results in formation of four daughter cells having reduced chromosomal number by half.
Homologous Chromosome Importance
All the cellular activities are regulated by genetic material i.e. chromosomes therefore chromosomes are crucial for all living beings. The trait of any organism is also determined by the chromosome that have the genetic information.
The homologous chromosomes also exchange their genetic material, which is essential to promote genetic variation. Because of the chromosomes, organisms can reproduce by sexual means to produce offspring that are not identical.
The differences between organisms increase their chances to survive in harsh conditions and prevent from wiped out by any viral disease or by any other method.
Homologous Chromosome Citations
- Molecular mechanisms of homologous chromosome pairing and segregation in plants. J Genet Genomics . 2014 Mar 20;41(3):117-23.
- Homologous chromosome pairing in Schizosaccharomyces pombe. Yeast . 2006 Oct 15;23(13):977-89.
- Homologous chromosome interactions in meiosis: diversity amidst conservation. Nat Rev Genet . 2005 Jun;6(6):477-87.
- A cohesin-based structural platform supporting homologous chromosome pairing in meiosis. Curr Genet . 2016 Aug;62(3):499-502.
- Homologous chromosome pairing in meiosis of higher eukaryotes-still an enigma? Genome . 2020 Oct;63(10):469-482.