Homologous Chromosome – Structure and Functions

Homologous chromosomes refer to paired chromosomes that essentially have the same gene sequence, gene position (loci), centromere location, and chromosomal length. Though they may have the same genetic sequence and loci they may differ in alleles. The pair is comprised of a paternal chromosome and a maternal chromosome.

Just like homologous chromosomes, there are heterologous chromosomes as well. So let us understand what a chromosome is and then further about homologous chromosomes, and how is it different from heterologous chromosomes. In this article, we will read about homologous chromosomes, their structure, functions, and much more.

Definition of Homologous Chromosomes

Homologous chromosomes are a pair of chromosomes which have the same loci, gene sequence, chromosomal length and centromere location. They have the same genes but slightly different DNA.

What is a Chromosome?

A chromosome is a thread-like structure formed by the DNA molecule that undergoes a packaging process within the nucleus of every cell. In the chromosome, the DNA is tightly coiled many times around proteins called histones that support its structure. The chromosomes are visible in the cell’s nucleus only when it divides.

Each chromosome has a centromere, which divides the chromosome into two sections or arms. The short arm of the chromosome is called the p arm. The long arm of the chromosome is called the q arm. The location of the centromere on each chromosome gives the chromosome its characteristic shape. According to the position of the centromere, chromosomes are of four types namely Metacentric, Submetacentric, Acrocentric, and Telocentric.

What are Homologous Chromosomes?

Homologous chromosomes are paired chromosomes that essentially have the same gene sequence, gene position technically called loci, centromere location, and chromosomal length. The pair consists of a paternal chromosome and a maternal chromosome. It is important to remember that the pairing of homologous chromosomes occurs only during meiosis and not during mitosis. The pairing is important for the accurate separation of chromosomes during meiosis.

Example of Homologous Chromosomes

Humans have 23 pairs of homologous chromosomes. One chromosome of each pair comes from the mother, while the other comes from the father. These chromosomes are carried in the haploid gametic cells called sperms and eggs. During fertilization, sperm and eggs fuse leading to the formation of a diploid organism. This zygote divides many times to create all of the cells of the human body. Each cell in the human body, thus, contains 23 pairs of homologous chromosomes, or 46 chromosomes in total.

Structure of Homologous Chromosomes

The basic parts of a homologous chromosome are centromere also called kinetochore, short arm p, and long arm q. The length of the arm and the position of the centromere have an important role in determining the homogeneity of the chromosomes. As stated above based on the position of the centromere there are four types of chromosomes, thus, the two chromosomes with similar structures come together to form homologous chromosomes. It should be noted that homologous chromosomes are not identical chromosomes like the sister chromatids.

Difference Between Homologous Chromosome and Heterologous (Non-Homologous) Chromosome

The difference between homologous and heterologous chromosomes are stated below:

Homologous Chromosome	Heterologous Chromosome (non-homologous)
Homo means same. So, homologous chromosomes are the same at least in terms of the gene sequence and loci.	Hetero means different. They are any two chromosomes that are different, such as in terms of gene sequence and loci.
Homologous chromosomes have alleles on the same genes located in the same loci.	Heterologous chromosomes have alleles on different genes.
During meiosis, homologous chromosomes may naturally exchange genetic material.	No genetic exchange takes place normally. But during translocation, an exchange of chromosomal parts may happen which is the result of mutation.
During the process of meiosis, they pair up.	They do not pair up during meiosis
Examples: 22 autosomal chromosomes, X and X sex chromosomes in females, etc.	Examples: X and Y sex chromosomes in males, etc.

Sister Chromatids vs Homologous Chromosomes

There lies a difference between homologous chromosomes and sister chromatids. As discussed above homologous chromosomes are pair of chromosomes that have the same structure and genes but may have different alleles of those genes. Whereas, sister chromatids are identical copies of a single chromosome. They are formed during the DNA replication process that occurs before cell division. Each chromosome consists of two identical sister chromatids which are connected by a protein complex called the centromere.

Functions of Homologous Chromosome

The functions of Homologous Chromosome are as follows:

• Genetic Diversity: During sexual reproduction, homologous chromosomes play an important role in ensuring that offspring inherits a genetic composition that is different from both of the parents.
• Recombination: During the process of meiosis, homologous chromosomes actively participate in gene exchange which is termed recombination. Recombination plays a significant role in the genetic diversity observed among offspring.
• Expansion of the Gene Pool: The genetic recombination between homologous chromosomes during sexual reproduction helps in the expansion of the gene pool.
• Adaptation to Dynamic Environments: The exchange of genes among homologous chromosomes causes variations in genes of an organism that help them during adaptation to changing environments.
• Diagnosis of Genetic Disorders: Errors during the pairing and separation of homologous chromosomes can cause genetic abnormalities. Identification of such chromosomes helps in the examination and understanding of genetic disorders.

Conclusion – Homologous Chromosome

Homologous chromosomes are chromosomes having the same genetic makeup. The homologous chromosomes are vital for diagnosing genetic disorders. Thus it can be concluded that homologous chromosomes are chromosomes that have the same genetic organization. They contribute to genetic diversity, recombination, exchange of genetic material, and much more. This leads to the enhancement of the gene pool and increases the adaptation ability of the organism towards changing environmental conditions for the survival of the organism.

FAQs on Homologous Chromosome

What are Homologous Chromosome ?

Homologous chromosomes are the chromosomes that have the same gene sequences and loci. Homologous chromosomes contribute to genetic diversity in offspring, genetic recombination, gene pool expansion, adaptation to dynamic environments, DNA repair, and genetic disorders.

The 23rd pair of chromosomes is Homologous or Heterologous?

The 23rd pair of chromosomes or sex chromosomes are homologous in women and heterologous in male.

What is the Significance of Homologous Chromosome?

Homologous chromosomes serve multifaceted roles such as genetic diversity in offspring, promoting genetic recombination, expanding the gene pool, enabling adaptation to dynamic environments, participating in DNA repair, and aiding in the diagnosis of genetic disorders resulting from irregular homolog pairing.

Are Homologous Chromosomes Identical?

No, homologous chromosomes aren’t identical. Since homologous chromosomes are derived from distinct parents, they are similar but not identical.

Do Men have Homologous Sex Chromosomes?

No. Men have a pair of two different chromosomes as a 23rd pair of chromosomes. These are X and Y chromosomes and have different genes.

What is Homologous vs Heterologous Chromosome?

A pair of chromosomes is referred to as “homologous chromosomes” if their structure and form are similar, and “heterologous chromosomes” if they differ.