Chromosome – Definition, Structure, Function, Examples

Chromosomes are thread-like structures present in the nucleus. They are important because they contain the basic genetic material DNA. These are present inside the nucleus of plants as well as animal cells. Chromosomes were first discovered by Strasburger in 1815  and the term ‘chromosome’ was first used by Waldeyer in 1888. Human beings have 46 chromosomes in their body.  These are arranged into 23 pairs. Let us discuss the chromosome structure in detail.

 

Definition of Chromosome

“A Chromosome looks like a  thread and is coiled material, made of proteins. Chromosomes are present in the nucleus of all the cells and contain the basic genetic material DNA, which passes from one generation to another”.

Structure: 

A chromosome has generally 8 parts; Centromere or primary constriction or kinetochore, chromatids, chromatin, secondary constriction, telomere, chromomere, chromonema, and matrix.

• Centromere or Kinetochore: It is the primary constriction at the center to which the chromatids or spindle fibers are attached. Its function is to enable movement of the chromosome during the anaphase stage of cell division.
• Chromatid: During cell division, a chromosome is divided into 2 identical half strands joined by a centromere. A chromatid is each half of the chromosome joined. Each chromatid contains DNA and separates at Anaphase to form a separate chromosome. Both chromatids are attached to each other by the centromere.
• Chromatin: It is a complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells. Nuclear DNA is highly condensed and wrapped around nuclear proteins in order to fit inside the nucleus. In other words, it is not present as free linear strands. The chromatin consists of DNA, RNA, and protein.
• Secondary Constriction: It is generally present for the nucleolar organization.
• Telomere: Telomere is the terminal region of each side of the chromosome. Ach chromosome has 2
• Chromonema: It is a threadlike coiled filamentous structure along which chromomeres are arranged. Chromonema controls the size of the chromosome and it acts as a site of gene bearing.
• Chromomeres: These are the bead-like structures present on threads or chromonema. These are arranged in a row along the length of chromonema. The number of chromosomes is constant and it is responsible for carrying the genes during cell division to the next generation.
• Matrix: Pellicle is the membrane surrounding each of the chromosomes. Matrix is the jelly-like substance present inside pellicle. It is formed of non-genetic materials.

Functions of Chromosomes

For the first time, Sutton and Bover suggested the role of chromosomes in heredity in 1902.

1. The most important function of chromosomes is to carry the basic genetic material – DNA. DNA provides genetic information for various cellular functions. These functions are essential for growth, survival, and reproduction of the organisms.
2. Histones and other proteins cover the Chromosomes. These proteins protect it from chemical (e.g., enzymes) and physical forces. Thus, chromosomes also perform the function of protecting the genetic material (DNA) from damage during the process of cell division.
3. During cell division, spindle fibers attached to the centromeres contract and perform an important function. The contraction of centromeres of chromosomes ensures precise distribution of DNA (genetic material) to the daughter nuclei.
4. Chromosomes contain histone and non-histone proteins. these proteins regulate gene action. Cellular molecules that regulate genes work by activating or deactivating these proteins. This activation and deactivation expand or contract the chromosome.

Examples of Types of Chromosomes

• Metacentric Chromosomes: Metacentric chromosomes have the centromere present exactly in the center. Both the sections are metacentric chromosomes are therefore of equal length. Example:  Human chromosome 1 and 3 are metacentric.
• Submetacentric Chromosomes: In Submetacentric chromosomes, the centromere is not present exactly at the center. The centromere is slightly offset from the center. Both the sections are therefore not of equal length or are asymmetrical. Example: Human chromosomes 4 to 12 are submetacentric.
• Acrocentric Chromosomes: Acrocentric chromosomes have a centromere which is highly offset from the center. Therefore, one of the strands is very long and one very short. Example: Human chromosomes 13,15, 21, and 22 are acrocentric.
• Telocentric Chromosomes: In telocentric chromosomes, the centromere is present at the very end of the chromosome. Telocentric chromosomes are present in species such as mice. Humans do not possess telocentric chromosomes.

Question on Chromosome Structure

Q:  How do genetic disorders occur?

Ans: When DNA sequence in chromosomes changes, genetic disorders occur. The mutation refers to a change in the DNA sequence. The monogenic disorder occurs when a mutation occurs in one gene. Multifactorial genetic disorder occurs when mutation occurs in multiple genes. Human beings have 23 pairs of chromosomes.  All the diseases have a genetic component and the mutations pass from one generation to the other. Cancer, diabetes, obesity are examples of such diseases.