There is a thread-like structure called a chromosome in the nucleus that transmits genetic information from generation to generation. Mutation, heredity, regeneration, and repair play a vital role in cell division. DNA molecules are organised into highly ordered chromosomes in eukaryotic cells, and histone proteins support the structure of DNA. Every species has a different number of chromosomes. 

Protozoa contain 1600 chromosomes in their cells, whereas nematodes contain only two. Generally speaking, the somatic cells of plants and animals have 8 -50 chromosomes. Chromosome numbers do not reflect a species’ complexity. There are 23 pairs of chromosomes in the human body (2n, 23*2 = 46), of which 22 are autosomes, and only 1 is a sex chromosome.

The chromosomal structure

The chromosomes that are homologous in each cell are known as homologous chromosomes. The DNA molecule and its proteins are contained in chromatin in chromosomes. Several proteins are precisely coded for in each chromosome. In cell division, the chromosomal structure can be observed best.

The important parts of chromosomes are as follows:

• Chromatid: In mitotic metaphase, each chromosome contains two structures called chromatids or sister chromatids. One DNA molecule resides in each chromatid. When sister chromatids separate during cell division of the mitotic phase, they migrate to exact opposite poles.

• Kinetochore and centromere: The sister chromatids are joined with the help of centromeres. When cells divide, spindle fibres attach to centromeres. Different chromosomes have different numbers and positions of centromeres that are called primary constrictions. The centromere divides the chromosome into two arms; the shorter arm is called the C-arm, and the arm is called the ‘q’ arm. Kinetochores are disc-shaped structures that contain specific DNA sequences and proteins bound to them. Kinetochores serve as centres for tubulin polymerization and microtubule assembly. Besides the centromere, chromosomes have secondary constrictions. During anaphase, secondary constrictions are identifiable by their bending at the centromere (primary constriction). The secondary constrictions that contain genes that form nucleoli are known as the nucleolar organiser.

• Telomere: Telomeres are the ends of chromosomes. The polarity of telomeres prevents chromosomal segments from fusing.

• Satellite:- At the chromosome’s secondary constriction, a satellite segment is present. Satellite chromosomes are often referred to as sat-chromosomes.

• Chromatin: Chromatin is found in chromosomes. This protein, DNA, and RNA compound constitute chromosomes. A chromosome can be seen as a thin chromatin fibre extending into the nucleoplasm at interphase. Chromatin fibres condense during cell division, and chromosomes become visible with distinct features. Heterochromatin is the dark, condensed region of chromatin. It contains firmly stuffed DNA, which is hereditarily latent. The light-stained, diffused locale of chromatin is known as euchromatin. It contains hereditarily dynamic and inexactly pressed DNA. The chromosomal material is apparent in prophase as flimsy fibres known as chromonemata. In interphase, dot-like designs are apparent; an aggregation of chromatin material is called chromomere. Chromatin, which has a chromomere, resembles a neckband with globules.

Chromosomal Functions 

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

The chromosomal structure contains the genetic material DNA, which is their most important function. DNA determines molecular functions. These functions are essential for the organism to grow, survive, and reproduce. 

Histones and other proteins protect chromosomes. Proteins like these protect cells from chemicals such as enzymes and physical forces. Chromosomes also protect DNA from damage during the division process.

An important function of spindle fibres attached to centromeres is to contract during cell division. Chromosomal centromere contraction ensures precise distribution of DNA (genetic information) to daughter nuclei. Chromosome 9 has a special function here–the earliest genetic events in both pathways of URCa development are chromosome 9 alterations, which set the stage for genetic instability and subsequent genetic events.

Histones and other proteins coexist in the chromosomal structure. Genes are controlled by these proteins. These proteins are activated or deactivated by cellular molecules that regulate genes. Chromosomes expand or contract as a result of this activation and deactivation.

Examples of chromosomes

A human chromosome 1 and a human chromosome 3 are metacentrics. A submetacentric chromosome is the fourth to the 12th chromosome of a person.

A chromosome with a centromere that is significantly offset from its centre is an acrocentric chromosome. As a result, one strand is very long while the other is very short. Acrocentric chromosomes are 13, 15, 21, and 22 on the human genome. In telocentric chromosomes, the centromere is accessible toward the completion of the chromosome. Telocentric chromosomes are accessible in species like mice. People don’t have telocentric chromosomes. 

Conclusion

Several processes in cellular life depend on the secondary structure of DNA. Several genes are regulated by changes in DNA structure or by differences in replication, transcription, or expression. A Holliday’s structure is formed by recombination, which leads to the rearrangement of genes. Furthermore, specific DNA structures lead to different kinds of mutations. We also saw the functions of chromosomes, like chromosome 9.

Proteins can recognize distinct regions of DNA in many ways, including by activating and inhibiting the expression of many genes, methylating DNA, and cleaving it at the base. Another method involves recognizing the differences in secondary structure between regional DNA fragments. Thus, in this article, we have some useful notes on the structure of the chromosome.