Chromosomes Structure and Functions

Chromosomes are thread-like structures found in the nucleus. They consist of proteins and DNA that are arranged into specific sequences for specific traits. Humans have 23 pairs of chromosomes, one from each parent. The most important function of chromosome is to carry the genetic material from one cell generation to the next. The basic shape of chromosomes is rod-shaped consisting of a centromere and arms. During cell division, chromosomes divide and ensure the distribution of genetic material to daughter cells.

The function of chromosome includes the transfer of genetic information from one generation to another, protein synthesis, cellular replication, and gene regulation. In this article, we will study What is chromosome, the discovery of chromosomes, the structure of chromatin, the structure of chromosomes, the function of chromosomes, types of chromosomes, Human chromosomes and Karyotypes, and Genetic mapping.

What is a Chromosome?

Chromosomes are thread-like structures that are made of proteins and carry a strand of DNA. It is found in the nucleus of eukaryotic cells. During cell division, chromosomes replicate and separate. This ensures that each daughter cell receives a complete set of genetic material. Genes are located on the chromosomes in a particular sequence. The presence of different genes on the same chromosome is called Linkage. The sex chromosomes, X and Y, determine an individual's biological sex: XX for females and XY for males in humans. Chromosomal aberrations occur when there is change in chromosome number or structure. It can lead to genetic disorders, such as Down syndrome or Turner syndrome.

Discovery of Chromosome

The chromosome was discovered in the late 19th century. German biologist Walther Flemming, in 1882, first observed thread-like structures during cell division in the cell nucleus, which he called "chromatin." The term "chromosome" was first coined by W. Waldeyer in 1888. Waldeyer contributed significantly to understanding cell division and the visible thread-like structures he observed during mitosis. In 1842, Karl Nageli observed the rod-shaped chromosomes in the plant cell and termed them transitory cytoblast, which were later identified as chromosomes. The discoveries laid the foundation for understanding the role of chromosomes in genetics.

Structure of Chromosome

The chromosome structure consists of the following parts:

• Chromatid: A chromatid is one of the two identical copies of DNA that form a replicated chromosome. Chromatids are connected at the centromere until they separate during cell division. Chromatid formation takes place in S-phase.
• Centromere: The centromere is a specific region on a chromosome where the sister chromatids are held together. Centromere is required for the movement of chromosomes towards the pole during cell division.
• Telomeres: Telomeres are protective caps located at the ends of chromosomes. They consist of region of repetitive DNA sequences towards the end of chromosome and help prevent the loss of genetic material during cell division and protect chromosomes from degradation.
• Kinetochore: The kinetochore is made up of protein and is located at the centromere. It facilitates the attachment of spindle fibers during cell division that help in the proper segregation of chromatids.
• Chromatin: Multiple chromatin fibers join and coil together to form the chromosome.
• Secondary constriction: It is usually present for the purpose of nucleolar organization.

Structural Organization of Chromatin

The chromosomes are present in the nucleus of the cell. The DNA along with the histone proteins forms a nucleosome and single nucleosome consists of eight histone proteins. These nucleosome are wrapped over DNA, to form a "beads on a string"-like structure. Multiple nucleosomes join together to form a chromatin fiber and Chromatin fibres together form a chromosome. To understand the difference between chromatin and chromosomes, please refer to the article 'Difference Between Chromatin and Chromosomes'.

Also read: What is Chromatin?

Functions of Chromosome

The functions of chromosomes are as follows:

• Chromosomes are segments of DNA which contains genetic information and act as a genetic blueprint.
• Chromosomes transmit DNA, the basic genetic material, from one generation to the next.
• Chromosome are responsible for regulation of gene expression.
• Sex chromosomes (e.g., X and Y in humans) determine an individual's biological sex.
• Chromosomes also carry out protein synthesis and cellular replication.
• It ensures the proper distribution of DNA (genetic material) to the daughter nuclei during processes like mitosis and meiosis.
• Chromosome play an important role in growth, repair, reproduction and regeneration.
• Chromosomes contribute to genetic diversity through processes such as recombination, which increases adaptability and contributes to evolution.
• It provides structural support to the DNA by condensing this genetic material. It prevent DNA from getting tangled or damaged.

Types of Chromosomes

Based on the position of the centromere, there are four types of chromosomes. These are as follows:

Metacentric Chromosomes

• The centromere is present in the center and divides it into two equal halves.
• During cell division, the chromosomes appear V- or X-shaped.
• Examples of such chromosomes are: 1^st, 3^rd, 16^th, 19^th, and 20th.

Sub-metacentric chromosomes

• The centromere is present near the middle of the chromosome and divides it into two unequal arms.
• During cell division, the chromosomes appear L-shaped.
• Most chromosomes in humans are submetacentric. For example, 2^nd, 4^th to 12^th, 17^th, 18^th, and X chromosomes.

Acrocentric chromosomes

• Centromere is present near the end of the chromosome.
• It forms a short p-arm and a long q-arm.
• Examples of such chromosomes are the 13th, 14th, 15th, 21st, 22nd, and Y chromosomes.

Telocentric chromosomes

• Centromere is present at the end of the chromosome, and they have only one distinguishable chromosome arm.
• Rarely observed in humans.
• During the anaphase phase of cell division, the shape of the chromosome is like the letter ‘i’.

Human Chromosomes

In humans, there are 23 pairs of chromosomes present, thus total 46. These 23 pairs of chromosomes can be divided into two types: autosomes and allosomes. The first 22 pairs are autosomes, and the last 23rd pair is the allosome, or sex chromosome. The sex chromosomes, X and Y, determine an individual's biological sex: XX for females and XY for males in humans. Each chromosome contains highly condensed and coiled DNA consisting of millions of gene sequences. Chromosomal aberrations occur when there is change in number or structure of chromosomes which can lead to various genetic disorders, such as Down syndrome, Klinefelter syndrome, and Turner syndrome.

Karyotypes and Genetic Mapping

Karyotype represents the complete set of chromosomes of an individual, organized by size, shape, and banding pattern. It provides details about the chromosome count of an organism and how these chromosomes appear under a microscope. Karyotyping is a test to examine chromosomes in cells. It creates a photograph of chromosomes during mitosis or meiosis, providing a snapshot of cell division called a karyogram. It also provides information regarding the number, structure, length, and position of the chromosome. It is also used to identify abnormalities in chromosomes like Down syndrome, Klinefelter syndrome, Philadelphia chromosome, Trisomy 18, and Turner syndrome.

Gene mapping refers to the techniques used to identify a gene’s location and distance between genes. The distances between various sites inside a gene can also be described through gene mapping. Genome mapping involves using several molecular markers at specific locations on the genome. It is of two types: Linkage mapping which show the location of each gene on a chromosome and their relative distances from one another and physical mapping provide the actual DNA base pair distances between landmarks.

Also Read:

• Difference Between Chromosome and Gene
• Chromosomal Theory of Inheritance
• Chromosomal Translocation: Definition, Occurrence, Types, and Examples
• Chromosomal Disorders