Chromosome

A chromosome is a microscopic thread-like structure that is made up of proteins and they are found in the nucleus of each cell of animals, humans and plants. They are the basic structures of all living species. In the nucleus of each cell, they are found in pairs. In humans, there are 23 pairs of chromosomes. The chromosomes are the carriers of the genetic, or precisely hereditary, information from one generation to another. German anatomist, Heinrich Wilhelm Waldeyer has coined this word and the word chromosome is the combination of two Greek words, ‘chroma’ means colour and ‘soma’ means body.

In humans, there are 23 pairs of chromosomes and are of two types, namely, autosomes and allosomes. Out of 23 pairs, 22 pairs are autosomes and one pair is allosomes or sex chromosomes. Autosomes contain information of heredity whereas sex chromosomes are the deciding factors of sex of the next generation.

Types of Chromosome

Chromosomes are made up of one molecule of DNA (Deoxyribonucleic Acid) and proteins. When the cell division happens these chromosomes are visible through a compound microscope. Each species in the world has specific numbers of chromosomes in the cell. The shape of these chromosomes is like dumbells which are joined at the centre.  This constriction region is ‘primary constriction’ or ‘centromere’, which divides the chromosomes into two parts. Each part of the chromosomes is termed as ‘Arm’. The position of centromere in the chromosomes varies and based on this characteristic, chromosomes there are divided into four different types.

1. Metacentric: In these chromosomes, the centromere is located exactly in the middle of the chromosome. And someone sees the chromosome from the centromere it looks like the English letter “V”. In these types of chromosomes, arms are of equal length.
2. Sub-metacentric: In these types of chromosomes, the centromere is located somewhat a little far from the centre of the chromosome. If someone sees the chromosome from the centromere it looks like the English letter “L”. In these types of chromosomes, one arm is shorter than the other.
3. Acrocentric: In this type of chromosome, the centromere is located near the end of the chromosome. If someone sees the chromosome from the centromere it looks like the English letter “J”. In these types of chromosomes, one arm is much smaller as compared to the other one.
4. Telocentric: In this, the centromere is exactly at the end of the arm. These chromosomes make a thread-like structure that has a knot in the middle. If someone sees the chromosome from the centromere it looks like the English letter “i”.

Based on the characteristic, chromosomes are divided into two parts, autosome and sex chromosomes. Humans have 23 pairs of chromosomes and out of these, 22 pairs are autosomes and one pair of chromosomes, sex chromosomes or allosomes, have two chromosomes. In the males, these sex chromosomes are X and Y whereas, in the females, there are two numbers of X sex chromosomes. In the off-springs, the pairing of sex chromosomes is the combination of these 4 chromosomes, i.e. 1 chromosome from the father and 1 from the mother makes 1 pair of sex chromosomes in the off-springs.

Diseases and Disorders

Gregor Johann Mendel has carried out experiments in plants and shown that there are two types of genes (segments of DNA molecule), dominant and recessive. Genes control the structure and function of the cell and in turn, the body. In human beings, there are 46 chromosomes, i.e. 23 pairs of chromosomes. In the offspring, if this number changes from 46 chromosomes, some diseases or disorders arise and these are rare genetic disorders. In a principal manner of heredity, genetic material transfers in equal quantity from each parent to child. Chromosomal abnormalities occur when there is a decrease or increase and deletion or translocation of any part of the chromosome. Some of these types of diseases are:

Down Syndrome

In this, the offspring has 47 chromosomes instead of 46 chromosomes. Infants suffering from this syndrome have one extra chromosome of the 21^st chromosome (autosome) in the cell of the body. Suffering from this syndrome, children are mentally retarded and a have short lifespan. Other symptoms of this disease are short height, flat nose, short wide neck, short fingers and a single horizontal crease on the palm.

Turner and Klinefelter Syndromes

These syndromes arise due to abnormalities in sex chromosomes. In Turner syndrome, which occurs in women, one X chromosome is missing and each cell contains 44 autosomes and only one X chromosome instead of two Xs. Such women are unable to have children as their reproductive organs are not fully developed. On the other hand, Klinefelter syndrome arises in those men who have one extra X chromosome. These types of men are sexually inactive as their sex organs are not developed fully.

Albinism

This disease occurs due to a mutation in a single gene. The person suffering from this disease cannot produce melanin. The symptoms are pale skin, white hair and eyes are pink due to the absence of melanin in the retina and sclera.

Frequently Asked Questions on Chromosome

Q.1: Who is responsible for gender in the offspring in humans?

Answer: In a human, the sex chromosomes in the man are X and Y whereas, in the woman, there are two X chromosomes. In the off-springs, one sex chromosome comes from each parent. At the time of reproduction or fertilization process, when man’s X chromosome combines with X chromosome of woman, a female zygote forms and when man’s Y chromosome combines with X chromosome that results in a male zygote. Thus, the deciding person for the sex of the offspring (child) is the man and not the woman. So as a social responsibility, we should spread this information to reduce the crimes against women.

Q.2: What is the difference between chromosomes and DNA?

Answer: Both the chromosomes and DNA are part of each cell of the living organisms. The DNA is a long fibre and it is made up of two long fibres intertwined together to form a double-helical like structure. When the cell divides they combine with proteins they form chromosomes. So it can be said that DNAs are part of chromosomes. So it can be said that the chromosomes are made when DNA combines with proteins and make up an important part of the human.

Q.3: Write a small note on chromosomes starting from DNA.

Answer: DNA is the building block of chromosomes. DNA, a double helical structure, wraps around the proteins (histones) many times to form nucleosomes. These nucleosomes are tightly tied up in the form of a coil-like structure (chromatin loops). These chromatin loops wrap each other to make a chromosome. Each chromosome has two short, p, arms and two long, q, arms that are joined together at the centromere. In humans, there are 23 pairs of chromosomes in each cell of the body and each parent contributes 23 chromosomes in their off-springs.

Q.4: What are polygenic disorders in humans?

Answer: As the name suggests, polygenic diseases occur due to the mutation in more than one gene. The severity of these disorders is being affected by environmental factors and as a result of the complex interaction between lifestyles and defects in genes. Some examples are diabetes, obesity, asthma, blood pressure, heart disorders, cleft lips, cleft palate, constricted stomach, etc. Mendel’s principle of heredity does not explain polygenic disorders.