Chromosomes: Definition, Types, Structure and Function

Chromosomes are DNA packages containing a part or all of the genetic material of an organism. In most chromosomes, the long thin DNA fibres are surrounded by packaging proteins called the histones. The histones bind to condense the DNA molecule to maintain its integrity. The chromosomes have a complex structure and play a role in transcriptional regulation.

This Story also Contains

1. What are Chromosomes?
2. Discovery of Chromosomes
3. Role of Chromosomes in Genetics
4. Composition of Chromosomes
5. Types of Chromosomes
6. How Many Chromosomes in Humans?
7. Chromosomal Disorders
8. Cell Division and Chromosomes
9. What is the Function of Chromosomes?
10. Methods for Chromosome Analysis
11. Chromosomes in Evolution and Speciation
12. Importance of Chromosomes in Medical Research
13. MCQs on Genetics

Chromosomes are generally visible under a light microscope during the metaphase stage of cell division. Each chromosome becomes duplicated before its entry, during the S-phase. The two copies are joined by a centromere, resulting in an X-shaped structure if the centromere is present at the centre. During the metaphase stage, they are duplicated and highly condensed. They reach their highest compaction level in anaphase during chromosome segregation. A chromosome is a topic of the chapter Molecular Basis of Inheritance. It is an important chapter in the Biology subject.

What are Chromosomes?

Chromosomes are long, thread-like structures found in the eukaryotic cell nucleus, composed of DNA and proteins. This chromosome plays a critical role in the storage and transmission of genetic information from generation to generation. It is important in inheritance because it ensures accurate copying and distribution of DNA at the time of cell division.

Discovery of Chromosomes

Matthias Schleiden and Theodor Schwann laid the foundation of cell theory, establishing that all living organisms are composed of cells, which are the basic units of life.

Gregor Mendel conducted pioneering work on heredity, laying the groundwork for understanding genetic inheritance through his experiments with pea plants.

Thomas Hunt Morgan studied fruit fly chromosomes, providing important evidence for the chromosomal theory of inheritance and demonstrating that genes are located on chromosomes.

Walter Sutton and Theodor Boveri developed the chromosomal theory of inheritance, explaining how chromosomes are responsible for gene regulation and the inheritance of traits.

Role of Chromosomes in Genetics

Chromosomes primarily work in genetics as they contain the genetic information in genes that give an organism its characteristics and control gene expression.

It plays a crucial role in proper inheritance, gene expression, and the evolutionary process to ensure that the information is passed from one generation to another. It is materialised through the proper transmission of genetic information.

Composition of Chromosomes

Chromatin is a DNA-protein complex in the interphase nucleus of the cell, presenting an amorphous, decondensed appearance.

At the time of cell division, however, chromatin condenses into visible, differentiated chromosomes so that the DNA can be separated in a highly ordered and precise manner.

Components of Chromosomes

• DNA: The DNA encodes the genetic blueprint of the organism.

• Histone Proteins: Histone proteins package and order the DNA in the form of organised units called nucleosomes; they help to organise and control the genetic material.

• Non-Histone Proteins: They play a variety of functions, including chromosome maintenance and gene regulation.

• Centromere: It is the middle part of a chromosome where sister chromatids are held, and also plays a critical role in the movement of chromosomes during cell division

• Chromatids: Duplicated halves of the same chromosome are divided during mitosis and meiosis

• Telomeres: Protective caps that feature at the ends of the chromosomes that help prevent their degradation and fusion with other chromosomes.

• Kinetochore: A complex of proteins attached to the centromere that attaches spindle fibres to the chromosomes during cell division.

Types of Chromosomes

There are different types of chromosomes: based on the number of centromeres, or based on the position of the centromere.

Based on the Number of Centromeres:

• Monocentric: It has only one centromere.

• Dicentric: Contains two centromeres.

• Acentric: Lack of centromere.

• Holocentric: Present with centromeres at all other places on the entire chromosome length.

Based on the Position of the Centromere:

• Metacentric: The centromere lies in the centre. Results in both arms are of the same length.

• Submetacentric: The centromere is found off-centre. Results in unequal length of the two arms.

• Acrocentric: Centromere near one end, so a very short p arm and a long q arm.

• Telocentric: Centromere at the end of the chromosome, thus no p arm.

How Many Chromosomes in Humans?

Humans have 23 pairs of chromosomes in total, a total of 46 chromosomes. Out of which, 22 pairs are autosomes, and the remaining pair consists of sex chromosomes.

Autosomes vs. Sex Chromosomes

Autosomes: These include the chromosome pairs 1-22. It has similar characteristics in both sexes and determines most of the traits of an individual.

Sex Chromosomes: This includes an X and Y chromosome, responsible for determining the sex of an individual. Females possess the XX chromosomes, while males carry the XY chromosomes.

Chromosomes vs Chromatids

Chromosomes: They are single, thread-like structures that contain DNA.

Chromatids: They are identical halves of a duplicated chromosome, connected at the centromere, and separated during cell division.

Chromosomal Disorders

There are various types of chromosomal mutations that are seen in the chromosomes. Some of them might result in an increase or decrease of the total chromosome number. Some of the disorders are listed below-

Trisomy 21: Also known as Down syndrome. This is the presence of an extra copy of chromosome 21.

Monosomy X: That is Turner syndrome, a condition of having only one X chromosome in females.

Klinefelter Syndrome: The condition in males where there is an extra X chromosome (XXY) that affects sexual development and fertility.

Types of Structural Abnormalities in Chromosomes:

Deletions: A chromosome segment missing from the genetic material of a person, which may cause genetic disorders.

Duplications: Doubling of a chromosome segment that might be the reason for many genetic imbalances.

Inversions: The reversal of a segment of a chromosome, which can impact gene function.

Translocations: Exchange of a part of a chromosome between two non-homologous chromosomes. Most often, such an exchange causes genetic disorders or cancer.

Cell Division and Chromosomes

Chromosomes either maintain the same amount of DNA content or reduce the DNA content, based on the kind of cell division they enter. The two different types of cell division are listed below-

Mitosis

Mitosis ensures that daughter cells both contain the same number of chromosomes. Phases of Mitosis:

• Prophase: Chromosomes condense and create the mitotic spindle.

• Metaphase: Chromosomes align along the equatorial plane of the cell.

• Anaphase: Sister chromatids separate to opposite poles.

• Telophase: Chromosomes start to de-condense, and nuclear membranes reform.

• Cytokinesis: The plasma membrane pinches in to divide the cell into two.

Meiosis

Reduction Division for Gametes: Meiosis reduces the chromosome number by half to produce gametes (sperm and eggs).

Stages of Meiosis:

• Meiosis I: Homologous chromosomes are separated into different cells, reducing the chromosome number by half.

• Meiosis II: Sister chromatids are separated into different cells, resulting in four genetically diverse gametes.

• Crossing Over: It occurs in Prophase I, that is, the genetic materials are exchanged between homologous chromosomes and thereby increase the genetic variation.

What is the Function of Chromosomes?

Chromosomes serve various functions, such as the storage of genetic information and the regulation of gene expression. The functions of chromosomes are given below:

Storage of Genetic Information

The chromosomes take, carry, and transfer genetic information from the DNA. This ensures that the genetic instructions regarding growth, development, and function are conserved and passed on to the next generation.

Gene Regulation

Chromosomes regulate gene expression by controlling the accessibility of DNA to the transcription machinery.

Cell Cycle Regulation

Chromosomes are involved in various phases of the cell cycle. It participates in replication of DNA during the S phase and chromosome segregation during both mitosis and meiosis.

Methods for Chromosome Analysis

Chromosomal analysis can be done by karyotyping, the FISH technique, and chromosome banding. Some of the methods of chromosome analysis are:

Karyotyping

Karyotyping is the process of aligning chromosomes in a standardised manner to detect abnormalities of chromosomes and diagnose genetic diseases.

Fluorescence In Situ Hybridisation (FISH)

FISH utilises fluorescent probes to hybridise with specific DNA sequences on chromosomes to facilitate the visualisation of chromosomal anomalies and gene localisation.

Chromosome Banding

• G-banding: It results in the generation of banding due to the use of Giemsa stain in staining, which further aids in the identification of the chromosomes

• Q-banding: It utilises quinacrine mustard dye to highlight the different chromosome regions

• R-banding: One can visualise reverse G-banding patterns by using heat and staining

Chromosomes in Evolution and Speciation

Chromosomal alterations create genetic diversity and evolve the species into various forms, thus affecting reproductive isolation. New species can arise due to duplication, deletion, and translocation of chromosomes that change the gene content and function. These changes may, at times, result in reproductive barriers.

Importance of Chromosomes in Medical Research

Chromosomes are manipulated in various medical conditions. They can be treated by gene therapy to reverse the condition. The importance of chromosomes in medical research is explained below:

Genetic Engineering and Gene Therapy

Chromosome manipulation is very important in the development of treatments for genetic diseases, including gene therapy to correct defective genes.

Cancer Research

The mechanism of cancer can be understood through chromosomal abnormalities. Targeted therapy was developed as a result of understanding the Philadelphia chromosome associated with chronic myeloid leukaemia.

MCQs on Genetics

Q1. What is the purpose of meiosis in cell division?

Option 1: Growth and tissue repair

Option 2: Production of gametes

Option 3: Genetic recombination

Option 4: DNA replication

Correct answer: 2) Production of gametes.

Explanation:

The purpose of meiosis in cellular division is to diminish the chromosome count by a factor of two, thereby producing gametes such as sperm and eggs in animals, or pollen and ovules in plants. This process guarantees that upon fertilization, where two gametes unite, the ensuing progeny will possess the precise number of chromosomes.

Principal Functions:
1. Genetic Diversity: Meiosis enhances genetic variation among offspring by employing mechanisms like crossing over and independent assortment.
2. Chromosome Number Decrease: It reduces chromosomes from diploid (2n) to haploid (n), thus averting chromosome duplication across generations.
3. Gamete Formation: This process generates haploid cells, namely sperm or eggs, which are vital for sexual reproduction.

Meiosis is indispensable for maintaining the genetic integrity and diversity of sexually reproducing organisms.

Hence, the correct answer is option 2) Production of gametes.

Q2. Assertion: Genetic information is passed from parent to offspring.

Reason: Offspring inherit half of their genetic information from each of their parents during sexual reproduction.

Option 1: Assertion and Reason are both true, and Reason is an accurate account of Assertion.

Option 2: Both Assertion and Reason are accurate, but Reason does not adequately explain Assertion.

Option 3: The assertion is correct, but the reasoning is incorrect.

Option 4: Both assertion and Reason are incorrect.

Correct answer: (1) Assertion and Reason are both true, and Reason is an accurate account of Assertion.

Explanation:

In sexual reproduction, the genetic information of two parents is combined to create a unique offspring. Each parent contributes half of their genetic material to the offspring, which determines the physical and biological characteristics of the offspring. The genetic material is carried in the form of DNA, which contains the genetic code that determines traits such as eye colour, height, and susceptibility to certain diseases.

Hence, the correct answer is Option (1) Assertion and Reason are both true, and Reason is an accurate account of Assertion.

Q3. Assertion: Genetic mutations can lead to changes in the traits of an organism.

Reason: Mutations can alter the DNA sequence, which can affect the expression of genes and lead to changes in physical or biological traits.

Option 1: Assertion and Reason are both true, and Reason is an accurate account of Assertion.

Option 2: Both Assertion and Reason are accurate, but Reason does not adequately explain Assertion.

Option 3: The assertion is correct, but the reasoning is incorrect.

Option 4: Both assertion and Reason are incorrect.

Correct answer: (1) Assertion and Reason are both true, and Reason is an accurate account of Assertion.

Explanation:

Genetic mutations can develop naturally or as a consequence of exposure to certain chemicals or radiation in the environment. These mutations can change the DNA sequence, which can impact how genes are expressed and eventually result in modifications to a person's biological or physical characteristics. While some mutations may have little to no impact on the organism, others may have a major one, such as the development of diseases or changes to the organism's appearance. In areas like medicine and agriculture, where researchers can use this knowledge to breed crops with desirable characteristics or develop treatments for genetic diseases, understanding genetic mutations is crucial.

Hence, the correct answer is Option (1) Assertion and Reason are both true, and Reason is an accurate account of Assertion.

Also Read:

• MCQ Practice on Genetics

• Mendelian Disorders

• Chromosomal Abnormalities