Chromosome

Chromosome is a thread-like structure composed of DNA and proteins, located in the nucleus of eukaryotic cells. It carries genetic information in the form of genes, which are essential for the growth, development, and reproduction of an organism. Chromosomes ensure that DNA is accurately replicated and distributed during cell division, maintaining genetic continuity and variation.

In 1988 the German anatomist Heinrich Wilhelm Waldeyer introduce the term chromosome which means "coloured body" for these structure. Chromosome number varies among different animals and plant species. Every species has a specific number of chromosome. Humans, for example has 46 chromosomes, where as butterfly have 268 chromosomes.

Structure of a Chromosome

Chromosomes are composed of several key components:

01. DNA

DNA is the primary molecule that makes up chromosomes. It carries the genetic information necessary for the development, functioning, and reproduction of organisms.

02. Histone Proteins

These proteins help package and organize DNA into a compact, dense structure. DNA wraps around a core of histone proteins, forming a structure called a nucleosomes, which look like "beads on a string." Each nucleosome consists of an octamer of histones (two each of H2A, H2B, H3, and H4) around which DNA is wound. This arrangement compacts the DNA into a dense, organized form, facilitating efficient storage and regulation of genetic information. The nucleosomes are further coiled and folded to form higher-order structures, ultimately creating the characteristic X-shaped chromosomes visible during cell division.

03. Non-Histone Proteins

These proteins have various functions, including regulating gene expression, assisting in DNA replication and repair, and maintaining chromosome structure.

04. Chromatin

Chromatin is the complex of DNA, histones, and non-histone proteins. Chromatin can be in a relaxed (euchromatin) or condensed (heterochromatin) state, affecting gene accessibility and activity.

05. Centromere

The constricted region of a chromosome where sister chromatids are held together. It is crucial for the proper segregation of chromosomes during cell division.

06. Telomeres

Repetitive DNA sequences at the ends of chromosomes that protect them from degradation and prevent them from fusing with other chromosomes.

Together, these components ensure that chromosomes can effectively carry and protect genetic information, regulate gene expression, and facilitate accurate DNA replication and distribution during cell division.

Types of Chromosomes

Chromosomes can be categorized based on several criteria, including their role in determining sex, their structure, and their location within cells. Here are the main types:

Based on Role in Determining Sex

01. Autosomes: These are non-sex chromosomes. Humans have 22 pairs of autosomes, which are the same in both males and females.

02. Sex Chromosomes: These determine the sex of an individual. Humans have one pair of sex chromosomes: females have two X chromosomes (XX), and males have one X and one Y chromosome (XY).

Based on Structure and Position of Centromere

01. Metacentric Chromosomes: The centromere is located in the middle, and the chromosome arms are of approximately equal length.

02. Submetacentric Chromosomes: The centromere is slightly off-center, creating one long arm and one short arm.

03. Acrocentric Chromosomes: The centromere is located near one end, producing one very long arm and one very short arm.

04. Telocentric Chromosomes: The centromere is at the very end of the chromosome, resulting in only one arm. 
(Note: Telocentric chromosomes are not found in humans but are present in some other species).

Based on Appearance During Cell Division

01. Homologous Chromosomes: Pairs of chromosomes (one from each parent) that have the same structure and gene sequence but may have different alleles.

02. Sister Chromatids: Two identical halves of a replicated chromosome, joined at the centromere, which separate during cell division.

Based on Location

01. Nuclear Chromosomes: Found within the nucleus of eukaryotic cells, these chromosomes contain the majority of an organism's genetic material.

02. Mitochondrial Chromosomes: Found in the mitochondria, these chromosomes are small and circular, carrying genes important for mitochondrial function. Mitochondrial DNA is inherited maternally.

Functions of Chromosomes

01. Genetic Information

Chromosomes carry genes, the units of heredity, which contain the instructions for building and maintaining an organism. Each gene is a specific sequence of DNA that encodes a particular protein or functional RNA.

02. Cell Division

During cell division (mitosis and meiosis), chromosomes ensure the accurate distribution of genetic material to daughter cells. In mitosis, each daughter cell receives an identical set of chromosomes. In meiosis, chromosomes undergo recombination and segregation, resulting in genetically unique gametes (sperm and egg cells).

03. Genetic Variation

Chromosomes undergo processes like crossing over during meiosis, which increases genetic diversity in offspring. This recombination of genetic material ensures variation within a species.



Read More:

SAQ 1

  1. What is a cell? What are the essential characteristics of cells?
  2. Explain the fluid mosaic model of the plasma membrane
  3. Which organelles are involved in photosynthesis?
  4. Why the mitochondria is called the powerhouse of the cell?
  5. Which organelle contains enzymes for cellular respiration?
  6. Why mitochondria and chloroplast are called semi-autonomous?
  7. Mention any two advantages of the extensive network of the endoplasmic reticulum
  8. What is the function of peroxisomes in plant cells?
  9. Explain the following terms: (a) chromatin network (b) chromosomes (c) Nucleosome (d) Solenoid Model
  10. What is the function of the nucleolus in the cell?

SAQ 2



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