Chromosome - 2 | Agriculture Optional Notes for UPSC PDF Download

Structural Organisation of Chromatin

Chromatin Composition: Chromatin is composed of DNA and associated proteins. DNA is meticulously organized within chromosomes.

• Nucleosomes as Basic Units: Nucleosomes serve as the fundamental units of chromatin and have a diameter of approximately 10 nanometers (nm).
  • DNA Packaging with Histones: DNA is compactly packaged with the assistance of proteins called histones. DNA is wound around histone proteins to create a nucleosome.
  • Types of Histone Proteins: Eukaryotic chromosomes contain five types of histone proteins, namely H1, H2A, H2B, H3, and H4.
  • Histone Charge and DNA Interaction: Histones carry a positive charge due to the presence of basic side chain amino acids, and they interact with negatively charged DNA, which contains phosphate groups.
  • Role of Histone Proteins in Gene Regulation: Histone proteins play a crucial role in regulating gene expression.
  • Nucleosome Structure: A typical nucleosome includes about 200 base pairs (bp) of DNA helix. The core portion of the nucleosome consists of approximately 146 bp of DNA coiled around a core made up of eight histone molecules (two molecules of four histone proteins). This core is connected by linker DNA of roughly 80 bp.
  • Preventing DNA Tangling: Nucleosomes serve to prevent DNA from becoming tangled.
• Linker DNA and the Fifth Histone (H1): Linker DNA and the fifth histone, H1, compact adjacent nucleosomes to form a 30 nm densely packed chromatin fiber.
• Formation of Extended Chromatin: These fibers create large coiled loops held together by non-histone proteins (such as actin, α and β tubulin, myosin) known as scaffolding proteins, resulting in extended chromatin with a diameter of 300 nm.
• Further Chromatin Condensation: Chromatin undergoes further condensation with the aid of a protein called condensin. Condensin binds to DNA and coils it into loops, resulting in the formation of compacted chromosomes.

Types of Chromosome

Based on the Number of Centromeres:

• Monocentric: Having a single centromere.
• Holocentric: Featuring a diffuse centromere with microtubules attached along the chromosome's length.
• Acentric: Chromosomes that may break and fuse together, lacking a centromere, and therefore cannot attach to the mitotic spindle.
• Dicentric: A chromosomal abnormality where chromosomes break and fuse with two centromeres. They are unstable as the two centromeres tend to migrate to opposite poles, leading to fragmentation.

Based on the Position of the Centromere:

• Telocentric: Rod-like chromosomes with the centromere located at the proximal end. They lack a "p" arm or short arm and are not found in humans.
• Acrocentric: Rod-like chromosomes with the centromere situated at one end, resulting in one very short arm and one exceptionally long arm.
• Submetacentric: L-shaped or J-shaped chromosomes, with the centromere positioned near the center, giving rise to two unequal arms.
• Metacentric: V-shaped chromosomes with the centromere located in the middle, leading to two equal arms.

Giant Chromosomes

Polytene Chromosomes:

• Balbiani's Discovery: A structure in the nuclei of midges' secretory glands was first discovered by Balbiani.
• Rediscovery in Drosophila: Painter, Heitz, and Bauer later identified these structures in the salivary gland of Drosophila and recognized them as a unique type of chromosome, often referred to as salivary gland chromosomes.
• Polytene Name Origin: Kollar named them "polytene" due to the presence of numerous chromonemata in these chromosomes.
• Occurrence in Dipteran Larvae: Polytene chromosomes are present in some cells of the larvae of Dipteran insects.
• Large Size: They are considerably large due to their high DNA content. For instance, the polytene chromosome of Drosophila's salivary gland contains 1000 DNA molecules, while Chironomus has 1600 DNA molecules in each polytene chromosome.
• Distinct Banding Pattern: Polytene chromosomes display a pattern of alternating dark and clear bands known as interbands.
• Chromosome Puffs or Balbiani Rings: These are regions where bands swell due to DNA unfolding into open loops. They are associated with intense transcription or mRNA formation.

Lampbrush Chromosomes:

• Origin in Salamander Oocytes: Lampbrush chromosomes were first discovered in the oocytes of salamanders.
• Resemblance to a Brush: They are named "lampbrush" because of their resemblance to a brush used for cleaning lamps and glass chimneys.
• Stage of Occurrence: Lampbrush chromosomes are found during the diplotene stage of oocytes in both vertebrates and invertebrates.
• Additional Occurrence: These chromosomes are also present in the spermatocytes of many animals and in the giant nucleus of an alga called Acetabularia.
• Structure: Lampbrush chromosomes are observed as bivalents with four chromatids. They feature a chromosomal axis formed from highly condensed chromatin, with lateral loops extending from a row of chromomeres.
• RNA Synthesis and Loop Formation: The symmetrical lateral loops of DNA are created as a result of intense RNA synthesis.
• Abundance of Loops: In salamander oocytes, there are approximately 10,000 loops present per haploid set of chromosomes.
• Centromere Exclusion: The centromere does not bear any loops.

Functions of Chromosomes

• Genetic Material Transmission: The primary role of chromosomes is to transport genetic material from one generation to the next.
• Guiding Role in Life Processes: Chromosomes serve as a crucial guiding influence in processes such as growth, reproduction, repair, and regeneration, which are vital for an organism's survival.
• DNA Protection: Chromosomes protect DNA from becoming entangled and damaged.
• Gene Expression Regulation: Histone and non-histone proteins assist in controlling the expression of genes.
• Chromosome Movement in Cell Division: Spindle fibers attached to the centromere facilitate the movement of chromosomes during cell division.
• Gene-Packed Information: Each chromosome contains a multitude of genes that precisely encode numerous proteins found within the body.