Chromosome - 1 | Agriculture Optional Notes for UPSC PDF Download

Chromosome Meaning and Discovery

The term "chromosome" is derived from the Greek words meaning "colored body," referencing their ability to be stained by specific dyes.

Karl Nägeli first observed the rod-like structures within the plant cell nucleus in 1842.

The term "chromosome" was coined by W. Waldeyer in 1888.

In 1902, Walter Sutton and Theodor Boveri proposed that chromosomes serve as the physical carriers of genes in eukaryotic cells.

The number of chromosomes remains constant in all cells of a species. Gametes (e.g., sperm and eggs) have half the chromosome count of somatic cells and are referred to as a haploid set due to meiosis during sexual reproduction. The preservation of chromosome number is crucial in the mitotic division of somatic cells, which is necessary for an organism's growth, repair, and regeneration.

The number of chromosomes can vary among different species. Nematode species have only two chromosomes in their cells, while some protozoans may have as many as 1600 chromosomes. Most plant and animal species typically possess between 8 and 50 chromosomes in their somatic cells. The number of chromosomes doesn't necessarily reflect the complexity of a species. For example, human cells contain a total of 23 pairs of chromosomes, with 22 being autosomes and one being a sex chromosome.

Karyotyping is a technique used to examine the structure of chromosomes within a species. It involves isolating, staining, and photographing the chromosomes. This method is valuable for detecting any chromosomal abnormalities.

Chromosome Structure

Chromosomes consist of histone proteins and DNA in terms of their chemical composition. Within each cell, there are pairs of these chromosomes, which are called homologous chromosomes. Chromosomes are composed of chromatin, a substance that encompasses a single DNA molecule along with associated proteins. Each chromosome contains numerous genes, often numbering in the hundreds or thousands, and these genes have the ability to code accurately for various proteins within the cell. The structure of a chromosome is most clearly observed during the process of cell division.

Main parts of chromosomes are:

• Chromatid: Every chromosome possesses two identical structures known as chromatids or sister chromatids, which become visible during the metaphase stage of mitosis.
  • Chromatid DNA: Each individual chromatid contains a single DNA molecule.
  • Sister Chromatid Separation: During the anaphase of mitotic cell division, the sister chromatids separate from each other and move toward opposite poles.
• Centromere and Kinetochore Connection: Sister chromatids are connected by the centromere.
  • Spindle Fiber Attachment: Spindle fibers during cell division attach to the centromere.
  • Centromere Variability: The number and location of the centromere vary among different chromosomes.
  • Primary Constriction: The centromere is often referred to as the primary constriction.
  • Centromere Division: The centromere divides the chromosome into two segments, with the shorter part known as the 'p' arm and the longer part as the 'q' arm.
  • Kinetochore Structure: The centromere contains a disc-shaped structure called the kinetochore, which possesses specific DNA sequences and is bound by special proteins.
  • Kinetochore Function: The kinetochore serves as the central point for the polymerization of tubulin proteins and the assembly of microtubules.
• Secondary Constrictions: Apart from the centromere, chromosomes have additional structures known as secondary constrictions.
  • Distinguishing Secondary Constrictions: Secondary constrictions can be distinguished from the centromere during anaphase because bending occurs exclusively at the centromere, also known as the primary constriction.
  • Nucleolar Organizers: These secondary constrictions, which contain genes responsible for forming nucleoli, are referred to as nucleolar organizers.
• Telomere: The end portion of a chromosome is called a telomere.
  • Polar Telomeres: Telomeres have a polar nature, serving to prevent the fusion of chromosomal segments.
• Satellite: An elongated segment is occasionally found on a chromosome at the secondary constriction.
  • Satellite Chromosomes: Chromosomes containing these elongated segments are referred to as sat-chromosomes.
• Chromatin Composition: Chromosomes are composed of chromatin, which consists of DNA, RNA, and proteins.
  • Interphase Chromatin: During interphase, chromosomes appear as thin chromatin fibers within the nucleoplasm.
  • Chromatin Condensation: When the cell undergoes division, chromatin fibers condense, and chromosomes become visible with distinct characteristics.
  • Heterochromatin: The densely stained, condensed portion of chromatin is referred to as heterochromatin. Heterochromatin contains tightly packed DNA and is typically genetically inactive.
  • Euchromatin: The lightly stained, diffuse section of chromatin is called euchromatin. Euchromatin contains loosely packed DNA and is usually genetically active.
  • Prophase Chromosomal Material: During prophase, the material that forms chromosomes is observed as thin filaments known as chromonemata.
  • Interphase Chromatin Structures: At interphase, there are bead-like structures visible, which are accumulations of chromatin material known as chromomeres. Chromatin with chromomeres resembles a necklace with beads.