Understanding Chromosomes

  • Chromosomes are long, thread-like structures located within the nucleus of animal and plant cells.
  • Each chromosome is made up of DNA tightly coiled many times around proteins called histones.
  • Chromosomes carry genes, segments of DNA, that are responsible for determining the genetic traits of an organism.

Number of Chromosomes

  • In human biology, the number of chromosomes is fixed; humans have 46 chromosomes or 23 pairs in each somatic (body) cell.
  • Among these, 22 pairs are identical and known as autosomal chromosomes, while one pair is the sex chromosomes (XX for females and XY for males).
  • Sex cells or gametes (sperm for males and egg for females) have a single set of 23 chromosomes as they are produced through a specialised cell division process called meiosis.

Importance of Chromosomes in Inheritance

  • Chromosomes play a key role in inheritance and deciding the genetic makeup of the new generation.
  • During reproduction, the egg cell from the mother and the sperm cell from the father combine their genetic material.
  • This forms a fertilised egg, or a zygote, that holds 23 chromosomes from each parent, making up the 46 chromosomes typical in humans.

Chromosomal Disorders

  • Sometimes during cell division, chromosomes can experience changes or mutations which may cause genetic disorders.
  • Down’s Syndrome, for example, is caused by an extra chromosome 21 (making three copies, hence it’s also known as trisomy 21).
  • Another example is Turner Syndrome where a female is born with one X chromosome missing or partially missing, leading to various developmental and infertility issues.