STRUCTURE AND PROPERTIES OF CHROMOSOMES

Biology

STRUCTURE AND PROPERTIES OF CHROMOSOMES

  • These are thread-like structures found in the nucleus.
  • They are normally very thin and coiled and are not easily visible unless the cell is dividing.
  • When a cell is about to divide, the chromosomes uncoil and thicken.
  • Their structure, number and behaviour is clearly observed during the process of cell division.
  • The number of chromosomes is the same in all the body cells of an organism.
  • In the body cells, the chromosomes are found in pairs.
  • Each pair is made up of two identical chromosomes that make up a homologous pair.
  • However sex chromosomes in human male are an exception in that the Y-chromosome is smaller.
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Number of Chromosomes

Diploid Number (2n)

  • This is the number of chromosomes found in somatic cells.
  • For example, in human 2n = 46 or 22 pairs (44 chromosomes) are known as autosomes (body chromosomes”)
  • While 1 pair is known as the sex chromosomes.
  • In Drosophila melanogaster, 2n = 8.

Chromosome Structure

  • All chromosomes are not of the same size or shape.
  • In human beings; each of the twenty¬ three pairs have unique size and structure.
  • On this basis they have been numbered 1 to 23.
  • The sex chromosomes form the 23rd pair.
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Properties of Chromosomes

  • Chromosomes are very long and thin.
  • They are greatly and loosely coiled and fit within the nucleus.
  • During cell division they shorten, become thicker and are easily observable.
  • Each consists of two chromatids.
  • The two chromatids are held at same position along the length, at the centromere.
  • Chromatids separate during cell division in mitosis and in the second stage of meiosis.
  • Chromosomes take most dyes and stain darker than any other part of the cell.
  • This property has earned them the name “chromatin material”
  • Each chromosome is made up of the following components:
  • Deoxyribonucleic acid (DNA) – this carries the genes.
  • It is the major component of the genetic material.
  • Protein e.g. histones.
  • Ribonucleic acid (RNA) is present in very small amounts.
  • Enzymes concerned with DNA and RNA replication – these are DNA and RNA polymerases and ligases.
See also  ACTIVE TRANSPORT

See also

GENETICS

Growth and Development in Animals

ROLE OF GROWTH & HORMONES IN PLANTS

PRIMARY AND SECONDARY GROWTH

TYPES OF GERMINATION

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