5.1.1

Chromosomes

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Chromosomes

Genetic information is stored in chromosomes. Chromosomes are made of DNA & protein. Chromosomes have specific structural elements.

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DNA

  • If the DNA from all 46 chromosomes in a human cell nucleus was laid out end to end, it would measure approximately two meters.
  • Considering that the size of a typical human cell is about 10 µm, DNA must be tightly packaged to fit in the cell’s nucleus.
  • At the same time, it must also be readily accessible for the genes to be expressed.
    • During some stages of the cell cycle, the long strands of DNA are condensed into compact chromosomes.
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Histone proteins & compaction

  • In the first level of compaction, short stretches of the DNA double helix wrap around a core of eight histone proteins.
  • This wrapping occurs at regular intervals along the entire length of the chromosome.
    • The DNA-histone complex is part of the chromatin.
  • Compaction continues as the DNA-histone complexes coil tighter.
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Chromosomes during the cell cycle

  • DNA replicates in the S phase of interphase.
  • During this phase, the chromatin condenses so much DNA becomes visible under a microscope as chromosomes.
    • After replication, the chromosomes are composed of two linked sister chromatids.
    • The connection between the sister chromatids is called the centromere.
    • Telomeres are highly coiled regions located at the end of the chromosomes.
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Telomeres

  • The ends of the linear chromosomes are known as telomeres. They are made by the enzyme telomerase.
    • These telomeres protect the genes from getting deleted as cells continue to divide.
    • This is because some DNA is lost from the end of chromosomes during each division.
  • Cells that undergo cell division continue to have their telomeres shortened.
  • This is because most somatic cells do not make telomerase.
    • So, telomere shortening is associated with aging.

Jump to other topics

1Cell Structure

2Biological Molecules

3Enzymes

4Cell Membranes & Transport

5The Mitotic Cell Cycle

6Nucleic Acids & Protein Synthesis

7Transport in Plants

8Transport in Mammals

9Gas Exchange

10Infectious Diseases

11Immunity

12Energy & Respiration (A2 Only)

13Photosynthesis (A2 Only)

14Homeostasis (A2 Only)

15Control & Coordination (A2 Only)

16Inherited Change (A2 Only)

17Selection & Evolution (A2 Only)

18Classification & Conservation (A2 Only)

19Genetic Technology (A2 Only)

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