Chromosomes
Biology: Cambridge International A Level, topic 5.1.1
Chromosomes
Genetic information is stored in chromosomes. Chromosomes are made of DNA & protein. Chromosomes have specific structural elements.
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.
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.
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.
Test yourself on Chromosomes
When does DNA replicate?