IB Biology SL

Revision Notes

3.1.6 Eukaryotic Chromosomes

Eukaryotic Chromosomes

  • Chromosomes are made of one very long, condensed DNA molecule associated with proteins (in eukaryotic cells)
    • The main proteins present are the large positively charged globular proteins called histones, their role is to organise and condense the DNA tightly so that it fits into the nucleus
    • The other proteins are enzymes used in copying and repairing the DNA
  • The tightly coiled combination of DNA and proteins is called chromatin – this is what chromatids, and therefore chromosomes, are made of

Chromatin and histone proteins, downloadable AS & A Level Biology revision notes

DNA is coiled around histone proteins to make chromatin

  • During interphase (S phase) the DNA replicates to create two identical strands of DNA called chromatids, joined together by a narrow region called the centromere
  • The two chromatids that make up the double structure of a chromosome are known as ‘sister chromatids’
  • It is important that the sister chromatids are identical (contain the same genes) because this is key to cell division, as during anaphase of mitosis one chromatid goes into one daughter cell and one goes into the other daughter cell, ensuring the daughter cells are genetically identical
    • After the centromere is split apart at the start of anaphase the chromatids are referred to as individual chromosomes again
  • Each chromatid is made up of one very long, condensed DNA molecule, which is made up of a series of genes

Diagram illustrating the structure of a chromosome at different stages of mitosis

Exam Tip

It is important to distinguish when the terms chromatid, sister chromatids and chromosomes are used.

Different Types of Chromosomes

  • Within the chromosomes of eukaryotic organisms there are many different type of genes that code for many different polypeptides
    • Humans have between 20,000 and 25,000 genes across all 46 chromosomes
  • All the different chromosomes contain different genes. For example:
    • The SRY gene found on the Y chromosome causes the development of male genitalia such as the testes
    • The genes that determine eye colour are located on chromosome fifteen
    • The gene with a faulty version that leads to the disease cystic fibrosis is located on chromosome seven
  • The location of a gene on a chromosome is called a locus (plural loci)
  • Chromosomes have a characteristic shape
    • They have a fixed length
    • The position of the centromere is in a particular location

Homologous Chromosomes

  • In diploid cells there are two complete sets of chromosomes in the nucleus
  • Homologous chromosomes:
    • Carry the same genes in the same loci
    • Are the same shape
  • During fertilization a diploid zygote is formed
    • In a zygote one chromosome of each homologous pair comes from the female gamete and the other comes from the male gamete
  • Having the same genes in the same loci helps homologous chromosomes line up alongside each other during metaphase one of meiosis
  • In photomicrographs chromosomes are often grouped into their homologous pairs

Human Karyogram showing Homologous pairs, downloadable AS & A Level Biology revision notes

Human Karyogram showing homologous chromosomes

Exam Tip

Although homologous pairs of chromosomes contain the same genes in the same order they don’t necessarily carry the same alleles (form) of each gene!

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