3.1.6 Eukaryotic Chromosomes

• Chromosomes in eukaryotic cells are one, very long DNA molecule associated with proteins
  • The main proteins present are the large, positively charged globular proteins called histones, their role is to organise and wrap 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 chromosomes are made of

DNA is coiled around histone proteins to form chromatin

The replication of chromosomes

• During interphase (the period before mitosis) 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 in order to produce genetically identical daughter cells via mitosis
  • During anaphase of mitosis one chromatid ends up in one daughter cell while the other chromatid ends up in the other daughter cell
  • After the centromere is split apart at the start of anaphase the chromatids are referred to as individual chromosomes again

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

• In a eukaryote species, there are different chromosomes that carry different genes
• During mitosis, chromosomes become denser by supercoiling, so are easier to observe than when they are in interphase
• Different types of chromosomes can be seen
  • They differ in length and the position of the centromere

• Humans have 23 types of chromosomes
  • The largest one is numbered 1
  • The smallest is numbered 22
  • Pair 23 is the pair of sex chromosomes (XX or XY)

• Humans have between 20,000 and 25,000 genes across all 46 chromosomes
• Specific genes always appear at the same locus (position) of a particular chromosome, 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 15
  • The gene with a faulty version that leads to the disease cystic fibrosis is located on chromosome 7

• In other words, each chromosome type contains specific genes arranged in a standard sequence
  • This property allows for the exchange of genetic material between chromosomes during meiosis

A pair of homologous chromosomes showing the loci of various alleles

• In diploid cells there are two complete sets of chromosomes in the nucleus
• Homologous chromosomes
  • Carry the same genes at the same loci
  • Are the same shape
  • Are not usually identical because they may be carrying different alleles to each other

• 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 1 of meiosis
• In photomicrographs, chromosomes are often grouped into their homologous pairs
  • These are shown in a picture format as a karyogram

Human karyogram showing homologous chromosomes