Chromosome Structure

  • Chromosomes are only visible as distinct structures when a cell is dividing
  • Through interphase, the chromosomes are free to move in the nucleus
  • The first time they are visible is during cell division in which they appear as 2 threads (chromatids) joined at a single point (centromere)
  • Each chromatid has already replicated to give two identical DNA molecule
  • The DNA is wrapped around histones (protein) molecules which are then further wrapped to form chromosomes

Homologous Chromosomes

  • All chromosomes occur in pairs, one from the mother (maternal chromosome) and one from the father (paternal chromosome)
  • These are known as homologous chromosome pairs and that total number is reffered to as the diploid number
  • A homologous pair is always two chromosomes that correlate into the same genetic characteristic however determining the same genetic characteristic is not the same as being identical e.g. both homologous pairs have the allele for hair colour, but the maternal chromosome might code for blond hair whilst the paternal chromosome codes for brown hair.
  • Through meiosis, the division of the number of chromosomes is carried out to ensure that each daughter cell receives one chromosome from each homologous pair. Each cell therefore has one chromosome for each characteristic of the organism. When these haploid cells combine, the diploid state with paired homologous chromosome is restored


  • A small section of DNA is a gene. Each gene exists in 2 (and sometimes more) forms. Each form is called an allele
  • Each cell has one allele from its maternal and paternal cells. These two alleles may be the same or different. If they are different each allele will code for a different polypeptide