3.2  Chromosomes

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Essential Idea:

Chromosomes carry genes in a linear sequence

that is shared by members of a species


  • Prokaryotes have one chromosome consisting of a circular DNA molecule
  • Some prokaryotes also have plasmids but eukaryotes do not
  • Eukaryote chromosomes are linear DNA molecules associated with histone proteins
  • In a eukaryote species there are different chromosomes that carry different genes
  • Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes
  • Diploid nuclei have pairs of homologous chromosomes
  • Haploid nuclei have one chromosome of each pair
  • The number of chromosomes is a characteristic feature of members of a species
  • A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length
  • Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex


  • Cairns' technique for measuring the length of DNA molecules by autoradiography
  • Comparison of genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens and Paris japonica
  • Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa and Parascaris equorum
  • Use of karyograms to deduce sex and diagnose Down syndrome in humans


  • Use of databases to identify the locus of a human gene and its polypeptide product