Chromosomes

Nature of science:

Developments in research follow improvements in techniques—autoradiography was used to establish the length of DNA molecules in chromosomes. (1.8)

• 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.

Applications and skills:

• Application: Cairns’ technique for measuring the length of DNA molecules by autoradiography.
• Application: Comparison of genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens and Paris japonica.
• Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.
• Application: Use of karyograms to deduce sex and diagnose Down syndrome in humans.
• Skill: Use of databases to identify the locus of a human gene and its polypeptide product.

• Sequencing of the rice genome involved cooperation between biologists in 10 countries.

Utilization:

• Syllabus and cross-curricular links:
• Biology
• Topic 1.6 Cell division

Aims:

• Aim 6: Staining root tip squashes and microscope examination of chromosomes is recommended but not obligatory.
• Aim 7: Use of databases to identify gene loci and protein products of genes.

• The terms karyotype and karyogram have different meanings. Karyotype is a property of a cell—the number and type of chromosomes present in the nucleus, not a photograph or diagram of them.
• Genome size is the total length of DNA in an organism. The examples of genome and chromosome number have been selected to allow points of interest to be raised.
• The two DNA molecules formed by DNA replication prior to cell division are considered to be sister chromatids until the splitting of the centromere at the start of anaphase. After this, they are individual chromosomes.