3.2: Chromosomes
Teaching time: 3 hours Practical time allocated: 0
Key vocabulary
Prior learning and retrieval practice
Review 3.1 Genes
Review 2.6 DNA & RNA structure
Prokaryotic Chromosomes
U1: Prokaryotes have one chromosome consisting of a circular DNA molecule.
U2: Some prokaryotes also have plasmids but eukaryotes do not.
Exercise 1: Draw and label a prokaryotic cell and then complete the following word cloze exercise.
Most prokaryotes store their genes in one __________________________________ which consists of a single circular ‘naked’ DNA molecule. The region of a prokaryote that contains the chromosome is called the _____________________________________. Some prokaryotes also contain ______________________________________. These are very short rings (tens to hundreds of base pairs long) of DNA that contain a few genes.
Label your diagram to show the location of the nucleoid and the plasmids
Eukaryotic Chromosomes
Essential idea: Chromosomes carry genes in a linear sequence that is shared by members of a species.
U3: Eukaryote chromosomes are linear DNA molecules associated with histone proteins.
U4: In a eukaryote species there are different chromosomes that carry different genes.
U8: The number of chromosomes is a characteristic feature of members of a species.
Exercise 2: Draw and label a Eukaryotic cell and complete the following word cloze exercise
Eukaryotic chromosomes are linear _____________________ molecules that are also associated with proteins called ____________________________. In any given eukaryote species there are different chromosomes that carry different genes. Therefore, chromosome 1 will carry___________________________ genes from chromosome 2. Eukaryotes never contain _______________________ but a tiny (0.1%) of their genes may be carried in the _____________________________. Different eukaryotic species have differing numbers of ________________________ and the number of chromosomes is a characteristic feature of members of a species.
Homologous Chromosomes
U5: Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.
U6: Diploid nuclei have pairs of homologous chromosomes.
U7: Haploid nuclei have one chromosome of each pair.
Guidance from the syllabus: 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.
Exercise 3: Draw a pair of Homologous Chromosomes. Label the centromere, telomere, sister chromatids, non-sister chromatids. Use the letters A, B & C to indicate the position of three genes and use capital or small letters to represent different alleles. Then answer the questions below.
Explain how these chromosomes are similar
Explain how these chromosomes are different
Define homologous chromosomes
What term do we apply to cells that contain homologous chromosomes
What term do we apply to cells that only contain one chromosome of each pair
What types of cells do not contain homologous chromosomes and why
Karyograms
U9: A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.
U10: Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.
Guidance from the syllabus: 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.
A3: Use of karyograms to deduce sex and diagnose Down syndrome in humans.
Exercise 4: Look at the following karyograms and for each one deduce whether the individual is male or female and whether that individual has down's syndrome.
Autoradiography and Cairn's technique
NoS: Developments in research follow improvements in techniques—autoradiography was used to establish the length of DNA molecules in chromosomes. (1.8)
A1: Cairns’ technique for measuring the length of DNA molecules by autoradiography.
Exercise 5: Outline the Cairn's technique for measuring the length of DNA molecules.
Practical work
A2: Comparison of genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens and Paris japonica.
A3: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.
S1: Use of databases to identify the locus of a human gene and its polypeptide product.
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.
