3.2 Chromosomes

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.

Q: List 3 characteristics of a plasmid.Some prokaryotes also have plasmids but eukaryotes do not.

Nature of science: Developments in research follow improvements in techniques—autoradiography was used to

establish the length of DNA molecules in chromosomes.

1. Cells were grown for two generations in a culture medium containing tritiated thymine.

2. Cells were gently burst to release their DNA.

3. A layer of photographic emulsion was applied to the DNA and it was left for two months.

4. At the end of two months the film was developed and a dark grain was left wherever a tritium atom decayed.

Application: Cairns’ technique for measuring the length of DNA molecules by autoradiography.

Eukaryote chromosomes are linear DNA molecules associated with histone proteins.

Q: Draw a simple representation of how DNA is associated with histones.

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.

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.

Haploid vs. Diploid

Diploid nuclei have pairs of homologous chromosomes.

Haploid nuclei have one chromosome of each pair.

Q: What types of cells in humans are haploid?

The number of chromosomes is a characteristic feature of members of a species.

Q: Copy the data table from page 154 illustrating the difference between the diploid chromosome number of five different species.Application: Comparison of diploid chromosome numbers of Homo sapiens,Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.

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.Q: What combination of sex chromosomes produces male and female humans?Q: Answer questions 1-4 on page 159.Application: Use of karyograms to deduce sex and diagnose Down syndrome in humans.

• Klinefelter syndrome is a chromosomal condition that affects male physical and cognitive development.

• Affected individuals typically have small testes that do not produce as much testosterone as usual. A shortage of testosterone can lead to delayed or incomplete puberty, breast enlargement (gynecomastia), reduced facial and body hair, and an inability to have biological children (infertility).

• Some affected individuals also have genital differences including undescended testes (cryptorchidism), the opening of the urethra on the underside of the penis (hypospadias), or an unusually small penis (micropenis).

• Older children and adults with Klinefelter syndrome tend to be taller than their peers. Compared with unaffected men, adults with Klinefelter syndrome have an increased risk of developing breast cancer and a chronic inflammatory disease called systemic lupus erythematosus. Their chance of developing these disorders is similar to that of women in the general population.

• Children with Klinefelter syndrome may have learning disabilities and delayed speech and language development. They tend to be quiet, sensitive, and unassertive, but personality characteristics vary among affected individuals.

Skill: Use of databases to identify the locus of a human gene and its polypeptide product.