DNA in Prokaryotes and Eukaryotes, Chromosome Condensation and Mutations

The structural organization of genetic material differs between organisms: DNA lies bare in the cytoplasm of prokaryotes, while in eukaryotes it is organised inside a defined nucleus bounded by a nuclear membrane and condensed by specialised proteins.

The three-dimensional structure of the double helix

• Two antiparallel sugar–phosphate backbones run 5′→3′ and 3′→5′.
• Complementary base pairing by hydrogen bonds: A=T with two bonds, G≡C with three.
• The helix shows a major groove and a minor groove along its length.

The total length of the DNA molecules in a single human nucleus is about two metres, while the diameter of that nucleus does not exceed 2–3 microns. To fit, the DNA must be shortened by a factor of 100,000 through precisely ordered stages:

1. Coiling around histone proteins — nucleosomes. Histones are rich in the two basic amino acids arginine and lysine and therefore carry positive (+) charges, while the phosphate groups of the sugar–phosphate backbone carry negative (−) charges. The charges attract, so the DNA strand winds around a disc of eight histone molecules forming repeating "beads-on-a-string" loops called nucleosomes — this shortens the DNA by only 10 times.
2. Coiling by non-histone structural proteins. The nucleosome threads coil tightly around one another with the help of non-histone proteins to form bands of condensed chromatin, then arrange into large loops attached to a cytoskeleton of non-histone structural proteins, shortening the molecule by 100,000 times until the chromosome appears in its condensed form at metaphase.

Functional note: regulatory non-histone proteins determine whether the code of a DNA molecule will be used to build enzymes and proteins or not.

The genome is the total set of genes present in the cell. Research has shown that:

• In prokaryotes, most of the DNA molecule represents codes for building proteins and enzymes.
• In eukaryotes, less than 70% of the DNA represents codes for building RNA and protein, while more than 30% represents parts that do not code.

Repeated sequences

• Repeated sequences occur in the genome: for example the base sequence AGACT repeats 100,000 times in the middle of one chromosome of the fruit fly Drosophila, and it does not represent a code.
• Some genes repeat hundreds of times to build enormous quantities of protein and RNA, as in the genes that build histones and rRNA.

Telomeres

Telomeres are structures at the ends of chromosomes that do not code; their function is to preserve the structural integrity of the chromosome and prevent its erosion.

A mutation is a sudden change in the nature of the factors that determine particular characteristics, which may result in a change in the characteristics of the organism.

• Give the reason: eukaryotic cells contain enormous amounts of histone proteins. Because histones are rich in the basic amino acids arginine and lysine, which carry positive charges, they attract the negative phosphate groups of the DNA strand electromagnetically at physiological pH (7.4).
• Give the reason: a mutation in reproductive cells is more dangerous than in body cells. Because gametic mutations in eggs and sperm pass directly to the embryo and to successive generations, causing chronic hereditary deformities, while somatic ones end with the death of the organism or removal of the affected part.
• Give the reason: the biological importance of polyploidy in plants. Polyploidy (3n, 4n, 6n) increases the size of plant cells and the thickness of their leaves, flowers and fruits and raises their sugar and protein content; it is common and desirable in crops such as wheat, cotton and strawberry.
• Give the reason: yeast is a special case in the classification of genetic material. It is a eukaryote, yet it is the only eukaryotic organism in the syllabus whose cytoplasm contains circular DNA plasmids.