Molecular Genetics: 1 The Structure and Function of DNA.

Overview of the DNA Model 2 polynucleotide chains that twist around each other to form a double helix. Each chain has an alternating phosphate group and deoxyribose sugar backbone. The two strands are complementary. A purine (adenine and guanine) is always paired with a pyrimidine (cytosine and thymine). A=T and G=C (Complementary Base Pairing) Hydrogen bonds link both pairs. (A=T have 2, G=C have 3) Two strands are anti-parallel (direction is opposite). At each end of the DNA strand you have the 3’ end (sugar with -OH group on the 3rd carbon) attached to the 5’ end (sugar carbon number 5 attached to a phosphate group). DNA sequence is always written in the 5’ to 3’ direction. A complete turn of the helix is 3.4nm The DNA double strand is 2nm in width

Ribbon Model of DNA

Space-Filling Model of DNA

Terminology Genome: the total genetic makeup of an organism; the entire set of DNA Gene: the basic functional unit of heredity that determines a genetic trait; a specific sequence of DNA that encodes for proteins and RNA molecules, and can contain sequences that influence the production of these molecules. Non-Coding DNA: a large amount of DNA that does not contain instructions.

DNA of Prokaryotes Circular, double stranded DNA. Bacterial chromosome is packed tightly in a specific region called the nucleoid by super-coiling. Amount of super-coiling is controlled by enzymes (some anti- bacterial drugs can target these enzymes). Prokaryotes might also have a plasmid Regulatory sequences are sections of DNA that determine when certain genes are activated.

DNA of Eukaryotes Total amount of DNA is much greater Contained in a nucleus Mitochondria and chloroplasts have their own DNA The DNA strand winds around proteins called histones to form a repeated series of structures called nucleosomes Each nucleosome has 146 base pairs of DNA and 8 histones. Individual nucleosomes are then attached through “linker DNA” Further compacting occurs by coiling the nucleosomes with more histones into a 30nm fibre More compacting of DNA happens when the 30nm fibres form loops that attached to a protein scaffold This is how the DNA sits in the nucleus normally (called chromatin). When cells are getting ready for cell division, though, they must form chromosomes. This is when the scaffolds further condense into chromosomes that have duplicated DNA and joined by a centromere in the middle-ish.

Variation in the Genomes The organization of genes on a chromosome can vary quite a lot. EG in humans, chromosome # 19 has 72 million base pairs and 1450 genes while chromosome #4 has 1.3billion base pairs and only about 200 genes. There is no correlation between an organism’s complexity and the number of protein-coding genes. EG a lungfish has 40 times more DNA than humans

Review Questions Page 218 Numbers 1-10 and 14.