Topic 7.1 (DNA structure - HL)

7.1.2: Outline the structure of nucleosomes

A nucleosome consists of DNA wrapped around eight histone proteins and held together by another histone protein

7.1.5: State that eukaryotic genes can contain exons and introns

A typical eukaryote gene consists of a series of exons and introns. Exons are sequences of bases that are transcribed and translated. Introns are sequences of bases that are transcribed, but not translated. Prokaryotes do not usually have introns.

7.1.4: Distinguish between unique or single-copy genes and highly repetitive sequences in nuclear DNA

Highly repetitive sequences (satellite DNA) constitutes 5-45% of the genome. The sequences are typically between 5 and 300 base pairs per repeat, and may be duplicated as many as 105 times per genome. A surprisingly small proportion of eukaryotic DNA is single copy, or unique genes.

7.1.3: State that nucleosomes help to supercoil chromosomes and help to regulate transcription

Nucleosomes have two functions: - they help to package up the DNA during mitosis and meiosis by the process of supercoiling - they can be used to mark particular genes, either to promote gene expression by transcription and translation, or to cause silencing of a gene by preventing transcription

7.1.1: Describe the structure of DNA, including the antiparallel strands, 3'-5' linkages and hydrogen bonding between purines and pyrimidines

The two strands have their 3' and 5' terminals at opposite ends - they are antiparallel. DNA replication can only occur in a 5'→3' direction so a different method is needed for the two strands. At one end of each strand is a phosphate linked to carbon atom 5 of deoxyribose. This is the 5' terminal. At one end of each strand is a hydroxyl group attached to carbon atom 3 of deoxyribose. This is the 3' terminal. Adjacent nucleotides are linked by a bond between the phosphate group of one nucleotide and carbon atom 3 of the other nucleotide. Hydrogen bonds link the bases. Two bonds form between adenine and thymine and three bonds between guanine and cytosine. Only these pairs can form hydrogen bonds. Two of the bases in DNA are purines: adenine and guanine. They have two rings in their molecules. Two of the bases in DNA are pyrimidines. Cytosine and thymine are pyrimidines. They have one ring in their molecule. Only a purine plus a pyrimidine will fit in the space between the sugar-phosphate backbones.