Nucleic acids, protein synthesis and cell division

DNA and RNA

• DNA: double helix of two antiparallel polynucleotide strands; nucleotides contain deoxyribose, phosphate and a base (A, T, C, G); complementary base pairing A–T and C–G by hydrogen bonds.
• RNA: single-stranded, ribose sugar, uracil instead of thymine; mRNA, tRNA and rRNA.

DNA replication

Semi-conservative: the helix unwinds (helicase), each strand acts as a template, free nucleotides pair up, and DNA polymerase joins them. Each new molecule has one old and one new strand (shown by Meselson and Stahl).

Protein synthesis

• Transcription (nucleus): RNA polymerase makes mRNA complementary to the template strand of a gene.
• Translation (ribosome): tRNA anticodons pair with mRNA codons; amino acids join by peptide bonds. The genetic code is a triplet, degenerate and universal.
• Gene mutations: substitution, insertion or deletion of bases (insertions and deletions cause frame shifts). Example: sickle-cell anaemia results from a single base substitution in the β-globin gene.

The cell cycle and mitosis

Interphase (growth and DNA replication), then mitosis (prophase, metaphase, anaphase, telophase) and cytokinesis. Mitosis produces two genetically identical diploid cells for growth, repair and asexual reproduction. Uncontrolled division leads to cancer.

Meiosis

Two divisions produce four haploid gametes. Variation arises from crossing over (prophase I) and independent assortment (metaphase I), plus random fertilisation.

Check yourself

1. Why is DNA replication described as semi-conservative?
2. State two features of the genetic code.
3. Give two sources of genetic variation in meiosis.