3 Control of gene expression
(a) The phenotype is determined by the proteins produced as the result of gene expression, influenced by intra- and extra-cellular environmental factors. Only a fraction of the genes in a cell are expressed.
Gene expression is controlled by the regulation of transcription and translation.
(b) Structure and functions of RNA. Single strand, replacement of thymine with uracil and deoxyribose with ribose compared to DNA. mRNA (messenger) carries a copy of the DNA code from the nucleus to the ribosome. rRNA (ribosomal) and proteins form the ribosome. Each tRNA (transfer) carries a specific amino acid.
(c) Transcription of DNA into primary and mature RNA transcripts to include the role of RNA polymerase and complementary base pairing. The introns of the primary transcript of mRNA are non-coding and are removed in RNA splicing. The exons are coding regions and are joined together to form mature transcript. This process is called RNA splicing.
(d) Translation of mRNA into a polypeptide by tRNA at the ribosome.
tRNA folds due to base pairing to form a triplet anticodon site and an attachment site for a specific amino acid. Triplet codons on mRNA and anticodons translate the genetic code into a sequence of amino acids. Start and stop codons exist. Codon recognition of incoming tRNA, peptide bond formation and exit of tRNA from the ribosome as polypeptide is formed.
(e) Different proteins can be expressed from one gene as a result of alternative RNA splicing and post-translational modification. Different mRNA molecules are produced from the same primary transcript depending on which RNA segments are treated as exons and introns.
Post translation protein structure modification by cutting and combining polypeptide chains or by adding phosphate or carbohydrate groups to the protein.
(f) Proteins are held in a three-dimensional shape — peptide bonds, folded polypeptide chains, hydrogen bonds, interactions between individual amino acids.
