(a) Gene expression involves the transcription and translation of DNA sequences

• Only a fraction of the genes in a cell are expressed.
• Transcription and translation involves three types of RNA (mRNA, tRNA and rRNA).
• RNA is single stranded and is composed of nucleotides containing ribose sugar, phosphate and one of four bases: cytosine, guanine, adenine and uracil.

• Messenger RNA (mRNA) carries a copy of the DNA code from the nucleus to the ribosome.
• mRNA is transcribed from DNA in the nucleus and translated into proteins by ribosomes in the cytoplasm.
• Each triplet of bases on the mRNA molecule is called a codon and codes for a specific amino acid.
• Transfer RNA (tRNA) folds due to complementary base pairing.
• Each tRNA molecule carries its specific amino acid to the ribosome.
• A tRNA molecule has an anticodon (an exposed triplet of bases) at one end and an attachment site for a specific amino acid at the other end.
• Ribosomal RNA (rRNA) and proteins form the ribosome.

(b) The role of RNA polymerase in transcription of DNA into primary mRNA transcripts

• RNA polymerase moves along DNA unwinding the double helix and breaking the hydrogen bonds between the bases.
• RNA polymerase synthesises a primary transcript of mRNA from RNA nucleotides by complementary base pairing.
• Uracil in RNA is complementary to adenine.

• RNA splicing forms a mature mRNA transcript.
• The introns of the primary transcript are non-coding regions and are removed.
• The exons are coding regions and are joined together to form the mature transcript.
• The order of the exons is unchanged during splicing.

(c) Translation of mRNA into a polypeptide by tRNA at the ribosome.

• tRNA is involved in the translation of mRNA into a polypeptide at a ribosome.
• Translation begins at a start codon and ends at a stop codon.
• Anticodons bond to codons by complementary base pairing, translating the genetic code into a sequence of amino acids.
• Peptide bonds join the amino acids together.
• Each tRNA then leaves the ribosome as the polypeptide is formed.

(d) Alternative RNA splicing

• Different proteins can be expressed from one gene, as a result of alternative RNA splicing.
• Different mature mRNA transcripts are produced from the same primary transcript depending on which exons are retained.

(e) Protein structure

• Amino acids are linked by peptide bonds to form polypeptides.
• Polypeptide chains fold to form the three-dimensional shape of a protein, held together by hydrogen bonds and other interactions between individual amino acids.