Explain how proteins are synthesised through the processes of transcription and translation.

To fully understand how proteins are synthesised, we first need to understand the fundamentals of protein structure. If this is a concept you're comfortable with, feel free to skip this paragraph. Proteins consist of one or more polypeptide chains. A polypeptide chain is a specific sequence of amino acids, and each amino acid is coded for by three nucleotide bases (A, T, C or G) in DNA. DNA has a double helix structure, with two nucleotide strands paired with hydrogen bonds. One strand, the coding strand, contains the sequence of bases coding for a polypeptide, whilst the other strand, the template, or non-coding, strand, is complementary to the coding strand. A is complementary to T, and C to G, meaning that if the coding strand reads ATG, the non-coding strand reads TAC. Each gene is a section of DNA which codes for one protein, meaning that the specific sequence of nucleotides codes for the amino acid sequence which makes up the polypeptide chain. There are three important types of RNA relevant to protein synthesis; rRNA, tRNA and mRNA. RNA is a single polynucleotide strand, which contains the nucleotide base uracil (U) instead of thymine (T). This allows cellular differentiation between RNA and DNA. rRNA is a component part in ribosome structure. mRNA is manufactured using DNA as a template, and therefore codes for a polypeptide in the same way as DNA; each codon (sequence of three nucleotides) in the mRNA codes for one amino acid. tRNA is a folded single strand of RNA. It has an anticodon, which is complementary to a specific mRNA codon and allows the tRNA to link to the mRNA. Each tRNA molecule is also specific for an amino acid, depending on the identity of the anticodon. The tRNA essentially acts as a link between an mRNA codon and the specific amino acid which it codes for. Transcription: The first step of transcription, which occurs in the nucleus, is the unwinding of the DNA helix and the breaking apart of hydrogen bonds holding the two strands together. RNA polymerase binds to the template strand of the DNA at the beginning of a gene, and synthesis of a complementary mRNA strand begins. Free RNA nucleotides in the nucleus form the mRNA by complementary base pairing with the template strand, and join together to form a sugar phosphate backbone by the formation of phosphodiester bonds. The DNA helix returns to its original shape following transcription by reforming of hydrogen bonds between the bases. The mRNA molecule, which is complementary to the template strand, and therefore identical to the coding strand of the DNA (except for U instead of T) leaves the nucleus via a nuclear pore. Using the example above, if part of the coding strand reads ATG and the non-coding strand reads TAC, the mRNA will read AUG. Translation: At the ribosome, the mRNA codons are 'read'- for each mRNA codon, a tRNA molecule with the correct complementary anticodon binds via hydrogen bonds, bringing the correct amino acid (which is specific for the tRNA anticodon and therefore for the mRNA codon), into the ribosome. A peptide bond forms between the new amino acid, and the previous one in the growing polypeptide chain, hydrogen bonds between the mRNA and tRNA molecule break, and the ribosome moves along the mRNA molecule by one codon. This process continues until a stop codon in the mRNA is reached and read by the ribosome. Stop codons do not code for an amino acid, but instead signal the end of a polypeptide chain. The polypeptide chain leaves the ribosome and is transported to the Golgi body for folding and further processing.