Translation of the Genetic Code: Protein Synthesis

• Messenger RNA is made in the nucleus by transcription from a DNA molecule. 
• A ribosome on the rough endoplasmic reticulum (RER) attaches to the mRNA molecule.
• A transfer RNA molecule arrives and brings an amino acid to the first three bases (codon) on the mRNA.
• The three unpaired bases (anticodon) on the tRNA link up with the codon.
• Another tRNA molecule comes into place bringing a second amino acid
• Its anticodon links up with the second codon (next three bases) on the mRNA
• A peptide bond is formed between the two amino acids
• The first tRNA molecule releases its amino acid and moves off into the cytoplasm
• The ribosome moves along the mRNA to the next codon
• Another tRNA molecules bring in the next amino acid into place
• A peptide bond joings the second and third amino acid to form a polypeptide chain
• The process continues - polypeptide chain gets longer, this continues until termination (stop) codon is reached then the polypeptide is complete

AS Biology: Proteins

Amino acids

• Proteins are made of monomers called amino acids.
• These monomers join together to forma long chain = polypeptide.
• Polypeptides can be cominted to form a protein. Polypeptides and proteins are polymers.
• There are twenty biologicall important amino acids.
• All amino acids (and so proteins) contain Carbon, Hydrogen, Oxygen and Nitrogen (some contains sulphur)

• Proteins are polymers of amino acids and are made up of a Amino group (NH2) , a carboxyl group (COOH), and the central carbon (α-carbon), hydrogen and a variable group.

Animals needs proteins in their diets. these are digested to amino acids and used to prouce proteins. Excess amino acids cannot be stored and their amino group makes them toxic. This is removed by deamination in the liver.

Plants make the amino acids they need. They use nitrate from the soil to produce amino groups. These are added tot he organic groups made from photosynthesis.

Aminos acids can link together by forming peptide bonds. a peptide bond is formed when the carboxyle group of one amino acids combine with the elimination of water. Therefore it is a condensation reactions. When to amino acids are joining by a peptide bond they form a dipeptide.

• Many amino acids can joing to form a polypeptide chain (series of condensation reactions) = polymerisation. 
• Polypeptides and proteins are synthesised on ribosomes - protein synthesis. It uses mRNA which puts the amino acids together in the right order- different mRNA molecules make different proteins.

Primary structure

This is the sequence of amino acids in a polypeptide molecules.

The sequence of amino acid is important as it determines the shape of the protein and ergo the function.

Secondary structure

This is a regular arrangement of polypeptide chains. The alpha-helix is where the polypeptide chain is loosely coiled in a regular spiral. in the beta-pleated sheet the polypeptide chains are more extended n than alpha helix.

Tertiary structure

This si further folding of the secondary structure which gives a compact 3D shape. It depends on the properties of the different R-groups in the polypeptide chain.

Collagen is a fibrous protein, it has three polypeptide chains and is twisted into a triple helix, polypeptides held together by hydrogen bonds between chains, this forms a collagen fibril, many fibrils form a fibre.

Haemoglobin has 4 polypeptide chains, 2 alpha and 2 beta. Each one has an iron prosthetic group attatched to it. It is a globular protein so it is soluble.