(i) Proteins - Proteins are molecules made of amino acids. They are coded for by our genes and form the basis of living tissues. They also play a central role in biological processes. For example, proteins catalyse reactions in our bodies, transport molecules such as oxygen , keep us healthy as part of the immune system and transmit messages from cell to cell.

Protein synthesis

A gene is a segment of a DNA molecule that contains the instructions needed to make a unique protein. All of our cells contain the same DNA molecules, buteach cell uses a different combination of genes to build the particular proteins it needs to perform its specialised functions.

Protein synthesis has 2 main stages. The 1 stage is known as transcription, where a messenger molecule (mRNA) is formed. This molecule is transcribed from the DNA molecule and carries a copy of the information needed to make a protein.

In the 2 stage, the mRNA molecule leaves the nucleus for the cytoplasm where the cell’s ribosomes read the information and start to assemble a protein in a process called translation.

During translation, the ribosomes read the mRNA sequence of bases 3 at a time.

These 3-letter combinations (called codons) each code a particular amino acid. For example, the base sequence TTT codes for the amino acid lysine.

There are 4 bases (adenine, thymine, guanine and cytosine) and therefore 64 (4) possible codons specified using some combination of 3 bases.

However, only 20 amino acids are required to build all of the proteins in our bodies (some amino acids are specified by more than 1 codon). It is the particular sequence of amino acids that determines the shape and function of the protein.

Protein synthesis, like many other biological processes, can be