(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 affected by environmental factors. These include maternal nutrition, temperature, stress, oxygen levels and exposure to chemicals.

Different types of proteins

There are many different types ofproteins in our bodies. They all serve important