The following is a brief overview of protein synthesis. For a more detailed description of how ribosomes
function,
click here.

See also:
protein structure and function and DNA structure.

Nucleotides are molecules that make up
nucleic acids like DNA (deoxyribonucleic acid) and RNA
(ribonucleic acid).

Nucleotides are made of a molecule called a
base, a ribose sugar molecule and a phosphate
(containing the element phosphorus, P). The base is the important bit for our purposes and so we shall
say that DNA and RNA are made up of bases.

The
genetic code is a set of instructions (or recipes) telling the cell how to make every protein and
RNA molecule that the cell can make. The genetic code is contained on molecules of DNA. Proteins are
the main building blocks of cells (along with sugars, nucleic acids and lipids).

The nucleus of the human cell contains 23 pairs of
DNA molecules. These DNA molecules are bound
to proteins to form
chromosomes. Chromosomes are made of chromatin (DNA + protein).

Bases form the ‘letters’ of the genetic code. There are four bases/letters in the genetic code: A, T,
C and G (adenine, thymine, cytosine and guanine).

These letters are arranged into ‘words’, each word is only three bases long, e.g. ATT, CGA, etc. These
words are called
codons. Each codon represents or encodes one particular amino acid.

Amino acids are molecules that are joined together in long chains to form proteins. The words of the
genetic code are telling the cell which amino acid comes next in the sequence of particular amino acids
that make up a specific protein. There are 20 amino acids to chose from, so the genetic code contains
only 20 different words plus some additional words that mean things like ‘start the protein here’ or ‘end
the protein here’.

Genes are the sentences of the genetic code. Each gene encodes one protein and is made of a series
of codons.

For example,
haemoglobin is a protein that gives red blood cells their red colour and which contains
iron and carries oxygen around the body. Each haemoglobin protein molecule is made up of four chains
of just over 140 amino acids each. These amino acids will be a particular sequence of the 20 amino
acids that is unique to haemoglobin. The amino acids give the protein its unique properties.

Where do the amino acids come from?

The amino acids are derived from the proteins in your foods and some can be synthesised by the cells.
Proteins in the stomach and small intestine are broken down into their constituent amino acids by
hydrolytic enzymes. The amino acids are absorbed and transported across the cell membranes of the
cells lining the gut and then transported into the
blood stream where they are carried to all the cells of
the body. The cells take in (import) the amino acids they need and the amino acids enter the cytosol.
Special RNA molecules called
transfer RNA, tRNA, transport the amino acids to the ribosomes. Each of
the 20 amino acids has its own special type of tRNA which has an
anticodon that recognises the codon
which has been transcribed onto the mRNA.
Nucleic acids and Proteins – Protein Synthesis
Problem:

The DNA containing the instructions to make proteins is in the cell nucleus. The ribosomes that
synthesise the proteins are in the cytoplasm outside the nucleus.

Ribosomes are small machines made from protein and RNA (of a type called
ribosomal RNA, rRNA).
The ribosome assembles proteins by joining amino acids together into a chain in the correct sequence.

How does the nucleus instruct the ribosomes which protein to make and how does the nucleus pass on
the blueprints (correct number and sequence of amino acids) for the protein to be made to the
ribosomes?