Making Protein
Genes
Life needs to be able to make different proteins at different times. For example, if its food changes it will need to make different enzymes. So it must be able to control their production, so that the right proteins can be made in the right amounts at the right time.
The reason that nucleic acid is important in life is that the bases carry information. The sequence of bases (TACG…) is like the sequence of 0s and 1s which a computer uses to store digital information.
Life can decode this information and use it as an instruction to build a protein. A piece of double-stranded nucleic acid which codes for a single protein is called a gene.
To build a protein, a copy of this information is made.
Size of Genes
If you imagine a soccer ball blown up to the size of the Earth (Soccearth) then a gene would be about 10 cm thick and about 20 m long, about the size of a tree trunk.
Copying the message
The first step in building a protein is to unzip the two halves of the gene and use one half as a template to make a copy of the information. This process is called transcription and is done by proteins called polymerases.
A polymerase transcribes a gene to produce a message
The copy of the gene is called messenger ribonucleic acid, mRNA. It is called "ribo" because the sugars it contains are different from the sugars in DNA: they are ribose which have not lost an oxygen atom.
Protein Synthesis
Next comes one of the most complex parts of life. An extraordinary group of molecules called a ribosome reads the information in the mRNA and uses it to build a protein.
A ribosome builds a protein
So the flow of information is:
Gene (DNA) 
message (mRNA) protein
The Ribosome
The ribosome is central to the process of life. In life today, ribosomes occur both as free particles within cells and as particles attached to membranes inside cells. A ribosome is made of about 40% protein and 60 % nucleic acid. It is composed of four nucleic acid molecules and about 70 different proteins.
Ribosomes are very numerous in a cell and account for a large proportion of its total nucleic acid. Every second in every animal cell, around 1 million amino acids are added to growing proteins. The ribosome is the molecular machine which makes this happen. It reads the information in messenger RNA (mRNA) to tell it which amino acids to add to the protein it is growing. One ribosome can add between 3 and 5 amino acids to the protein it is growing every second.
A ribosome consists of two parts, a larger one and a smaller one, each of which has a characteristic shape.
Details of a ribosome in action
The ribosome has two parts, the large and small subunits. Each of these subunits is made of ribosomal RNA (rRNA) and about 25 proteins.
The RNA and proteins which make ribosomes is different in bacteria than in more advanced cells (eukaryotes). Also it varies between species of eukaryotes. However the structure and function is always the same. We conclude that this machinery appeared only once, and has been modified round this common theme during evolution.
As protein is made it passes through a hole in the large sub-unit. The exact way the ribosome works is not understood.
A ribosome reads three bases at a time, and uses this as a code for choosing the next amino acid to add to the growing protein. So the sequence of the bases in a nucleic acid controls which protein the ribosome builds.
Size of Ribosome
On Soccearth (a soccer ball blown up to the size of the Earth) a ribosome would be about 1 meter across - about the size of a washing machine.
