1.5 - Protein Synthesis

The genetic code

• A gene is a section of DNA. The order of the bases along the gene is a code for building a protein.
• A protein is a chain of small molecules called amino acids. What makes one protein different from another is the number of amino acids and the order they're joined in.
• The order of the bases in the gene sets the order of the amino acids, so the base sequence decides which protein is made.

Reading the code in threes

• The bases are read in groups of three. Each set of three bases is called a base triplet (also called a codon).
• Each triplet codes for one amino acid. The triplets are read one after another, with no overlapping - the first triplet fixes the first amino acid, the next triplet the next, and so on along the whole gene.
• Only about 20 different amino acids are used, but they can be arranged into the many thousands of proteins a body needs - like a small alphabet spelling a huge number of words.
• Four bases read three at a time give 4 × 4 × 4 = 64 possible triplets. That's more triplets than amino acids, so some amino acids are coded for by more than one triplet.

From chain to protein

• Once the amino acids are joined in the order the gene sets, the chain folds into a particular 3D shape.
• It's this shape that lets the finished protein do its specific job.
• A different base sequence gives a different amino-acid sequence, which folds into a differently shaped protein - so each gene makes its own specific protein.

Jobs that proteins do

• Structural proteins - Such as collagen, they give tissues their shape and strength
• Enzymes - Speed up chemical reactions
• Hormones - Such as insulin, they carry signals around the body
• Antibodies - Help defend against pathogens
• Receptors - Sit on the cell surface and let the cell detect chemical signals

Making the protein - mRNA and the ribosome

• The instructions for a protein are held in a gene, part of the DNA inside the nucleus of the cell.
• DNA is far too big to leave the nucleus, so the gene stays there.
• Proteins are assembled out in the cytoplasm, on small structures called ribosomes.
• So the coded information has to be carried from the nucleus to the ribosome.

Messenger RNA carries the code out

1. A complementary copy of the relevant gene is made in the nucleus as a short strand of messenger RNA (mRNA).
2. Because it's only one gene long and much smaller than the whole DNA molecule, the mRNA can leave the nucleus and move out into the cytoplasm to a ribosome, carrying the code with it.
3. There the mRNA passes through the ribosome.

Assembling the amino acids

• As the mRNA passes through, the ribosome builds the protein up one amino acid at a time.
• The amino acids are brought to the ribosome by carrier molecules.
• The order they're joined in is not random - it's set by the sequence of bases along the mRNA, which was copied from the gene.
• So the base sequence of the gene, copied into the mRNA, fixes the order of amino acids in the protein.
• Once the whole chain is assembled in that order, it folds into the shape of the finished protein.