3.6 - 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

Transcription and translation

• Making a protein from a gene happens in two stages: transcription and translation.
• Proteins are assembled on ribosomes in the cytoplasm.
• DNA is too big to leave the nucleus, so the gene's code is first copied onto a messenger molecule that can - messenger RNA (mRNA).

Transcription - Copying the gene into mRNA

1. RNA polymerase (an enzyme) binds to a region of non-coding DNA just in front of the gene.
2. The two DNA strands unzip.
3. RNA polymerase moves along one strand of the gene, the template strand, using it as a template to build a complementary mRNA strand - each base added to the mRNA pairs with the matching base on the DNA.
4. Once complete, the mRNA leaves the nucleus and attaches to a ribosome.

Like DNA, mRNA is made of bases, but it is:

• A single, shorter strand.
• Made with the base uracil (U) in place of thymine (T).

Translation - Building the protein at the ribosome

• The mRNA's bases are read in triplets called codons. Each codon codes for one amino acid.
• Amino acids are brought to the ribosome by transfer RNA (tRNA).
• Each tRNA molecule carries a particular amino acid and has a set of three bases called an anticodon, which is complementary to a codon on the mRNA.
• As the ribosome moves along the mRNA, each codon pairs with the matching tRNA anticodon, so each amino acid is delivered to the right place in the sequence.
• The ribosome links the amino acids together in the order the codons set, building a chain called a polypeptide, which then folds into the finished protein.