Basic Biology - How genes encode proteins

Translation Translation is the process where mRNA molecules are translated into proteins at the ribosomes. The nucleotides of the mRNA molecule are read by the ribosome so that each set of three nucleotides called a codon, specifies a single amino acid. Therefore, the first three nucleotides of the mRNA will encode the first amino acid, the second three bases the second amino acid and so on. The rules by which the base sequence of the mRNA molecule is translated into the primary amino acid sequence of a protein are called the genetic code. There are 64 different possible codons (this is because there are 4 bases: A, U, C, G, and each codon has 3 bases, so 43 = 64) and 20 amino acids. Some codons code for the same amino acid and therefore the genetic code is said to be degenerate. No codon codes for more than one amino acid. An example of the standard genetic code that is used by nearly all living things is shown here. Three of the codons do not specify the incorporation of any amino acids. These are known as the stop codons - UAA, UAG, UGA. They are found at the end of the mRNA coding sequence and they tell the ribosome to stop translating the message and release the protein. The mRNA is translated from the 5' end and read one codon at a time to the 3' end. Translation usually starts at a start codon (AUG) which codes for methionine. Each successive codon is read and the amino acid incorporated into the protein chain until a stop codon is encountered. The codons in a mRNA molecule do not directly recognise the amino acids that must be incorporated. Instead this process is directed by a group of adapter molecules called transfer RNAs (tRNAs). Every codon, except the stop codons, has its own tRNA molecule. A tRNA molecule has an anti-codon end, which is made of a set of three base pairs. These base pairs can base pair with the complementary codon in the mRNA. The 3' end of a tRNA molecule is attached to an amino acid. In the translation process, a ribosome reads a mRNA molecule codon by codon. At each codon, a tRNA molecule with an anti-codon complementary to that codon attaches to the mRNA. It brings with it the appropriate amino acid that is then incorporated into the growing polypeptide chain. Once the amino acid has been added, the tRNA molecule is released and the ribosome moves onto reading the next codon in the mRNA chain. This process continues until the ribosome reads a stop codon. At this point the ribosome releases the mRNA molecule and the completed protein. The tRNA molecule functions as an interpreter reading codons in the mRNA molecule and translating them into amino acids. In this way, the sequence of base pairs in a given gene determines the amino acid sequence of the protein. In this next section we will look at genes & disease >>>