Python Programming for Biology: Bioinformatics and Beyond

Figure 11.3.  An overview of transcription and translation processes. When a protein

is made in a cell its sequence is determined by the coding sequence contained in a

messenger RNA molecule in a process called translation. This RNA is created from a gene

region in the DNA, which is itself part of a chromosome, in a process called transcription.

In most cases an initial RNA transcript, effectively a copy of part of one of the DNA

strands, must be processed by splicing, to remove non-coding introns.

With four types of base in RNA there are 64 (4×4×4) possible combinations of triplet

nucleotide sequence that can make a codon. A few of the codons will cause the production

of  a  protein  to  stop,  but  most  will  specify  the  inclusion  of  one  specific  amino  acid.

Because there are more codons than amino acid types, an amino acid is often specified by

several different codons. The relation between the triplet of bases in codons and the amino

acid that is added to a protein is known as the genetic code (see

Table 11.1

). This genetic

code may be different in different kinds of organism, but only slightly so.