Biology Reference
In-Depth Information
that is handed on during cell division while RNA is part of the mechanism by which
that genetic information is expressed. The only exceptions are some viruses that use
RNA, not DNA, as their genetic material.
How a gene determines the amino acid sequence of a protein chain is the
most complicated biochemical process yet discovered. This complexity is required
because one chemical language - the base sequence of DNA - has to be translated
into another chemical language - the amino acid sequence of protein chains. It is
important to realise that this process does not involve a conversion of the bases
themselves into amino acids. What flows from bases to amino acids is sequence
information, not material.
The mechanism of gene expression is outlined in Fig. 4.5. DNA acts as a template
that allows the sequence of bases in one strand of the double helix of each gene to be
copied in the form of RNA. This process is called
transcription
, and produces from
each gene many copies of a RNA molecule called messenger RNA (mRNA). Each
molecule of mRNA is the about same length as the gene that acts as a template for
its synthesis, and the enzyme that carries out this process is called RNA polymerase.
The order of bases in mRNA is the same as the order of bases in one strand of the
double helix of DNA from which it is copied. The mRNA in turn acts as a template
for a second process called
translation,
in which amino acids are joined together
in a linear order determined by the order of bases in the mRNA. Each molecule of
mRNA can programme the synthesis of many molecules of the protein chain that it
encodes. The rules governing which bases determine which amino acids are joined
together constitute the
genetic code
.
Fig. 4.5
