Biology Reference
In-Depth Information
You may like to think of gene expression in terms of computer terminology. The
nucleic acids represent the software - the memory that contains the information to
make proteins. The proteins are the hardware -they constitute the physical apparatus
that executes the programme that stored in the software. You may have heard media
people say that DNA is the “blueprint” for each organism, but this is incorrect. The
term “blueprint” is traditionally used for a physical description of the parts of a
given structure, such as a ship or a camera, but DNA contains no such description of
an organism. A better analogy is that DNA is a recipe - it contains the information
to make all the proteins of the organism, and it is the interaction of these proteins
that produce the structure of the organism, just as it is the mixture of flour, butter,
raisins etc that makes the structure of a cake.
The process of translation requires many interacting components. Ribosomes,
you will remember, are the structures inside each cell that decode the base sequence
information in each molecule of messenger RNA. By “decode”, we mean that
the ribosome is essentially a device for converting the base sequence information
in each messenger RNA into the amino acid sequence information of the pro-
tein encoded by each gene, just as during the Second World war, the scientists at
Bletchley Park in England built a machine that was able to decode the secret mes-
sages sent out by German military forces. Recall that each gene is defined by its
unique sequence of bases, while each polypeptide is defined by its unique sequence
of amino acids. Thus ribosomes use the base sequence in messenger RNA to join
amino acids together in the correct order.
Ribosomes are complex organelles consisting of 50-70 protein molecules bound
to 2 or 3 RNA molecules, depending on the species. A human liver cell contains
several million ribosomes. Each ribosome consists of two subunits, called large and
small. These subunits are separate when they are not synthesizing protein chains, but
are joined together when the ribosome attaches to one end of a mRNAmolecule. The
ribosome then moves along the mRNA molecule, joining amino acids together as it
goes, in the order specified by the order of bases in the mRNA. Several ribosomes
can attach to a single molecule of mRNA, forming a structure called a polyribosome,
or polysome. The existence of polysomes allows many copies of the same protein
chain to be made rapidly. Figure 4.6 shows the front and back ends of a polysome
isolated from the salivary glands of an insect; the middle section on the left hand
side is missing.
Why is decoding by ribosomes absolutely vital for life? It is vital because it is
the sequence of amino acids in each linear chain of protein that determines how that
chain folds into the compact three-dimensional structure that enables that protein
to carry out its specific function. Remember that it is proteins that are the “action
molecules” of the organism - that is, the molecules that carry out all the thousands
of different functions necessary for life. Genes by comparison, are inert and rather
boring - all they do is contain the sequence codes for proteins; some genes encode
RNAmolecules that are not messenger RNAs to make proteins, but RNAs that either
regulate transcription and translation or form part of the structure of ribosomes
themselves. You might like to think of this arrangement by analogy with a tape
recorder. The tape is like the DNA; it is essential because it contains the information
