Biology Reference
In-Depth Information
Figure 2.7 Micrograph of polysomes from salivary gland cells of a midge larva ( Chironomus ). Here,
the ribosomes are moving in order along the mRNA molecule, gradually extending their individual
protein chains. More than one ribosome can be attached to the mRNA so that multiple polypep-
tides are produced. The start of the polysome is at the bottom and the end is at the top.
ribosome and another ribosome can attach and a second polypeptide can begin
to form. When the second ribosome has joined 25 amino acids, a third ribo-
some can attach to the initiation site. This process can result in mRNA molecules
with many ribosomes, all of which are moving in the same 5 to 3 direction.
Such a large unit is called a polysome . Figure 2.7 illustrates a polysome isolated
from a midge larva ( Chironomus ). This electron micrograph, magnified 140,000
times, shows the start of an mRNA molecule on the bottom right ( Kiseleva
1989 ). The structure at the top shows the end of the molecule, with the growing
proteins shown attached to the ribosomes.
Proteins are linear chains of amino acids that adopt unique three-dimensional
structures that allow them to carry out their biological functions. A central tenet
until recently has been that the function of a protein depends entirely on its
fixed three-dimensional shape ( Chouard 2011 ). However, new studies indicate
that, although many proteins immediately and spontaneously fold into appro-
priate three-dimensional shapes, some protein segments can only function when
able to change shape, and this unstable region may be essential to its function.
However, truly malformed or defective proteins can be dangerous to organisms.
It is thought that all the information needed to specify a protein's three-
dimensional structure is contained within its amino-acid sequence ( Denton and
Marshall 2001 ), but the process of prediction remains complex and the subject
of intense research ( Chouard 2011 ). Protein folds are the basic units of proteins,
each consisting of between 80 and 200 amino acids. Some proteins consist of
a single fold, but most are a combination of two or more folds. Given suitable
conditions, most small proteins fold spontaneously into their three-dimensional
Search WWH ::




Custom Search