Biology Reference
In-Depth Information
against animals that prey on them by making them too big and prickly to swallow.
But sticklebacks that live in shallow freshwater lakes today lack such spines because
the larger predator problem is much less in that environment, and the spines are a
disadvantage against another predator found only in freshwater - dragonfly larvae
that grasp the spines. Sticklebacks from the ocean will reproduce with sticklebacks
from freshwater lakes so experiments are possible to determine the genetic differ-
ence between the two types. Molecular techniques have been used to identify a gene
that affects where in the body of the fish the genes for these spines are expressed.
This regulatory gene is normally expressed in the head, trunk, pelvis and tail because
the products of the genes that it regulates are required for other processes in these
regions, but in the freshwater fish, the gene is no longer expressed in the pelvis. So
the change in anatomy has been produced by changes in gene regulation and not in
the genes determining the structure of the spines.
Principle 4: The genes in each organism are handed on to the next generation.
In animals such as humans, each generation is formed from a zygote, a cell
formed by the genetic material of a sperm entering and fusing with the genetic mate-
rial in an ovum. All the information to make another human is contained within this
zygote, but it is a common misconception to conclude that all that is inherited is
DNA. The DNA contains all the information to make all the proteins required to
construct an organism but this information requires a pre-existing cell in which it
can be utilised. Life is a cellular phenomenon, and so the continuity of life from its
origin some four billion years ago resides in cells, not in DNA. This conclusion is
summarized in the adage “All cells from cells”.
The Tree of Life
I said earlier that all organisms are genetically related. An example of a gene
sequence that is highly conserved in all forms of life is shown in Fig. 4.7. This
diagram compares the amino acid sequence of part of a protein involved in control-
ling protein synthesis by ribosomes in organisms from all three domains of life. This
protein is called an elongation factor, because without it the addition of amino acids
by the ribosomes to the growing chains of protein stops. Each amino acid (there are
twenty different ones in proteins) is represented by a letter e.g. A stands for alanine.
You can see that there are several regions where the sequence of amino acids is
identical between very different species - from humans to bacteria.
There is fossil evidence that bacteria have been on the Earth for at least 3 billion
years, so the interpretation is that these sequences have been conserved over that
huge length of time because they are essential for the elongation factor to help the
ribosome to carry out its job of making proteins. Because the ribosome is such a
vital component of all cells, its structure is highly conserved in all organisms. This
is true for both the protein and the RNA molecules that make up the ribosome. So
it is not surprising that the base sequence of the RNA component of the ribosome is
also highly conserved in all organisms - too much variation would run the risk that
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