Explaining the significance of non-coding DNA.
In humans the coding for proteins is only about
1.5% of our DNA involving 20 000 to 25 000 genes, most of the rest is
non-coding DNA, but it isn't there just to make up the numbers!
It appears that many sections of DNA are described
as 'non-coding', meaning they do not code for any proteins.
However
these non-coding sections of DNA perform other essential functions
including switching genes 'on and off'.
This means whether or not a gene
is expressed.
The word 'expression' in genetics means that that gene is
switched on and used to make a protein that contributes to a
phenotype - what is produced in a particular characteristic.
Therefore, any mutations in this non-coding DNA
may prevent the synthesis of protein and the lack of this protein may
adversely affect the organism's phenotype - the gene expression.
Some specific examples
Fruit flies have an enzyme XDH which is
involved in producing a red pigment.
Fruit flies with normal XDH enzyme
activity have red eyes.
Fruit flies with no XDH activity have
brown eyes because no red pigment is produced.
So there are parts of
the DNA strands that do NOT code for any proteins, but they are
of great importance.
More and more scientific research is showing that some of these non-coding sections
switch genes on and off, in other words, they control whether or
not a gene is expressed to make a protein.
Therefore some of these non-coding regions
of the DNA are involved in protein synthesis.
Before transcription can occur, the RNA
polymerase has to bind to a non-coding section of DNA adjacent to
the specific gene (for a specific protein).
If a mutation has occurred in this section of
the DNA it can affect the ability of the RNA polymerase to bind to
it - it might be harder or easier (or no effect).
The quantity and accuracy of how much mRNA is
transcribed depends on how well this binding takes place - and
therefore affects how well the protein is produced.
Therefore the production of the protein may be
affected, and, depending on its function, that specific phenotype
may also be affected.
This means that genetic variants in non-coding
regions of DNA can affect the phenotypes exhibited by an organism,
despite the fact that these non-coding sections of DNA done code for
proteins themselves.
A summary from
DNA and RNA structure and Protein Synthesis gcse
biology revision
