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6. Extra notes on non-coding DNA

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(6) Extra notes on non-coding DNA

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



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