Genetic Variants in Non-coding DNA
- During transcription, part of a DNA molecule unwinds, exposing the gene to be transcribed (the gene from which a particular protein will be produced)
- An enzyme known as RNA polymerase then binds to a region of non-coding DNA just in front of this gene
- Non-coding means that the DNA bases do not code for the production of an amino acid
- RNA polymerase makes a complimentary copy of the gene by building a single-stranded nucleic acid molecule known as mRNA (messenger RNA)
- However, genetic variants (different versions of genes caused by mutations) can be produced if a mutation occurs in the non-coding DNA of a gene
- A mutation is a random change to the DNA base sequence
- Such genetic variants can affect phenotype by influencing the binding of RNA polymerase and altering the quantity of protein produced:
- If a mutation happens in the non-coding region to which RNA polymerase attaches, this can affect the ability of RNA polymerase to bind to it
- In some cases, this might make it easier for RNA polymerase to bind. In other cases, it might make it more difficult
- If it makes it more difficult for RNA polymerase to bind, less mRNA will be transcribed from the gene and therefore less protein (that the gene codes for) will be produced
- Depending on the function of this protein, the phenotype of the organism may be affected by how much of it is produced
- In this way, genetic variants in the non-coding DNA of a gene can affect phenotype, even if the coding region of the gene is completely normal
Genetic Variants in Coding DNA
- Genetic variants (different versions of genes caused by mutations) can also be produced if a mutation occurs in the coding DNA of a gene
- If a mutation happens in the coding region of a gene, the gene may code for a different sequence of amino acids
- As the amino acid sequences are highly specific and form specific shapes and types of proteins, a change in the sequence may change the shape of the final protein and therefore may affect how it functions
- For example, if its amino acid sequence is changed, the activity of a protein such as an enzyme may be increased, decreased or stopped altogether
- This, in turn, could change the phenotype (the characteristics) of the organism
- For example, XDH is an enzyme found in fruit flies. Fruit flies with normal genes coding for XDH show normal levels of XDH activity and have red eyes. Fruit flies with genetic variants of the genes coding for XDH (caused by mutations in the coding DNA) show decreased levels of XDH activity and have brown eyes, as they cannot produce the red eye pigment
