Question

What steps of transcription (initiation, elongation, termination) might be affected by a DNA mutation in the template strand upstream of the promoter sequence? What about downstream?

Answer 1

The effect of mutation depends upon the type of mutation.For the start, there are two type of mutations possible in template strand-

1.Substitution- In this kind of mutation one nitrogen base is replaced by another nitrogen base.This kind of mutation can affect Pribnow box in prokaryotes and Hogness box in Eukaryotes.

Pribnow box is basically a sequence of 6 base pairs (TATAAT) which is located 10 Nitrogen bases upstream of starting point in prokaryotes. It plays an important role in initiation step.

And hogness box is sequence of 7 base pairs (TATAAAA) which is located 20 Nitrogen bases upstream of starting point.It plays a similar role to pribnow box.

The substitution mutation in any of these sequence will affect the initiation step.

2. Frame Shift or Gibberish Mutation - It is loss of one or rarely more than one Nitrogen base.It shifts the whole sequence of bases.

So this kind of mutation will affect all three steps of transcription

The mutation downstream can cause change in terminator site and the Rho factor might not be able to detect the terminator site.

Rho factor ( factor ) is specific protein which helps RNA polymerase enzyme to recognize the terminator site.

And effect on the amino acids will again be decided by three type of mutations -

1. Mis-Sense Mutation- When change in nucleotide causes change in amino acid of polypeptide chain.

2. Non-sense mutation- When change in nucleotide causes termination of polypeptide synthesis.So it will affect the translation.

3.Non-Sense mutation-When change in nucleotide does not change amino acid.Because both codons code the same amino acid.