Question

DNA repair errors include losing or inserting random nucleotides at the cut site. Explain how these changes might inactivate a gene.

Answer 1

Answer-

Mutation is hereditary change in the genetic make up of an individual other than that which may be caused by simple recombination of genes. These include changes in gene structure or composition and the changes in the chromosomes either in structure or number. So, mutation is the process by which the sequence of base pairs in DNA is altered. The mutation caused by the addition (insertion) or deletion (losing) of nitrogenous bases in the DNA or mRNA are known as Frame-shift mutations, because these shift the reading frame of codons from the site of change onward. Frame-shift mutations are of 2 types:

  1) Deletion mutation- These mutations are caused due to the loss or deletion of one or more nucleotides.   2) Insertion mutation- These mutations are caused by the addition of one or more extra nucleotides in a DNA molecule at one or more places.

If mutation produced, codes for one of the terminator or nonsense codon (UAA, UAG, UGA), the reading of RNA would stop at that point. This would end up with inactivation of that particular gene i.e. this would prevent a complete polypeptide chain from being formed. This usually results in the formation of non-functional proteins. Several intercalating agents such as acridine, proflavin, Ethidium bromide cause the frame-shift mutation which sometimes, results in inactivation of genes,

Picture of nonsense mutation is given below:

5- ATOQTAATCCTCGGGATA GCA -3' DNA 3- TACCATTAGS AS COCTATCGT -5' Normal MRNA- 5- AUGGUAAUCCUCGGG AUAGCA - 31 T 2 3 4 5 6 7 Polypeptide-N-met-val- Ile-leu-Gly- Ile-Ala-c Functional V Deletion of from 5'3' DNA strand 5-ATS TAATOC TOSEGATA CCA - 3 DNA 3'-TAC ATTAGE ASCCCTATCGT - 5 Non sense mutation MRNA S-AUGURA UCC U CGGGAUAGCA - 3' Y by deletion. polypeptide - met 1 Nonfunctional stop Insertion of ca at 5' 3' strand of DNA just after tst coding sequence. S-ATQCCCTAATCCTCGECATA SC A -37 DNA - 3-TACGCCATTAGE AGCCCTATCGT -S Monsense MRNA S-AUGCaGUA A UccucaręAUAGCA -3° 6 mutation by insertion. stop polypeptide - Met-Aseg. - Nonfunctional.