Molecular mechanism of Mutations

Presented by: Anirudh Acharya,

Department of Zoology,University of Mysore.

Contents:• Introduction to mutation• Types of Mutations• Molecular mechanism• Base Substitution mutations

• Ex: Sickle Cell Anemia • Frameshift mutation• Ex: Huntington’s Disease - Insertion• Cystic fibrosis – Deletion

• Induced mutations• Tautomers • Base Analogs• Alkylating agents• Intercalating agents• Deamination and free radicals

• Summary

Introduction to mutation:• Sudden change in genetic material.

• Term ‘Mutation’ introduced by Hugo de Vries in year 1900.

• Hermann Muller – X rays mutation in Drosophila.

• Bruce Ames – 1970s mutagenicity tests

Types of Mutation:• Spontaneous Mutations

• Induced Mutations1. Radiations2. Temperature3. Chemicals

Molecular mechanism: • DNA at molecular level is made up of four molecules called

nucleotides

• They are Purines : Adenine & GuaninePyrimidines : Cytosine & Thymine Linked with sugar and phosphate group.

• They exist in form of triplet code, to synthesize particular amino acids i.e, during protein synthesis, they are called ‘CODONS’

• At molecular level mutation occurs by change in nucleotide sequence in the genetic material.

• Based on the molecular change there are:1. Base substitution mutations• Transitions• Transversions

1. Neutral mutations2. Silent mutations3. Missense mutations4. Nonsense mutation

2. Frameshift Mutations• Addition mutations• Deletion mutations

Base Substitution Mutations• Point mutation in which, one base pair is replaced by another.

• Transition: Replacement of a base by other base of same chemical category.• Purine replaced by another purine. • Pyrimidine replaced by another pyrimidine.

• Transversion: Replacement of base of one chemical category by a base of other category.• Purine replaced by pyrimidine

• Based on the consequence of mutation, the substitution mutation may be grouped into neutral, silent, missense

and nonsense mutations.

Sickle Cell Anemia• The allele responsible for

sickle-cell anaemia is autosomal recessive and can be found on the short arm of chromosome 11.

• The sickle-cell disease occurs when the seventh amino acid, glutamic acid, is replaced by valine to change its structure and function.

Frameshift Mutations:• Reading frame is altered, either by insertion or deletion.

Addition or Insertion Mutations: One or more nucleotides are inserted into a sequence. If the number of inserted bases is not a multiple of 3, it will cause frameshift, resulting in serious consequences. As shown in the following table, non-frameshift insertions may also cause diseases.

Huntington’s Disease:“Huntington’s is an inherited disease, characterized by the aggregates of the

misfolded protein, huntingtin. Normal huntingtin’s function is not clearly known, but when patients have more than 36 repeats of the nucleotide

sequence, CAG, they then acquire the mutant form of huntingtin. Although the huntingtin protein is found throughout the body, the neuronal

degeneration is selective to the corpus striatum. This exploration into the unanswered mysteries of Huntington’s disease tries to find the answer as to

why mutant huntingtin only kills striatal cells. In 2009, Srinivasa et al., discovered Rhes, a striatal specific protein, that they found played a

significant role in the pathway of Huntington’s disease. Rhes proteins have a strong bond with mutant huntingtin, and in order for the aggregation of

mutant huntingtin to take place, this bond must be intact. Srinivasa et al., also demonstrated that cell survival is completely dependent on the

aggregation of mutant huntingtin.”

- Eukaryon Journal, March 01, 2011.

Deletion Mutations• Involves elimination of one or more nucleotides from a DNA

sequence. It may cause frameshift, producing a non-functional protein.

• Real examples of deletion mutations which cause diseases. (a) Deletion of "T" from the sequence "TTTTT" in the CFTR gene.

(b) Deletion of "AT" from the sequence "ATAT" in the CFTR gene. (c) Deletion of "TTG" from the sequence "TTGTTG" in the FIX gene. (d) Deletion of "ATAG" from the sequence "ATAGATAG" in the APC gene.

Cystic Fibrosis• Cystic Fibrosis Transmembrane conductance Regulator (CFTR gene).

1. Cytogenetic Location: 7q31.22. Molecular Location on chromosome 7: base pairs 117,120,016 to

117,308,718

• Deletion in CFTR protein resulting abnormal channel breaks down shortly after it is made, so it never reaches the cell membrane to transport chloride ions.

• Lungs, pancreas, and other organs produce mucus that is abnormally thick and sticky. The abnormal mucus obstructs the airways and glands, leading to the characteristic signs and symptoms of cystic fibrosis.

Induced Mutation:• Chemical mutagens• Radiations

Tautomerization:• Change in nitrogenous base, ie hydrogen from adjacent

carbon atom shifts to keto group to make it an enol form.

Normal Base Pairing:

Tautomeric shifts:

Base Analogs: • A molecule similar enough to a purine or pyrimidine base to

substitute for the normal bases, resulting in abnormal base pairing, only during replication.

Alkylating agents:• These agents can mutate both replicating and non-replicating

DNA. Each of these classes of chemical mutagens has certain effects that then lead to transitions, transversions, or deletions.

EX: EMS, MMS etc.

Intercalating agents:

• Acridines (e.g., proflavin) are positively charged molecules. They may be inserted between two DNA strands, thereby altering DNA's structure and rigidity. As a result, DNA replication will not be faithful.

Deamination and Free Radicals• Nitrous acid is a agent that converts cytosine to uracil,

adenine to hypoxanthine, and guanine to xanthine. The hydrogen-bonding potential of the modified base is altered, resulting in mispairing.

• Hydroxylamine and free radicals may modify base structures, resulting in mispairing.

Summary:• Thesunwashotbuttheoldmandidnotgethishat.

The sun was hot but the old man did not get his hat.– Codon

• The sun was was but the old man did not get his hat – Base Substituion

T hes unw ash otb utt heo ldm and idn otg eth ish at. - Frameshift

Thank you.