Dr. Vijay Marakala, MBBS, MD.

Senior Lecturer

BIOCHEMISTRY

IMS, MSU.

Consequences of DNA replication errors

Chemical agents acting on the DNA

UV light imparting energy into DNA molecule

Spontaneous changes to the DNA

Types of damage to DNA

Single base alteration

•Depurination

•Deamination

Two base alteration

•UV light induced pyrimidine dimer

Chain break

•Oxidative free radical formation

• Ionizing radiation

Cross linkage

• Between bases or DNA and protein

• Damaged DNA must be repaired

• If the damage is passed on to subsequent generations, then we use the evolutionary term -mutation. It must take place in the germ cells - the gametes - eggs and sperm

• If damage is to somatic cells (all other cells of the body bar germ cells) then just that one individual is affected.

In most cases, DNA repair is a multi-step

process

An irregularity in

DNA structure is

detected

The abnormal

DNA is removed

Normal DNA is synthesized

Mismatch repair

Base excision-repair

Nucleotide excision repair

Double-strand break repair.

• The replication process should be carried out with high fidelity, otherwise the information is altered.

• However, there do occur replication errors

• Mismatch repair corrects errors made when DNA is

copied.

• For example, cytosine (instead of thymine) could be incorporated opposite to adenine

• GATC sequences which occur approximately once every thousand nucleotides

• GATC- are methylated on the adenine residue

Identification of the mismatched strand

• An endonuclease nicks the mismatched strand, mismatched base(s) is/are removed

• Gap is filled by DNA polymerase III and DNA ligase

Repair of damaged DNA

• The bases of DNA can be altered

• Thus cytosine, adenine and guanine bases spontaneously form uracil, hypoxanthine and xanthine respectively.

Spontaneously

Cytosine which slowly undergoes deamination to

form uracil

Chemical agents

Deaminating and alkylating

compounds

Removal of abnormal bases

• Specific DNA-glycosylases can remove these abnormal bases.

• This leaves an apyrimidinic site (or apurinic, if a purine was removed), referred to as an AP-site.

Recognition and repair of an AP-site

•Apurinic endonucleases excise this abasic sugar. Proper base is then added

• The DNA damage due to ultraviolet light, ionizing radiation and other environmental factors often results in the modification of certain bases, strand breaks, cross-linkage etc.

• Nucleotide excision-repair is ideally suited for such large-scale defects in DNA.

• Identification of the defective piece of the DNA

• An excision nuclease cuts the DNA on either side (upstream and downstream) of the damaged DNA.

• Gap filled up by DNA polymerase which gets ligated by DNA ligase

Nucleotide excision repair

Nucleotide excision repair

XerodermaPigmentosum

• Defective NER mechanism

• Sensitivity to UV light

• Skin cancers

Hereditary Polyposis Colon Cancer

• Mismatch repair is defective