Bio TUT 6 DNA Mutation and Repair PDF

Data Zone 44 Biochemistry Bio TUT 6: DNA Mutation and repair Mutation= permenant change(s) in the nucleotide/base sequence of DNA. may occur due to errors in DNA replication or due to the impacts of chemicals (free radicals...

Data Zone 44 Biochemistry Bio TUT 6: DNA Mutation and repair Mutation= permenant change(s) in the nucleotide/base sequence of DNA. may occur due to errors in DNA replication or due to the impacts of chemicals (free radicals& alkylating agents) or physical (X-Ray & UV radiation) injury to the DNA molecule. Mutation may result in coding sequences for new amino acids in proteins or not! Types of mutation I. Point mutations II. Insertion/Deletion of bases One or more base Three or multiples of three (not multiples of three) 1. Point mutations affect single sites on DNA Substitution of 1 base for another Results of point mutations: a. Silent mutations b. Missense mutations c. Nonsense mutations 1  Data Zone 44 Biochemistry Sample outcome of DNA code Methionine, proline, threonine, arginine, stop Silent mutation Due to redundancy of Genetic Code, no change in amino acid sequence is produced!! Missense mutation produces a change (substitution) in amino acid sequence in protein product (Histidine in for Arginine) may change function of protein or may not! Nonsense mutation produces a STOP codon within the mRNA transcript leading to a truncated protein. 2  Data Zone 44 Biochemistry 2. Insertion/deletion mutations Results of insertion/deletion mutations 1. Frame shift mutation 2. Insertion / deletion of codon or whole gene due to addition / deletion of one may result from insertion/deletion or more bases (not multiples of three) of triplet bases (or multiples of three) to a coding DNA region affecting the coding region Frame shift mutation: - Alteration of reading frame. - Introduction of a STOP codon. - New amino acid may be added - Amino acid is missing 3  Data Zone 44 Biochemistry DNA repair Definition Outline Definition - DNA repair: refers to the processes by which the Importance of DNA repair cell identifies and corrects any damage of its DNA. Causes of DNA damage DNA repair mechanisms Importance of DNA repair Defective DNA repair mechanisms 1- Damaged DNA must be repaired. 2- 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 3- If damage is to somatic cells (all other cells of the body bar germ cells) then just that one individual is affected. Damage from where? Causes of DNA damage include 1. DNA replication errors. 2. Chemical agents acting on the DNA e.g Nitrous oxide causes deamination of C to U, alkylating agents and free radicals 3. Physical agents: radiation as UV causes pyrimidine dimers and x-rays 4  Data Zone 44 Biochemistry Deamination of DNA - An amino group of Cytosine is removed, and the base becomes Uracil. - An amino group of Adenine is removed, and the base becomes Hypoxanthine. - An amino group of Guanine is removed, and the base becomes Xanthine. DNA repair mechanisms The correction (DNA repair mechanisms) include: 1) base-excision 2) nucleotide-excision 3) mismatch repair 4) double strand break repair Basic steps are the same for the first three repair mechanisms: 1) Identify damaged segment 2) Remove damaged region 3) Resynthesis of DNA 4) Ligate 1. Base excision repair a. DNA glycosylase recognizes & removes damaged base, leaving deoxyribose sugar thus producing an Apurinic–Apyrimidinic site (Ap-site). b. AP endonuclease cuts phosphodiester backbone. c. A deoxyribose phosphate lyase (dRP Lyase) removes the single free sugar-phosphate moiety. d. DNA polymerase replaces missing nucleotide. e. DNA ligase seals nick. 5  Data Zone 44 Biochemistry 2. nucleotide-excision repair Same as previous except that - It recognizes more varieties of damage - Remove larger segments of DNA (10 -100s of bases) Usually is used for replacement UV-damaged DNA (Covalent joining of two adjacent pyrimidines, usually thymines), producing a dimer. a large UV-specific endonuclease scans the DNA strand for abnormalities upon detection it cuts the strand on both sides (5’ &3’) of the damage and removes the oligonucleotide the gap is repaired by DNA polymerase and DNA ligase enzymes 3. Mismatch repair Special enzymes scan the DNA for bulky alterations in the DNA double helix. Sometimes replication errors escape the proofreading function during DNA synthesis, causing a mismatch of one to several bases (AG, AC and CT). These are excised and the DNA repaired. 1. Mut proteins identify the mismatched strand. 2. Endonuclease nicks the strand with the mismatch. 3. The mismatched nucleotide(s) is/are removed by an exonuclease. 4. DNA polymerase fills the gap, and ligase seals the nick. 6  Data Zone 44 Biochemistry Defective DNA repair mechanisms DNA Damage Repair Mechanism Disorder due defective mechanism Alkylated, deaminated Base excision repair Point mutation and/or depurinated bases Pyrimidine Nucleotide excision repair Xeroderma pigmentosum (thymine) dimers Mismatched bases Mismatch repair Hereditary non-polyposis colorectal cancer Double strand breaks Double strand breaks repair Immunodeficiency disorders & cancer e.g Ataxia Telangectasia Xeroderma Pigmentosum (XP) Symptoms include: 1. Extreme sensitivity tosunlight 2. dermatitis (skin inflammation) with spots and skin lesions 3. Early onset of skin cancer (8years) 7  Data Zone 44 Biochemistry ❖ Causes of DNA mutation include the following, except: 1. UV radiation 2. Alkylating agents 3. Replication error 4. Oxygen ❖ In base –excision repair, the site that is produced by removal of the damaged base is called: 1. Ap site 2. N site 3. Ad site 4. AE site ❖ DNA repair mechanisms include the following, except: 1. Base-excision repair 2. Nucleotide-excision repair 3. Nonsense repair 4. Mismatch repair 8