Variation Mutations DNA repair

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Presentation transcript:

Variation Mutations DNA repair

Variation Variation is any difference between organisms Variation Non-heritable Heritable - Recombination - Mutations

Mutations (H. de Vries, 1901) Mutations are permanent changes in the genetic material.

Classification of mutations By inheritance: Somatic (occur in body cells) Germinal (occur in gametes) By expression in a heterozygote: Dominant Recessive

By effect on a wild-type (the most common phenotype): Forward (wild-type → mutant (new) phenotype) Reverse (mutant phenotype → wild-type) Suppressor (mutant phenotype → wild-type) 4. By effect on phenotype: Lethal Morphological Biochemical etc.

5. By causes: Spontaneous (result from abnormalities in cellular processes) Induced (caused by environmental agents) 6. By effect on fitness: Beneficial Neutral Harmful 7. By effect on genome: Single-gene (small changes in DNA structure that occur within a particular gene) Chromosome (changes in chromosome structure) Genome (changes in chromosome number)

Mutagens Mutagens are agents that can cause mutations. Physical Ionizing radiation (X rays, gamma rays) UV light Chemical Base analogous (bromouracil, aminopurine) Alkylating agents (ethyl methanesulfonate, nitrogen mustard) Deaminating agents (nitrous acid, nitrites) Intercalating agents (acridine dyes, proflavin, ethidium bromide) Dioxins etc. Biological Mobile genetic elements Viruses

Gene mutations Base-pair substitutions A transition: A↔G, T↔C A gene mutation is a permanent change in the DNA sequence that makes up a gene. Base-pair substitutions A transition: A↔G, T↔C A transversion: A↔C, A↔T, G↔C, G↔T Insertions of nucleotides Deletions of nucleotides Intragenic inversion Dynamic mutations (trinucleotide repeat expansion) frameshift mutations

Missense mutations cause a codon to code for a different amino acid Nonsense mutations change an amino acid codon into a stop codon Silent (same-sense) mutations do not alter the amino acid sequence of the polypeptide because of redundancy in the genetic code

Mechanisms of Mutation Induction 1. Errors in DNA replication 2. Tautomeric shift (the spontaneous isomerization of a nitrogen base to an alternative hydrogen-bonding form, possibly resulting in a mutation) 3. Base-analogue-induced mutagenesis 4. Deamination 5. Depurination 6. Formation of pyrimidine dimers 7. Alkylation 8. Intercalating agents 9. Breaking of phosphodiester bonds

DNA repair DNA repair is correction of DNA damages Types of Repair: Direct repair Excision Repair Recombinational repair SOS-repair

Direct repair These systems act directly on damaged nucleotides, converting each one back to its original structure (reversing). DNA polymerase proofreading Photoreactivaton is a process that directly reverses pyrimidine dimers by the action of the enzyme photolyase. This enzyme mechanism requires visible light. Repair of alkylation damage by the special methyltransferases. Nicks can be repaired by a ligase. Repair of AP-sites by base-insertion enzymes (insertases).

Excision repair The damage is repaired by excision and replacing it with new DNA synthesized using the complementary strand as template Excision repair of modified bases Excision repair of pyrimidine dimers Mismatch repair

Steps of excision repair: Damage recognition Damage excision DNA repair synthesis DNA ligation

Excision repair of modified bases Modified or damaged base is removed by a DNA glycosylase AP-deoxyribose is then released by AP endonuclease and phosphodiesterase Missing nucleotide replaced by DNA polymerase The nick is sealed by DNA ligase

Excision repair of pyrimidine dimers (prokaryotic cells) UvrABC endonuclease complex (excinuclease) recognizes the DNA damage. Excinuclease cuts the damaged strand at two sites separated by 12 or 13 bases. Helicase UvrD unwinds the damaged region. DNA polymerase I fills the gap. DNA ligase seals the nick.

DNA mismatch repair Proteins MutS и MutL binds to a mismatched bp Endonuclease MutH cleaves the error strand Helicase separates the strands Exonuclease removes the error strand DNA polymerase fills the gap DNA ligase seals the nick

Recombinational repair This pathway allows a damaged chromosome to be repaired using a sister chromatid or a homologous chromosome as a template.

SOS-repair SOS-repair occurs when cells are overwhelmed by DNA damages This is induced by RecA binding to ssDNA. RecA protein mediates proteolytic cleavage of LexA protein. LexA is a repressor of 43 genes involved in DNA repair. Transcription of umuC and umuD genes are activated. Protein complex UmuCD serves as DNA polymerase that can operate on damaged or imperfect DNA templates. This is error-prone replication.