Mutations

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Base Analogs

Base similar to bases normally found in DNA so can be incorporated into DNA in place of normal bases since they are so similar. Exist in normal and rare tautomeric states. These mutations depend on replication

Base modifying agents

Chemicals that act as mutagens by modifying the chemical structure and properties of bases. These mutations can induce mutations at any state in cell cycle.

Thymine dimers

Common lesion in DNA caused by UV radiation, where adjacent thymines in same strand are linked in an abnormal way that distorts the double helix at that site. Induced mutation where covalent bonds form between thymines instead of at bottom. Reversed by UV radiation

Chemical mutagens

Include both naturally occurring chemicals and synthetic substances. Grouped depending on basis of their mechanism of action.

Intercalating Agents

Used to stain DNA in gel electrophoresis. Inserts themselves between adjacent bases in one or both strands of the DNA double helix causing the helix to relax. Can result in base-pair addition or deletion, resulting in frameshift mutation.

Ames test

Screens for potential carcinogens. Assays the ability of chemicals to revert mutant strains of the bacteria salmonella typhimurium to the wild-type using S9.

S9

Liver enzymes that can turn thing into a mutagen. Many chemicals start out as not mutagens but are convert into one due to liver enzymes in the body or other tissues.

Mismatch repair in Bacteria

In E. coli, methyl-directed mismatch repair is used when error remains in bases even after proofreading mechanism. Using this type of repair, mismatches are recognized, incorrect bases are excised, and repair synthesis is carried out. To identify erroneous base, you have to distinguish between the two strands. Usually the parental strand is the good strand. You can identify parental strand by finding the methylated A nucleotide at the beginning of the sequence. Nucleotides should be methylated but it takes sometime after replication for methylation to occur. Once unmethylated, erroneous nucleotide found, excision and repair can occur.

Direct Reversal of DNA Damage

Mismatch repair by DNA Polymerase proofreading. DNA polymerase adds wrong nucleotide. Error detected by polymerase, halting activity. Exonuclease removes nucleotide and adds correct one. DNA polymerase resumes DNA synthesis.

Repair of UV induced Pyrimidine Dimers

Photreaction or light repair. Occurs when photolyase enzyme activated by photon of light splitting dimer apart. Only seen in plants and other eukaryotes other than humans.

Base excision Repair

Commonly repair damage by removing base or nucleotide and replacing it.

Nucleotide Excision Repair/Dark Repair

Photoreactive repair can't function in dark. UVRAB scans and finds damage, then releases A's. UVRC binds and then cuts are made of about 4 nucleotides on 3' side. UVRD binds and unwinds regions between cuts, releasing damaged segment and DNA polymerase fills gaps. DNA Ligase then fills gaps and repair is complete.

Point Mutations

A change of one or a few base pairs. Will not change the phenotype of organism unless it occurs with in a gene or in sequence regulating gene.

Gene Mutations

Point mutation of interest by geneticists. Affects the functions of genes and can alter the phenotype by changing the function of a protein.

Mutation

Process where the sequence of base pairs in a DNA molecule is altered change in DNA base pair of chromosome.

Somatic Cell mutation

Mutant characteristics affect only the individual the mutation is in and is not passed on.

Germ Line Mutations

Mutations in germ cells may be transmitted by the gametes to the next generation, producing an individual with the mutation in both somatic and germ cells.

Chromosomal Mutations

Changes involving whole chromosomes or sections of them.

Base-Pair Substitution Mutation

Involves a change in the DNA such that one base pair is replaced by another. Can result in no change to protein, insignificant change, noticeable change.

Missense Mutation

Gene mutation where a base-pair change in the DNA changes an mRNA codon so that a different amino acid is inserted into the polypeptide. Phenotypic change may or may not result.

Nonsense Mutation

Gene mutation where a base-pair change in the DNA changes an mRNA codon for an amino acid to a stop codon. Instead of changing outcome, new mRNA codon is nonsense and codes for nothing. Causes premature chain termination.

Neutral mutation

Gene mutation where new codon codes for a different amino acid that is chemically equivalent to the original so it doesn't affect the proteins function.

Silent Mutation

Gene mutation that changes a base-pair in a gene but the altered codon in the mRNA specifies the same amino acid in the protein.

Frameshift Mutation

When one or more base pairs are added or deleted causing the reading frame to shift. mRNA's move by one base so that incorrect amino acids are added to the polypeptide chain after mutation site. Can result in a nonfunctional protein. May generate new stop codons, resulting in shortened protein. May result in longer proteins from reading through stop codons. Worse when toward start.

Transversion Mutation

Base-substitution mutation from one purine-pyrimidine base pair to a pyrimidine-purine base pair.

Transition Mutation

Base-substitution mutation from one purine-pyrimidine base pair to other purine-pyrimidine base pair.

Reverse Mutation

Change genotype from mutant to wild-type or partially wild-type. Reversion of nonsense mutation occurs when a base-pair change results in a change of the mRNA nonsense codon to a codon for an amino acid.

True Reversion

When goes back from mutation

Partial Reversion

When reversion is to some other amino acid and complete or partial function is restored depending on change.

Supressor Mutation

Class of Point mutation. Mutation at a different site from that of original mutation. Effects of a mutation can be diminished. Masks or compensates for effects of initial mutation but doesn't reverse original mutation. May occur within same gene or not.

Intragenic Suppressor

Occurs within the same gene as original mutation but different site. Operates by altering a different nucleotide in the same codon in which the original mutation occurred.

Intergenic Suppressor

Occurs in different genes. Occurs as a result of a second mutation in another gene.

Suppressor Genes

Genes that cause suppression of mutations in other genes. Work by changing the way mRNA is read. Each one can suppress the effects of only one type of mutation.

Spontaneous Mutation

Occurs naturally at DNA replication, G1 phase, or G2 phase. All types of point mutations can occur spontaneously. Most corrected by cellular repairs. Occurs at a rate of 10 -4 and 4x10 -6 per gene per generation.

Forward Mutations

Change the genotype from wild type to mutant

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