II. Reverse Mutations Exact reversion - base pair change in the mutant type to the original wild type AAA GAA AAA

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Vivien Small
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1 3/7/02 Reminders: PT school presentation, 12:30, MO 157 Our faculty research interests, 12:30, MM#2 Quiz on Tues., Mar. 12 Types of mutations (Table 15-1 on p. 464) I. Forward mutation - single* base-pair substitution (* see frameshift mutations) Transitions AT GC; pur pur; pyr pyr Transversion AT CG; pur pyr; pyr pur Silent mutation = Synonymous substitution - codon change, but no a.a. change (e.g. AGG AGA are Arg) Missense mutaion = Nonsynonymous substitution - any base change that causes an a.a. change Neutral mutation - a.a. change, but no change in protein functionality Null mutation - a.a. change leading to loss of function Nonsense mutation - codon changed to a stop codon; most nonsense mutations are also null mutations Frameshift mutation - insertion or deletion of one or more base pairs (not a multiple of three) within the translated portion of a gene causing the triplet reading frame to shift resulting in different a.a. from that point forward (i.e., downstream) or until chain termination. II. Reverse Mutations Exact reversion - base pair change in the mutant type to the original wild type AAA GAA AAA forward reverse Equivalent reversion - UCC (Ser) UGC (Cys) AGC(Ser) forward reverse CGC Arg, basic CCC Pro, nonbasic CAC His, basic forward reverse

2 Another categorization of mutations Spontaneous mutation - change that occurs naturally Induced mutation - change that occurs following exposure to mutagenic agent How common are mutations? Mutation rate = the number of mutations occurring in some unit of time Mutation frequency = the frequency at which a specific kind of mutation (or mutant type) is found in a population of cells or individuals In the example, only one mutation has occurred over seven cell divisions, therefore mutation rate is 1/7. Shouldn' t this be 1/3? In the example, in the final population of 8 cells there are 2 mutants, therefore the mutation frequency is 2/8

3 Mechanisms of gene mutation Spontaneous mutaion (Replication errors) Tautomeric shift (base shifts to a different isomeric configuration with different pairing properties). This can result in a mispair. Examples:

4 CG C*A replication results in C*A and TA (transition) TA T*G replication results in T*G and CG (transition) GC G*T replication results in G*T and AT (transition) AT A*C replication results in A*C and GC (transition) Transversion also may occur by mispairing of a pur with a pur or pyr with a pyr, but these are more rare. Frameshift mutation - seem to occur more often at repeated sequences (see Fig. 16-4)

5 Spontaneous mutations (spontaneous DNA damage) Depurination - spontaneous loss of a purine (either A or G) off the DNA backbone. Repair enzymes available to fix this damage. However, if replication follows before repair occurs, a random nucleotide can be inserted across from the apurinic site resulting possibly in a mutation. Deamination - Spontaneous loss of the amine (-NH2) from cytosine or 5-methlycytosine. C U - deamination of cytosine creates uracil

6 5MC T - 5MC is simply a methylated form of C that has the pairing properties of C. Methylation is thought to play in role in regulating transcription. Deamination of 5MC creates T. Uracils are usually remove by a specific repair enzyme. This leaves an unpaired G. Another repair enzyme then inserts C. The net result of the cytosine deamination and repair restores the original sequence. But for the other deamination, T, of course, is not recognized as an error. Therefore, 5MC residues act as hot spots of mutation. (C T transitions). Induced Mutations (base analogs) 5-BU and 2-AP 5-BU is an analog of T. Once it pairs with C, it can frequently change to an isomer that mispairs with G. AT GC or TA CG unless it is ionized first then it is an analog of cytosine and causes GC AT 2-AP is an analog of A. Once it pairs with T, it can be protonated and mispair with C. AT GC or TA CG transition unless it is protonated first thereby becoming an analog of G and causing GC AT transistion

7 Induced Mutations (base altering agents) Alkylating agents - EMS and NG EMS adds an ethyl group (and NG a methyl group) to guanine causing it to mispair with thymine. GC AT or CG TA transition (rarely causes AT GC transition) HA (NH 2 OH) HA hydroxylates cytosine causing it to mispair with adenine. GC AT or CG TA transitions only. NA NA changes cytosine to uracil which mispairs with adenine. GC AT or CG TA transitions. NA can also deaminates adenine to form HX which mispairs with cytosine. AT GC or TA CG transitions. Intercalating agents (proflavin, acridine orange, ICR compounds) These agents are planar polycyclic molecules that are about the same width of the double helix diameter. They can slip inbetween (intercalate) the paired bases at the center or the DNA molecule resulting in a insertion or deletion - frameshift mutation. Ethidium bromide is an intercalating agent, too.

The sequence of bases on the mrna is a code that determines the sequence of amino acids in the polypeptide being synthesized:

The sequence of bases on the mrna is a code that determines the sequence of amino acids in the polypeptide being synthesized: Module 3F Protein Synthesis So far in this unit, we have examined: How genes are transmitted from one generation to the next Where genes are located What genes are made of How genes are replicated How