3/12/02
Reminders:
Thursday, March 14, 12:30, MO 157, Tulane School of Public
Health presentation
Next Thursday, March 21, exam 2
Cleaning up last Thursday's lecture:
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 à U - deamination of cytosine creates uracil
• 5MC à 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 à T transitions).
Induced Mutations (base analog mutagens)
5-BU and 2-AP
• 5-BU is an analog of T. Once it pairs with A, it can frequently
ionize to an isomer that mispairs with G. AT à GC unless it
starts as the ionized form first, then it is an analog of cytosine
and causes GCà AT
• 2-AP is an analog of A. Once it pairs with T, it can be
protonated and mispair with C. AT à GC transition unless it is
protonated first thereby becoming an analog of G and causing
GCà AT transistion
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 à AT transition (rarely
causes AT à GC transition)
HA (NH2OH)
• HA hydroxylates cytosine causing it to mispair with adenine.
GC à AT transition only.
NA (nitrous acid)
• NA deaminates cytosine to uracil which mispairs with adenine.
GC à AT transition.
• NA can also deaminates adenine to form HX which mispairs
with cytosine. AT à GC or TA à 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 an insertion or deletion - frameshift
mutation. Ethidium bromide is an intercalating agent, too.
Reversion Analysis - method to evaluate the nature of the mutation
or the action of a mutagen. (Table 16-1)
Relative potency of different mutagens (ad-3 mutants are purple
and require adenine):