Lecture 13b Flashcards
(23 cards)
What is a point mutation?
Alteration of a single base pair of DNA
What are the two types of point mutations?
1) base substitutions (bp is replaced by another)
2) base insertions or deletions
What are the two subtypes of point mutations?
1) Transition: purine for purine OR pyrimidine for pyrimidine
2) Transversion: purine for pyrimidine OR pyrimidine for purine
What is a synonymous mutation?
Change in bp of DNA, but still codes for the same amino acid therefore protein is not altered (silent mutation).
What is a missense mutation?
Change in bp of DNA leads to different amino acid therefore protein is altered.
What is a nonsense mutation?
The codon for one amino acid is changed into a translation-termination (stop) codon.
What is a frameshift mutation?
The addition or deletion of a single base pair of DNA changes the reading frame for the remainder of the translation process. Typically results in loss of normal protein structure and function.
How can point mutations alter mRNA splicing?
Change in a bp can lead to a new splice site which leads to loss of coding regions. Change in bp can eliminate a splice site therefore introns are retained in the mRNA.
How can point mutations alter regulation?
Change in bp can alter binding of regulator proteins, effect is variable.
What would a northern (RNA) and western (protein) blot look like for a missense, nonsense, frameshift, regulatory region mutation?
Missense: N=WT, W=WT
Nonsense: N=WT, W=shorter
Frameshift: N=WT, W=variable
Regulatory region: N=no mRNA, W=no protein
Are mutations random and pre-existing or do they arise in response to a selective agent?
Random, selection occurs after a mutation. Fluctuation experiment demonstrated this.
What was the fluctuation experiment done by Luria and Delbruck?
Experiment done to see if mutations are random or induced. Design: Grew 20 small cultures of bacteria in separate tubes. Also grew one culture of 20x that volume in single flask and divided into 20 tubes. They were all plated in the presence of phage. If the phage were inducing mutations, then all 40 tubes should have approx. same number of mutants (not seen). If mutation occurred randomly then tube 1-20 would have higher fluctuation (seen).
How do spontaneous mutations arise?
Error in DNA replication (transition/transversion/frameshift/repeats) or spontaneous lesions (depurination, deamination, oxygen radicals)
What is replication slippage?
Indel mutations arise when loops in single-stranded regions are stabilized by the “slipped mispairing” of repeated sequences in the course of replication. The newly synthesized strand can slip (addition) or the template strand can slip (deletion).
What is depurination?
Loss of purine base. Loss of A or G.
What is deamination?
H20 added, NH3 lost. Amine is lost, ketone is formed. C to U.
What is a mutagen?
Anything that can cause a change to DNA sequence. Mutagens are found in the environment.
What are the different mechanisms of mutagenesis?
1) Incorporation of base analogs
2) Specific mispairing
3) Intercalating agents
4) Base damage
What are base analogs?
Some chemical compounds are very similar to the normal nitrogen bases of DNA therefore they occasionally are incorporated into DNA in place of normal bases. They have pairing properties unlike those of normal bases. They can incorrect nucleotides to inserted across from them.
Example: the base analog 2-AP for Adenine can bp with Thymine when unprotonated and with Guanine when protonated.
What is specific mispairing mutagens?
Some mutagens are not incorporated into the DNA but
instead alter a base in such a way that it will form a specific mispair. Alkylating agents do this by changing a base to cause mispairing.
What are intercalating agents?
These agents are planar molecules that mimic base pairs and are able to slip themselves in (intercalate) between the stacked nitrogen bases at the core of the DNA double helix. They lead to insertions and deletions of base pairs.
What is base damage?
Damage to bases causes DNA block because DNA pol does not which base to add to complement the damaged base. Ex. UV light causes thymine dimers
What is the Ames Test?
It helps reveal mutagenic compounds.
Design: Solubilized liver enzymes (S9) are added to a suspension of auxotrophic bacteria (His-) in a solution of the potential carcinogen (X). The mixture is plated on a medium containing no histidine. The presence of revertants indicates that the chemical is a mutagen and possibly a carcinogen as well.
