Prokaryotic genetics- 47 Flashcards
Mutagenesis and mutations (41 cards)
Why are we interested in mutants?
-Finding and characterising mutants has been essential for developing our understanding of biochemistry and cell biology.
-Mutants are essential for our biotech exploitation of bacteria and an issue for controlling disease.
-Mutation in bacteria are caused by the same mechanisms as in our cells i.e. we can find mutagens, which are likely to be carcinogenic.
-Mutants can be spontaneous or result of exposure to mutagens, compounds which increase likelihood of mutagens.
How can bacteria protect their DNA?
Bacteria have different enzymes that can repair their DNA.
Bacteria can produce melanin and other pigments to protect from radiation damage.
How often does DNA polymerase make a substitution?
Approximately once every 10^7 bases- more often than eukaryotic DNA polymerase.
Genome is 5.4 x 10^6 base pairs. After 2 generations 3.2 x 10^7 base pairs synthesised and probably 3 substitutions made.
There can be millions of E. coli in a ml of culture. But DNA repair will ensure some are corrected.
What is a replication error?
When the wrong base is inserted by DNA polymerase.
What is a tautomer?
Isomers that exist in equilibrium.
Tautomer of the bases have different H-bonding pattern. If it is in a tautomer form in DNA replication then the tautomer base pairing will occur:
-minor tautomer of T and G leads to GT base pairing.
-minor tautomer of A and C leads to AC base pairing.
What is base pair slipping?
Repeat nucleotides can lead to frameshift mutations.
What are mutagens? Give examples.
Mutagens are chemical or physical agents causing damage to DNA.
Mutagens can increase mutation rate by orders of magnitude.
E.g. nitrous acid, reactive oxygen species, alkylating agents, intercalating agents (inserting themselves between base pairs), UV light.
What are intercalating agents?
Insert themselves between base pairs.
Usually have flat multiple ring structures.
Binds between base pairs.
DNA tries to bind with intercalating agent.
Distorts the helix.
Can also lead to frameshift mutations.
E.g. Ethidium bromide. Notice extended flat ring system- flatness allows it to be inserted between base pairs. Used to be added to DNA gels.
What is point mutation?
A change to one base pair. They have different effects at a protein level, therefore varying consequences.
E.g. Substitution, deletion and insertion.
Insertions and deletions can also be called indels.
What can the segregation of mismatched base pairs result from?
Could result from the error in replication, tautamerisation or damage, such as deamination.
E.g. Hypoxanthine- deamination of adenine, H base pairs with C.
Therefore, culture will be a mixture of different genotypes and potentially phenotypes.
What are the consequences of point mutations?
Some mutations are lethal, cells will die and hence not inherited.
Non-coding: may have no consequence at all, but non-coding DNA can have other functions e.g. promoters and other regulatory sequences.
Promoter: can effect transcription (up or down) but may have no consequence at all.
Genes under control of promoter: transcribed to mRNA- depends, can affect sequence of protein or regulation of translation.
What can be the effect of point mutations in protein coding sequence?
Substitutions in coding region leads to:
-silent
-missense
-nonsense
What are the consequences of mutations?
Third base pair substitution often, but not always silent.
Most first and second base pair substitution result in missense. Some third base pair substitutions also do.
Some substitution in 1st, 2nd and 3rd position can result in nonsense mutations. No translation continued, due to stop codon.
What do insertions and deletions result in?
Frameshift mutations.
Frameshift mutations are better tolerated when at the end, because less is affected.
What happens to the protein after silent, missense and nonsense & frameshift mutations?
Silent: nothing. Genotype changed but not phenotype.
Missense: often nothing but can be detrimental. Genotype changed and phenotype may be changed.
Nonsense and frameshift: usually detrimental (because you lose everything after the mutation) but can be tolerated close to C-terminus. Genotype and phenotype changed.
What is the effect of a large scale mutation- deletion?
Can remove kilobases and lose several genes or be just a single base pair.
Can result in frameshift mutations one of few bases are lost within a gene affecting coding or regulation.
What is the effect of large scale mutations- inversions?
Can flip kilobases and several genes or be much shorter.
Can disrupt genes (no longer functional) or just invert them (not an issue).
What is the effect of large scale mutations- tandem repeats?
Part of genome is duplicated.
Can lead to overproduction of proteins encoded in the duplicated region.
Also leads to evolution of proteins.
What is the effect of large scale mutations- transposons?
Transposons are nucleotide sequences that are able to move themselves around.
Can disrupt genes.
What is reversion of mutations?
A point of mutation resulting in restoration of the original sequence.
What is intragenic supression?
2 mutations in the same gene and causes a change in each gene.
What are suppressor mutations?
A second mutation happens that results in the original phenotype being restored.
E.g. original salt bridge
Detrimental mutation, protein destabilised Suppression mutation, salt bridge reversed, protein is stabilised.
What happens in the suppression of frame shift?
Wild type, then addition occurs. Then intragenic suppression- downstream removal of base pair.
