BIOL 438 exam 2 lecture 22

What are mutations

a heritable change in the sequence of the genetic material

What can count as a mutation?

single bp changes,

deletions or insertions

chromosomal rearangements

gain or loss of plasmids

When can mutations occur

they can occur in any cell at any time

What can the cause of a mutation be

internal, spontaneous (e. g. mistakes during replication of DNA).

external, mutagen-induced (e. g. radiation, chemicals).

do all mutations have phenotypic consequences

no, Occationally, mutations can be beneficial

What mutations are acted on by natural selection

deleterious (decreases an organism's fitness) and beneficial mutations

Where can mutations be

in protein-coding genes or in regulatory sites (then they have important consequences) , or in non functional parts

What do mutations offer to species

the ability for species to adapt to changing conditions and challenges

they in turn serve as the as the driving force for evolution and biological diversity

Wild type

a strain isolated from nature (reference strain).

Mutant

an organism whose genome carries a mutation

Phenotype

the observable property of an organism

Genotype

the genetic makeup of an organism; the genetic identity (ultimately the nucleotide sequence) of an organism associated with a given phenotype.

Alleles

variants of a gene that differ in their sequence

What are the causes of Spontaneous mutation

result of exposure to natural radiation (cosmic rays, etc.) or oxygen radicals.

When do the majority of spontaneous mutations occur

during DNA replication as a result of errors in the pairing of bases.

Spontaneous mutation rate

frequency of mutation within a gene per cell division.

WHat is the typical frequency of a mutation occurring in a given gene

in the range of 10 -4 to 10-8 per generation.

when do the majority of mutations occur

during DNA replication as a result of errors in the pairing of bases.

Induced mutations

caused by mutagens (chemical, physical, or biological agents that increase the mutation rate).

Chemical induced mutations

nucleotide base analogs, intercalating agents

physical induced mutations

nonionizing radiation (ultraviolet radiation), or ionizing radiation (X-rays)

Forward mutation

wild type sequence → mutant sequence

Reversion

restoration of a mutant sequence to the wild-type sequence

Suppressor mutation

correction of the effects (phenotypes) of one mutation by a second mutation at some other site, which could be in the original gene or in another gene

What are the types of mutations

forward,

reversion

supressor

What are point mutations

Single base-pair change mutations

silent point mutations

point mutations with no phenotypic effect

missense point mutations

point mutations with a single amino acid substitutions; may or may not have phenotypic effects.

nonsense point mutations

point mutations that cause premature translation termination; these usually cause drastic defects in the affected proteins.

frameshift point mutations

point mutations that cause insertion or deletion of a single base pair; these are also usually cause drastic defects in the affected protein.

What are essential genes

genes that encode for proteins that are essential for growth,

e.g. cells cannot exist without DNA polymerase, RNA polymerase, ribosomal proteins

how much essential genes does ecoli have

about 400 essential genes (out of a total of about 4400 genes)

Can you isolate a mutation that inactivates an essential gene at all times

no, because such a cell would be dead

how can you isolate conditional mutations in essential genes

by having mutations that inactive the protein under some conditions, the so called non-permissive conditions but permit function under others, the so-called permissive conditions.

permissive and non-permissive conditions

When trying to isolate a conditional mutation in essential genes, mutations that inactive the protein under some conditions, the so called non-permissive conditions but permit function under others, the so-called permissive conditions.

What are the most well known conditional mutations

temperature sensitive mutations

what are Temperature sensitive mutations

special missense mutations that inactivate (unfold) the affected protein at one temperature (the non-permissive temperature) but do not interfere with its structure/function at some other temperature (the permissive temperature)

What are the two types of temperature sensitive mutations

heat sensitive (hs), which unfold or otherwise inactive the protein at high temperature (say 42o C) but permit protein function at a lower temperature (say 30o C)

cold sensitive (cs), which inactivate the protein at a low temperature (say 30o ) but permit function at a higher temperature (say 42o )

How can ts mutations in essential genes be observed

the mutant phenotype can be analyzed by shifting to the non-permissive temperature

What are the types of Base Substitutions

Transitions and Transversions

transtions

base substitution in which one purine (A or G) is substituted by another purine, or one pyrimidine (C or T) is substituted for another pyrimidine.

Transversions

base substitutions in which a purine is substituted by a

pyrimidine or vice versa

Base analogs

cause of induced mutations,

structurally similar to normal bases

mistakes occur when they are incorporated into growing polynucleotide chain

DNA modifying agents

cause of induced mutations,

alter a base causing it to mispair

intercalating agents

cause of induced mutations,

distort DNA to induce single nucleotide pair insertions and deletions