Molecular Bio Ch. 13

Mutations that affect phenotype are usually _&_. They mostly result in an _change that is deleterious to _

deleterious & recessive amino acid change / protein function

Stress does not _ but rather _ rare pre-existing mutations that result in resistance phenotypes

does not direct; it selects

Mutations that affect phenotype are usually deleterious because amino acid sequence highly adapted to specific _&_

environment & function

examples of induced mutations

exposure to physical or chemical agents (mutagen) Ionizing radiation, ultraviolet light, a variety of chemicals Mutation Induced by Transposable Genetic Elements

change from wild-type to mutant phenotype

forward mutation (normal mutation)

1 (or more) base pair addition or deletion in gene(alter reading frame: these alter the codons and thus the resulting amino acid in translation)

frameshift mutation

Only mutations that occur in _ cells will be passed onto progeny.

germ (Germ-line) cells

cells that give rise to gametes:_ all other cells:_

germ cells somatic cells

Mutations that result from a known cause

induced mutations

X-rays induce mutations through _. Ultraviolet (UV) light induces mutations through _.

ionization excitation

Enzymes important in metabolizing certain chemical and creating mutagen compound in the context on human.

liver extract

when the change of a single base pair causes the substitution of a different amino acid in the resulting protein. This amino acid substitution may have no effect, or it may render the protein nonfunctional.

missense mutation

Viruses that can integrate into the genome are also _ agents.

mutagenic agents

organism that exhibits a novel phenotype resulting from a mutation

mutant

Change in the genetic material and the process by which the change occurs

mutation

ultimate source of all genetic variation

mutation

Because of the degeneracy and order in the genetic code, many mutations (_ mutations)have no effect on the phenotype of the organism.

neutral (or silent) mutations

Insertion of transposon into an expressed gene or its regulatory elements will often render gene _

non-functional

Mutation usually is a _ process in which an environmental stress simply selects organisms with preexisting, randomly occurring mutations.

nonadaptive (environmental stress selects for organisms with pre-existing mutations)

Exposure to antibiotic preserves a pre-existing and random mutagenic event is an example of _

nonadaptive mutation

change of nucleotide to form a termination codon - stops translation in the middle - not a functional protein

nonsense mutation (nonsense mutations generate nonfunctional proteins, whereas missense mutations may not necessarily lead to nonfunction)

mutation of a specific position within a gene

point mutation

Testing for mutagens identifies what?

possible carcinogens

helps to ensure accurate replication of DNA from generation to generation

proofreading (however, mistakes occur)

restoration of wild-type phenotype

reverse mutation

Mutation can provide a selective advantage, such as in the disease _

sickle cell anemia (protects against malaria)

Base pair substitutions that are "silent" - often referred to as _

single nucleotide polymorphisms (SNPs) Evidence they are not be entirely silent: Translation kinetics Risks of disease

Occur without a known cause, rare -Inherent errors in replication -Unknown agents in the environment -Problem with DNA repair and proofreading

spontaneous mutations

Reverse mutation in which second mutation anywhere in genome compensates for effect of first mutation

suppressor mutation

can increase in copy number and cause inherited diseases

tandem repeats/trinucleotide repeats

In the base analog 5-bromouracil, a _ is more likely due to the Br. This results in _

tautomeric shift transitions (5-bromouracil causes transitions)

Transient shift from stable keto or amino to less stable (rare) enol or imino forms

tautomeric shifts

Frequency of mutation depends on what?

the agent can increase rate dramatically if potent mutagen Up to 1% of all genes can be mutated in an organism

Cross-linking of adjacent thymine forms _, which block DNA replication and activate error-prone DNA repair mechanisms.

thymidine dimers

replacement of purine with a purine (C<-->T, G<-->A) or pyrimidine with a pyrimidine

transition

Point mutations are of 3 types:

transitions, transversions & frame shifts

DNA fragments that can move from one site in a genome to another site

transposons (ex: Mendel's wrinkled pea allele)

replacement of purine ( C<-->A, G<-->T) with a pyrimidine or pyrimidine with purine

transversion

Rate of mutations in prokaryotes: rate in eukaryotes:

~ 10-8 to 10-10 detectable mutations /bp/generation ~ 10-7 to 10-9 detectable mutations /bp/generation

Examples of spontaneous mutations

-Inherent errors in replication -Unknown agents in the environment -Problem with DNA repair and proofreading

Mechanism for addition or deletion mutations

1) Newly synthesized strand loops out, resulting in the addition of 1 nucleotide on new strand 2) Template strand loops out, resulting in the omission of one nucleotide on the new strand

The compound _ is an example of an analog to thymine. It undergoes a tautomeric shift to base pair with guanine instead of adenine

5-bromouracil

Intercalates into DNA Increases rigidity Induces bends/kinks Tends to cause a greater likelihood for slippage during replication - results in addition/deletions Nonfunctional proteins

Acridine dyes

Transfer Alkyl group to base at N or O Alter base pairing potential Induce transitions, transversions, frameshifts Can cause cross linking of DNA and induce chromosomal breaks

Alkylating agents (mustard gas, EMS, EES - induce all 3 point mutations & can cause cross linking of DNA and induce chromosomal breaks)

What is the difference between an auxotroph & a prototroph?

An auxotroph needs some nutrient that the wild type strain (prototroph) can make for itself.

Structures similar to normal bases Incorporated into DNA during replication, causing increased mispairing

Base analogs (analogous to bases/similar but slightly different)

Tautomeric shifts affect _

Base-pairing; thus, formation of A:C and G:T base pairs possible ---THIS IS A PROBLEM FOR DNA REPLICATION

Hydroxylates amino group of cytosine Resulting hydroxylaminocytosine bps with Adenine Net Result: C->T transitions G/C changed to A/T basepair and vice versa

Hydroxylating agents - Hydroxylamine (NH2OH)

causes increased reactivity of bases; breaks chromosomes and can cause deletions, duplications, inversions, and translocations

Ionizing radiation (ex: x-rays, gamma rays & cosmic rays)

created screen to identify lethal mutations on X-chromosome of Drosophila

Muller

Deaminates amino groups Acts both on replicating and non replicating DNA Results in transitions A->G (purine to purine) C->T (pyrimidine to pyrimidine) G->G (no mutation)

Nitrous acid

"excites" bases causing pyrimidine dimers and pyrimidine hydrates

Non-ionizing radiation: UV radiation

Shifting of H atoms from one position to another (resonance structure)

Tautomeric shifts of common bases (found by Watson & Crick)

What are the limitations of the Ames Test?

Thalidomide & other mutagens - IT IS NOT FOOLPROOF

Rapid, Inexpensive and sensitive method for testing the mutagenicity of chemicals

The Ames Test

The Ames Test uses _ that are _ for histidine, Media lacking histidine -> no growth (his-) Media with histidine -> growth (his+)

a Salmonella strain (S. typhimurium) auxotrophs:a mutant organism (typically a bacterium or fungus) that requires a particular additional nutrient that the normal strain does not.

A delicate balance of mutation is required, as they have positive (ex:_) and negative (ex:_) effects

ability to respond to environment compromise faithful transfer of genetic info from generation to generation (lethal)

Mutagenic event which arises due to exposure to antibiotic is an example of _

adaptive mutation

Tautomeric shifts can do what?

alter the H-bonding of potential bases; they affect base-pairing

the increased severity of disease or earlier age of onset in successive generations as the trinucleotide copy number increases

anticipation

The Ames Test monitors reversion of_to_ (his- -> his+)

auxotrophs to prototrophs

Reverse mutation in which second mutation is at the same site as first

back mutation (completely restores original mutation)

There is a high correlation between mutagenicity & _

carcinogenicity

any change in the normal structure or number of chromosomes; often results in physical or mental abnormalities

chromosomal aberrations