Microbiology E2 Ch8 Pt2

Mutation

permanent change in the base sequence of DNA (harmful or beneficial)

Natural selection

survival of new genotypes

Point mutation (aka base substitution mutation)

single DNA base pair is altered

Missense mutation

mutation that results in the substitution of an amino acid in a protein (type of point mutation)

Explain process of a point mutation (3)

- An incorrect nucleotide is paired into an incorrect base pair (A-G, T-C, G-T, etc)

- Following replication, an incorrect base pair will be in place of the intended base pair (instead of G-C, it may become A-T)

- during translation, a codon will code for a different amino acid resulting in a new or inactive protein

Frameshift mutation

DNA base pairs are added/removed from the sequence; causes a shift in the sequence reading

Explain process of a frameshift mutation (3)

- DNA base pair(s) are added/removed

- 3 pair groupings are altered and result in different amino acids due to the nucleotide sequence shifting

- an inactive protein will be produced

How does a frameshift mutation works

offsets 2 strands of DNA, leaving a gap in one strand to be altered

Nonsense mutation

a base substitution resulting in a nonsense codon

Spontaneous mutation (2)

- occur in the absence of any mutation-causing agents

- mutations occur because of occasional mistake during DNA replication

Mutagens

agents in the environment that directly/indirectly bring about mutation

Nitrous acid

mutagen that changes DNA's base pairs (A can pair with C, G can pair with T, etc)

How does nitrous acid affect nucleotides

- reshapes a nucleotide to pair with an incorrect pairing

How does nitrous acid alter DNA following replications (2)

- the altered-reshaped nucleotide will pair with the incorrect base pair because of its reshaping

- can turn into a mutated DNA if the altered nucleotide becomes a different nucleotide to match the incorrect base pairing (exp: adenine shaped to pair cytosine; mutated DNA pairs the cytosine with guanine)

Nucleoside analog

chemical structurally similar to normal nucleotide but with altered base-pairing properties

What is the nucleoside analog of adenine & what is different about it

- 2-aminopurine nucleoside

- adenine pairs with cytosine (mutated DNA can change AT pair to CG pair)

What is the nucleoside analog of thymine & what is different about it

- 5-bromouracil nucleoside

- thymine pairs with cytosine (mutated DNA can change AT pair to GC pair)

Mutation rate

probability that a gene will mutate when a cell divides (very rare)

Radiation

mutagens with the ability to ionize atoms & molecules

Effect of ultraviolet (UV) light

forms harmful covalent bonds (dimers) between pyrimidine bases (Thymine, Cytosine, Uracil)

Dimers

result from adjacent (thymine, cytosine, or uracil) in a DNA strand cross-linked to form distortions

UV light formation process (3)

- exposed to UV light

- adjacent (T, C, or U) become cross-linked

- dimer forms & disrupts normal base pairing

UV light dimer repair process (nucleotide excision repair) (3)

- endonuclease cuts DNA & exonuclease removes damaged DNA

- DNA polymerase fills gap by synthesizing new DNA

- DNA ligase seals gap by joining old & new DNA

endonuclease

enzyme that cuts DNA

exonuclease

removes damaged DNA

Photolysases

light repair enzymes; separate dimer back to original 2 nucleotides

Feedback inhibition

stops a cell from performing unneeded chemical reactions (stop enzymes that have been synthesized already)

60%-80% of genes are not regulated but ____ (their products are constantly produced at a fixed rate)

constitutive

Cyclic AMP (cAMP)

substance derived from ATP, serving as a cellular alarm signal in response to environment/nutritional stress (alarmone)

Catabolite repression

presence of preferred carbon source like glucose inhibits the expression of genes of other carbon sources

Is glucose or lactose the faster carbon source

glucose

when enzymes run out of glucose, what happens

cAMP accumulates

where does cAMP bind to

allosteric site of catabolic activator protein (CAP)

where does the CAP bind to

lac promoter (initiating transcription)

transcription of lac operon require what (2)

presence of lactose & absence of glucose (glucose = no cAMP)

process of positive regulation of lac operon

- glucose used up as carbon source until no more

- short lag time (cAMP levels increase, lac operon transcribes to synthesize enzyme to break down lactose)

- lactose is used as carbon source

Lac operon function

produce large amounts of mRNA for synthesis of lactose-digesting enzymes

if lactose & glucose are present, what happens to cAMP levels & lac operon

cAMP levels are low; lac operon is inactive

if lactose is present, but glucose is not, what happens to cAMP levels & lac operon

cAMP levels are high; lac operon is active

Epigenetic control

eukaryotes & prokaryotes can turn genes off by methylating certain nucleotides (not permanent, can be passed on {epigenetic inheritance})

Riboswitch

part of mRNA molecule that binds to a substrate and changes the mRNA structure

purpose of a riboswitch

stop protein synthesis (translation) after transcription occurs

if a riboswitch makes a change, what happens

translation can either be initiated or stopped

microRNAs (miRNAs)

single-stranded RNA molecules that inhibit protein production in eukaryotes

how do miRNAs inhibit protein sysnthesis

by binding to mRNA to delay or destroy it from being translated