ch. 16: mechanisms of genetic variation (memorize)

what is the difference between spontaneous and induced mutations

• spontaneous = natural errors (DNA replication, chemical changes)

• induced = caused by external factors (UV, chemicals)

what is the most common cause of spontaneous mutations

error during DNA replication (despite proofreading)

how do spontaneous mutations benefit bacteria

create genetic diversity → some mutations help survival in changing environments

do antibiotics cause mutations

no - they select for already existing resistant mutants

what is point mutation

a change in a single base in dna

what is a silent mutation

base change → same amino acid → no protein change

what is a missense mutation

base change → different amino acid

what is a nonsense mutation

base change → creates stop codon → shorter protein

what is a transition mutation

purine ←> purine OR pyrimidine ←> pyrimidine

what is a transversion mutation

purine ←> pyrimidine

what is an insertion or deletion mutation

addition or removal of DNA bases

what is a frameshift mutation

insertion/deletion shifts reading frame → changes all downstream amino acids

what is a reversion mutation

a mutation that restores the original function (reverses effect of prior mutation)

if a point mutation occurs in gene A of an operon, does it affect gene B

usually no - only gene A protein is affected

if an insertion mutation occurs in gene A, does it affect gene B

yes - frameshift can disrupt downstream genes

what is vertical gene transfer

DNA passed from parent → offspring

what is horizontal gene transfer

DNA transferred between unrelated bacteria

what are the three main mechanisms of horizontal gene transfer

transformation, conjugation, transduction

what is transformation

uptake of free DNA from environment

what is conjugation

DNA transfer via direct cell contact using a pilus

what is transduction

DNA transfer by bacteriophages (viruses)

what type of DNA is transferred in transformation

free DNA fragments

what type of DNA is transferred in conjugation

plasmid DNA (F plasmid)

what type of DNA is transferred in transduction

bacterial DNA carried by a phage

where does transferred DNA go after entering a cell

• integrated into chromosome

• exists as plasmid

• degraded if unusable

what is natural competence

ability of bacteria to take up DNA from environment

what is the benefit of transformation

gain useful genes (antibiotic resistance, survival traits)

how do transformation occur in S, pneumoniae

• pilus binds DNA → pulls it in

• one strand degraded

• other enters cytoplasm via ComEC

what is the F plasmid

fertility plasmid that enables conjugation

what is an F+ cell

has F plasmid → can donate DNA

what is an F- cell

lacks F plasmid → received DNA

what is the role of the sex pilus

connects F+ to F- cell for DNA transfer

what does relaxase do in conjugation

cuts one DNA strand at oriT and transfers it

what is oriT

origin of transfer - starting point for DNA transfer

what is the relaxosome

protein complex that prepares DNA for transfer

what happens after DNA enters the recipient cell during conjugation

complementary strand is synthesized → F- becomes F+

what is the key differences between HGT mechanisms

• transformation = free DNA

• conjugation = plasmid + pilus

• transduction = phage-mediated

what protects bacteria from foreign DNA

CRISPR/Cas system → degrades foreign DNA