Microbiology Lecture 7: Genetic Variation

what are the five types of spontaneous DNA mutations?

1. missense point

2. nonsense point

3. insertion frameshift

4. deletion frameshift

5. inversion

what is a missense point mutation?

one nucleotide is exchanged for another and results in a different amino acid

when is a missense point mutation most common?

when the 2nd or 3rd nucleotide changes

what is a nonsense point mutation?

one nucleotide exchanges for another and a stop codon is produced

what is an insertion frameshift mutation?

adding a nucleotide and shifting the entire reading frame of the mRNA sequence

what is a deletion frameshift mutation?

deleting a nucleotide and shifting the entire reading frame of the mRNA sequence

what is an inversion mutation?

when the DNA sequence flips and reverses itself resulting in the production of different amino acids

what are tautomers?

structural isomers that cause abnormal base pairing; ex: A binding to G instead of T

what causes pyrimidine dimer formation?

UV light

what is a pyrimidine dimer?

the locking of pyrimidines in the DNA backbone; makes the DNA stiff and nonfunctional

what are the three types of horizontal gene transfer?

1. transformation

2. conjugation

3. transduction

is transformation density dependent?

yes; the more nearby cells, the higher the uptake of genetic material there will be

what are competence factors?

autoinducer proteins that trigger transformasome expression

what is transformasome?

a giant trans-envelope protein that allows a cell to bring in outside DNA from the environment

what is cell competence?

the ability of a cell to take in DNA from the outside environment

are all cells competent?

no

can all cells experience transformation?

no, but noncompetent cells can be artificially manipulated to do so

describe the process of transformation

1. active competence factor is secreted by the cell

2. CF activates ComD sensor kinase

3. genes for sigma factor H are transcribed

4. SigH directs transcription of transformasome components

5. transformasome binds extracellular DNA

6. one strand is transported and the other is degraded

is recombination related to reproduction in prokaryotes?

no

is horizontal gene transfer species specific?

no

what is an F-factor?

a plasmid that codes for the ability to participate in conjugation

is conjugation related to reproduction?

no

what are F+ cells?

cells that contain F-factor

what are F- cells?

cells that do not have F-factor

which cells act as donors? which act as recipients?

F+: donor

F-: recipient

what connects two cells during conjugation?

a sex pilus

describe the steps of the conjugation process

1. sex pillus from F+ donor attaches to F- receptor

2. pilus contracts and forms relaxosome bridge

3. 5’ end of the F factor begins transfer to recipient

4. remaining strand in donor is replicated 

5. in the recipient, the transferred strand cyclizes and replicates

6. recipient is now a donor

what is the purpose of the relaxosome bridge?

keeps the two cells participating in conjugation close

what is an F’ plasmid?

a plasmid containing the F-factor and some linear chromosomal DNA

what is the result of an F’ plasmid forming?

increased chromosomal recombination, possibly at the expense of the host cell

what is Hfr?

high frequency recombination; a cell with an F’ plasmid; allows for greater genetic recombinance

what phage cycle correlates to generalized transduction?

the lytic cycle

describe the steps of generalized transduction

1. phage infects host cell and makes subunit components of more phages

2. DNA packaged into capsid heads; some contain phage some contain host 

3. cell lyses, phages released

4. phages containing host DNA inject into a new cell where they may recombine into its genome 

is horizontal gene transfer species specific?

no; but some phages may be

describe the steps of specialized transduction

1. phage DNA integrates with host DNA in prophage state

2. both viral and host DNA together are excised from the genome and form a specialized transducing DNA molecule

3. chimeric phage DNA can continue on into another cell

what is used to digest/recycle/cut DNA?

restriction endonucleases

how are restriction endonucleases useful when it comes to viruses?

viral DNA is often recognized as foreign, so the restriction endonucleases cut it up and degrade it

what are CRISPR repeats?

small runs of phage DNA; don’t code for any proteins

what are CAS?

CRISPR associated sequences

what do CRISPR sequences code for?

guide RNA

what do CAS code for?

enzymes

how do CRISPR and CAS work together?

the guide RNA from CRISPR hybridizes with live phage DNA, leading the CAS enzymes to the phage DNA and destroying it

what are transposons?

segments of DNA that can relocate within a genome

describe the steps of basic transposition

1. transposase makes cut in target sequence

2. insertion sequence is moved into the target site

3. replication occurs to fill staggered ends

what is the difference between nonreplicative and replicative transposition?

nonreplicative: the transposable element jumps from one site to another

replicative: the transposable element is copied before moving sites; one copy in original site and one copy in new site