Bacterial Genetics and Gene Regulation- Steinauer

Question 1

What are the 2 components of prokaryotic DNA?

Answer 1

oHaploid with single circular chromosome. (no dominant/recessive case here)
oPlasmid (smaller, circular, self-replicating DNA)

Question 2

What are the 3 mechanisms for producing genetic change?

Answer 2

1. Mutations
2. Transposons
3. Horizontal Gene Transfer

Question 3

How do mutations contribute to genetic change? Are they common? What are the 2 types of point mutations and what do they result in? What is inversion? What is insertion/deletion and what does it result in?

Answer 3

•Genetic change passed to next generation.
•Not very common (0.003 mutations/genome/generation)
Point mutations: (at a single base)
1. Silent: no change in translated protein.
2. Missense: amino acids are changed in translated protein.
•Inversion: segment of DNA is reversed but stays in the same location.
•Insertion/deletion: single base added or deleted, resulting in frame shift.
•Results in:
oMissense:
oNonsense: a protein that terminates prematurely due to the addition of a stop codon.

Question 4

What are transposons? How can they be recognized? What are the 2 types of transposition?

Answer 4

•Mobile DNA that moves among sites within the same chromosome or between DNAs of the chromosome, plasmids, and bacteriophages.
•They can be recognized because they are flanked by inverted repeats, regulatory elements, contain the gene for transposase and can also contain other genes like those that confer drug resistance.
•Are mutagens.
1. Direct transposition: “cut and paste”
2. Replicative transposition: “copy and paste” (gene is not “lost”)

Question 5

What are the 3 mechanisms of horizontal gene transfer?

Answer 5

1. Transformation
2. Conjugation
3. Transduction

Question 6

What is transformation? Describe how it works.

Answer 6

• Taking up free DNA usually from dead bacteria.
• A recipient cell must be competent to take up free DNA.
• DNA taken up with DNA translocase.
• The DNA gets incorporated into the recipient bacterial chromosome by homologous recombination.

Question 7

Describe how conjugation works. What is high frequency recombination?

Answer 7

Two cells:
1. F+: has F plasmid (fertility) with fertility factor and donates some of the plasmid to the F- bacteria.
2. F-
•Conjugation pilus → mating bridge.
•High frequency recombination:
oFor some reason the F plasmid is incorporated into the chromosome so entire chromosome could be transferred between cells. (usually takes too long to transfer the whole chromosome)
oYou get a lot more DNA transfer than just plasmid.

Question 8

Describe transduction. What are the 2 stages of the lifecycle of a bacteriophage? Describe them. Give an example.

Answer 8

Transfer by viral delivery.
Life cycle of bacteriophage:
1. Lytic:
•Phage attaches to bacterial cell. Injects DNA into cytoplasm. DNA circularizes. Uses cellular machinery to replicate its DNA. Get all these baby viruses. Lyses the cell and releases everything.
•As the cell is being broken apart, the bacteria DNA is getting all broken up. It can then be incorporated into the virus and then transferred to other bacteria.
2. Lysogenic:
•The DNA from the virus is incorporated into the bacterial genome. (called a prophage) Lies dormant for a while and then switches to the lytic cycle.
•Example: shiga toxin triggers lysogenic conversion. (Phenotypic change induced by prophage integration.)

Question 9

What are genomic islands/pathogenicity islands and how are they a consequence of change?

Answer 9

•clusters of genes that are absent in closely related strains or species
•Contain a variety of genes that enable bacteria to colonize and invade host tissues and to avoid or overcome host defenses.
•Large > 10 Kb
•Inserted near tRNA encoding genes
•Different G +C content than rest of genome
•Repeat structures
•Fragments of other mobile elements
oCan develop different levels of pathogenicity.

Question 10

What is constitutive gene expression? What is inducible/repressible gene expression?

Answer 10

oConstitutive: genes continuously expressed.
oInducible/repressible:
•Expressed only when needed.
•Binding of regulatory proteins to activate or repress transcription.

Question 11

What is an operon? What is negative gene regulation? What is positive gene regulation?

Answer 11

Operon: a single transcriptional unit that includes a series of structural genes, a promoter and an operator.
Negative gene regulation: binding of a repressor that stops transcription. Promoter alone is sufficient for transcription.
Positive gene regulation: need regulator protein to bind for transcription to occur.

Question 12

What type of control is the Lac operon? Describe the Lac operon example of gene regulation. What is LacI? What happens when lactose is present? What can we learn from this?

Answer 12

Lac operon (negative control): lactose metabolism
•3 structural genes (lacZ, lacY and lacA)
•Promoter: where RNA polymerase binds.
•Operator: where regulator binds.
oUpstream (lacI): codes for regulator protein.
•LacI repressor binds to operator and prevents RNA polymerase from doing its thing.
•If lactose is present, it binds to the repressor and makes it fall off, so RNA polymerase can bind and do transcription.

Question 13

What is global regulation?

Answer 13

One single regulatory protein can regulate many different operons

Question 14

What is quorum sensing?

Answer 14

•Density dependent gene regulation.
•Bacterial cells communicate with each other in order to produce gene products.
•Genes are only turned on if you have a specific density of cells.
•Communal behavior:
1. Toxin production
2. Biofilm formation

Question 15

What are programmed rearrangements?

Answer 15

• Can change their antigenic structures to evade the immune system.
• Change happens in a sequential manner.
• Expression locus (site of expression)
• Silent storage site: has other genes in reserve that can be moved to the expression site.