Microbial Genetics

Question 0

How does semi-conservative replication work?

Answer 0

Two strands of double helix unwind and separate - helicase
Free nucleotides line up along the parental strands at complimentary base pairs
Nucleotides linked enzymatically - DNA polymerase
Two replicas result, each containing a parental DNA strand and a newly formed strand

Question 1

How do prokaryotes replicate DNA?

Answer 1

Replication bubble
DNA trade allows unwinding of supercoiled DNA
Vertical transmission of genes to daughter cell
Changes in chromosome stably inherited by daughter cells = mutations

Question 2

Why and how is gene expression regulated in bacteria?

Answer 2

Protein synthesis metabolically expensive- cells only make what is needed
60-80% of genes constitutively expressed - “housekeeping genes”
Genes not involved in normal or continuous processes have expression regulated
Feedback inhibition regulates enzymes that have already been synthesised
Genetic control mechanisms control synthesis of new enzymes

Question 3

What two mechanisms are used to regulate transcription of mRNA and thus control enzyme synthesis?

Answer 3

Induction - mechanism that turns on the transcription of a gene and thus translation of its enzyme product
Repression - mechanism that inhibits gene expression thus decreasing synthesis of corresponding enzyme

Question 4

What is an operon?

Answer 4

Clusters of related genes for synthesis and control of enzymes

Question 5

What does the lac operon consist of?

Answer 5

3 structural genes - lacZ, lacY, lacAe
A promoter
A terminator
A regulator 
An operator

Question 6

What are plasmids?

Answer 6

Small circular 1/10th the size of a chromosome
Double stranded DNA 
Free in cytoplasm
Can link into chromosome
Not essential for normal metabolic growth or cell reproduction
Replicate independently
Can be removed by curing
Exploited in recombinant DNA technology

Question 7

What roles do plasmids play?

Answer 7

R plasmids - resistance genes
Fertility factor - sex pillus - conjugation
Virulence plasmids
Metabolic plasmids
Bacteriocins - antimicrobials proteins targeting other bacteria

Question 8

Features of transposons

Answer 8

Jumping genes
700-40,000 base pairs in length
Move from one location to another, either Same or different molecule
Found in many if not all prokaryotes and eukaryotes
Occurs between plasmids and chromosome
Occur within and among chromosome
Can be carried by plasmids
Inserion sequences are simplest transposons

Question 9

Describe the process of recombination

Answer 9

Horizontal/lateral gene transmission from a donor to a recipient
Transformation - uptake of DNA from environment
Conjugation - transfer of DNA through sex pillus
Transduction - transfer of DNA by bacteriophage
Allows genetic variation
Not observed in all bacteria
Bacterial cells must be competent

Question 10

What are the features of the two strains of S.pneumoniae?

Answer 10

Smooth (S) strain - capsulated
- capsules protect against phagocytes
- virulent
Rough (R) strain - mutant strain
- no capsule (cannot synthesise capsule carbohydrate)
- avirulent - killed by host cells

Question 11

What did Griffith’s experiments show in regards to S strain and R strain?

Answer 11

Injected varies strains into living mice
Live S strain killed mouse
Live R strain mouse survived
Heat killed smooth strain mouse survived
Heat killed smooth strain + live rough strain mouse died

Question 12

Describe the experiment carried out by Avery, Macleod and McCarty (1944)

Answer 12

1 - lipids and carbohydrates were removed from a solution of Heat killed S cells - proteins, RNA and DNA remain
2 - subject solution to treatment to destroy proteins, RNA or DNA
3 - add a small portion of each sample to culture R cells and observe wether transformation has occurred by testing for the presence of virulent S cells
4 - conclusion - transformation cannot occur unless DNA is present

Question 13

What is conjugation?

Answer 13

Exchange in genetic material involving physical contact between bacterial cells
Donor strains contain thenF plasmid (F factor) F+
Recipient strains do not contain f plasmid F-

Question 14

Characteristics of bacteria genders?

Answer 14

Contain a plasmid - sex factor
Free in cytoplasm = F+ male
Integrated into chromosome = Hfr male
Female bacterium = F-

Question 15

What is the purpose of the F plasmid?

Answer 15

Codes for formation of sex pilus
Promotes conjugation
Promotes fertility
Codes for its own replication and transfer to recipient
May contain extra genes (eg resistance genes)
Two fertile crosses in conjugation - F+ x F- and Hfr x F-

Question 16

How does F+ x F- mating occur?

Answer 16

F plasmid transfers from F+ to F-
F- gains all genes on F plasmid
F- becomes F+
F plasmid replicates by rolling circle mechanism where a strand is nicked and extended by replication enzymes then a complimentary strand is synthesised
No chromosomal gene is transferred

Question 17

How does Hfr x F- mating occur?

Answer 17

F plasmid integrated with chromosome of Hfr cells
Hfr = High frequency recombination (ie high frequency of chromosomal genes being transferred from donor to recipient resulting in recombinant cell)
Sex pilus is relatively fragile connection therefore the entire chromosome is rarely transferred
Recipient cell remains F- (since some of th F plasmid genes are last to be transferred)
Chromosomal genes replicate by rolling circle mechanism, genes transfer to the recipient in a specific order entire chromosome takes around 100 minutes

Question 18

What does the female recipient gain or not gain in an Hfr x F- cross?

Answer 18

Remains female
Does not gain sex factor
May gain useful genes for synthesis of amino acids
May gain useful genes for use of sugars such as lactose
May gain resistant genes

Question 19

What does it mean if the F plasmid is F’?

Answer 19

F plasmid can deintegrate from chromosome and may carry chromosomal genes with it
F’ = F plasmid which contains additional genes (eg lac)
F’ cells can conjugate with F- cells and the specific chromosomal genes in the F’ plasmid will always be transferred
- do. It have to be incorporated into chromosome to be expressed - sexduction
Among gram negative organisms this is a major way bacterial genes are transferred
Extremely important in microbial resistance