Genetics of Bacteria

Question 1

What is a gene?

Answer 1

smallest region of DNA (or RNA) that encodes a polypeptide or is transcribed (RNA)

Question 2

What is an allele?

Answer 2

an alternate form of a gene

Question 3

What is a Locus?

Answer 3

Location of a gene on the chromosome and often refers to a group of related genes

Question 4

What is a wild type organism?

Answer 4

an organism that carries standard/reference gene which is usually but not always functional

Question 5

What is a mutant organism?

Answer 5

carries altered form of gene(s)

Question 6

What is a genotype?

Answer 6

genetic or allelic composition of a strain

Question 7

What is a phenotype?

Answer 7

the observable properties of a strain

Question 8

What is the structure of the bacterial genome?

Answer 8

Circular chromosome (usually one)
No nucleus, histones, mitosis, meiosis
Genome is highly dense in coding sequences (lack introns)
Number of self replicating plasmids

Question 9

What is sequencing used for?

Answer 9

• understanding how genomes work
• identification of pathogenicity and resistant genes
• epidermiology

Question 10

What is the core genome?

Answer 10

the group of essential genes and other genes found in all representatives of the species

Question 11

What is the accessory genome?

Answer 11

genes present in some members of a species but not all e.g antibiotic resistance on some plasmids

Question 12

What are the 3 stages of chromosome replication in bacteria?

Answer 12

Initiation
Elongation
Termination

Question 13

Describe how the initiation occurs in replication of the bacterial chromosome

Answer 13

Initiation:

• replication is always initiated at the same segment of DNA: oriC which is fixed to the cell membrane
• initiation is started by the accumulation of initiation protein DnaA to oriC
• DnaA melts the `DNA then DnaB is recruited (a helicase) to unwind DNA
• DnaB opens replication forks in both directions (bidirectional replication)

Question 14

Describe elongation in chromosome replication

Answer 14

the process of elongation involves the movement of replication forks and synthesis of new DNA complementary to the melted template strands

• primase (DnaG) lays down an RNA primer and DNA polymerase III copies the DNA
• okazaki fragments allow the lagging strand to be synthesised
• DNA ligase joins Okzaki fragments, together in the lagging strand

Question 15

Describe termination in chromosome replication

Answer 15

it occurs when the replication forks reach the part of the chromosome opposite oriC at ter sites (which prevent helicase activity)
-completed chromosomes are interlinked and need to be resolved by topoisomerase to separate them

Question 16

How are bacterial genes often organised?

Answer 16

in polycistronic operons consisting of genes of related function

Question 17

What is the structure of a bacterial gene?

Answer 17

Operator
Promotor (-35, -10)
Transcription start site
Ribosome binding site
Gene A
Gene B
Gene C
Terminator

Question 18

How does bacteria control gene expression?

Answer 18

by controlling transcription

Question 19

In what ways can you influence the rate of transcription?

Answer 19

• promoter structure (strength)
• operators/enhancers
• operons/regulons
• transcriptional regulators (+ve and -ve)

Question 20

What are promotors?

Answer 20

Upstream of a genes coding sequences that are binding sites for RNAP
Consist of -10 and -35 sites
Binding is via sigma factor

Question 21

What does the sequence of a promoter region influence?

Answer 21

How well RNAP binds (binding strength) and the rate of transcription

Question 22

How do transcriptional regulators effect the rate of transcription?

Answer 22

-by helping/hindering the binding of DNA to turn genes on or off in response to environmental change

Question 23

What is a regulon?

Answer 23

When genes with similar functions and therefore similar sequences can be turned on or off together

Question 24

Why do bacteria evolve fast?

Answer 24

• rapid generation rates
• very large populations
• programmed mutations

Question 25

How does evolution occur?

Answer 25

when an alteration to the genome occurs, followed by selection

Question 26

What do DNA changes result from?

Answer 26

• mutation

- acquisition of new DNA

Question 27

What is a programmed mutation?

Answer 27

When some genes are turned on or off at high frequency

genes known as phase variable

Question 28

What are the mechanisms of phase variation?

Answer 28

• genomic variation
• strand slippage
• methylation

Question 29

What are the three mechanisms that DNA can be exchanged by? (horizontal gene transfer)

Answer 29

conjugation
transduction
transformation

Question 30

What are the two ways that transposition can occur?

Answer 30

• copy and paste mechanism (class II transposition) sequence coped to RNA and then into DNA (by reverse transcriptase) and is inserted elsewhere
• cut and paste mechanism (class II transposition) transposon cut out of DNA and inserted elsewhere by resolvase

Question 31

How are transposons related to antibiotic resistance?

Answer 31

-capture resistance genes and transfer them between chromosomes/plasmids and therefore between species/strains

Question 32

How does transduction occur?

Answer 32

• phage infects bacterial cell
• DNA injected and inserts randomly in host chromosome
• when lytic cycle s initiated phage DNA excised +/- some host DNA
• new phage particles synthesised
• bacterial cell lyses releasing particles
• infect new cell and may carry DNA from previous host

Question 33

What is transformation?

Answer 33

-uptake of naked DNA from the environment by competent bacteria

Question 34

How does conjugation occur?

Answer 34

1. Plasmid replicates in the cytoplasm
2. F pili promote cell-cell contact between 2 bacteria of the same generation
3. A the new copy of the plasmid, generated by replication is transferred to a recipient cell along the conjugation tube
4. A second copy of plasmid remains in the donor cell