Bacterial Genetics

Question 1

Phenotype

Answer 1

Identifiable characteristics of a cell that can be altered by mutation
- results from gene expression

Question 2

Genotype

Answer 2

Sequence of nucleotides in the DNA of a cell (genes)

- bacteria are haploid, so every mutation is automatically expressed

Question 3

Mutant

Answer 3

Bacteria with altered genetic sequence

- may or may not have altered phenotype

Question 4

Isogenic mutant

Answer 4

Bacteria with only a single gene difference compared to parent strain
- could lead to antibiotic resistance

Question 5

Bacterial chromosome

Answer 5

Usually a single chromosome, double stranded DNA, circular, covalently closed, and extensively coiled

• has an origin of replication
• not associated with histones, no nuclear membrane
• appears as an amorphous, less densely packed region in the cell (nucleoid)

Question 6

Gene organization

Answer 6

• genes have no introns
• several genes can be expressed on a single mRNA molecule –> operon
• gene expression is regulated by specific sequences and gene products
• sets of genes and operons are coordinately regulated in response to the environment

Question 7

Plasmids

Answer 7

Smaller DNA molecules that are separate and replicate independently from the chromosome

• closed circular, supercoiled, double stranded DNA molecules, similar to chromosome except smaller
• each has its own origin of replication and replicates independently of the chromosome
• plasmids typically carry genes that ensure copies are maintained in each daughter cell as the parent divides
• do not carry the genes needed for survival!*

Question 8

How many plasmids are found?

Answer 8

Depeding on plasmid, each cell can contain one copy, a few, or many
- a bacterial strain can carry more than one type of plasmid

Question 9

What type of genes do plasmids carry?

Answer 9

Genes that offer a selective advantage for the host bacteria under certain environmental conditions (virulence genes, antibiotic resistance)

Question 10

Bacteriophage

Answer 10

Bacterial virus

- similar to eukaryotic viruses, can replicate in lytic phase or enter quiescent lysogenic phase

Question 11

Lytic phase

Answer 11

Bacteriophage directs the host cell to manufacture copies of the phage, and the host cell is lysed

Question 12

Lysogenic phase

Answer 12

Bacteriophage inserts in host DNA

• can be subsequently induced to enter lytic phase
• virus lives in bacteria until the right conditions occur for lysis

Question 13

Transposons

Answer 13

Jumping genes, genes that can move from one location in bacterial DNA and insert randomly in the same or a different DNA molecule

• can be carried in chromosome or plasmid (can jump from one to the other) –> becomes permanent once in the chromosome
• when transposition occurs, the gene removes itself and randomly inserts in either the same DNA molecule or a different one

Question 14

Insertion sequence

Answer 14

Simple form of a transposon

- can cause an insertion mutation if it inserts in a gene

Question 15

Composite transposons

Answer 15

Carry genes that offer a selective advantage

• ex: antibiotic resistance
• typically 2 transposons that flank a larger sequence

Question 16

Integrons

Answer 16

Mobile DNA elements with the ability to capture genes, notably those encoding antibiotic resistance, by site-specific recombination

• a single integron can capture many antimicrobial resistance genes
• integrons can be carried in the chromosome (permanent) or plasmid
• will only capture genes with a certain sequence

Question 17

Features of bacterial reproduction

Answer 17

• asexual reproduction: progeny are clones
• short generation time
• haploid: mutations have immediate effect

Question 18

Mutations

Answer 18

• some are silent
• some are lethal
• some cause a detectable phenotypic change: nutrient requirement (auxotrophy), failure to grow under certain environmental conditions, adaptation to new environment, resistance

Question 19

How do bacteria adapt when they reproduce asexually?

Answer 19

Have the ability to control gene expression in response to their environment

• genes are located close together and do not have interrupting sequences (introns)
• no nuclear membrane (transcription and translation occur simultaneously)
• adapt to their environment (salvage nutrients when possible)
• random mutations and short generation time
• exchange of DNA between bacteria

Question 20

Evidence for DNA exchange between bacteria

Answer 20

• genomic sequencing of bacteria has revealed large differences in gene makeup even within bacterial species
• genomic islands: clusters of genes, located near phage remnants or other evidence of lateral exchange that have different G+C content or codon usage than the rest of the chromosome

Question 21

Transformation

Answer 21

Uptake of naked DNA

- natural transformation occurs only in a few bacterial species

Question 22

Transduction

Answer 22

Transfer of DNA by bacteriophage
- sometimes bacteriophage can carry chromosomal DNA from an infected bacterium and transfer it into an uninfected bacterium

Question 23

Conjugation

Answer 23

Transfer of plasmid DNA (can sometimes chromosomal DNA) by mating of 2 bacterial cells

• DNA is transferred from a donor bacteria to a recipient
• host range: some plasmids are narrow (1 bacterial species or family), some are broad (large number of species)

Question 24

How is exchanged DNA maintained?

Answer 24

Exchanged DNA must have its own origin of replication that is recognized by the host strain (plasmid) OR, it must be inserted into the chromosome

• transposition (transposons only)
• lysogeny (phage only)
• integron integration
• homologous recombination (allelic exchange)

Question 25

Allelic exchange

Answer 25

- bacteria have recombinases (RecA) that mediate this process - homologous recombination requires a similar sequence already in the chromosome

Question 26

What are 4 mechanisms of getting things into the chromosome?

Answer 26

- lysogeny - transposons - integrons - homologous recombination

Question 27

What are the 4 major forms of extrachromosomal DNA

Answer 27

- plasmids - bacteriophage - transposons - integrons