Genetic transfer and linkage mapping in bacteria: Important concepts

Question 1

Getting the phenotype you want - Screen for phenotype

Answer 1

• Everything grows and colonies with desired phenotypes are picked
• However, difficult due to sheer numbers

Question 2

Selecting for Phenotypes

Answer 2

• Only bacteria with desired phenotype grows
• e.g. select for Arg+ bacteria on minimal plates

Question 3

Genetic Linkage

Answer 3

• Alleles that tend to co-inherit during independent assortment in gametes

deviation from predicted ratio (9:3:3:1) in punnet square = genes are linked

Question 4

Relationship between homologous recombination + linkage

Answer 4

• During meiosis, the 2 diff chromosomes in an eukaryote recombined at a holiday junction
• creates 2 new strands of DNA different from both parent chromosomes

Question 5

Distance + homologous recombination

Answer 5

• The closer 2 genes are together, the more linked they are and the more likely they are coinherited

Question 6

Bacteria’s recombination issues

Answer 6

• Only 1 chromosome - all genes are linked
• Haploid so reproduction is asexual
• Chromosomes are circular so issues with topology

Question 7

Homologous recombination with circular chromosomes

Answer 7

• Must have even number of recombination events so the chromosome stays a circle
• Is a little less common as 2 evens has to occur for DNA to be viable

Question 8

Conjugation - genetic exchange

Answer 8

• Transfer of genes through cell-cell contact

Question 9

Transduction - Genetic exchange

Answer 9

Transfer of genes through bacteriophages (viral) vector

Question 10

Transformation - Genetic exchange

Answer 10

Direct uptake of DNA

Question 11

Conjugation - Conjugable plasmids

Answer 11

Plasmids are small circular piece of DNA
- Can contain other genes for things like antibiotic resistance

• Codes for a p

Question 12

Integration of an F-plasmid onto the chromosome

Answer 12

• Results in an Hfr strain (high freq. conjugation) + the result of homologous recombination with host bacteria + f plasmid
• This occurs with IS elements (transposons that leave parts of DNA and move about leaving a copy of itself behind)

Question 13

High freq. transfer of chromosomal genes

Answer 13

• By oriT, genes are transferred to other bacteria
• The further away from oriT a gene is, the longer before it will transfer to recipient
• Whole chromosome is not transferred but a fragment is
• Fragment inserts into DNA

Question 14

Hfr Mapping - Time of Transfer

Answer 14

The further a gene is from the oriT, the longer it will take to detect transconjugants

e.g. f plasmid transfers antibiotic resistance - when varying time is given for this + is put of a plate with antibiotic, you can tell the freq. of transfer over time

Question 14

Measure of high freq. transfer of chromosomal genes

Answer 14

in minutes
- would take 100 minutes to transfer whole chromosome
- linkage map of E. coli divides chromosomes into 100 centisomes

Question 15

Hfr mapping - gradient of transfer

Answer 15

• based on freq. of transfer, you can tell the distance from oriT
• Lower frequency = further away
• Can make a ‘map’ of minutes based on distance from oriT

Question 16

F’-Plasmids

Answer 16

F-plasmids recombine out of an Hfr chromosome and take some with it
- Excision chromosomal genes result in homologous recombination with different elements
- These F’-plasmids can generate partial diploids
- Can be used for complementation studies or genetic mapping

Question 17

Selection and counter selection in conjugation