Mutation And Bacterial Genetics Flashcards
What effect does changes within the genome have on the bacterial cell ?
Microbial diversity
Habitat adaptation
The evolutionary process is driven
How do alterations in the bacterial genome occur ?
- Mutations
- Exchange of genetic material between bacterial cells - horizontal gene transfer
What is a mutation ?
A permanent change in the nucleotide base sequence of a genome
What is a wild type strain ?
An organism isolated from the environment
What is a mutant ?
A cell derived from the wild type that carries a change in the nucleotide sequence.
The change can either be beneficial, detrimentally or have no effect on the mutant.
Mechanisms of mutation - Spontaneous mutation
Occasional errors during DNA replication
Mechanisms of mutation - Induced mutations
This is caused by agents in the environment:
- Natural radiation e.g, X-rays, gamma radiation, ionising radiation and UV light
- Exposure to certain chemicals called mutagens e.g, ethidium bromide and aflatoxin
Impacts of mutation
- Alter the product of the gene
- Prevent the gene from working correctly
- Entirely prevent gene function
- Have no effect on gene function
Type of mutation - Point mutations (base substitution)
This is when a single base pair in the DNA is affected.
- Will change the genotype
- Will not always change the phenotype depending on where the mutation occurs — will only affect if it happened in a region that codes for protein
Type of mutation - Silent mutation
This is when there is a single base pair substitution in the DNA.
- DNA sequence has chnaged but polypeptide = same .
- Phenotype = the same.
Type of mutation - Missense mutation
This is when there is a single base pair substitution in the DNA.
- DNA and polypeptide sequence change
- Protein = inactive or have reduced activity
Type of mutation - Nonsense mutation
This is when there is a single base pair substitution of DNA
- Stop codon formed
- Premature termination of translation
- Truncated proteins (shortened)
Frameshift mutations
This is due to a deletion or addition of a base(s).
= alteration in the way in which nucleotides are read
= alternation of the gene product
What is a plasmid ?
A circular piece of DNA that carries extra genes such as antibiotic resistance genes, or metabolic genes
What is a transposon?
A small piece of DNA that jump around the chromosome or transfer from bacterium to bacterium
What is horizontal gene transfer?
The movement of DNA between cells that are not directly related.
- Donor cell contributes to part of its genome.
- Recipient cell receives the DNA
- Received DNA may be degraded but often incorporated into the recipient cell’s chromosome by recombination
- Recombinant cell formed
Mechanisms of horizontal gene transfer - Transformation
The process by which free DNA is incorporated into a recipient cell’s chromosome and brings about a genetic change.
Step 1 of Transformation
Competent bacteria reversible bind DNA to their surface through DNA binding protein - becomes irreversible. DNA binding protein drives the DNA through the Gram-negative outer membrane or Gram-positive wall
Step 2 of Transformation
- Either dsDNA or ssDNA enters the cell.
- Competence specific proteins bind to their surface through donor DNA - protects against other degrading nucleases
- DNA integrates into the recipient chromosome by recombination
Step 3 of Transformation
Some bacterial cells can be treated to become artificially competent
- Chemically
- Electrically
- Temperature
Mechanisms of horizontal gene - Transduction
The bacteriophage mediated transfer of DNA from one cell to another.
- Generalised transduction — any gene from the donor chromosome can be transferred to a recipient
- Specialised transduction — selective transfer of small, specific sections of the chromosome
Process of transduction
- Bacteriophage injects its DNA into host bacterial (donor) cell
- Phage enzymes degrade the host cell
- Cell synthesises new phages & some host DNA mistakenly enters into phage.
- Transducing phage injects donor DNA.
- Donor DNA is incorporated into recipient’s chromosome by recombination
Mechanisms of horizontal gene transfer - Conjugation
A plasmid enclosed mechanism that mediates transfer of DNA between closely related or more distantly related cells.
— a conjugation pilus connects the donor and recipient cells allowing the transfer of DNA
Process of conjugation
- Donor cell attaches to a recipient cell with its pilus
- Pilus may draw cells together
- One strand of ‘F plasmid DNA’ transfers to the recipient
- The recipient synthesises a complementary strand (by rolling circle method of DNA replication) to become a F+ cell with a pilus. The donor cell synthesises a complementary strand, restoring its complete plasmid.
High frequency of recombination cells
The F plasmid integrates into the bacterial chromosomes with the cells being called Hfr.
Hfr cells conjugate with F- cell:
- Part of F plasmid, followed by a chromosome
- Pilus breaks before transfer is complete
- Recipient is composed of a portion of F plasmid, some chromosomal genes
- Recombinant remains F-
Hfr conjugation
- F+ cell has donor chromosome and pilus
- F plasmid integrates into chromosome by recombination
- Cells join via conjugation pilus - F+ cell (Hfr) recipient
- A portion of F plasmid partially move into recipient cells = strand of donor’s DNA trailed.
- Conjugation ends with pieces of F plasmid and donor DNA in recipient cell — cells synthesise complementary DNA strands
- Donor DNA and recipient DNA recombine, making a recombinant F- cell.
Structure of a simple transposon
- There are different types of transposon but all have a palindromic sequence at both ends
- A simple transposon has inverted repeats and a transposase enzyme.
- Complex transposons carry other genes not connected with transposition e.g, antibiotic resistance genes
Why is gene transfer important ?
Aids the movement of:
- Antibiotic resistance genes
- Metabolism genes - when adapting to a new energy source
- Virulence genes
