BIOS 44 - Bacterial Genetics

Lecture 7

Bacterial Diversity

When bacteria divide, genetic changes can cause diversity in the bacteria. This can happen for various reasons;

1. Genetic mutations

2. Gene transfer

3. Natural Selection

Bacterial Mutation

Random DNA changes that introduce new traits to the bacteria, can be beneficial or harmful

3 Types of Bacterial Mutations:

1. Spontaneous

   • Base pair mutations

     1. Missense

     2. Nonsense

     3. Silent

   • Frameshift Mutations

     • Addition of a nucleotide

     • Deletion of nucleotide

2. Induced

   • External influences

   • Mutagens (chemicals, radiation, etc.)

Base Pair Mutations

Base pair mutations occur when a base in a DNA strand changes, or is altered. In the wild type DNA, the amino acid remains the same- along with the complementary strand.

There are 3 Types of base pair mutations:

1. Silent Mutation

   • The base changes, but the amino acid remains the same. This is due to codon degeneracy- many codons can code for the same amino acid, and this provides some protection against mutations and such.

2. Missense Mutation

   • Missense mutation base changes result in the coding of a different/new amino acid.

3. Nonsense Mutation

   • Occurs when the mutation codes for a stop codon. The

Frameshift Mutation

Addition or deletion of nucleotides causes a shift in the DNA frame, resulting in mutation.

Induced Mutations

External influences that cause mutations in the DNA, such as exposure to chemicals, radiation, or mutagens, agents that induce mutations.

Induced mutations via mutagens:

1. Chemicals/

   • Base Analogs

     • Base analogs are chemicals that imitate the structure of normal DNA or RNA nucleic acids, but alter the function.

     • In addition to chemically mimicking bases, these mutagens incorporate themselves into the DNA; so for example, 5’-bU replaces thymine.

     • Once the incorporation/replacement is complete, the DNA replicates with the 5’-bU, and further replication cycles continue with bU pairs

2. Radiation

   • UV light

   • Ionizing mutagens

     • X-rays, gamma rays

Causes cross linking between nucleotide bases, forming covalent bonds between thymine- creating a thymine dimer. This bends the sugar phosphate backbone!

Gene Transfer

The movement of genes between organisms.

1. Horizontal gene transfer

   • Gene transfer between 2 unrelated organisms (usually observed in bacteria)

2. Vertical gene transfer

   • Parent to offspring

Methods of Gene Transfer

1. Transformation

2. Transduction

3. Conjugation

Gene Transfer - Transformation

Transformation is a method of horizontal gene transfer that occurs primarily between bacteria.

The transfer of DNA happens between 2 bacterial cells:

1. Donor cell lyses (bursts) and releases NAKED DNA

2. The competent/recipient cell receives the naked DNA, but only ONE STRAND OF NAKED DNA
   

This singular strand is then integrated into the competent cells chromosomes. This method of transfer has the potential to share antibiotic resistance DNA.

KEEP IN MIND:

The naked DNA must have an origin of replication; otherwise, it cannot replicate in the cell. It will NOT be integrated into the chromosomes.

If the DNA DOES have an origin of replication, homologous recombination occurs, meaning the DNA will now be replicated and passed onto daughter cells. All daughter cells will have a copy of this DNA.

Plasmids do not need homologous recombination

Homo= Same

Logos: Relation, reason, proportion

Recombination: “reunite again” (DNA transfer)

Plasmids are self replicating and already carry their own origins of replication (oriV) versus chromosomal origin (oriC).

Gene Transfer - Transduction

Transduction is a method of horizontal gene transfer.

Transduction occurs when a bacteriophage, a virus that infects and replicates within bacteria, accidentally pick up bacterial DNA.

First, a transducing particle is needed to begin transduction.

1. A bacteriophage attaches to a host cell, and empties the phage DNA into the cell.

2. The phage genomes cut up the bacterial DNA so it can replicate its own phage DNA instead- beginning replication.

3. New bacteriophages are synthesized inside the cell, alongside the DNA.

4. During synthesis of the new viruses, bacterial DNA can mistakenly enter a virus protein coat that carries the bacterial DNA instead of the phage DNA. This produces a transducing particle.

Transduction

1. The transducing particle containing the bacterial DNA attaches to a host cell.

2. The bacterial DNA is injected, and integrates via homologous recombination.

3. The bacteria then multiply with the new DNA.

Gene Transfer - Conjugation

Conjugation is a method of horizontal gene transfer. Conjugation is the transfer of plasmids via the sex pilus.

1. The donor cell, F+, extends the F pilus to the recipient cell, F-.

2. The plasmid unwinds at the origin of transfer, and a singular strand of the plasmid gets transferred to the cell.

3. The F- recipient cell is now an F+ cell

Transposons; Jumping Genes

Jumping genes. Cells already containing plasmids can still receive donor plasmids.

Eg. a vancomycin resistant plasmid enters the cell of a non vancomycin resistant cell via conjugation.

The V resistant gene jumps to the non resistant plasmid. Tada and then it becomes resistant