L15 - Bacteriophages & Transposons

Bacteriophages

Viruses that use bacteria as their host

Structure of Bacteriophage

• Head (capsid)

• Collar

• Sheath

• Base plate

• Tail fibres

• Tail pins

Types of infections caused by bacteriophages

• Virulent phages

• Temperate phages

Virulent phages

Phages that cause lysis of bacterial cells immediately upon infection, leading to cell death.

Temperate phages

Phages that can either cause lysis or enter a lysogenic cycle, integrating their DNA into the host genome.

Prophage

A bacteriophage genome that has integrated into the host bacterial genome, remaining dormant until it enters the lytic cycle.

Transduction

Accidental transfer of bacterial DNA by bacteriophages

Generalised transduction

The process in which a bacteriophage transfers any bacterial DNA from one host to another during infection, allowing for genetic recombination.

Specialised transduction

Transduction that only transfers specific portions of the genome in temperate phages, resulting in the transfer of particular genes during lysogenic infection.

Stages of CRISPR

1. Adaptation

2. Expression

3. Interference

Adaptation stage

The first stage of CRISPR where new DNA sequences from invading viruses are incorporated into the CRISPR array, creating a memory of previous infections.

PA sequence

A short, palindromic DNA sequence that is part of the CRISPR system, recognised and processed during the adaptation stage to guide the immune response against specific viral infections.

Expression stage

The second stage of CRISPR where the integrated DNA sequences are transcribed into RNA, forming CRISPR RNA (crRNA) that guides the targeting of specific viral DNAs.

Interference

The third stage of CRISPR where the crRNA and associated proteins form a complex that identifies and cleaves the complementary viral DNA, effectively neutralizing the threat.

Cas9

A CRISPR-associated protein that acts as a molecular scissors to introduce double-strand breaks in target DNA, guided by crRNA.

Transposons

Genetic elements that can move within the genome, often referred to as "jumping genes".

Inverted repeats

Sequences of nucleotides that are the same on both strands of DNA but run in opposite directions, often flanking transposons.

Types of transposition

• Cut & paste transposition

• Replicative transposition

Transposable elements

Segments of DNA that can change their position within the genome, including transposons and other mobile genetic elements.

Insertion sequences

Simplest transposable elements that contain only the necessary information for their own transposition. They typically consist of a short DNA sequence flanked by inverted repeats.

Simple transposition

A mechanism of transposition in which a transposable element is excised from one location and inserted into another, with no replication of the element.

2 genes involved in replicative transposition

• Transposase

• Resolvase

Replicative transposition

A method of transposition where a transposable element is duplicated during its movement to a new location in the genome. This results in the element being present at both the original and new site.

Horizontal Gene Transfer

Transfer of genes from 1 mature independent organism to another

How HGT contributes in plasmids

• Conjugation

• Transformation

How HGT contributes in phages

• Transduction

How HGT contributes in transposons

• Conjugation

• Transformation

Colistin

An antibiotic used to treat infections caused by multidrug-resistant Gram-negative bacteria and is nephrotoxic to humans.