MICR2000 Module 3 L1: Gene transfer between bacteria

How are new genes so easily introduced into bacterial genomes without disrupting processes

bacteria are able to accept new genes bacause of how easily they are able to divide and grow.

how do bacteria spread their antibiotic resistance

Conjugation, transposition, integrons, gene cassettes

Plasmids

circular pieces of DNA which are the reason bacteria can spread resistance so quickly - involved in conjugation

transposable elements

dna sequences which can jump from one position to another or from one dna molecule to another - the two major types are transposons and insertion sequences

Insertion sequences

smallest and simplest transposable elements. Has a gene encoding the transposase protein. DNA which contains the info to cut and copy itself in a new location

transposons

made up of an insertion sequence+extra genes. these extra genes are often made of antibiotic resistance genes. they can insert into chromosomal DNA or plasmids. if one is inserted into a conjugative plasmid (one which can be transferred between bacteria) then the transposon can also be spread.

what are the two mechanism of transposition

Conservative transposition: Cut/Paste - transposable element is excised from one location and reinserted. Replicative Transposition: copy/paste - new copy of transposon is produced and inserted in new location

conservative transposition (e.g. IS1)

• Transposase makes staggered nicks in target DNA.

• The IS element integrates into the nicked site.

• Host DNA polymerase and ligase fill the gaps.

• This generates direct repeats of the target sequence flanking the inserted IS element.

replicative transposition (e.g., Tn3).

Replicative transposition makes a copy of the transposon instead of moving it.
In Tn3:

1. Transposase links donor and target DNA → forms a cointegrate.

2. DNA replication duplicates the transposon.

3. Resolvase separates donor and target DNAs.
   👉 Both DNAs end up with a copy of Tn3.

How do transposons acquire multiple antibiotic resiatance genes

By replicative transposition and recombination — transposons can pick up resistance genes when they move between plasmids and chromosomes, and can integrate additional resistance cassettes or other transposons, creating composite transposons with multiple genes.

Integron

It is found within a transposons and is a genetic unit capavbe of the capture and expression of genes that are contained in mobile elements called gene cassetes. It also provides a promoter - thus acting as natural cloning and expression vector for gene cassettes

Essential features of integrons

Attachment site - recognised by integrase and acts as acceptor site for cassettes.
A site specific recombinase enzyme (cuts and rejoins specific dna sequences) which is responsible for inserting/removing gene cassettes as the chosen region

Promoter - drives expression of inserted sequence

Approaches to reduce ths spread of antibiotic resistance

stop innapropriate use of antibitotics to reduce selective pressure for antibiotic resistance. remove ineffective antibiotics from use.