Horizontal Gene Tranfer

Overview of Horizontal Gene Transfer (HGT) in Bacteria

This note explains how bacteria change and evolve through a process called Horizontal Gene Transfer (HGT), which is different from how most living things pass on genes.

How Genes Transfer:

  • Vertical Gene Transfer (VGT): This is the usual way genes are passed down. A parent cell makes an exact copy of its DNA and gives it to its offspring (daughter cells) when it divides. Think of it like inheriting traits from your parents.

  • Horizontal Gene Transfer (HGT): This is different. Bacteria can get completely new DNA from other sources, not just from their parent cell. It's like a cell suddenly gaining a new skill or feature that wasn't in its original blueprint.

How a Cell's DNA Changes:

  • Mutation: This is when small errors or changes happen within a cell's existing DNA. It's like a typo in a book that's already written.

  • Horizontal Gene Transfer (HGT): This is when a cell brings in new pieces of DNA from outside, adding brand new information to its genome.

Main Ways HGT Happens:

  1. Transformation: Bacteria pick up "naked" DNA (DNA that's floating freely) from their surroundings. Imagine a cell vacuuming up loose genetic material.

    • For this to happen, a cell needs to be competent, meaning it's ready and able to take in outside DNA. Some bacteria are naturally competent, while others can be made competent in a lab.

    • Competent cells create a special transporter called a Transformasome to pull in single strands of DNA.

  2. Conjugation: This is like bacterial "sex" or direct sharing. Two bacterial cells make physical contact, and one cell shares DNA with the other.

    • A conjugation pilus (a small tube-like structure) forms between the donor and recipient cells.

    • While entire chromosomes are rarely transferred, plasmids (small, circular pieces of DNA that carry extra genes) are commonly moved this way.

    • F+ cells are donors with the F factor plasmid, F- cells are recipients without it, and Hfr cells have the F factor integrated into their main chromosome.

  3. Transduction: Viruses that infect bacteria, called bacteriophages (or phages), accidentally carry bacterial DNA from one bacterium to another.

    • Generalized Transduction: A phage mistakenly packages random pieces of bacterial DNA into its virus shell instead of its own viral DNA.

    • Specialized Transduction: This happens when a phage's DNA (called a prophage) is integrated into the bacterial chromosome. When it leaves the chromosome, it sometimes takes adjacent bacterial genes with it.

What Happens to the New DNA?

  • For the new DNA to stick around, it either needs to have its own way to copy itself (like a plasmid) or it has to join the cell's main genome through a process called recombination.

    1. Homologous Recombination: This requires the new DNA to have similar sections (sequences) to the existing bacterial DNA. A protein called RecA helps these similar pieces find each other and combine.

    2. Site-specific Recombination: This happens at very specific short DNA sequences, using special enzymes to insert the new DNA, like how a prophage inserts itself.

Mobile Genetic Elements (The "Jumping" DNA):

  • Bacteria often have a core set of genes, but HGT adds accessory genes (extra genes) that make them unique. These extra genes are often found on Mobile Genetic Elements, which are pieces of DNA that can move around.

  • These include: Phage DNA, Plasmids, Transposons, and Genomic Islands.

Transposons:

  • These are "jumping genes"—segments of DNA that can move from one place to another within the same genome, or even jump to other pieces of DNA like plasmids.

  • They usually contain a gene for an enzyme called Transposase (which helps them jump) and special inverted repeat sequences at their ends.

  • Composite transposons also carry other genes, like those for antibiotic resistance.

R-factors:

  • These are special types of plasmids that carry genes for antibiotic resistance (making bacteria immune to certain drugs). They often have their own genes for conjugation, meaning they can easily spread resistance to other bacteria.

Why This Matters:

Horizontal Gene Transfer and these mobile genetic elements are super important because they help bacteria evolve quickly, share new traits (like antibiotic resistance), and adapt to different environments. Understanding these processes is crucial for fighting infections and other applications in microbiology.