Lecture 7 Flashcards
What is vertical gene inheritance?
Vertical gene inheritance occurs via the asexual reproduction via cell division (parental genes get passed on), this happens through duplication of the initial chromosome, continued growth of the cell and then division.
What is horizontal gene transfer? What are three example methods?
Horizontal gene transfer (lateral gene transfer), organism acquires genes directly from another cell and incorporates them into its genome, this is responsible for the spread of fitness-enhancing traits e.g antibiotic resistance, and provides an awesome mechanism for ongoing adaptive evolution. This process can occur via transformation (uptake of naked DNA into a competent recipient cell), conjugation (DNA transfer through cell-to-cell contact mediated by a plasmid or transposon) or transduction (DNA transfer is mediated by a bacteriophage.
How do the three horizontal gene transfer methods occur?
Transformation occurs via: cell lysis leading to free DNA fragments, these fragments get taken up by a new bacterium and then recombination occurs to incorporate the DNA into the chromosome.
Conjugation occurs via: F factor plasmid integrates with chromosome in order to allow part of the genome to transfer via single strand, recipient bacteria then takes this into its chromosome.
Transduction occurs via: bacteriophage infects bacterium, new virus particles are made and cell lysis occurs, a few viruses will have fragmented chromosome from the host cell and will infect the next cell with its bacterial DNA, which will then be incorporated via recombination.
What are the key points shared by all horizontal gene transfer methods?
Gene transfer is unidirectional in bacteria (father to mum), from donor to recipient cell, only part of the donor genome is transferred, in most casses the transferred DNA consists of linear fragments that cannot be replicated autonomously. Consequently, for transferred genes to be stably inherited they must be recombined, permanently changing the recipient.
How was transformation found and how was the transferred part found?
Transformation was found by Griffith’s experiments: adding dead smooth strain bacteria to R strain bacteria(which could not kill) and transferring them to a live mouse afterwards, the mouse died. The Avery-Macleod-McCarty Experiment found it was the DNA which facilitated this, as all other parts were removed.
Does transformation occur in all bacteria, what does it depend on and what is important in many bacteria for this?
Transformation occurs naturally in some bacteria, but not all. It is dependent on specialised cell state called competence. In lab cultures, part of culture becomes competent at a specific growth stage for a short period. In some cases the DNA uptake is highly-specific to ensure only certain bacterial genomes are incorporated through common specific base pairs, this can be important because transformation is by homologous recombination, replacing the original host DNA.
How does a cell get into a competence state?
A pilus reaches out and grabs the DNA during transformation, it is a single strand which gets coded by the recA protein into a recombinogenic form which will then insert itself in the DNA. This competence is dependent on extracellular concentration of a small secreted peptide, this can also be induced by stresses like antibiotic treatment. This allows sensing of population density to know when more potential donor cells are available. The competence state also triggers lysis and DNA release for other cells in the environment.
How does transformation help aid against the immune system?
Capsules can be changed via transformation (this is what our immune system detects), this change can aid in immune evasion and generation of “vaccine-escape” strains and antibiotic resistant strains in vivo. Interspecies transformation is responsible for the appearance, in the penicillin-resistant isolates, of mosaic pbp genes, which encode proteins with reduced affinity for penicillin.
