Extrachromosomal replication and plasmids Flashcards
How many eukaryotic species are present on the planet ?
Around 8.7±1.3 million eukaryotic species exist on the planet, most of which have never been cultivated, many of which are still unknown.
How many prokaryotic species are present on the planet ?
10^7 - 10^9
How many bacterial species does a gram of soil contain ?
Up to 53,000.
How many species does 1L of sea water contain ?
20,000
How many microbial species does the human body carry ?
About 10,000.
How many cells Vs bacteria make up the human body ?
37.2 trillion human cells vs. 10 times as many bacteria
What are the two main proposed scenarios for the evolution and eukaryotes and prokaryotes ?
The 2 domain Vs 3 domain model:
i. Classical view-three primary domains (left): Archea & Eukarya have common anchestor (undetermined).
ii. Two primary domains (2D) scenario (right): Archaea and the Bacteria are the two primary domains, whereas the Eukarya is a secondary domain that arose from the merging of an archaeon and a bacterium.
What is horizontal gene transfer ?
Gene transfer that is divorced from reproduction.
What are the 3 main mechanisms of horizontal gene transfer ?
Define these mechanisms.
Transformation : uptake of short fragment of naked DNA by transformable bacteria
Transduction : transfer of DNA from one bacterium into another via bacteriophages (λ-phage)
Conjugation : transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells
Who were the 2 scientists that discovered conjugation ?
When did this happen ?
J. Lederberg and E. Tatum in 1946
What experiment allowed the discovery of conjugation ?
Lederberg and Tatum plated bacteria into dishes containing only unsupplemented minimal medium. Some of the dishes were plated only with strain A bacteria, some only with strain B bacteria, and some with a mixture of strain A and strain B bacteria that had been incubated together for several hours in a liquid medium containing all the supplements. No colonies arose on plates containing either strain A or strain B alone, showing that back mutations cannot restore prototrophy, the ability to grow on unsupplemented minimal medium. However, the plates that received the mixture of the two strains produced growing colonies at a frequency of 1 in every 10,000,000 cells plated (in scientific notation, 1 × 10−7). This observation suggested that some form of recombination of genes had taken place between the genomes of the two strains to produce prototrophs.
Which scientists ruled out the theory of “cross-feeding” between bacteria and how ?
Bernard Davis constructed a U-tube in which the two arms were separated by a fine filter. The pores of the filter were too small to allow bacteria to pass through but large enough to allow easy passage of the fluid medium and any dissolved substances. Strain A was put in one arm; strain B in the other. After the strains had been incubated for a while, Davis tested the content of each arm to see if cells had become able to grow on a minimal medium, and none were found. In other words, physical contact between the two strains was needed for wild-type cells to form. It looked as though some kind of gene transfer had taken place, and genetic recombinants were indeed produced.
Can gene transfer in bacterial conjugation occur in both directions ?
No !
It can only occur in one direction, with 1 donor cell and an recipient cell.
Why is the analogy between bacterial conjugation and sexual reproduction inaccurate ?
Gene transfer is not true sexual reproduction, where two organism donate equally. Instead during the bacterial gene transfer the recipient receives genetic information from the donor and is thereby changed.
How is the fertility of E. Coli regulated ?
The fertility of E. coli can be lost and regained rather easily and the donor ability is itself a hereditary state imposed by a fertility factor (F).
Donor = F+ = male
Recipient = F- = female
Consequence : F+ do not attach to F+
Which scientists introduced the term plasmid ?
What does it mean ?
The term “plasmid” was introduced 1952 by Joshua Lederberg as a generic term for any extrachromosomal hereditary genetic particle.
What are episomes ?
Episomes are special form of plasmid that can also traffic in and out of chromosomes.
Can plasmids be found in archea ?
In eukaryotes ?
Plasmids can be found in all domains of life: eubacteria (true bacteria), archea, yeasts (eukaryotes)
What is the F episome ?
The F episome is an episome harboring the F factor.
It was the first episome ever discovered.
How big is the F episome ?
100kb (5% of E. Coli genome size)
What are the two origins of replication of the F episome ?
oriV: used when plasmid is “free” – one copy per bacterial chromosome
oriT: cis-acting element located at the beginning of the transfer region and the location where the transfer of the F plasmid is initiated
How big is the transfer (tra) region of the F episome ?
How many genes are contained in this region and what are their function ?
Tra region ~ 33kb, 40 genes
Most of these genes are involved in DNA transfer and replication. At least 12 genes are required for modification and assembly of pilin in the sex pilus, and the stabilization of the Type 4 Secretion System (T4SS).
