Genetic Recombination Flashcards
Why is variation good
It helps an organism survive because of natural selction
Genetic recombination
Cutting and pasting DNA backbones
Genetic recombination requires
At the simplest 2 unique DNA strands, something to bring DNA close, and enzymes to cut, exchange, and paste DNA
Sugar phosphate backbone
Winds around base pairs connected by weak hydrogen bonds, It is held together by strong covalent bonds
Homologues
Similar but not identical, allows for precise recombination
Recombination general process
Homologues regions pair,
Enzymes break 4 phosphate bonds
Free ends are exchanged in DNA, then recombined
There are x backbones
4/every recombination event
Recombination in bacteria
Uses DNA from other cells because it produces asexually
How is E.Coli la grown
In a medium containing water, an organic carbon source, and an inorganic salts (1 including a nitrogen base), agar
Cultures
Lots of clone bacteria. Made by spreading bacteria on the agar gel so a colony can form
Prototrophs
Strains of ecoli that can make amino acids
Auxotrophes
Mutant strains that cannot make amino acids. Cannot make arginine
ArgA
Gene that governs if a cell can make arginine
E. coli study
Proto and auxotroph’s were placed near each other and were observed for DNA mixing
2 strains of DNA from x-ray mutating
They were extracted from replica plating. Fount that colonies grew when nothing was present, suggesting the 2 parental types combined to create healthy bio+ met+ leu+ thr+ thi+
1st strain
Needed biotin and the amino acid methionine to grow
bio- met- leu+ thr+ thi+
2nd strain
Needed the amino acids leucine and threonine and vitamin thiamine. bio+ met+ leu- thr- thi-
Mutate back
Didn’t happen when the cells were alone therefore it was unlikely to have occurred when the cells were together
How did the cells DNA mix
Conjugation
Conjugation
Cells contact each other along the sex pilus and form a cytoplasmic bridge.
Copied part of the DNA flows through and non directional transfer and gene regulation occurs
The F factor and conjugation
Initiated by a small bacteria with small circle DNA called plasmids
Fertility plamsid (vertical interference)
Carries genes and a replication origin allowing DNA copies to be passed on, proteins that encode for the sex pili
Horizontal interference
An F factor is copied and passed on to a recipient cell
F+
Donor cell (containing the F factor). Can mate with each other
F-
Recipient cell that lacks the F factor
Rolling circle
How the F factor replicates DNA
Steps to rolling circle DNA replication
1 DNA strand is broken at the replication site
The strand is pulled through the cytoplasmic bridge while the other strand rolls
DNA synthesis fill in complimentary base pairings
Recombination and rolling circle
Does not occur between the 2 cells DNA
Hfr cells
Cells that donate DNA and make sex pili and conjugate with F cells
F+ DNA combination
2 DNA circles recombine to become 1 large circle
In Hfr recombination
DNA is only altered in the one specific cell, the F factor is integrated, but gene expression still occurs
Conjugate bridge
Easily breaks-Very rare for a whole chromosome to be transfered
F factor in Hfr
1/2 is the leading head of DNA, and the other 1/2 is the end of the DNA strand.
If the full strand is transferred the recipient cell becomes Hfr
Partial diploid
More common. 2 copies of the genes that came through
Enzymes in the recipient cell
Degrade the linear Hfr chromosome after recombination.
Cells produce creating clones of the new DNA
Mapping enes by conjugation experiment
Matin Hfr and F cells with different number of alleles
They were broken apart at different stages
Mapping genes by conjugation results
The longer they waited the more donor genes that entered the recipient, producing recombinants
Mapping genes by conguation application
This is how the genes for E. Coli were mapped
Transformation and transduction
Other ways SNA can be transferred, can use living or dead organisms
Transformation
Bacteria take up DNA pieces released into the environment by other cells
The transfer occurs the same as conjugation
Artificial transformation
Because not all cells can do it naturally. Uses chemical or electrical currents
Transduction
DNA transfer occurs inside the head of an infecting bacterial virus
Bacteriophages
Cycle of viruses that infect bacteria
Transduction process
Phages assemble in a cell
Viral/hosts DNA is incorporated and the host is killed
Phages take over the cell and spread their DNA the same way conjugation does
Bacterial DNA from a host cell
May be altered but does not kill the cell it enters
2 types of transduction
Arise from different infection cycles. General and specialized transduction
General transduction
All donor genes are equally likely to be transferred. Associated with virulent phages
Host chromosomes destroyed for building of new chromosomes
Virulent phages
Kill the host cell every lytic (infection) cycle
If some genes are missed in general treansduction
It becomes a transducing phage, and manually infects cells.
The new recipient cell survives
Specialized transduction
A mistake in a different infection cycle. Involves temperate bacteriophage
Temperate bacteriophage
Determines if a host will be a host for a long time.
If temperate bacteriophage says no
the cell makes new phages and then explodes
If temperate bacteriophage says yes
A single recombination event occurs
Phphage
A state similar to the integration of an F factor. Is passed on to all daughter cells
Specialized transduction mistake
When a prophage is excreted from the chromosome.
It can be imprecise, leaving bacteria DNA behind. The host cell is not killed
Conjugation, transformation and transduction similarities
DNA from 2 bacteria cells is brought together, creating diversity
