Download Concept 18.3. How get genetic variation in prokaryotes: • E. coli is

Concept 18.3. How get genetic variation in prokaryotes:
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E. coli is the lab rat of molecular biology.
DNA is ds, circular and associated with proteins = 1mm length.
Eukaryotic DNA is linear and associated with lots of proteins.
4.6 million bases = 4,400 genes, 1/1000th DNA in Human somatic cells.
DNA fills nucleoid-dense region of DNA.
In addition have plasmids ( several dozen genes).
Divide by binary fission.
Fig. 18.14 Replication of Bacterial DNA-single origin of replication and synthesis in
both directions.
Bacteria can divide up to every 20mins. Lower in gut.
Binary fission is asexual –clones.
Mutations give rise to genetic variations.
Spontaneous mutations occur 1:10million in specific gene.
Increase in genetic diversity as reproductive rates high due to short generation
time.
Humans have long generation time therefore little effect of spontaneous mutations.
Instead, variation from recombination during sexual reproduction.
Bacteria also have genetic recombination-genetic material from 2 separate bacterial
cells. Evidence Fig. 18.15.
Eukaryotes
Meiosis and fertilization
Prokaryotes ( Bacteria)
Transformation
Transduction Fig. 18.16.
Conjugation Fig. 18.17.
Transformation:
• Bacteria have cell surface proteins that recognize and transport DNA from closely
related species into cell and are incorporated into genome.
• Environment of high Ca2+ conc. Artificially stimulates E. coli to take up DNA. Used
to get Human insulin gene, growth hormone gene into E.coli.
• Random process.
Transduction:
• Phages carry bacterial genes from one host cell to another.
• Two typesa) Generalized- Fig. 18.16. Lytic cycle, virulent phage
- Transfer of random pieces of host DNA to recipient cell packaged with phage capsid
- DNA may recombine with recipient DNA.
b) Specialized – Fig. 18.7. Lysogenic cycle, temperate phage.
- A prophage picks up a few adjacent genes as it leaves and transfers to a new host.
- Transfer only of adjacent genes.
Conjugation: Fig. 18.17.
• Direct transfer of genetic material between two bacterial cells.
• Cells are temporarily joined by a sex pilus.
• Transfer is one-way ( one donar and one recipient).
• Bacterium needs F factor (segment in DNA or plasmid) to form pilus and donate
DNA.
• Any genetic element that can replicate as part of host DNA or independently is
called an episome ( plasmids, temperate phage-lambda).
Plasmids
Small, circular ds DNA.
Self-replication
Separate from bacterial DNA
Does not exist outside cell
No protein coat
Can confer advantages to host
Phages
DNA or RNA ds/ss, non-circular
Need cellular machinery.
Can incorporate into host DNA
Can exist outside cell
Has protein coat
Can confer advantages to host
Conjugation and Recombination Fig. 18.18.
a) F Plasmid-Plasmid form of F factor.
- About 25 genes.
- Most for the production of pili
- Replicates in sync. with chromosomal DNA.
b) F+ cell- Cells containing F plasmid.
- DNA donors during conjugation.
- Binary division of F+ cells gives two F+ cells.
- Mating of F+ and F- results in only F plasmid being transferred.
c) Hfr cell- High frequency recombination cell.
- DNA donors during conjugation.
- Has F factor built-in into its chromosome.
- DNA replication initiated on specific point on integrated F factor DNA.
- Single strand of F factor DNA moves into F- cell along with adjacent chromosomal DNA.
- Movement of bacteria tends to disrupt conjugation early before whole strand of Hfr
passed to F- cell.
- The Hfr’s DNA stays the same.
- F- cell gets new DNA, temporarily diploid.
- If new DNA lines up with homologous regions of F- chromosome get exchange of DNA
segments = recombination.
- DNA outside of F- chromosome is eventually degraded.
- Recipient cell is F- as no F factor.
Also have R-plasmids- Confer antibiotic resistance.
- Have genes for pili.
- Enable plasmid transfer by conjugation.
- Up to 10 genes for resistance to 10 antibiotics !
- How did they get so many??
Transposable Elements ( “jumping genes”)
– Never exist independently.
- Part of plasmid/chromosomal DNA.
- Moves by a type of recombination from one site to “target” site.
- Move within chromosome, plasmid to chromosome, plasmid to plasmid.
- DO NOT detach from DNA.
- Sites are brought together by folding.
- Cut/paste or copy/paste.
- Two types: Insertion sequences and Transposons.
a)
b)
-
Insertion Sequences-Fig. 18.19a.
Simplest transposable elements.
Only in bacteria.
Single gene coding for transposase.
Transposase catalyses movement of insertion sequence.
On either side are pair of noncoding DNA ( 20-40 bases) = inverted repeats.
Enzyme molecules recognize these as boundaries of insertion sequences and bind
inverted repeats and to target site and catalyze cutting and resealing.
If sequence goes into coding region of a gene or region required for regulation then
mutation results.
1 every 10 million generations. Same as for other sources of mutations.
Make up 1.5% of E. coli genome.
No real benefit to bacteria.
Transposons-Fig. 18.19b.
Longer and more complex cf insertion sequences.
Include extra genes ex. Antibiotic resistance.
Some have insertion sequences on either side.
Help bacteria adapt to new environments.
This is how R plasmids have multiple antibiotic resistance genes.
Important in Eukaryotes too! – retrotransposons ( use an RNA intermediate Fig.
19.16).
Alu elements family make up about 10% of Human and other primate genome.