bacterial transformation
Post on 05-Jan-2016
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Bacterial transformation
All living things use the same genetic code
Each codon corresponds to a specific amino acid, regardless of the species
We can take a gene from one species and insert it into a different one and still get the same protein (same amino acid sequence)
the genetic code is universal
In addition to the nucleoid DNA, E. coli bacteria contain small circles of DNA called plasmids;
plasmids
http://en.wikipedia.org/wiki/Plasmid
A plasmid is a small, circular piece of double-stranded DNA
plasmids
Often, the genes carried in plasmids provide bacteria with genetic advantages, such as antibiotic resistance.
Plasmid DNA contains coding sequences (genes) which are expressed by the bacterium (the bacterium produces the corresponding proteins ;
The cell that receives the piece of DNA (plasmid) is called transformed cell
Bacterial Transformation:
The introduction of a piece of DNA, like a plasmid, into a bacterial cell
Transformation rarely occurs naturally;
By subjecting bacteria to certain artificial conditions, we can enable many of them to take up DNA;
When bacterial cells are in a state in which they are able to take up DNA, they are referred to as competent
Competent cells
a plasmid contains:
• an origin of replication
• a gene for resistance to an antibiotic
• Color marker gene
• a sequence called polylinker (inside the coding sequence of the color marker gene)
Color marker gene
Plasmid: origin of replication
When a bacterium divides, all of the plasmids contained within the cell are copied;Each daughter cell receives a copy of each plasmid;
Plasmid: gene for antibiotic resistance
This gene is useful to “select” the transformed cells (cells that contain the plasmid)
Bacterium without plasmid
Bacterial DNA
Bacterium with plasmid
(plasmids contain gene for antibiotic resistance)
In the presence of antibiotic ……..
In the presence of antibiotic ……..
Selection
Transformed cells contain the plasmid with ampicillin resistance gene
Non-transformed cells do not contain the plasmid
(agar plate)
How ampicillin works?
Ampicillin is a member of the penicillin family of antibiotics;
Like other antibiotics, it works by keeping a bacterium from building a cell wall;
Without the cell wall, the bacterium cannot live (the membrane bursts)
Ampicillin (like other penicillin antibiotics) contains a chemical group called a beta-lactam ring;
Bacteria build cell walls by linking molecules together: beta-lactams block this process.
Beta-lactam ring
The ampicillin-resistance gene encodes for a protein called beta-lactamase;
This is an enzyme that destroys the activity of ampicillin by breaking down the beta-lactam ring.
The ampicillin (penicillin)-resistance gene
Penicillin resistance
Thus, bacteria expressing beta lactamase gene can resist the effects of ampicillin and other beta-lactam antibiotics (penicillin);
These bacteria can grow in the presence of ampicillin
The beta-galactosidase gene (sometimes called lacZ gene) encodes a protein, called beta-galactosidase;
This is an enzyme that normally cleaves the disaccharide sugar lactose into its two constituent sugars, galactose and glucose.
Color marker gene: beta-galactosidase
lactose galactose + glucose Beta-galactosidase
Plasmid containing beta-galactosidase gene
Color marker gene = beta-galactosidase
However, beta-galactosidase can also cleave a synthetic
analog of lactose called X-gal;
X-gal is colorless, but when it is cleaved by beta-
galactosidase, one of the products is dark blue;
Color marker gene: beta-galactosidase
X-gal identify cells with beta-galactosidase
When bacteria expressing beta-galactosidase are grown on a agar plate containing X-gal, the enzyme digests X-gal and produces a blue compound;
The colonies will be bright blue
If the bacteria do not produce beta-galactosidase, the colonies will be white
Origin of replication
a gene for resistance to an antibiotic (ampr)
Color marker gene (beta-galactosidase)
a sequence called polylinker
pBLU
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