Processes Leading to Mutagenesis

• Removal of an incorrectly inserted base is prevented.
• Tautomerization – an inserted base changes its base pairing   properties
• Base is altered and has a new base pairing property.
• A base is skipped during replication.
• MUTAGENS INCREASE THE RATE OF MUTAGENESIS

Classes of Mutagens

• Chemicals -
• Base analogs
• Intercalating Agents
• Chemicals that react with DNA like alkylating agents
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• Physical -
• Radiation including UV usually causing thymine dimers to form
• Most Carcinogens are mutagenic.  

DNA Repair Mechanisms 

• Direct DNA repair is the direct enzymatic removal of dimers.
• Excision repair is the excision of a DNA segment containing the damaged DNA followed by synthesis of a new strand. Bacteria and people can do this as evidenced by the affect of mutations.
• Recombination or postreplication repair is the retrieval of missing information by genetic recombination when both strands are damaged.
• The SOS response is the induction of many genes after DNA damage or interruption of DNA replication.
• Error-prone repair is the last resort before it dies. 

Plasmids

Classification of Plasmids

• Over 1,000 different plasmids
• Two Main Groups: Large plasmids that are self-transmissible and small plasmids that nonconjugative.
• Copy number per cell varies
• Plasmids can produce different sex pili
• Plasmids can be in the same incompatability group  

Phenotypic Effects of Plasmids

• Code for bacterial toxins
• Code for enzymes that metabolize unusual substrates
• Code for antibiotic resistance (The R plasmids)     

1.      R plasmids have two parts: resistance transfer  factor (RTF) and the R determinant that encodes resistance to antibiotics

• The F plasmid can integrate into the bacterial chromosome to form an Hfr strain.

Conjugation 

• Most frequent form of gene transfer and occurs between many different bacterial species.
• Involves cell-cell contact and the presence of a sex pili.
• F-plasmid is conjugative and encodes the conjugation apparatus that includes pili and tra genes. This plasmid can integrate into the chromosome to form Hfr strains.
• Some non-conjugative plasmids can be transferred along with the conjugative plasmid

R Plasmids

• R plasmids can encode resistance to multiple antibiotics. These resistance cassettes can be acquired from the bacterial chromosome by transposition.
• They can be transferred rapidly by conjugation.
• Inappropriate use of antibiotics has led to an increase in antibiotic resistant bacteria.
• R plasmids are unstable and can be rapidly lost if the selection pressure is lessened.
• R plasmids have a resistance transfer region (RTF) and a resistance determinant (r-determinant).

Why Clone DNA?