Endonuclease vs. Exonuclease

Question 1

Nucleases

Answer 1

Enzymes that catalyze the hydrolysis of phosphodiesters in nucleic acids.

Question 2

Exonuclease

Answer 2

Catalyze they hydrolysis of phosphodiester linkages to release nucleotide residues from only one end of a polynucleotide chain.

Question 3

Endonuclease

Answer 3

Catalyze the hydrolysis of phosphodiester linkages at various sites within a polynucleotide chain.

Question 4

Restriction endonuclease

Answer 4

Certain bacteria can block bateriophage (virus) infections by specifically destroying the incoming bateriophage.

Many species of bacteria synthesize restriction endonucleases that bind to and cleave foreign DNA.

These endonucleases recognize specific DNA sequences and cut both strands at a binding site that produces large fragments that are rapidly degraded by exonucleases.

Question 5

Type I

Answer 5

Catalyze both the methylation of host DNA and the cleave of unmethylated DNA at a specific recognition sequence.

Question 6

Type II

Answer 6

Simplier in that they can only cleave double stranded DNA at or near an unmethylated recognition sequence.

Question 7

How do host cells protect its own DNA from clevage by restriction endonucleases

Answer 7

By covalently modifying the bases that make up the potetntial restriction endonuclease binding site.
MOST COMMON: specific methylation of adenine or cytosine residues

Question 8

Methylation

Answer 8

The presence of methylated bases at the potential binding sites inhibits the clevage of the host DNA by the restriction endonuclease. Methylation is catalyzed by a specific methylase that binds to the same sequence of DNA recognized by the restriction endonuclease.

Question 9

Sticky Ends

Answer 9

Some restriction endonucleases such as EcoRI, BamHI, and HindIII catalyze staggered clevage, producing DNA fragments with single stranded extensions.

Question 10

Blunt Ends

Answer 10

Other enzymes such as HaeIII and SmaI, produce blunt ends with no single stranded extensions.