Question

1. Which is most likely to have a high processivity?

all of these

a SSB

a DNA polymerase that is a main polymerizing enzyme

a DNA polymerase that is primarily a repair enzyme

a DNA ligase

2. For Messelson-Stahl experiments: Which type of DNA forms a band at the top of the centrifuge tube after ultracentrifugation in a CsCl gradient?

DNA containing the isotope 14N only

DNA containing both 14N and 15N

DNA containing the isotope 15N only

none of the above

Answer 1

SSB are single-strand binding proteins which bind to the single-stranded DNA and protect it from degradation, DNA ligase link two DNA fragments to each other, DNA polymerizing enzyme has to remain bound to the DNA template for a long time, highly processive DNA polymerase increases the speed of polymerization.

processivity is the ability of the enzyme to catalyze the reaction continuously without falling off, so the answer is a DNA polymerase that is a main polymerizing enzyme

2) Messelson and Stahl grew bacteria in the medium containing N15 for many generations then bacteria were transferred to medium containing light nitrogen, after one generation there were only DNA with intermediate density, as the bacteria the bacteria grows in the N14 media 2 bands of DNA can be obtained, one with light nitrogen N14 and one with intermediate density, the one with light nitrogen forms a band at the top of the centrifuge tube after ultracentrifugation in a CsCl gradient.

so the answer is  

DNA containing the isotope 14N only