Unit 10

Question 1

What are the 3 hypotheses of DNA replication test by Meselson and Stahl?

Answer 1

-Conservative replication
-Dispersive replication
-Semi-Conservative replication

Question 2

What is semi-conservative replication?

Answer 2

The two nucleotide strand separate and each serves as a template for a new strand

Question 3

What is a replication fork?

Answer 3

Point at which a double-stranded DNA molecule separates into two single strands that serve as templates for replication

Question 4

What end of a DNA strand is used by Pol I or Pol III to add new nucleotides?

Answer 4

3’

Question 5

What are the differences between leading and lagging strands of DNA? Why are they necessary?

Answer 5

-The leading strand goes through continuous replication while the lagging strand goes through discontinuous replication.
-This is necessary because the lagging strand runs out of room and has to start over at the origin of replication creating Okazaki fragments

Question 6

What are Okazaki fragments?

Answer 6

-The short lengths of newly synthesized DNA produced by discontinuous replication on the lagging strand
-After Reiji Okazaki

Question 7

Why is an RNA primer necessary to initiate DNA synthesis?

Answer 7

They provide a 3’ -OH group for the attachment of a DNA nucleotide at the initiation of replication.

Question 8

What happens when DNA polymerase selects the wrong nucleotide to insert?

Answer 8

Polymerase III uses its 3’ - 5’ exonuclease activity to back up and remove the incorrect nucleotide

Question 9

What prevents the ends of a linear chromosome from getting shorter each time the chromosome is copied?

Answer 9

The G-rich overhang can be extended by telomerase

Question 10

How can eukaryotic cells generate a large amount of DNA in a small amount of time?

Answer 10

Thousands of origins of replication

Question 11

Initiator proteins

Answer 11

Proteins that bind to the origin of replication and causes a short section of DNA to unwind

Question 12

Helicase

Answer 12

Breaks the hydrogen bond that exist between the bases of the two nucleotide strands of a DNA molecule

Question 13

Single-Strand binding proteins

Answer 13

Protein that attaches tightly to the exposed single-stranded DNA during replication and prevents the formation of secondary structures that would interfere with replication

Question 14

Topoisomerase (DNA gyrase)

Answer 14

Enzyme in E. coli that relieves the torsional strain that builds up ahead of the replication fork

Question 15

Primase

Answer 15

An enzyme that synthesizes a short stretch of RNA on a DNA template; functions in replication to provide a 3’ -OH group for the attachment of a DNA nucleotide

Question 16

DNA Polymerase III

Answer 16

Bacterial DNA polymerase that synthesizes new nucleotide strand by adding new nucleotides to the 3’ -OH group provided by the primer

Question 17

DNA Polymerase I

Answer 17

Bacterial DNA polymerase that removes RNA nucleotides and replaces them with DNA nucleotides

Question 18

DNA ligase

Answer 18

Enzyme that catalyzes the formation of a phosphodiester bond between adjacent 3’ -OH and 5’ phosphate groups in a DNA molecule without adding another nucleotide to the strand

Question 19

What gradient was used in the Meselson and Stahl 1958 experiment?

Answer 19

A cesium chloride gradient

Question 20

Explain the Meselson and Stahl experiment:

Answer 20

1. First, E. coli were grown on a 15N “heavy” medium as well as a lighter 14N medium.
2. A centrifuge tube is filled with a heavy salt solution and DNA fragments containing both 14N and the 15N
3. It is then spun in a centrifuge at high speeds for several days
4. A density gradient develops within the tube. Heavy DNA ( with 15N) will move toward the bottom and the lighter 14N will be at the top
5. They DNA is then goes through a single replication and is promptly measured to see the proportions of 15N and 14N

Question 21

What technique did Meselson and Stahl use?

Answer 21

Equilibrium density gradient centrifugation

Question 22

What is the function of the conserved DnaA Box within OriC, the origin of replication in bacterial DNA?

Answer 22

It is the binding site, conformation change of chromosome opens up the double helix and “brings in” DnaB and DnaC helicase proteins that break the hydrogen bonds to “unzip” DNA

Question 23

What is the function of the AT-rich region of the conserved site within the OriC origin of replication?

Answer 23

Weaker interaction, open up DNA with conformation change

Question 24

What structure is made during DNA replication of circular DNA?

Answer 24

Theta structure

Question 25

What is the equation for elongation?

Answer 25

ndNTP > DNA polymerase, Mg2+, DNa template > (dNMP)n + nPPi

Where n = # of A, C, G, or T

Question 26

Which exonuclease activity is the proofreading activity?

Answer 26

3’ > 5’

Question 27

What is the DNA sequence called on the opposite site of the chromosome from the origin where the Tus protein binds to and stops the replication fork?

Answer 27

ter sequence

Question 28

What is the function of the Tus protein?

Answer 28

It is a protein (terminus utilization substance) that binds to the ter DNA sequence and stops the replication fork.

Question 29

What happens when concatemers split?

Answer 29

The end product is two circular chromosomes