四、问答题：

1. Describe three features of replication origin.

2. In the graphic below, assume that replication begins at the vertical lines.  Draw the leading and lagging strands (with arrows as we did in class), label the ends of EVERY DNA fragment (either 5' or 3') and indicate the direction that the replication forks are moving.

3. Draw a diagram of one of the replication forks of the E. coli genome during replication. Assume that replication has already begun and that leading and lagging strand synthesis is well underway. Diagram how the system accomplishes leading and lagging strand synthesis.(What happens during DNA replication in prokaryotes)?

4. Explain the role of the following enzymes in DNA replication: (a) helicase;  (b) topoisomerase; (c) primase;  (d): DNA polymerase.

5. For each of the enzymatic activities involved in replication (below), name the enzyme the fulfills that function MOST OF THE TIME.

Example:  Prevents renaturation of the DNA strands    Answer:  SSB

                         i.              Proofreads：

                       ii.              Removes the RNA primer：

                      iii.              Separates the DNA strands：

                     iv.              Synthesizes DNA：

                       v.              Links Okazaki fragments:

6. For each of the enzymes or enzymatic activities involved in replication (below), give the function of that enzyme IN THREE WORDS OR LESS.

Example:  DNA gyrase    Answer:  Relieves supercoiling

         A)  3' > 5' exonuclease activity of DNAP I

         B)  5' > 3' exonuclease activity of DNAP I

 C)  DNA helicase

 D)  DNA ligase

 E)  n' protein

 F)  SSB

7. For each of enzymatic activities involved in replication (below), give the function of every subunit in DNA polymerse III holoenzyme:

       α,θ,ε,β,γ,δ,τ.

8. Diagram the rolling circle replication mechanism.

9.      How does DNA replication take place in eukaryotes(for example SV40 or yeast).

10. What are telomeres? How are RNA primers at the ends of linear eukaryotic chromosomes replaces by DNA.

习题五

一、名词解释：

Mutagenesis，Indirect-acting carcinogens，Holliday model，Site-specific recombination，The UvrABC system，ORF

二、选择题：

1. Which of the following does NOT cause a frameshift?（  ）

(a) Insertion of one base into a coding region

(b) Deletion of five bases from a coding region

(c) Insertion of three bases into a coding region

(d) Deletion of two bases from a coding region

(e) Insertion of three bases into a regulatory region

2. A base analog is a chemical that: （  ）

(a) Converts one base to another by altering amino groups to hydroxy groups

(b) Hydrolyses pyrimidine bases off from the deoxyribose to which they were attached

(c) Distorts a DNA double helix by binding to base pairs

(d) Is mistakenly incorporated into DNA by DNA polymerase

(e) Absorbs ultraviolet radiation and reacts with DNA

3. Base deamination can cause single base pair mutations because（  ）

(a) deaminated G structurally resembles U.

(b) deaminated G can base pair with U.

(c) deaminated C structurally resembles A.

(d) deaminated C can base pair with A

4. The three enzymes that play a role during excision repair are, in order（  ）