8. Using the following letter codes, put the steps of translation elongation in order:

A) Peptidyl transfer (peptide bond formation);B) Entry of incoming charged tRNA;C) Translocation

_____ >> _____ >> _____

9. Immediately before translocation, the empty tRNA is located in the      site of the ribosome whereas immediately following translocation, the empty tRNA is located in the             site        of the ribosome.

四、问答题：

1. Write the consensus Kozak sequence below and label the translation initiation site.

2. DRAW what is meant by the sentence:  "Transcription and translation are coupled in prokaryotes."

3. How does IF-3 participate in ribosome dissociation?

4. What are the two methionyl-tRNAs called? What are their roles?

5. Define what the term "wobble" base pairing refers to. List the possible "wobble" base pairing combinations.

6. Summarize the translation initiation process in E.coli.

7. Diagram the translation elongation process in prokaryotes.

8. When a peptidyl-tRNA translates from the A to the P site on a ribosome. It leaves the A site empty. What events must occur before an AA-tRNA enters the vacant site

9. DRAW a graphic depicting the initiation complex for translation. On your drawing, label the following: 30S subunit, 50S subunit, A site, E site, P site, initiator tRNA, and mRNA with start codon.

10. What are the roles of RF-1, RF-2 and RF-3.

11. Draw a diagram of the scanning model of translation initiation.

12. Can a eukaryotic mRNA molecule code for more than protein?

13. What are the roles of eIF 4A and 4B?

14. What are constitutive genes? Are they all transcribed at the same rate?

习题十一

一、名词解释：

constitutive mutation，operon，operator，antitermation，enhancer，negative control, negative superhelix, Sigma factor, attenuator

二 选择题：

1. An operon consists of:  （   ）

a) All the genes for one biosynthetic pathway.

b) A cluster of genes which are transcribed into a single mRNA.

c) Several ribosomes attached to the same mRNA molecule.

d) The two nearby sequences on DNA where RNA polymerase binds.

e) The group of codons which all code for the same amino acid

2. In positive regulation: （   ）

a) an activator protein binds DNA at a site called the operator.

b) a gene is always transcribed .

c) constitutively expressed genes are turned off .

d) an activator protein helps RNA polymerase bind to the promoter .

e) none of the above is true

3. Regulation of β-galactosidase synthesis is （   ）

a) under neither positive nor negative control.

b) under only positive control.

c) under only negative control.

d) under both positive and negative control.

4. In the presence of glucose and the absence of lactose: （   ）

a) Both the LacI repressor and the Crp protein will bind to the DNA.

b) Neither LacI nor Crp protein will bind to the DNA.

c) Crp protein will not bind to DNA but the LacI repressor will.

d) Crp protein will bind to DNA but the LacI repressor will not

5. Which of the following statements is NOT true: The Crp Protein （   ）

a) Forms dimers in its DNA-binding form.

b) Binds to the DNA only in the absence of cyclic AMP.

c) Changes shape when it binds cyclic AMP.

d) Regulates many different genes.

e) Does not bind glucose.

6. Which of the following statements describes a step in the regulation of the lac operon: （   ）

a) A cyclic AMP-CAP complex binds to the CAP site.

b) The repressor binds to CAP to form an inactive repressor.

c) Cyclic AMP competes with the repressor for binding to the operator.

d) The repressor undergoes a conformational change upon binding cyclic AMP.

7. All the following mutations can result in a deduction of β-galactosidases except: （   ）

 a) A mutation in adenylate cyclase.

b) A mutation in CAP.

c) A mutation in the CAP site in the lac control region.

d) A mutation in the repressor binding site in the operator.

8. Stable RNAs are called "stable" because: （   ）