MIC205B Exam 3

December 10, 2007

Instructor: Dr. Stephen Billington

 

                                                        Name:_______________________________________

 

                                                Student ID:_______________________________________

 

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Part 1 MULTIPLE CHOICE: Read each question and the answers carefully. Circle the correct answer on the test paper. Each correct answer is worth 3 points, for a Part 1 total of 45 points.

 

JUSTIFICATIONS: If you are unsure of an answer, you may write a short explanation next to the question on the test paper (this may earn you points if your answer is wrong, but you can justify it).

 

1.      The genetic code is degenerate because

 

a. a single codon codes for each amino acid

 

b. multiple amino acids are coded for by a single codon

 

c. Some amino acids are coded for by more than one codon

 

d. None of the above

 

2.      The sigma factor of RNA polymerase functions in

 

a. recognizing promoter sequences

 

b. polymerizing ribonucleotides into RNA

 

c. terminating transcription

 

d. synthesis of RNA primers during DNA replication

 

3.      Which of the following is INCORRECT about operons?

 

a. Multiple genes are transcribed on the same mRNA transcript

 

b. Occur only in prokaryotes

 

c. Allow the regulation of multiple genes by a single promoter

 

d. None of the above

 

4.      Which of the following is TRUE about genes that contain introns?

 

a. Common in both eukaryotic and prokaryotic organisms

 

b. The introns must be spliced from the RNA prior to translation

 

c. Only the exons are transcribed into RNA

 

d. All of the above

5.      During assembly of the 30S initiation complex, the 16S rRNA binds to which part of mRNA?

 

a. Ribosome binding site

 

b. Start codon

 

c. Terminator

 

d. Stop codon

 

6.      The enzyme which unwinds and separates the two DNA strands prior to DNA replication is called

 

a. DNA ligase

 

b. DNA polymerase

 

c. DNA helicase

 

d. single stranded DNA binding protein

 

7.      N-formylmethionine is

 

a. the first amino acid incorporated into bacterial proteins

 

b. one of the 20 common amino acids

 

c. an amino acid used for the synthesis of peptidoglycan

 

d. the initiating amino acid in eukaryotic protein synthesis

 

8.      The enzyme DNA gyrase performs what function in DNA replication?

 

a. Polymerizes the leading strand

 

b. Unwinds the DNA strands

 

c. Relaxes the supercoils in DNA

 

d. Synthesizes RNA primers on the lagging strand

 

9.      An mRNA codon

 

a. codes for a single amino acid

 

b. is composed of two nucleotides

 

c. codes for a protein

 

d. catalyzes peptide bond formation

 

10.    The formation of a peptide bond during translation is catalyzed by the enzyme peptidyl transferase. What sort of molecule is this enzyme?

 

a. Carbohydrate

 

b. Protein

 

c. Lipid

 

d. RNA

 

11.    Which of the following is NOT a function of DNA polymerase III in DNA replication?

 

a. Polymerizing DNA in the 5’-3’ direction

 

b. Removing RNA primers and resynthesizing as DNA

 

c. Synthesis of the leading strand

 

d. Synthesis of Okazaki fragments on the lagging strand

 

12.    Semi-conservative replication refers to

 

a. each nitrogenous base binding at carbon 1 of the sugar

 

b. a purine always hydrogen bonding with a pyrimidine

 

c. a parent strand and a newly synthesized strand comprising a new DNA molecule

 

d. the two DNA strands being antiparallel

 

13.    During bidirectional replication of bacterial chromosomes

 

a. One strand is replicated 5’-3’, while the other is replicated 3’-5’

 

b. A single replication fork forms with the leading and lagging strands

 

c. Two replication forks form each with leading and lagging stands

 

d. RNA primers are not required to initiate DNA synthesis at the origin of replication

 

14.    Telomers on eukaryotic chromosomes

 

a. Allow bidirectional DNA replication

 

b. Assist in the replication of linear chromosomes

 

c. Must be spliced from transcribed RNA

 

d. Function in the 5’ capping of RNA

 

15.    Which of the following occur in prokaryotic cells?

 

a. Simultaneous transcription and translation of the same mRNA

 

b. Translation of a single mRNA by multiple ribosomes

 

c. Simultaneous mRNA transcription and translation by multiple ribosomes

 

d. All of the above

 

Part 2 SHORT ANSWER: Read each question carefully and answer each question in the space provided. Each correct answer is worth 5 points, for a Part 2 total of 45 points.

 

JUSTIFICATIONS: Again, if you are unsure of an answer, you may write a short explanation next to the question on the test paper.

 

16.    Fill in the blanks. In the Central Dogma, the information in DNA is TRANSCRIBED into RNA, and the information in RNA is TRANSLATED into protein.

 

17.    a.   True OR False? In bacteria, a single RNA polymerase transcribes the three major types of RNA, mRNA, rRNA and tRNA, but in eukaryotic cells these forms of RNA are transcribed by three separate RNA polymerases.

 

TRUE

 

         b.   True OR False? Bacterial and eukaryotic RNA polymerases recognize the same promoters sequences.

 

FALSE

 

18.    a.   Which of the following organisms can’t translate mRNA as it is being transcribed? Prokaryotes OR Eukaryotes?

 

EUKARYOTES

 

      b.   Why can’t they?

 

IN EUKARYOTES, TRANSCRIPTION OCCURS IN THE NUCLEUS AND TRANSLATION

 

 OCCURS IN THE CYTOPLASM. THEREFORE, MRNA MUST BE TRANSLOCATED TO

 

THE CYTOPLASM BEFORE IT CAN BE TRANSLATED.

 

19.    Name the three types of posttranscriptional modification of eukaryotic mRNAs, that do not occur with prokaryotic mRNA.

 

5’ CAPPING

 

3’ POLYADENYLATION

 

INTRON SPLICING

 

20.    Fill in the blanks. Following peptide bond formation during translation; the ribosome moves along the mRNA by one CODON. The tRNA containing the growing peptide moves into the P site, while the empty tRNA moves to the E site before being ejected from the ribosome. The A site is now available to accept a new charged tRNA.

 

21.    a.   True OR False?. Both DNA and RNA polymerases have proof reading ability.

 

FALSE

 

         a.   True OR False?. The 3’-5’ exonuclease activity of DNA polymerases is important for their proof reading ability.

 

TRUE

 

22.    a.   Fill in the blank. A codon within a mRNA transcript is recognized by the ANTICODON of the tRNA through complementary base pairing.

 

         b.   Fill in the blank. Aminoacyl tRNA synthetases are responsible for adding the AMINO ACID to the 3’ end of tRNA molecules. This reaction requires energy in the form of ATP to form a high energy bond. This high energy is broken during translation and the energy released is used drive PEPTIDE bond formation.

 

23.    True OR False? Rolling circle replication is a unique form of DNA synthesis that occurs only in eukaryotic cells

 

FALSE

 

24.    In the following mRNA sequence, what are the names of the underlined sequences represented by the numbers 1, 2 and 3?

 

5’-AGCAGGAGCUAGAUGGGCGGAGAUUGGACGGACGGAACGAUGACGGAUUUCAAGUGGCGCUAACC-3’

             1                 2                                                                                                       3

 

1. RIBOSOME BINDING SITE

 

2. START CODON

 

3.STOP CODON

 

Part 3 SHORT ESSAY: Answer TWO of the following questions in the space provided. Each correct answer is worth ten points, for a total of 20 points for Part 3.

 

25. Describe the elongation step during translation, including the enzymes and elongation factors involved.

 

A newly charged tRNA is delivered to the A site. This requires energy which is provided by hydrolysis of GTP by EF-Tu.

 

Peptide bond formation then proceeds with the transfer of the growing peptide chain from the tRNA in the P-site to the amino acid attached to the tRNA in the A site. This energy is provided by the hydrolysis of the high energy bond linking the amino acid to the tRNA. Peptidyl transferase (23S rRNA) catalyzes this reaction.

 

The ribosome then moves one codon along the mRNA. This translocation requires energy which is provided by hydrolysis of GTP by EF-G. The tRNA carrying the peptide chain moves from the A site to the Psite, the empty tRNA moves the E site and is then ejected from the ribosome. The A site is now empty and ready to receive the next charged tRNA.

 

 

 

 

 

 

 

 

 

26. Describe synthesis of the lagging strand during DNA replication. Include each of the enzymes involved and how they function in DNA replication.

 

Following formation of the replication fork, the lagging strand is synthesized discontinuously.

 

Primase synthesizes short RNA primers complementary to the lagging strand template.

 

DNA polymerase III uses the 3’ ends of these primers to synthesize Okazaki fragments (~1000 bp).

 

The RNA primers are then removed by DNA polymerase I , which then uses the 3’ ends of the Okazaki fragments and adds deoxynucleotides to resynthesize these segments as DNA.

 

The gaps between the discontinuously synthesized fragments are then sealed by DNA ligase to complete synthesis of the lagging strand

 

 

 

 

 

 

27. Describe the functions of the three major types of RNA in prokaryotic cells.

 

mRNA (messenger RNA) provides the template for protein synthesis. A codon in mRNA codes for an amino acid in the corresponding protein.

 

rRNA (ribosomal RNA) is a structural and functional portion of the ribosome. 16S rRNA binds to the mRNA during formation of the initiation complex. 23SrRNA has peptidyl transferase activity which leads to peptide bond formation

 

tRNA (transfer RNA) is the translator molecule between the sequence of codons in mRNA and the sequence of amino acids in the protein. An amino acid is attached to one end of the tRNA, an the corresponding anticodon is at the other end