Biology 50, Second Midterm Exam

Prof. J. Dangl / Prof. K. Bloom

Tuesday Oct. 19, 1999, Coker 201

 

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PLEDGE: I neither gave nor received help on this exam.             SIGNED:

 

Please read ALL questions CAREFULLY before starting. Answer in as short a manner as possible--writing down a lot of words won’t

necessarily help. Note that a Genetic Code table is provided on the last page.

 

1.         (4 pts.) An auxotrophic mutant of E. coli cannot synthesize the amino acids alanine and methionine (ala-, met-). What do you

need to add to the minimal media in order for this strain to grow?

 

 

 

2.         (5 pts.) What feature of generalized phage transduction distinguishes it from specialized transduction.

 

 

 

 

3.         (5 pts.) Consider the following Hfr x F- cross:

 

Hfr genotype:     a+  b+  c+  str-s

F- genotype:      a-  b-  c-  str-r

 

where the order of transfer is:                     a          9 minutes

                                                            b            11 minutes

                                                            c            30 minutes

 

Recombinants are selected by palting on a medium that lacks particular nutrients (a, b, c) and contains Streptomycin. Which of the

following are TRUE and which are FALSE:

 

a) a+ str-r  colonies are approx. equal to b+ str-r colonies

 

b) a+ str-r colonies > c+ str-r colonies

 

c) b+ str-r colonies < c+ str-r colonies

 

d) Among the total colonies selected for b+ str-r , the a+ b+ str-r are approx. equal to those selected for

a- b+ str-r.

 

e) Among the total colonies selected for a+ c+ str-r, the a+ b+ c+ str-r < a+ b- c+ str-r.

 

 

 

4.         (10pts.) For the aspects of transcription and translation listed below, write “E” if the statement applies to Eukaryotes ONLY

“P” if the statement applies to Prokaryotes ONLY and “BOTH” if it applies to both Prokaryotes and Eukaryotes and “NEITHER” if the

statement is false.

 

a) Translation begins at an AUG codon.

 

b) mRNA molecules are usually the products of splicing of a longer primary transcript.

 

c) The mRNA has a 5’ capped nucleotide.

 

d) One polycistronic mRNA can encode many different proteins.

 

e) Translation requires ribosomes.

 

f) After transcription, a run of “A” ribonucleotides is added to the mRNA.

 

g) RNA polymerase recognizes promoter elements in the DNA to begin transcripton.

 

h) Transcription and translation are coupled.

 

i) Charged tRNA molecules are used to add amino acids to growing polypeptides.

 

j) Uncharged tRNAs released from the ribosome are degraded.

 

 

 

 

5.         (9pts.) If the DNA molecule below is transcribed left to right,

 

                 5'-TCGACTTGCAGGACCCTGCGGTAC-3'

                 3'-AGCTGAACGTCCTGGGACGCCATG-5'

 

then what is:

 

a) the mRNA sequence?

 

 

b) the amino acid sequence? Assume that one does not need a typical start codon.

 

 

c) What would be the effect of a C-->A mutation at position #9 in the DNA sequence?

 

 

 

 

6.         (4 pts.) When Crick and Brenner performed the experiments which led to the conclusion that the Genetic Code is a

non-overlapping triplet code, they started their analyses with a mutation caused by Proflavin. What is the key phenotypic feature of this

mutation, and what conclusion drawn from this feature, were critical to the success of their experiments?

 

 

 

 

 

7.         (4pts.) Why is attenuation not used as a regulatory mechanism in Eukaryotes?

 

 

 

 

 

8.         (10pts.) For the following genotypes and conditions (+ or - lactose), predict with a checkmark in the correct column, whether

functional enzyme (b-galactosidase) or no enzyme is made:

 

Genotype                                 Lactose?            Functional            No enzyme?

                                                                        enzyme?          

I+ O+ Z+                                       -    

I+ O+ Z+                                       +                

I- O+ Z+                                        -                

I- O+ Z-                                        +                

I- O+ Z+  / F’I+                            +                

I+ OC Z+  / F’O+                          -                 

I+ O+ Z-  / F’O+Z+                     +                

I- O+ Z-  / F’ I+ O+ Z+             -                

IS O+ Z-  / F’ OC Z+                     -                 

I+ OC Z+ / F’ O+ Z+                    +                

 

9.         (10pts.) The tryptophan (trp) operon is controlled at various levels.

            a) When free tryptophan levels are high in a bacterium, how is transcription of the trp operon regulated?

 

 

 

 

 

            b) When free tryptophan is low, but charged tRNA-trp is still present, how is transcription regulated?

 

 

 

 

 

            c) Diagram the situation reflected in part (b).

 

 

 

            d) Diagram the situation when charged tRNA-trp levels are very low, or absent.

 

 

 

            e) What is the end outcome of the situation you drew in (D)?

 

 

10.        (8pts.) Genes A and B  map very close to each other. In a deletion mutation, both A and B enzyme activity are missing and

neither A nor B protein are made. Instead, a novel protein is made in which the 30 amino acids at the amino terminus derive from B

and the 30 amino acids of A at the carboxyl terminus derive from A.

 

a) What respect to the 5’ to 3’ orientation of the coding strand of DNA, what is the gene order of A  and B?

 

 

b) What inference can be made regarding the number of bases deleted in this mutant?

 

 

 

11.        (6 pts.) The gene was thought to be indivisible until Benzer’s rII experiments. Why did he recover plaques on E. coli B when he

infected with two single mutant phage, which each carried a different (non-deletion) mutant allele of rII?

 

 

What feature of his results suggested that the gene was divisible?