Biology 50, Second Midterm Exam

Prof. J. Dangl

Tuesday, Oct. 13, 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.         (6 pts.) In a cross between Hfr ala+ str-s x F- ala- Str-r, what medium should the mating pairs be plated to select for

Ala+ Str-r recombinants? Which are the selected and counterselected markers? Which strain is a prototrophic for alanine production

and which is auxotrophic?

 

 

 

 

 

 

2.         (8 pts.) A lambda phage mutant was discovered lacking a functional integrase gene. Could this mutant still function as

a specialized transducing phage? Would it still infect E. coli? Explain.

 

 

 

 

 

 

 

3.         (10 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.         (5pts.) In a mating of Hfr pro+ tpr+ ala+ tet-s x F-  pro- trp- ala- gly+ tet-r, cells are interrupted after several minutes

and PLATED ON MEDIA CONTAINING THE AMINO ACIDS SHOWN BELOW. Based on the number of colonies, what is the

order of gene transfer? What is the counterselectable marker?

 

proline + alanine                  290 colonies

tryptophan + proline               120 colonies

tryptophan + alanine               45 colonies

 

 

 

5.         (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.

 

 

6.         (6pts.) 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?

 

 

 

 

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

 

7.         (2pts.) What structural interaction of charged tRNA and mRNA molecules determines whether tRNA will stay bound to the

ribosome “A site” long enough for a peptide bond to be formed?

 

 

 

 

8.         (8pts.) In studies on a hypothetical protein, you identify the mutants below which have no activity.  What is the mutation(s)

responsible for each amino acid sequence? Show the DNA sequence of each.

Wild type protein                        Thr     Val    Tyr    Glu   Asn    His    Ser   Lys

Wild type         DNA                   ACC GTC TAC GGA AAC CAC AGC AAA

 

Mutant #1                                  Thr    Val

                                                ACC GTC

 

Mutant #2                                  Thr    Val   Ser Glu Thr Thr Ala

                                                 ACC GTC

 

Mutant #3                                  Thr    Val   Tyr    Ala Asn His Ser Lys

                                                ACC GTC TAC G

 

Mutant #4                                  Thr    Gly  Leu  Arg  Asp His Ser Lys

                                                ACC G

 

 

 

9.         (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?

 

 

 

 

10.        (4pts.) Why is attenuation not used as a regulatory mechanism in Eukaryotes?

 

 

 

11.        (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+ OC Z+                                      +

I- O+ Z+                                        -

I- O+ Z-                                        +

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

I+ OC Z+  / F’O+                          -

I+ O+ Z-  / F’I+ O+ Z+            +

I- O+ Z-  / F’ I+ O+ Z+             -

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

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

12.        (12pts.) 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)?

 

 

 

 

            f) A mutant which is constitutive for production of Trp operon mRNA maps to the attenuator region, the region which normally

encodes the short leader peptide. What is the probable nature of this mutation?

 

 

 

 

 

13.        (9pts.) When glucose is present in an E. coli cell, is the concentration of free cAMP high or low? Can a mutant with either

an inactive adenyl cyclase gene or an inactive CRP  gene synthesize B-galactosidase? Does the binding of cAMP-CRP to DNA effect

the binding of Repressor?

 

 

 

 

14.        (6pts.) 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?