First Exam Fall 1999 Biology 50 Dr. Jeff Dangl

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examination.

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1. (5pts.) What is the chemical basis for base-pairing of nucleotides on opposite strands of a DNA double helix and

why does this chemical basis make G-C base pairs thermally more stable than A-T?

2. (5 pts.) The combination of Chargaff’s rules AND two features of regularity in the helix observed in the DNA crystal

informed Watson and Crick’s model of DNA structure. What are Chargaff’s rules and what ONE feature of the structure,

combined with those rules, were the keys to the discovery?

3. (5 pts.) What is the key difference between leading strand DNA synthesis and lagging strand DNA synthesis?

What is the feature of DNA polymerase which determines this difference?

4. (10 pts.) Outline the experiment using Rough (R) and Smooth (S) bacteria that demonstrates the principal of

transformation. How was this experimental design extended to prove that DNA was the “transforming material”.

5. (12 pts.) Which of following occur during (or by) Mitosis, Meiosis, or Both?

No crossing over

Division in somatic cells

Chromosomes line up at an equatorial plane.

The end products are gametes.

Reductional division.

Homologous chromosome pairing.

6. (12 pts.) You are hiking in the mountains and find two stands of true breeding plants of the same

species. You harvest seed from one which is TALL and has YELLOW flowers, and one which is DWARF and

has BLUE flowers. You cross them and the F1 progeny are DWARF and have YELLOW flowers. (use H

and h for dominant and recessive height alleles; F and f for flower color).

A) What are the genotypes for each phenotype of the two plants you found and the genotype of the F1?

B) You want to take an F1 plant and test cross it. What are the genotype and phenotype of the

test-cross parent you should use, and is it one of the ones you collected?

C) What is the expected frequency of DWARF and BLUE plants in the F2 following self fertilization of

the F1 you made in part (A)?

7. (8 pts.) Some tree species occur with a variety of chromosome numbers. Individuals with

the somatic chromosome numbers of 28, 56, 70 and 84 were identified. What is the designation, in terms of

chromosome number, for each of these?

8. (6 pts.) Why were radioactive P and S ideal tracers for the experiments of Hershey and Chase,

where they infected bacteria with phage to address whether DNA was the genetic mateiral? What results

of their experiment strongly suggested that DNA was the genetic material.

9. (9 pts.) Twenty spontaneous white-eye (w) mutants are examined individually to determine their

reversion frequencies. Of these, 12 spontaneously revert at high frequencies of 1 / 1000 to 1 / 10,000 (CLASS A).

Another 6 do not revert spontaneously, but can be induced to revert by treatment with alkylating agents at

frequencies of around 1 per million (CLASS B). The remaining 2 can never be made to revert (CLASS C).

Which class of mutants probably result from insertion of transposons?

Which class are probably missense mutations?

Which class are probably deletions?

10. (6 pts.) Phenylketonuria is a recessive disease resulting in mental retardation. Female II-3 (filled circle) is

affected by this disease. (use P and p to denote your alleles)

What are the genotypes of the parents of II-3?

If first cousins III-1 and III-2 produce children, what is the probability that those children will have the disease?

Assume that II-1 and II-5 are homozygous for the normal allele.

11. (9 pts.) You have found three mutant corn kernels which are blue, and because you like blue tortilla

chips, you grow plants from the blue kernels, self pollinate them, and notice that all the kernels are blue.

You also outcross them to the normal yellow kerneled corn, and the progeny make yellow kernels. You cross

the blue mutants together in an intercross the blue mutants with each other, and all of the resulting progeny are

blue.

Are the mutations dominant or recessive?

Which cross or crosses represent the complementation test?

How many loci are mutated in your collection of three blue mutants and why?

12. (6 pts.) What is the role of Primase during DNA replication? Does primase do this job more often on

the leading strand or the lagging strand?

13. (7 pts.) What gametes can be formed in an individual organism of genotype

AaBBCc?