Molecular Genetics and Biotechnology Practice Test (page 2)

By — McGraw-Hill Professional
Updated on Aug 23, 2011

Molecular Genetics and Biotechnology Questions

  1. The buoyant density (ρ) of DNA molecules in 6M CsCl solution increases with the molar content of G + C nucleotides according to the following formula:
  2. ρ = 1:660 + 0:00098 (G + C)

    Find the molar percentage of (G + C) in DNA from the following sources:   (a) Escherichia coli: ρ = 1.710   (b) Streptococcus pneumoniae: ρ = 1.700   (c) Mycobacterium phlei: ρ = 1.732.

  3. Given two dsDNA molecules, the overall composition of which is represented by the segments shown below, determine which molecule would have the highest melting temperature. Explain.


  4. The primary mRNA transcript for chicken ovalbumin contains seven introns (light, A–G) and eight exons (dark) as shown below.
  5. Molecular Genetics and Biotechnology Supplementary Problems

    If the DNA for ovalbumin is isolated, denatured to single strands, and hybridized with cytoplasmic mRNA for ovalbumin, how would the hybrid structure generally be expected to appear in an electron micrograph? Note: Double-stranded regions appear thicker than single-stranded regions.

  6. About half the weight of RNA synthesized at any given time within a bacterial cell is rRNA. The 30S subunit of bacterial ribosomes contains one 16S rRNA molecule (1.5 kb); the 50S subunit contains one 23S rRNA (3 kb) and one small 5S rRNA (0.1 kb). Hybridization tests of 16S and 23S rRNAs with complementary single strands of DNA reveal that about 0.14% of DNA is coding for 16S rRNA and about 0.18% for 23S rRNA. Estimate the relative activity of rRNA genes as transcription templates compared with the average gene of the bacterial genome that gives rise to mRNA. Note: Assume that the amount of DNA allocated to 5S rRNA synthesis is negligible; likewise for all kinds of tRNAs.
  7. Some bacterial proteins are normally secreted from the cell. If transgenic rat insulin protein could be attached by genetic engineering to such a secreted bacterial protein, it too might be secreted from the cell. Suppose that you are given an agar plate containing several recombinant bacterial clones known to contain the gene for insulin. Propose an autoradiographic method for identifying those clones that are secreting this protein. (Hint: antibodies can be attached to certain kinds of plastic in a way that leaves their antigen-combining sites free to react.)
  8. Restriction endonuclease EcoRI makes staggered cuts in a 6-nucleotide DNA palindrome; restriction endonuclease HaeIII cleaves at one point in the middle of a 4-nucleotide palindrome. If different aliquots of a purified DNA preparation are treated with these enzymes, which one would be expected to contain more restriction fragments? Explain (give the rationale for) your choice.
  9. Only about 200 molecules of phage lambda repressor are made by bacteria when lambda is integrated at its specific attachment site between E. coli genes gal and bio. Some bacterial genes such as lac can be induced to produce more than 20,000 molecules of an enzyme per cell. If you could cut and splice genes and regulatory regions at your discretion, how would you design a bacterial cell for maximum synthesis of lambda repressor protein?
  10. The goal is to clone a specific human gene. Human DNA is isolated and cut into ~ 15-kb fragments that are spliced into phage vectors (shotgun method). The phage are introduced into recipient bacterial cells.   (a) How many kinds of cells exist? List their characteristics.   (b) How many recombinant phage would have to be screened in order to find the DNA fragment of interest (the human genome is ~ 3 × 106 kb)? (Hint: See Solved Problem 12.3.)
  11. The polymerase chain reaction (PCR) was originally performed with a DNA polymerase from the bacterium E. coli, a common inhabitant of the human gut (37°C). Each cycle of heating denatured the enzyme added during the previous cycle. In order to reduce costs and automate the PCR, another source of the enzyme had to be found. Where is the most likely place to find this alternative source?
  12. Protein P is synthesized in relatively high amounts in the mouse pancreas. This protein has been isolated and purified and the sequence of 6 amino acids from the N-terminal end has been determined. If the gene for protein P is desired to be cloned for recombinant expression in a bacterial host system:
    1. How can a probe be prepared to identify the gene for protein P?
    2. Which is the best type of library (genomic, cDNA or expression) to construct for cloning this gene?
  13. A cDNA library is being constructed from mouse pancreas tissue in order to clone the protein P gene.
    1. Outline the steps in cDNA production.
    2. Which type of vector is most appropriate to use in this case: plasmid pBR322 or phage lambda. Why?
    3. One method for cloning these cDNAs into a vector is to ligate short linker molecules on to the ends of each cDNA clone. The linkers each contain the recognition sequence for a particular restriction enzyme. For example, the linker sequence, 5' GCTGCAGC-3' contains the PstI restriction site (underlined). Now all the cDNA clones constructed contain PstI sites on each end. Outline the steps involved in ligating these clones into a vector, such as plasmid pBR322 (assume that pBR322 has a unique PstI site in the ampR gene).
    4. After transforming E. coli with the plasmids, how can the cells that contain a recombinant plasmid (i.e., one containing any DNA insert) be identified?
    5. If one million ampicillin-sensitive, tetracycline-resistant clones are grown on nutrient agar plates, how are we going to detect the rare clone or clones that carry the gene for protein P?
    6. After selecting several clones identified as carrying the gene for protein P, recombinant cells from each clone are propagated to high density in nutrient broth. The recombinant plasmids (presumably containing a fragment of the gene for protein P) are then extracted and purified from the rest of the cellular DNA. How can the gene be isolated from the plasmid?
    7. How can we demonstrate that the gene we have isolated is indeed the one for protein P?
  14. The gene for protein P is isolated and its DNA sequence is determined. It is found to contain 1000 bp of coding sequence and two EcoRI restriction enzyme sites, -GAATTC-, one at 150 bp internal to the start and one at 150 bp internal to the stop codon of the normal gene (see illustration below). A defective protein P has been discovered and its gene has also been cloned a nd sequenced. In the abnormal gene, one of these sequences has been changed to - GCATTC-.
  15. Molecular Genetics and Biotechnology Supplementary Problems

    We want to find out if fetal cells contain the normal or the abnormal gene. So DNA from fetal cells is cleaved with EcoRI and the fragments are separated on an agarose gel and then transferred to a nylon membrane. A probe for the normal gene hybidizes to the internal 700-bp EcoRI fragment and will also hybridize with the abnormal gene sequence in this same region. The size of the fetal DNA fragments can be estimated by running DNA fragments of known sizes on the same gel. What band pattern is expected if the fetal cells contain the abnormal gene?

  16. Diagram the electrophoretic pattern expected from a triple digest of the plasmid in Solved Problem 12.2 by restriction enzymes A + B + C.
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