BIO431- Molecular Genetics Exam I
Questions with Complete Solutions
What is a purine? - ANSWER-a double-ringed nitrogenous base; ex. adenine and
guanine
What is a pyrimidine? - ANSWER-single-ringed nitrogenous base; ex. thymine and
cytosine
Structure of adenine - ANSWER-
Structure of guanine - ANSWER-
Structure of cytosine - ANSWER-
Structure of thymine - ANSWER-
Structure of Uracil - ANSWER-
What bond allows for a trinucleotide? - ANSWER-Phosphodiester bonds between
the three phosphate groups of the nucleotides
What was Avery's, MacLeod's, and McCarty's experiment in 1944? - ANSWER-
Avery, MacLeod, and McCarty determined the identity of the transforming substance
by expanding the bacterial transformation. Took heat-killed S strain and injected to
each cellular component (Polysaccharides, Lipids, RNA, Protein, and DNA) and
coinjected with the live R cells. all except DNA had R colonies, while with DNA, there
was the R colonies and S colonies. Concluded that the transforming substance was
DNA. To further prove, they took S strain extract and R strain and destroyed each
component of the S strain and injected into mice. When DNA was destroyed, the
mouse lived and no live S strain was recovered. But for all the other components, the
mouse died and live S strain was recovered because DNA was still present.
What was Hershey's and Chase's "Blender Experiment'? - ANSWER-Used T2
bacteriophage (virus); the last experiment to prove that DNA was the transforming
agent; (1) mixed radioactively labeled (with sulfur) phages with bacteria and the
phages infected the bacterial cells; (2) agitated in a blender to separate phages
outside the bacteria from the bacterial cells and their contents; (3) centrifuged,
measured the radioactivity in the pellet and the liquid (the bacteria will be in the pellet
and most radioactivity was in the liquid). Did the same experiment with radioactive
phosphorus and most of the radioactivity was in the pellet
, What is Chargaff's rule and ratios? - ANSWER-Content of purines equaled the
content of pyrimidines; A=T and C=G; later explained by Watson and Crick with
complimentary base-pairing
How was the structure of DNA first identified? - ANSWER-Rosalind Franklin and
Maurice Wilkins used X-ray diffraction technology in 1952; X-ray crystallography
revealed the basic helical nature of DNA structure (repeated sugar backbone
structures)
What was the triple helix model? - ANSWER-Linus Pauling proposed a model of
DNA with the sugar-phosphate backbone in the center and the nucleotides on the
exterior of the helix (not possible because of all the negative charges in the center)
What is the bond that is between a base pair? - ANSWER-Hydrogen bond (non-
covalent)- weak electrostatic interactions between opposite dipoles
What is included in the DNA's repeating structure? - ANSWER-Nucleotide subunit,
pyrimidine base, sugar-phosphate backbone, and purine base
Is the interior of DNA (between the nitrogenous bases) hydrophobic or hydrophilic? -
ANSWER-Interior of DNA is hydrophobic
Which model best represents DNA replication? - ANSWER-Semiconservative
replication (template strand and new daughter strand)
How does the double helix form? - ANSWER-Two polynucleotides with
complimentary nucleotide sequences (when anti-parallel) can form hydrogen bonds
What is a "pitch" in the DNA helix? - ANSWER-10 base pairs per turn
What is a "turn" in the DNA helix? - ANSWER-+360 degrees. One turn would be one
major groove to another major groove; 24 A per helical turn
What does the minor groove do? - ANSWER-Minor groove helps with interacting
with bases
What does the major groove do? - ANSWER-Major groove allows proteins to interact
with DNA
What phase of the cell cycle does DNA synthesis take place? - ANSWER-During S
phase
What is the Meselson-Stahl experiment? - ANSWER-Came up with 3 models for
DNA replication: semiconservative model, conservative model (in the second
generation, one set of daughter strands and one set of daughter strand parental
strand) and , and dispersive model (DNA gets broken and pieced together); used
N15 (heavy) and N14 (light) and took photographs of DNA bands; using a cesium
chloride gradient; there was a shift towards the lighter (N14) and the heavier (N15)
kept halving; proving semiconservative replication
Questions with Complete Solutions
What is a purine? - ANSWER-a double-ringed nitrogenous base; ex. adenine and
guanine
What is a pyrimidine? - ANSWER-single-ringed nitrogenous base; ex. thymine and
cytosine
Structure of adenine - ANSWER-
Structure of guanine - ANSWER-
Structure of cytosine - ANSWER-
Structure of thymine - ANSWER-
Structure of Uracil - ANSWER-
What bond allows for a trinucleotide? - ANSWER-Phosphodiester bonds between
the three phosphate groups of the nucleotides
What was Avery's, MacLeod's, and McCarty's experiment in 1944? - ANSWER-
Avery, MacLeod, and McCarty determined the identity of the transforming substance
by expanding the bacterial transformation. Took heat-killed S strain and injected to
each cellular component (Polysaccharides, Lipids, RNA, Protein, and DNA) and
coinjected with the live R cells. all except DNA had R colonies, while with DNA, there
was the R colonies and S colonies. Concluded that the transforming substance was
DNA. To further prove, they took S strain extract and R strain and destroyed each
component of the S strain and injected into mice. When DNA was destroyed, the
mouse lived and no live S strain was recovered. But for all the other components, the
mouse died and live S strain was recovered because DNA was still present.
What was Hershey's and Chase's "Blender Experiment'? - ANSWER-Used T2
bacteriophage (virus); the last experiment to prove that DNA was the transforming
agent; (1) mixed radioactively labeled (with sulfur) phages with bacteria and the
phages infected the bacterial cells; (2) agitated in a blender to separate phages
outside the bacteria from the bacterial cells and their contents; (3) centrifuged,
measured the radioactivity in the pellet and the liquid (the bacteria will be in the pellet
and most radioactivity was in the liquid). Did the same experiment with radioactive
phosphorus and most of the radioactivity was in the pellet
, What is Chargaff's rule and ratios? - ANSWER-Content of purines equaled the
content of pyrimidines; A=T and C=G; later explained by Watson and Crick with
complimentary base-pairing
How was the structure of DNA first identified? - ANSWER-Rosalind Franklin and
Maurice Wilkins used X-ray diffraction technology in 1952; X-ray crystallography
revealed the basic helical nature of DNA structure (repeated sugar backbone
structures)
What was the triple helix model? - ANSWER-Linus Pauling proposed a model of
DNA with the sugar-phosphate backbone in the center and the nucleotides on the
exterior of the helix (not possible because of all the negative charges in the center)
What is the bond that is between a base pair? - ANSWER-Hydrogen bond (non-
covalent)- weak electrostatic interactions between opposite dipoles
What is included in the DNA's repeating structure? - ANSWER-Nucleotide subunit,
pyrimidine base, sugar-phosphate backbone, and purine base
Is the interior of DNA (between the nitrogenous bases) hydrophobic or hydrophilic? -
ANSWER-Interior of DNA is hydrophobic
Which model best represents DNA replication? - ANSWER-Semiconservative
replication (template strand and new daughter strand)
How does the double helix form? - ANSWER-Two polynucleotides with
complimentary nucleotide sequences (when anti-parallel) can form hydrogen bonds
What is a "pitch" in the DNA helix? - ANSWER-10 base pairs per turn
What is a "turn" in the DNA helix? - ANSWER-+360 degrees. One turn would be one
major groove to another major groove; 24 A per helical turn
What does the minor groove do? - ANSWER-Minor groove helps with interacting
with bases
What does the major groove do? - ANSWER-Major groove allows proteins to interact
with DNA
What phase of the cell cycle does DNA synthesis take place? - ANSWER-During S
phase
What is the Meselson-Stahl experiment? - ANSWER-Came up with 3 models for
DNA replication: semiconservative model, conservative model (in the second
generation, one set of daughter strands and one set of daughter strand parental
strand) and , and dispersive model (DNA gets broken and pieced together); used
N15 (heavy) and N14 (light) and took photographs of DNA bands; using a cesium
chloride gradient; there was a shift towards the lighter (N14) and the heavier (N15)
kept halving; proving semiconservative replication
