BCH5413 EXAM 1 UPDATED ACTUAL QUESTIONS AND
CORRECT ANSWERS
Question:
1. In the experiment described that supports DNA as the
genetic material:
Why does the mouse die if the researcher mixes the
complete dead pathogenic strain with live non-
pathogenic strain bacteria and then inject it into the
mouse? (The first mouse in the figure).
Answer:
The mouse dies because the R strain has been transformed and is now an S strain,
capable of producing the virulence factors that make the S strain deadly to mice.
This is because as the pathogenic strain is killed, its DNA is released, and now
floating freely can go into the R strain and transform it.
Question:
2. In the experiment described that supports DNA as the
genetic material:
What was the only scenario in which the mouse lives?
Why?
Answer:
The only scenario in which the mouse lives is when DNA is destroyed. This is
because it's DNA that contains the genetic information that codes for the
virulence factors that kill mice when they become infected.
Question:
3. In DNA, C1 is bonded to...
This is important because...
Answer:
a nitrogenous base (adenine, guanine, cytosine, or thymine)
Importance - this allows the nitrogenous bases to form hydrogen bonds (A-T, C-
G) between the 5`-3` strand and the 3`-5` strand.
Question:
4. In DNA, C2 is bonded to...
This is important because...
Answer:
H
Importance - C2 does not have a hydroxyl group, which distinguishes it from RNA.
,Question:
5. In DNA, C3 is bonded to...
This is important because...
Answer:
OH
Importance - The C3 of one sugar forms a phosphodiester bond with the 5`
phosphate of another sugar.
Question:
6. In DNA, C4 is bonded to...
Answer:
C5
Question:
7. In DNA, C5 is bonded to...
This is important because...
Answer:
phosphate
Importance - The C5 can be the lead sugar or form phosphodiester bonds with
the C3 of a second sugar. This gives DNA the 5` to 3` linkage.
Question:
8. Describe why DNA has directionality.
Answer:
The 5' end has a free phosphate group and 3' end has a free hydroxyl (OH) group.
Nucleotide strands form hydrogen bonds between the nitrogenous base off the 1'
Carbon. DNA is always synthesized in the 5'-to-3' direction, which means that
nucleotides are added only to the 3' end of the growing strand. In DNA, G forms
3 hydrogen bonds to C, A forms two hydrogen bonds to T.
Sequence of DNA has an effect on the structure.
Question:
9. What is the meaning of the term antiparallel in the
structure of DNA?
Answer:
Antiparallel in the structure of DNA refers to how each strand of the double helix
is parallel to each other, but their orientation is opposite; their 5' - 3' polarities are
in opposite directions.
, Question:
5. What does the Meselson and Stahl experiment
confirm?
Answer:
In 1958 Meselson and Stahl published their experiment of DNA replication in
Escherichia coli. In this experiment they started their DNA molecule with heavy
nitrogen bases and when they are separated the new strand made was with light
nitrogen bases and the new daughter strand each was found to have one heavy
and one light strand which proved the DNA replication mechanism as
semiconservative DNA replication model as proposed by Watson and Crick.
Question:
11. True/False: Most biological DNA is positively
supercoiled.
Answer:
False, Biological DNA is negatively supercoiled
Question:
12. True/False: GC rich DNA has a lower melting temperature
than AT rich DNA
Answer:
False, GC rich DNA has a higher melting temperature because it forms 3 hydrogen
bonds compared to the 2 hydrogen bonds by AT.
Question:
13. True/False: Chargaff's rule: A=G and T=C
Answer:
False
The rule is that DNA from any organism should have a stoichiometric relationship
of 1:1 for pyrimidines : purines or stated mathematically C + T (pyrimidines) = A + G
(purines).
The number of pyrimidines (C, T) however does not need to be equal (C=T), or
purines for that matter.
Question:
14. True/False: Increased salt concentration increases the
stability of complimentary base pairing
Answer:
True
CORRECT ANSWERS
Question:
1. In the experiment described that supports DNA as the
genetic material:
Why does the mouse die if the researcher mixes the
complete dead pathogenic strain with live non-
pathogenic strain bacteria and then inject it into the
mouse? (The first mouse in the figure).
Answer:
The mouse dies because the R strain has been transformed and is now an S strain,
capable of producing the virulence factors that make the S strain deadly to mice.
This is because as the pathogenic strain is killed, its DNA is released, and now
floating freely can go into the R strain and transform it.
Question:
2. In the experiment described that supports DNA as the
genetic material:
What was the only scenario in which the mouse lives?
Why?
Answer:
The only scenario in which the mouse lives is when DNA is destroyed. This is
because it's DNA that contains the genetic information that codes for the
virulence factors that kill mice when they become infected.
Question:
3. In DNA, C1 is bonded to...
This is important because...
Answer:
a nitrogenous base (adenine, guanine, cytosine, or thymine)
Importance - this allows the nitrogenous bases to form hydrogen bonds (A-T, C-
G) between the 5`-3` strand and the 3`-5` strand.
Question:
4. In DNA, C2 is bonded to...
This is important because...
Answer:
H
Importance - C2 does not have a hydroxyl group, which distinguishes it from RNA.
,Question:
5. In DNA, C3 is bonded to...
This is important because...
Answer:
OH
Importance - The C3 of one sugar forms a phosphodiester bond with the 5`
phosphate of another sugar.
Question:
6. In DNA, C4 is bonded to...
Answer:
C5
Question:
7. In DNA, C5 is bonded to...
This is important because...
Answer:
phosphate
Importance - The C5 can be the lead sugar or form phosphodiester bonds with
the C3 of a second sugar. This gives DNA the 5` to 3` linkage.
Question:
8. Describe why DNA has directionality.
Answer:
The 5' end has a free phosphate group and 3' end has a free hydroxyl (OH) group.
Nucleotide strands form hydrogen bonds between the nitrogenous base off the 1'
Carbon. DNA is always synthesized in the 5'-to-3' direction, which means that
nucleotides are added only to the 3' end of the growing strand. In DNA, G forms
3 hydrogen bonds to C, A forms two hydrogen bonds to T.
Sequence of DNA has an effect on the structure.
Question:
9. What is the meaning of the term antiparallel in the
structure of DNA?
Answer:
Antiparallel in the structure of DNA refers to how each strand of the double helix
is parallel to each other, but their orientation is opposite; their 5' - 3' polarities are
in opposite directions.
, Question:
5. What does the Meselson and Stahl experiment
confirm?
Answer:
In 1958 Meselson and Stahl published their experiment of DNA replication in
Escherichia coli. In this experiment they started their DNA molecule with heavy
nitrogen bases and when they are separated the new strand made was with light
nitrogen bases and the new daughter strand each was found to have one heavy
and one light strand which proved the DNA replication mechanism as
semiconservative DNA replication model as proposed by Watson and Crick.
Question:
11. True/False: Most biological DNA is positively
supercoiled.
Answer:
False, Biological DNA is negatively supercoiled
Question:
12. True/False: GC rich DNA has a lower melting temperature
than AT rich DNA
Answer:
False, GC rich DNA has a higher melting temperature because it forms 3 hydrogen
bonds compared to the 2 hydrogen bonds by AT.
Question:
13. True/False: Chargaff's rule: A=G and T=C
Answer:
False
The rule is that DNA from any organism should have a stoichiometric relationship
of 1:1 for pyrimidines : purines or stated mathematically C + T (pyrimidines) = A + G
(purines).
The number of pyrimidines (C, T) however does not need to be equal (C=T), or
purines for that matter.
Question:
14. True/False: Increased salt concentration increases the
stability of complimentary base pairing
Answer:
True
