«Chapter 5 Questions on p. 65 – November 5, 2012»
1) With reference to Figure 17, answer the following questions.
a) What part of the nucleotide is labelled A? The sugar.
b) What kind of bond forms between the structures labelled B? Hydrogen
bonds.
c) What kind of bond is indicated by label C? Covalent bond.
d) What sub-unit is indicated by label D? The phosphate group.
e) What sub-unit is indicated by label E? The nitrogenous base.
2) Figure 18 shows a short section of DNA before and after replication. If the
nucleotides used to replicate the DNA were radioactive, which strands in the
replicated molecules would be radioactive? II and III
3) Complete the table by filling in the missing base percentages.
Species
Base composition /%
A
C
G
T
I
30
30
20
20
II
28
28
22
22
III
24
24
26
26
IV
26
26
24
24
4) Give a synonym for the term “nucleoside monophosphate”. Nucleotide.
5) State the name of the enzyme with each of these functions.
a) Creates an RNA primer during replication. Primase.
b) Removes the primer at the end of replication. DNA polymerase I.
c) Seals gaps between fragments on the lagging strand. DNA ligase.
d) Unwinds DNA prior to replication. DNA helicase.
6) Refer to Figure 19 when answering the following questions.
a) State what molecule is represented. Sugar.
b) State whether the molecule would be found in DNA or RNA. RNA
c) State the part of the moleucle to which phosphates bind. V
d) Identify the part of the molecule that refers to the 3’ end. III
7)
a) Outline the steps involved in DNA replication. DNA replication is
semi-conservative; the original strands of the DNA are used to make two DNA
molecules that each contain an original strand and a new tDNA helicase unwinds DNA,
after which nucleotides with bases that are complementary to the bases (adenine to
thymine and guanine to cytosine) on one of the strands bind them through hydrogen
bonds. The phosphate group and the sugar of the nucleotides are covalently bonded
by the DNA polymerase.
b) Describe the structure and function of nucleosomes. Nucleosomes are
composed of eight histone proteins arranged in an octomer. They help prepare DNA
for mitosis/meiosis and they help regulate the expression of genes within the cell.
c) Compare RNA and DNA. The nucleotides in RNA and DNA are composed of
a phosphate group, a sugar, and a nitrogenous base. The sugar in RNA is ribose,
whereas the sugar in DNA is deoxyribose. Three of the bases – adenine, cytosine, and
guanine – are the same, but the fourth is thymine in DNA and uracil in RNA. Finally,
DNA is double stranded and RNA is single stranded.
8) State two functions of nucleosomes. Their two functions are to help supercoil the
DNA and the regulate transcription.
9) Figure 20 shows the structure of part of a nucleic acid molecule.
a) State, with a reason, whether the molecule is part of RNA or DNA. RNA –
the sugar is ribose and the molecule is single stranded.
b) State, with a reason, whether I is the 3’ or 5’ terminal. III – C5 is the one
that has the base attached to it, and it decreases counterclockwise (which means that
III is C3).
c) II is guanine. State whether it is a purine or a pyrimidine base. Purine.
«DBQ on p. 63 – November 5, 2012»
Explain why the data in Figure 14 suggests that replication is semi-conservative and
why the other models are not supported.
The data in Figure 14 suggests that replication is semi-conservative because
it explains why the densities of the DNA. Because the density is high in the 15N
medium and low in the 14N medium, it makes sense that a molecule of DNA made
using 15N would have an intermediate density when it is replicated using 14N. Also,
because one strand would be made of the denser 15N and the other would be made of
the less dense 14N in both of the newly synthesized DNA, the densities would be the
saem for both. This is not the case for (b), conservative replication, because then there
would need to be a high density curve and a low density curve; one of the newly
synthesized DNA molecules would be completely composed of the less dense 14N and
the other of the dense 15N.
This means that the remaining options are (a) dispersive and (c)
semi-conservative. However, a fourth graph is provided which shows the densities in
the next step – still in the 14N medium – when two newly synthesized DNA also
replicate. The graph shows two curves: one showing low density and another showing
medium density. Because there are two different curves, this means that the DNA
molecules have different densities. This means that dispersive replication is not a
viable option – even after they were separated all of the strands be composed of the
same amount of 15N and 14N, so the densities would continue to stay the same if they
were to be replicated.
This leaves semi-conservative replication. In this type of replication, one
strand in each DNA molecule would be composed of 15N and the other of 14N.
Therefore, if they were synthesized again using 14N, it would result in two DNA
molecules completely composed of 14N and two that have one 15N strand and 14N
strand. This would explain why the two curves show low density (because the DNA is
completely composed of 14N, the less dense one) and intermediate density (because
the DNA is composed of both 14N and 15N).