The following questions have been selected to test and consolidate your learning in this area of the CfE Higher syllabus.  They will also help you understand the style of question you may encounter in your final exam.  The answers for each question can be accessed by clicking to reveal. At the bottom of the section are the relevant homework sheets for this section. The answers can be accessed by clicking the marking guide link. Remember, practice in applying your learning is key to building your confidence and knowledge and improving your exam success.

1.  Read through the following passage on the structure of DNA, then choose the most appropriate word or words to complete the passage.

A DNA molecule consists of two strands of nucleotides. Each of these strands is twisted around the other, forming a ……………………..……… .

Each nucleotide consists of a pentose sugar called ……………………. , a base and a ….…………………………… . In each strand, the nucleotides are held together by …………………………………… bonds.
The two strands are held together by complementary base pairing. Adenine bonds with …………………………………. and cytosine bonds with ……………………………. .
The name of the bond that forms between these bases is a ……………………………………... bond. A DNA molecule that is composed of 34% adenine will be composed of …………… % cytosine.

//

Answers

a) What part of DNA is represented by the letter X?
b) What part of DNA is represented by the letter Y?
c) Name the type of bonds holding the two strands of DNA together.
d) A culture of bacteria had its DNA labelled with the heavy isotope of nitrogen (¹⁵N).
The diagram below shows the position of the DNA band in the centrifuge tube when the DNA was labelled with the heavy isotope of nitrogen, ¹⁵N.

The bacterial culture was then allowed to reproduce using nucleotides containing the normal isotope of nitrogen (¹⁴N).
(i) Which of the diagrams below show the correct pattern of DNA after the bacteria have divided once?

(ii) Which of the diagrams below show the correct pattern of DNA after the bacteria have divided twice?

e) Name the place in a eukaryotic cell where messenger RNA will be synthesised.

//

Answers

3. DNA is a very important molecule in living organisms as it carries the genetic code that controls all characteristics. When a cell divides, the DNA molecule replicates so that each resulting daughter cell is genetically identical to the original parent cell.
The diagram below shows part of this process of DNA replication.

For each of the statements below, put a cross in the box that corresponds to the
correct statement about DNA structure or DNA replication.
(a) What is the structure labelled J?
(b) What is the structure labelled K?
(c) Name the bond labelled L.
(d) What type of molecule is the structure labelled M?
(e) If the base labelled N on the parent DNA molecule is adenine, the base labelled O on the new DNA molecule is what?
(f ) The bond labelled P is a

//

Answers

HBiol Sheet 1_1

The Marking Guide has been made available. Use this where you are having difficulty, to help you understand the question better. Simply copying the answers from the Marking Guide will not help you learn, using it to help you understand the questions/ answers will.

You will need your Glow login to access the Marking Guide.

1. The diagram below shows part of a DNA molecule.

a) From the diagram, draw a single nucleotide in your jotter.
b) Name parts X and Y.
c) Name the two DNA bases not shown in the diagram.
d) What sequence would the complementary strand be?
e) Redraw the diagram in your jotter and label the 5’ and 3’ ends of the strand.
f) DNA is normally double stranded. What term is used to describe the shape of this double stranded molecule?

2. The percentage of adenine (A) bases in a double strand of DNA is known. For how many of the other bases could the percentage be calculated?

Explain your answer.

3. In analysing DNA, a technique called gel electrophoresis can be used to separate different strands of DNA.

a) What property of DNA allows the molecules to be separated using electrophoresis?
b) Having separated DNA molecules of different lengths using gel electrophoresis, where would the largest DNA strands be found? Explain why.
c) Explain why the smaller DNA strands travel faster through the gel.
d) DNA is an extremely large molecule, how can it be broken into smaller fragments?

4. A DNA molecule contains 8000 bases and 1800 of these are thymine (T). What percentage of the bases would be guanine (G)?

Remember to show your working in your jotter.

5. In 1952, Hershey and Chase used a type of virus, called a phage, grown in medium containing either radioactive phosphorous (32P) or sulphur (35S), to infect bacteria. The phage attach to the outside of the bacteria and pass their genetic material inside the bacteria, where new phage are then made.

Following infection, the bacterial cultures were agitated in a blender to remove the phage from the surface of the bacterial cells. Centrifugation was then used to separate bacteria from the phage, which remained in the liquid.

The diagram below shows details of the blender experiment, carried out by Hershey and Chase in 1952.

(A video of the details of the experiment can be viewed from here)

a) Radioactive nitrogen was also available to Hershey and Chase. Explain why radioactive nitrogen was not suitable for use in this experiment.
b) Explain why the use of radioactive sulphur and radioactive phosphorous allowed the researchers to distinguish between DNA and protein in their experiment.
c) For each of the treatments, Batch 1 and Batch 2, where was the radioactivity found after centrifugation; the pellet or the liquid? Explain these results?
d) What conclusion can be drawn from the results of this experiment?

HBiol Sheet 1_2

The Marking Guide has been made available. Use this where you are having difficulty, to help you understand the question better. Simply copying the answers from the Marking Guide will not help you learn, using it to help you understand the questions/ answers will.

You will need your Glow login to access the Marking Guide.

1. The diagram below shows part of a DNA molecule.

a) Name components X and Y.
b) Name the type of bond shown at Z.

2. The arrangement of DNA in bacteria, yeast (fungi) and eukaryotic cells differs. In your jotter construct a table to compare the organisation of DNA in prokaryotic and eukaryotic cells. Your table should include information about genetic material outside the nucleus, refer to the manner in which DNA is packaged, the role and type of proteins involved in packaging and the shape of any chromosomes.

Use a separate column for bacteria, fungi and animal/ plant cells.

3. The diagram below summarises the original experiment performed by Griffiths to investigate the molecules which carry information. The rough strain of pneumococcus bacteria is non-virulent, but the smooth strain is lethal. An interactive animation has been prepared detailing the experimental method. This will help you answer some of the questions that follow. Click on the diagram below to access the interactive presentation.

Griffiths concluded from his experiments that a transforming principle was responsible for the passage of virulence. Avery et.al. subsequently isolated this transforming principle.In their experiments they separated a lysate, containing the transforming principle from other cell debris. They used different enzyme treatments to determine which of the components of the lysate were responsible for the transformation of the R into S strain.

Answer the following questions:
a) What is the structural difference between the rough and smooth strains of pneumococcus?
b) How might they have separated the lysate from the cell debris?
c) Separate samples of the lysate were treated with different enzymes. Why would the use of enzymes allow the scientists to determine which of the components was responsible for the transformation?
d) Explain why the treatment of the lysate with enzyme SIII failed to prevent the lysate from transforming the R strain into virulent S train bacteria.
e) Avery et. al. used alcohol to isolate the nucleic acids. How did they manage to distinguish between the role of DNA and RNA?
f) What did Avery et.al. conclude from their experiments?
g) Bacteria can exchange DNA naturally. What structure in a bacterium allows them to exchange DNA?
h) What problem is facing us, partly as a result of this ability of bacteria to exchange DNA?

HBiol Sht 1_3

The Marking Guide has been made available. Use this where you are having difficulty, to help you understand the question better. Simply copying the answers from the Marking Guide will not help you learn, using it to help you understand the questions/ answers will.

You will need your Glow login to access the Marking Guide.

1. In 1957, Meselson and Stahl conducted experiments to investigate the process of DNA replication. Their experiment attempted to distinguish three hypotheses regarding the way DNA replication proceeds:

i.conservative (one of the daughter molecules is the parent one),

ii.semi-conservative (both daughter molecules contain a strand belonging to the parent molecule and a newly synthesised strand),

iii.or dispersive (both strands of both daughter molecules contain fragments of the parent molecule and fragments which are newly synthesised).

In their experiment, Meselson and Stahl followed the protocol shown below.

Following a period of growth in flask 2, the DNA of the bacteria was analysed. To determine the density of the DNA strands in the bacteria.

a) What method could used to separate the DNA strands of differing densities?
b) Into what part of a DNA nucleotide was the nitrogen incorporated?
c) The results of the experiment indicated that DNA replication was semi-conservative. In your jotter draw a test tube with two labelled bands to represent the positions of DNA made with only 15N and a second band for DNA containing only 14N. Add a third band to show where the DNA formed by semi-conservative replication would be positioned.
d) Redraw a second test tube in your jotter and show the results that would have been expected if DNA replication was conservative.
e) Describe the expected results of the CsCl density gradient centrifugation if DNA replication was dispersive.
f) What property of DNA lead Watson and Crick in 1953 to predict semi-conservative replication as the most likely copying mechanism?

2. A single strand of a DNA molecule has 6000 nucleotides of which 24% are adenine and 18% are cytosine.

a) Calculate the combined percentage of thymine and guanine bases on the same DNA strand.
b) How many guanine bases would be present on the complementary strand of this DNA molecule?

3. The diagram below shows part of a DNA molecule during replication and other molecules associated with it.

a) Name molecule X which is associated with the tightly coiled DNA.
b) Name the type of bond shown at Y.
c) Name base Z.
d) What feature of an organism is determined by the sequence of its bases?
e) In nucleic acids, the phosphate group is attached to which carbon of deoxyribose sugar?
f) Copy and complete the following, choosing the appropriate word from each bracketed pair.

4. A DNA strand has the sequence A-C-A-G-C-C-G-T-A. What would the sequence of its complementary strand be?

HBiolSheet 1_3A

The Marking Guide has been made available. Use this where you are having difficulty, to help you understand the question better. Simply copying the answers from the Marking Guide will not help you learn, using it to help you understand the questions/ answers will.

You will need your Glow login to access the Marking Guide.

1. Decide if each of the following statements about DNA replication is True or False. If the statement is False, rewrite the statement in your jotter, providing the correct word to replace the word underlined in the statement.

a. During DNA replication hydrogen bonds between bases break.
b. During the formation of a new DNA molecule, base pairing is followed by bonding between deoxyribose and bases.
c. As a result of DNA replication, the DNA content of a cell is halved.

2. The flowchart below describes steps in the process of DNA replication.

a) In your jotter, describe what happens at Step 2 and Step 4.
b) Other than the original DNA strand and free DNA nucleotides, give one substance needed for DNA replication.
c) State the importance of DNA replication to cells.

3. The diagram below shows the first two nucleotides in a DNA strand.

a) Which nucleotide, 1 or 2 is the 3’ end, and which is the 5’ end of this strand of DNA? Explain your answer.
b) To which end of the DNA molecule will new nucleotides be added when the strand of DNA above has new nucleotides added to it?
c) What type of bond is bond A?
d) A full DNA molecule consists of two DNA strands. Describe two ways in which the second strand of this DNA molecule will differ from the one drawn here.

4. The genome of a typical bacterium contains about 5 x 10⁶ base pairs and can be replicated in 20 minutes. The human genome is larger (3 x 10⁹ base pairs); yet the entire human genome can be replicated in 5 hours.

a) How many times larger is the human genome compared with the bacterial genome?
b) At what rate (in base pairs per minute) is the bacterial genome replicated?
c) How long would it take to replicate the human genome at this rate?
d) According to the text, at what rate (in base pairs per minute) is the human genome replicated?
e) How many times faster is the human genome replicated?
f) Explain how the replication of the human genome can occur so much faster.