Structure of Nucleic Acids & Replication of DNA (CIE A Level Biology)

Fig. 1 shows the structure of a small section of DNA in a cell during transcription.

Fig. 1

Identify the bases Q, R and S as indicated in Fig. 1

State the importance of hydrogen bonding in the structure of a DNA molecule.

ATP is the energy-carrying molecule which fuels chemical reactions within the cell.

Compare the structure of ATP with the structure of a DNA nucleotide.

Scientists analysed the nucleotide base sequence of a DNA molecule extracted from a human cheek cell. It was found that 29% of the nucleotides in this DNA molecule contained the nitrogenous base adenine.

Calculate the percentage of nucleotides that would contain the nitrogenous base cytosine.

Show your working.

The biochemist, Erwin Chargaff, did much of his research on the structure of DNA. From the research he conducted in the 1950s, he was able to determine the relative quantities of the four bases found in a DNA molecules of different organisms. Chargaff's work provided evidence which assisted in the development of later proposals on the structure of DNA.

Some of Chargaff's data is shown in Table 1.

Table 1

Explain how the data in Table 1 helps to confirm the arrangement of bases in a DNA molecule.

Describe three ways in which the structure of DNA differs from the structure of RNA.

Fig. 1 shows the structure of ATP.

Fig. 1

Explain why ATP can be described as a 'phosphorylated nucleotide'.

Messenger RNA (mRNA) plays an important role in protein synthesis by transcribing the genetic code and bringing it to the ribosomes for translation.

Fig.1 shows the structure of part of an mRNA molecule.

Fig. 1

Explain how the structure of mRNA enables it to perform its function.

Table 1 contains a variety of statements that apply to different biological molecules.

Identify which of the statements apply to DNA, messenger RNA and proteins by placing a tick (√) in the box.

Table 1

A section of a DNA molecule contains 124 nucleotides.

Calculate the maximum number of hydrogen bonds that could be present in this section of DNA.

Show your working.

Describe the role of covalent bonds in linking nucleotides together to form a polynucleotide, such as DNA.

DNA replication is described as being 'semi-conservative'.

Explain what this means.

Explain how the structure of DNA facilitates semi-conservative replication of the molecule.

The new DNA molecules that form during semi-conservative replication are identical to each other.

Discuss the importance of this statement.

Matthew Meselson and Franklin Stahl did an experiment in 1958 to investigate the replication of DNA in E. coli. They grew E. coli in a medium that contained only a heavy isotope of nitrogen (¹⁵N) for several generations. They extracted the DNA and placed it in a centrifuge to determine the density of the DNA based on the nitrogen that was incorporated into the DNA during replication.

They transferred E. coli cells that only contained ¹⁵N in their DNA to a growth medium that contained only ¹⁴N, which is a lighter isotope of nitrogen. They allowed the E. coli to divide for several generations, measuring the change in DNA density after each division by centrifugation.

The results of their experiment is represented in Table 1.

Table 1

[2 marks]

Define the term 'leading' and 'lagging' strand in DNA replication.

Describe the structure of a monomer of a DNA molecule.

State the names of two purines and two pyrimidines in RNA.

Describe how a DNA molecule replicates.

Fig. 1 shows the structure of a prokaryotic cell.

Fig. 1

Fig. 1 has not been fully labelled to confirm that the cell is prokaryotic.

State what other information could be added to two of the labels to confirm that this cell is prokaryotic and not eukaryotic.

Some prokaryotes are plant pathogens.

Liberibacter is a group of prokaryotic plant pathogens that causes severe damage to a variety of plant crops across the world.

Scientists made observations about plants infected with these pathogens compared to uninfected plants:

• • starch accumulates in the leaves
  • starch does not accumulate in roots and other storage organs
  • fruits are smaller
  • the pathogen is widely distributed throughout the plant and is found in a number of different organs including the root and leaf.

The scientists deduced that the pathogen infected the phloem tissue.

Suggest why the scientists were able to deduce that the pathogen infected the phloem tissue.

DNA and RNA both contain nucleotides with adenine.

Complete Table 1 to compare:

• • a DNA nucleotide with adenine
  • an RNA nucleotide with adenine
  • ATP

Table 1

A group of scientists studied the replication of DNA in Escherichia coli bacteria.

During their investigation, radioactive nucleotides were added to DNA that was actively replicating in a short pulse of about 5 seconds. This allowed the radioactive nucleotides to be incorporated into the new DNA strands. 

This was followed by a "chase" period, during which an abundance of unlabelled nucleotides was added to the DNA for different amounts of time, between 7 and 120 seconds. After the isolation and centrifugation of the DNA molecules, the results were obtained.

Fig. 1 below shows the results of their investigation.

Fig. 1

Contrast the results obtained at a "chase" period of 7 seconds with those obtained at 120 seconds.

Explain the results obtained at a "chase" period of 60 seconds.

Suggest a possible explanation for the low number of small fragments present at 120 seconds.

Sketch a line on the graph of the predicted results that could be obtained at a "chase" period of 150 seconds.

]

Table 1 shows the DNA base composition of different organisms. Note that for E. coli, the %C and %T has deliberately been left out.

Table 1

State two deductions that can be made from the data.

Calculate the possible value of X in the table in part (a).

Show your working.

The results from part (a) are similar to those first obtained by Erwin Chargaff in the 1950s. 

Suggest how Chargaff's research may have impacted the work of Crick and Watson.

Identify the pyrimidine and purine bases in DNA and RNA and explain their base-pairing characteristics.

The two strands of DNA are described as 'antiparallel'.

Explain the meaning of 'antiparallel' in this context.

The ends of a DNA strand are referred to as the 3' ('3-prime') end and the 5' ('5-prime') end.

Explain what this means by referring to the structure of a DNA strand.

Adenine / thymine and guanine / cytosine are well known to form hydrogen bonds with each other in complementary base-pairs within the DNA double helix. These bases can also form hydrogen bonds with other molecules in order to carry out specific functions.

Suggest two examples of how such molecules can bind to the bases and explain how this adds functionality.

Compare and contrast the structures of DNA and RNA.

Meselson and Stahl were two scientists who investigated DNA replication during the 1950s. 

They grew E. coli bacteria in dishes containing culture media. 

• Some bacteria were grown in a culture containing an isotope of nitrogen with a higher molecular weight (¹⁵N) than normal nitrogen (¹⁴N)
• Some bacteria were first grown in the (¹⁵N) culture and then transferred to the normal (¹⁴N) culture
• Some bacteria were grown in the normal (¹⁴N) culture

The bacteria started incorporating the nitrogen isotopes from the culture media into their DNA. Bacterial colonies that incorporated (¹⁵N) culture became denser than bacteria incorporating normal (¹⁴N) culture.

After letting the bacteria grow for several generations, the scientists extracted and centrifuged the DNA. During centrifugation, the heavier, denser molecules settled towards the bottom of the tube. Figure 1 shows some of their results.

Key:

A Bacteria grown in a (¹⁵N) culture 

B Bacteria first grown in a (¹⁵N) culture and then transferred to the normal (¹⁴N) culture (contents not shown)

C Bacteria grown in a (¹⁴N) culture

Fig 1

Some bacteria were first grown in the (¹⁵N) culture and then transferred to the normal (¹⁴N) culture (tube B). They were left there with enough time to replicate once before the DNA was extracted. 

Suggest where a band would appear on this tube (B) in comparison to tubes A and C. 

[2]

Suggest how the appearance of tube B would change if the E. coli bacteria were left in the (¹⁴N) culture to reproduce for one additional generation. 

[2]

Justify your answer to part (a). 

Explain how Meselson and Stahl’s experiment demonstrated the semi-conservative replication of DNA, concerning the results shown in Figure 1.

Another theory for how DNA replicates is called conservative DNA replication.

Conservative replication was disproven by this experiment. The theory suggested that when DNA replicates the original DNA strand is fully conserved and the new strand is entirely made up of new DNA. 

Describe how the outcome of the experiment from part (a)(i) would be different if the DNA replication were fully conservative. 

Even the smallest DNA molecules are very long.

• A kilobase (Kb) is a unit equivalent to 1000 base pairs of a DNA molecule
• One Kb of double-stranded DNA has a length of 0.34 μm

The DNA in the nucleus of a cell from a fruit fly (Drosophila) is 5.6 cm long.

Calculate the number of Kb in the DNA of the fruit fly. Give your answer to the nearest whole number.

Other than for use in replication, explain one advantage of DNA molecules having two strands.

The image below shows a section of the skeletal formula of a DNA molecule.

Number the carbon atoms of all the pentose sugars shown in the image using the standard numbering format.

A section of DNA contains 1,200 base pairs.

• The number of guanine molecules on strand one was counted as 156.
• The number of cytosine molecules on strand one was counted as 209.
• The number of adenine molecules on strand two was counted as 264.

Complete the table below to include the total number of each base present in the section, and the % composition of each base.