7.1 DNA Structure & Replication

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.

The graph below shows the results of their investigation.

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

[2 marks]

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

[2 marks]

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

[1 mark]

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

[2 marks]

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

State two deductions that can be made from the data.

[2 marks]

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

Show your working.

[2 marks]

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.

[2 marks]

A mutation of the HTT gene may increase the risk of a person developing Huntington disease later in their lives. Blood samples from four different individuals were taken and the base sequences of a part of their HTT gene were sequenced using the chain termination method. These were compared to the normal base sequence of the HTT gene by reading from the smallest to the largest fragments.

The results from the gel electrophoresis are shown in the diagram below.

Identify the base sequence of the normal gene.

[1 mark]

Using the information in part a), deduce which patient(s) would have a higher risk of developing Huntington disease.

[1 mark]

During the chain termination method, modified nucleotides called dideoxynucleotides are used.

Contrast the structure and function of a dideoxynucleotide with a regular nucleotide.

[2 marks]

Suggest what would happen to DNA polymerase once a dideoxynucleotide is incorporated into the growing DNA strand.

[1 mark]

One mark is available for clarity of communication throughout this question.

Compare and contrast DNA profiling with the chain termination method of DNA sequencing.

[6 marks]

The structure of DNA was discovered due to the research and input from several different scientists.

Outline how the discovery of the structure of DNA led to the development of the theory of semi-conservative DNA replication.

[4 marks]

Discuss the formation of Okazaki fragments during the process of replication on the lagging strand of a DNA molecule.

[5 marks]