State the purpose of DNA replication.
The diagrams below show two models of DNA replication.
State, with a reason, which diagram, A or B, is correct.
Identify two enzymes that are involved with the process of DNA replication.
2 marksCalculate the amount of the total DNA that consists of the original DNA strands after 3 full cycles
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Both DNA and RNA contain pentose sugars in their sugar-phosphate backbones.
Define the term pentose in reference to sugar molecules.
During DNA replication the new bases are added to the new strand by the enzyme DNA polymerase in the 5' to 3' direction.
Use your knowledge of enzymes to explain why it would not be possible for DNA polymerase to add the new bases in the 3' to 5' direction.
When bases are bonded to the new DNA strands during replication they undergo a condensation reaction.
Describe the events that occur during a condensation reaction.
In a length of DNA 1 000 nucleotides long there are 382 guanine nucleotides in one of the strands.
Explain why it is not possible to calculate the number of guanine nucleotides in the opposite strand from the information provided.
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Describe how the structure of a tRNA molecule contributes to the accuracy of the translation.
Draw a labelled diagram of two nucleotides bonded together within the same DNA strand.
Outline the three stages of polymerase chain reaction (PCR).
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The diagram below shows the process of DNA replication.
Identify whether strands X and Y are leading or lagging strands of the DNA molecule.
DNA replicates in a semi-conservative way.
Define the term 'semi-conservative' with regards to DNA replication.
One of the enzymes involved with DNA replication is DNA primase.
Describe the role of DNA primase during DNA replication.
DNA replication can only occur in the 5' to 3' direction in the new strand.
Explain the reason for this.
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A crime was committed and the DNA profiles of the victim and a drop of blood found at the crime scene were constructed. These were compared to the DNA profiles of three possible suspects, as seen in the diagram below.
Identify the suspect that most likely committed the crime.
Variable number tandem repeats (VNTRs) are short, non-coding regions of DNA that can be used in DNA profiling.
Explain the use of VNTRs in DNA profiling.
The diagram below represents the structure of a chromosome.
Label parts R and S of the chromosome.
R and S from the chromosome at part c) represents non-coding regions of DNA.
State the function of R and S in a chromosome.
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In the electrophoretogram, the DNA has moved from the negative cathode to the positive anode.
State the property of DNA which results in movement from cathode to anode.
State two uses of gel electrophoresis.
In some situations, gel electrophoresis cannot be carried out as the DNA samples are not collected in a large enough quantity.
Identify the process used to amplify the DNA in order to supply enough DNA for electrophoresis to be successful.
The stages of electrophoresis are shown below.
Construct a logical sequence of events in gel electrophoresis by numbering the steps 1–4. The first one has been done for you.
A sample of DNA is collected from an individual 1. DNA fragments are loaded into wells in the agarose gel using a micropipette. An enzyme is used to create fragments of the DNA in the sample. An electrical current is applied to the tank
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The molecules named below are all required during the process of PCR.
Identify the function for each of the molecules named by drawing a line between the boxes to show
State the property of Taq polymerase that makes it suitable for use in PCR.
The graph shows how the quantity of DNA increases over several cycles of PCR
Identify the stage of the graph where replication is exponential.
The electrophoretogram shows a pattern produced from the DNA of two people.
State how the electrophoretogram shows that the two people are identical twins.
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During electrophoresis, molecules travel through an agarose gel.
State two properties of the molecules that determine how far through the gel they can travel.
Outline the process used to determine paternity using electrophoretograms produced in gel electrophoresis.
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Describe the structural features of a DNA molecule.
Describe the process of semi-conservative replication of DNA in eukaryotes. In your answer, include details of any molecules, bonds or enzymes involved.
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The diagram below shows one cycle of the polymerase chain reaction (PCR).
Outline the events that are taking place during stage 1 in the diagram.
Molecule X in the diagram shown in part (a) is a DNA primer.
State the role of a DNA primer in PCR.
Stage 3 in PCR involves an enzyme.
Explain how the enzyme is suitable for its role in PCR.
State two applications of PCR.
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The diagram below illustrates a small section of a DNA molecule from the nucleus of a eukaryotic cell.
State the structures labelled X and Y.
A repetitive sequence of DNA occurs at the ends of eukaryotic chromosomes, called a telomere.
Explain the role of a telomere.
Most of the DNA in an organism is contained within the nucleus. Some of this DNA is unique, whilst some is made up of non-coding DNA called highly repetitive sequences.
Contrast unique and highly repetitive sequences of DNA
DNA was originally thought of as a protein. In the 1950s, Alfred Hershey and Martha Chase showed that DNA is a factor of heredity responsible for carrying genetic information from one generation to another.
Describe their experiment.
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State two features that allow nucleic acids or proteins to be separated by gel electrophoresis.
During gel electrophoresis of a sample of DNA fragments,
(i) Identify which electrode the DNA would move towards. Justify your answer.
(ii) State which component of DNA gives it its charge.
State which class of enzymes can be used to cleave DNA into fragments prior to gel electrophoresis.
Enzymes and gel electrophoresis can be used in genetic screening to identify genes associated with a disease. The mutation of the Beta-globin gene, which gives rise to sickle-cell anaemia, removes a recognition site of the restriction enzyme DdeI as shown in the diagram below. The lengths of some fragments are shown in base pairs (bp).
(i) Deduce the size of fragment X.
(ii) DdeI digested DNA from an individual who was a carrier for the sickle-cell beta-globin gene was analysed with gel electrophoresis as shown below. Draw and label the DNA fragments that would result from a normal individual.
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Explain why only bases that are complementary to the bases on the template strand can be added to the new DNA strand during DNA replication.
Ultraviolet exposure can cause guanine to be oxidised to 8-oxyguanine, which is no longer complementary to cytosine. Instead, during replication, 8-oxyguanine can form bonds with adenine, resulting in a base pair.
Outline the possible consequences of this change.
1 markIn the absence of mutagens, the rate of mutations during DNA replication is very low, approximately 160 bases per cell cycle.
Given that the human genome contains 3.2 billion base pairs, calculate the percentage copying error rate of each cell cycle.
Explain the formation of Okazaki fragments during the process of replication on the lagging strand of a DNA molecule.
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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.
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.
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3 marksOne round of PCR takes approximately 75 seconds.
Calculate how many copies of DNA would be produced after a single fragment of DNA has been in the thermal cycler for 1.5 hours. Give your answer in standard form.
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The diagram below shows the processes involved in the amplification of a sample of viral RNA.
Compare and contrast the process in the image with the process used in the amplification of a fragment of DNA.
RNAase is an enzyme that digests RNA. The addition of RNAase enzyme occurs between stages 3 and 4 shown in the diagram from part a).
Suggest why this is a necessary part of the process.
Seven skeletons were discovered in a house in Pompeii, three of which were children. It is believed they were inhabitants and workers within the house when Mount Vesuvius erupted in 79 AD.
Researchers were able to isolate very small amounts of DNA from these skeletons. The DNA obtained was used in the polymerase chain reaction (PCR). Genetic fingerprinting was then carried out on this DNA to identify the skeletons.
The image below shows some of the results of the genetic fingerprinting of the three children and four adults.
Explain why the researchers used PCR in their investigation.
It was determined that the three children were siblings and shared the same biological parents. Their mother is Adult B.
Identify, with a reason, which of the other adults was the children’s father.
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During PCR DNA is heated to 94 °C and DNA primers, nucleotides and thermostable enzymes are added to the mixture.
Explain why:
(i) DNA is heated to 94 °C.
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(ii) DNA primers are used.
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(iii) Nucleotides are used.
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(iv) Thermostable enzymes are used.
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