ScienceLinks - 3.1.5.2 DNA Replication

ScienceLinks

DNA Replication

Q1. Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication. (1)

Watson and Crick

Q2. Explain why the replication of DNA is described as semi-conservative. (2)

1. each strand is copied or acts as a template

2. daughter DNA has one new strand and one original (parent) strand

Q3. Give two features of DNA and explain how each one is important in the semi-conservative replication of DNA. (2)

1. Weak / easily broken hydrogen bonds between bases allow two strands to separate / unzip

2. Two strands, so both can act as templates

3. Complementary base pairing allows accurate replication

Q4. Name two enzymes involved in the semi-conservative replication of DNA. (2)

1.DNA helicase

2.DNA polymerase

Reject RNA

Accept phonetic spellings

Q5. DNA helicase is important in DNA replication. Explain why. (2)

1.Separates / unwinds / unzips strands / helix / breaks H-bonds

Q Neutral: strands / helix split

Accept: unzips bases

2.So nucleotides can attach / are attracted / strands can act as templates

Q Neutral: bases can attach

Neutral: helix can act as a template

Q6. In the process of semi-conservative DNA replication, the two strands within a DNA molecule are separated. Each then acts as a template for the formation of a new complementary strand.

Describe how the separation of strands occurs. (2)

1. DNA helicase

2. Breaks hydrogen bonds between complementary bases (AT and GC)

OR Breaks hydrogen bonds between polynucleotide strands

Reject hydrolysis of hydrogen bonds

Q7. Describe the role of DNA polymerase in the semi-conservative replication of DNA. (2)

1.Joins adjacent DNA nucleotides

Reject suggestions that it forms hydrogen bonds or joins complementary bases.

Reject ‘nucleotide bases

2.Catalyses condensation reactions

Catalyses formation of phosphodiester bonds between adjacent nucleotides

Q8. Describe the role of two named enzymes in the process of semi-conservative replication of DNA. (3)

1. DNA helicase causes breaking of hydrogen/H bonds between DNA strands

Reject ‘helicase hydrolyses hydrogen bonds’

2.DNA polymerase joins the DNA nucleotides

Reject if suggestion that DNA polymerase joins the complementary nucleotides or forms H bonds.

Reject if joining RNA nucleotides or forming RNA.

3.Forming phosphodiester bonds

Q9. Describe the process of semi-conservative replication of DNA. (5)

1.DNA helicase unwinds DNA/double helix

OR DNA helicase breaks hydrogen bonds

2.Both strands act as templates

Accept description of ‘template’, e.g. exposed bases on single polynucleotide strands

3.Free DNA nucleotides line up in complementary pairs/A-T and G-C

4. DNA polymerase joins nucleotides of new strand

Reject forms hydrogen bonds/joins bases

5.Forming phosphodiester bonds

6. Each new DNA molecule consists of one old/original/template strand and one new strand

Q10. Use your knowledge of semi-conservative replication of DNA to suggest the role of the single-stranded DNA fragments (2)

1.Template

2.Determines order of nucleotides / bases

Q11. Use your knowledge of semi-conservative replication of DNA to suggest the role of the DNA nucleotides. (1)

Form complementary pairs such as A – T and G - C

OR Form complementary DNA strand

Ignore forms complementary bases

Accept sequence/ chain for strand

Q12. Describe and explain how the structure of DNA results in accurate replication. (4)

1. two strands therefore semi-conservative replication possible

2. base pairing / hydrogen bonds holds strands together

3. hydrogen bonds weak / easily broken, allow strands to separate

4. bases sequence exposed so act as template / can be copied

5. A with T, C with G / complementary copy

6. DNA one parent and one new strand

Knowledge Application

Q13. The diagram shows the process of DNA replication. The horizontal lines represent the positions of bases.

a. What is represented by the part of the DNA molecule labelled W? (1)

sugar or phosphate / nucleotide chain / backbone / original / parent DNA

In the diagram, A represents adenine and C represents cytosine.

b. Name the bases found at position X, Y and W. (3)

X thymine

Y guanine

Z adenine

Q14. The scientists determined the percentage of heart cells undergoing DNA replication by using a chemical called BrdU. Cells use BrdU instead of nucleotides containing thymine during DNA replication.

Describe how BrdU would be incorporated into new DNA during semi-conservative replication. (5)

1.DNA helicase

2.Breaks hydrogen bonds between 2 DNA strands

Reject ‘hydrolyses hydrogen bonds’

Accept H bonds for hydrogen bonds.

3.BrdU complementary to adenine on template strand

OR BrdU forms hydrogen bonds with adenine on template strand

Accept H bonds for hydrogen bonds.

4.DNA polymerase joins adjacent nucleotides to incorporate BrdU into the new DNA strand

Reject if DNA polymerase catalyses complementary base pairing or if DNA polymerase catalyses nucleotides joining to template strand.

5.Phosphodiester bonds form between nucleotides

The Meselson-Stahl experiment

Q15. Bacteria require a source of nitrogen to make the bases needed for DNA replication.

In an investigation of DNA replication some bacteria were grown for many cell divisions in a medium containing 14N, a light form of nitrogen.

Others were grown in a medium containing 15N, a heavy form of nitrogen.

Some of the bacteria grown in a 15N medium were then transferred to a 14N medium and left to divide once.

DNA was isolated from the bacteria and centrifuged.

The DNA samples formed bands at different levels, as shown in the diagram.

a. What do tubes A and B show about the density of the DNA formed using the two different forms of nitrogen? (1)

15N / tube B DNA, more / greater density

reject heavier

b. Explain the position of the band in tube C. (2)

1.DNA with one heavy and one light strand

2.new / synthesised strand, made with 14N / light strand

Q16. The bases in DNA nucleotides contain nitrogen.

Researchers grew bacteria on a medium containing 15N (‘heavy’ nitrogen) for several generations. They then transferred the bacteria to a medium containing 14N (‘ordinary’ nitrogen). They analysed DNA from the bacteria at three stages:

1. whilst the bacteria were growing on the 15N medium

2. after one division of the bacteria on the 14N medium

3. after two divisions of the bacteria on the 14N medium

The diagram shows their results.

a. Describe how the proportion of DNA that contained 15N changed at each division when bacteria were grown on the 14N medium. (2)

1.Decreases by 50%

2.Per generation / per division

15N makes up ½ after 1 division

Makes up ¼ after 2nd division

Only accessible if linked to first marking point

b. The change in the proportion of DNA containing 15N is due to the way in which DNA replicates. Explain how. (2)

1.In DNA replication strands separate
2.Each acts as template for formation of new strand

1.One strand in each new molecule / semi-conservative replication
2.New strands made using 14N

Q17. Scientists investigating DNA replication grew bacteria for several generations in a nutrient solution containing a heavy form of nitrogen (15N). They obtained DNA from a sample of these bacteria.

The scientists then transferred the bacteria to a nutrient solution containing a light form of nitrogen (14N). The bacteria were allowed to grow and divide twice. After each division, DNA was obtained from a sample of bacteria.

The DNA from each sample of bacteria was suspended in a solution in separate tubes. These were spun in a centrifuge at the same speed and for the same time. The diagram shows the scientists’ results.

The table shows the types of DNA molecule that could be present in samples 1 to 3.

Use your knowledge of semi-conservative replication to complete the table with a tick if the DNA molecule is present in the sample. (3)

Sample 1: 15N / 15N

Sample 2: 15N / 14N

Sample 3: 15N / 14N and 14N / 14N

Spaced practice

Q18. The DNA-replication enzymes of a human cell make copies of the human papilloma virus genome.

Name two enzymes that are involved in replicating the DNA of the human papilloma virus and describe their roles in the replication process. (3)

1. DNA helicase and DNA polymerase

Accept DNA ligase for either enzyme.

2. Helicase breaks hydrogen bonds to unwind DNA

Reject ‘hydrolyse hydrogen bonds’

3. Polymerase condensation reactions to join adjacent nucleotides

OR Polymerase forms phosphodiester bonds between adjacent nucleotides

Reject mp3 if polymerase forming hydrogen bonds or joining complementary base pairs

Accept DNA ligase joins DNA fragments

Note: Incorrect/no enzymes named but both roles outlined = 1 mark

Q19. The enzymes DNA helicase and DNA polymerase are involved in DNA replication.

Describe the function of each of these enzymes. (2)

1.DNA helicase – unwinding DNA and breaking hydrogen bonds / bonds between chains / bases / strands

Accept H bonds.

Accept hydrolyses for breaks

2.DNA polymerase – joins adjacent nucleotides OR forms phosphodiester bond / sugar-phosphate backbone

Reject forms hydrogen bonds between nucleotides / bases

Q20. Describe how DNA is replicated. (6)

1.Strands separate / H-bonds break

Q Neutral: strands split

Accept: strands unzip

2.DNA helicase involved

3.Both strands / each strand act(s) as (a) template(s)

4. Free nucleotides attach

Neutral: bases attach

Accept: nucleotides attracted

5.Complementary / specific base pairing / AT and GC

6.DNA polymerase joins nucleotides on new strand

Reject: if wrong function of DNA polymerase

7. H-bonds reform

8. Semi-conservative replication / new DNA molecules contain one old strand and one new strand

Reject: if wrong context e.g. new DNA molecules contain half of each original strand

Q21. A scientist replicated DNA in a test tube. To do this, he mixed an enzyme with identical single-stranded DNA fragments and a solution containing DNA nucleotides.

Name the enzyme used in this DNA replication. (1)

DNA polymerase

Correct spelling

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