Biology  by Bradford
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D1.1 DNA replication: revision map

The subtopic in the middle and ten tasks round it: label five diagrams, complete two tables, put a process in order and fill 50 gaps. Every standard level statement has a box; the four boxes for higher level only are dashed and badged HL, and drop off for an SL class. Type into the blanks and check them, or print it on A4, Letter or A3 with an answer key.

D1.1 practice questionsThe whole D1 map

D1.1

DNA replication

Molecules · Continuity and changeName

D1.1.1Why copy DNA?

DNA replication makes two DNA molecules whose base sequences are . All organisms need it to ; multicellular organisms also need it for and for tissue .

D1.1.3At the replication fork

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One enzyme unwinds the helix and separates the two strands by breaking the between their bases. The other builds a new strand along each separated one, adding units that pair with the exposed bases: adenine with , cytosine with .

D1.1.5Making a DNA profile

Number the steps in order.

  1. Compare the pattern of bands with other profiles.
  2. Separate the copied fragments by gel electrophoresis.
  3. Collect cells, for example from blood, saliva or a hair root.
  4. Extract the DNA from the cells.
  5. Copy chosen markers many times over by PCR.

Profiles are compared in paternity tests and in investigations. Comparing more markers makes a false match , so the test is more .

D1.1.8Enzymes at the forkHL

EnzymeWhat it does
DNA primaseMakes the short stretch of that each new strand starts from
DNA polymerase IIIAdds DNA nucleotides one by one, building the new strand in the direction
DNA polymerase IRemoves the and fills each gap with DNA
DNA ligaseJoins the by sealing gaps in the sugar-phosphate backbone

D1.1.4PCR in three steps

StepTemperatureWhat happens
Denaturationabout °CHydrogen bonds break and the two strands separate
Annealingabout 55 °C bind to each strand at the ends of the target sequence
Extensionabout 72 °C builds a new strand on each template

The polymerase comes from a bacterium of hot springs, so it is . Each cycle the amount of the target sequence.

D1.1.6, D1.1.7Copying both strandsHL

3′5′5′3′123
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DNA polymerase can only add nucleotides to the end of a strand. So one new strand is built , towards the fork, while the other is built , away from the fork, and has to be started again and again.

D1.1DNA replication

D1.1 examA replication fork in a prokaryoteHL

A replication fork in a prokaryote1234
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D1.1.2Two from one

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This way of copying DNA is called replication. Each base can pair with only one other, so every new strand is to the strand it was built on, and copies are very .

D1.1.4, D1.1.5Reading a gel

MCAB123
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DNA's groups carry a negative charge, so it moves through the gel towards the electrode, and the fragments separate by . Lane M is a mother, C her child, and A and B two men. Every band of the child's matches one of the mother's or one of the father's, so the father is man .

D1.1.9Checking the copyHL

DNA polymerase III checks each nucleotide it adds. If the one at the end of the new strand carries a base, the enzyme it and adds a correctly paired nucleotide instead. This keeps copying errors very rare.

Word bank 3′5′ to 3′95accurateBcomplementarycontinuouslydiscontinuouslyDNA ligaseDNA polymerasedoublesforensicfree nucleotidegrowthguanineheat-stablehelicasehydrogen bondhydrogen bondsidenticallargest fragmentsleading strandless likelymismatchednew strandOkazaki fragmentOkazaki fragmentsparental strandphosphatepositiveprimersPrimersproofreadingreliableremovesreplacementreproduceRNARNA primersemi-conservativesizesmallest fragmentsTaq polymerasetemplate strandthyminewell

Answer key

Every box on the sheet with its gaps filled in, as the subtopic reads in full.

  1. GapsWhy copy DNA?

    DNA replication makes two DNA molecules whose base sequences are identical. All organisms need it to reproduce; multicellular organisms also need it for growth and for tissue replacement.

  2. LabelTwo from one

    1. parental strand
    2. new strand
    3. hydrogen bond

    This way of copying DNA is called semi-conservative replication. Each base can pair with only one other, so every new strand is complementary to the strand it was built on, and copies are very accurate.

  3. LabelAt the replication fork

    1. helicase
    2. DNA polymerase
    3. free nucleotide
    4. template strand

    One enzyme unwinds the helix and separates the two strands by breaking the hydrogen bonds between their bases. The other builds a new strand along each separated one, adding units that pair with the exposed bases: adenine with thymine, cytosine with guanine.

  4. TablePCR in three steps

    StepTemperatureWhat happens
    Denaturationabout 95 °CHydrogen bonds break and the two strands separate
    Annealingabout 55 °CPrimers bind to each strand at the ends of the target sequence
    Extensionabout 72 °CTaq polymerase builds a new strand on each template

    The polymerase comes from a bacterium of hot springs, so it is heat-stable. Each cycle doubles the amount of the target sequence.

  5. LabelReading a gel

    1. well
    2. largest fragments
    3. smallest fragments

    DNA's phosphate groups carry a negative charge, so it moves through the gel towards the positive electrode, and the fragments separate by size. Lane M is a mother, C her child, and A and B two men. Every band of the child's matches one of the mother's or one of the father's, so the father is man B.

  6. OrderMaking a DNA profile

    1. Collect cells, for example from blood, saliva or a hair root.
    2. Extract the DNA from the cells.
    3. Copy chosen markers many times over by PCR.
    4. Separate the copied fragments by gel electrophoresis.
    5. Compare the pattern of bands with other profiles.

    Profiles are compared in paternity tests and in forensic investigations. Comparing more markers makes a false match less likely, so the test is more reliable.

  7. LabelCopying both strands HL

    1. leading strand
    2. Okazaki fragment
    3. RNA primer

    DNA polymerase can only add nucleotides to the 3′ end of a strand. So one new strand is built continuously, towards the fork, while the other is built discontinuously, away from the fork, and has to be started again and again.

  8. TableEnzymes at the fork HL

    EnzymeWhat it does
    DNA primaseMakes the short stretch of RNA that each new strand starts from
    DNA polymerase IIIAdds DNA nucleotides one by one, building the new strand in the 5′ to 3′ direction
    DNA polymerase IRemoves the primers and fills each gap with DNA
    DNA ligaseJoins the Okazaki fragments by sealing gaps in the sugar-phosphate backbone

  9. GapsChecking the copy HL

    DNA polymerase III checks each nucleotide it adds. If the one at the 3′ end of the new strand carries a mismatched base, the enzyme removes it and adds a correctly paired nucleotide instead. This proofreading keeps copying errors very rare.

  10. Exam diagramA replication fork in a prokaryote HL

    1. helicase
    2. RNA primer
    3. Okazaki fragment
    4. DNA ligase