5 (a) James Watson and Francis Crick built a model that showed that DNA has a double helix structure - Edexcel - GCSE Biology Combined Science - Question 5 - 2019 - Paper 1
Question 5
5 (a) James Watson and Francis Crick built a model that showed that DNA has a double helix structure.
(i) Which statement about DNA is correct?
A each pair of base... show full transcript
Worked Solution & Example Answer:5 (a) James Watson and Francis Crick built a model that showed that DNA has a double helix structure - Edexcel - GCSE Biology Combined Science - Question 5 - 2019 - Paper 1
Step 1
Which statement about DNA is correct?
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Answer
The correct statement is A: each pair of bases is joined by hydrogen bonds. This reflects the essential structure of DNA, where nitrogenous bases (adenine, thymine, cytosine, and guanine) are paired specifically through hydrogen bonding.
Step 2
Describe how this data provides evidence for base pairing in DNA.
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Answer
In Figure 6, the data shows that the percentages of adenine (A) and thymine (T) are equal, as well as the percentages of cytosine (C) and guanine (G). This suggests that A pairs with T and C pairs with G, providing strong evidence for complementary base pairing in DNA.
Step 3
Calculate the mass of DNA the scientist should obtain from a haploid human cell.
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Given that a diploid human cell contains 0.0062 nanograms of DNA, a haploid cell, which has half the amount of DNA, would contain 0.0031 nanograms. Converting this to picograms, we multiply by 1000:
0.0031extngimes1000=3.1extpicograms
Step 4
State why ethanol is used.
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Ethanol is used to precipitate the DNA, as it is not soluble in ethanol, allowing the DNA to become visible and separating it from other cellular components.
Step 5
State two variables the student needs to control when using this method to compare DNA from these two fruits.
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Mass of fruit
Volume of buffer
Step 6
Give one reason why.
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The student may have repeated the experiment to ensure the results are reliable and consistent, minimizing the impact of random errors.
Step 7
Compare the outcomes of mitosis and meiosis.
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Mitosis results in two genetically identical diploid daughter cells, used for growth and repair, while meiosis results in four genetically diverse haploid gametes, used for sexual reproduction. This highlights the different purposes and processes of each form of cell division.
