(a) James Watson and Francis Crick built a model that showed that DNA has a double helix structure - Edexcel - GCSE Biology - Question 8 - 2019 - Paper 1

The data in Figure 13 shows the percentages of nucleotide bases A, T, G, and C in human DNA. The observation that the percentages of adenine (A) and thymine (T) are approximately equal, as well as the percentages of guanine (G) and cytosine (C), supports Chargaff's rules. This indicates that A pairs with T and G pairs with C, providing compelling evidence for the complementary base pairing in the DNA double helix structure.

A diploid human cell contains two sets of chromosomes, so to find the mass for a haploid cell, we divide the mass obtained by 2. Given the mass of DNA is 0.0062 ng, the calculation is:

rac{0.0062 ext{ ng}}{2} = 0.0031 ext{ ng}

To convert nanograms to picograms:

$0.0031 ext{ ng} imes 1000 = 3.1 ext{ picograms}$

Thus, the mass of DNA from a haploid human cell is 3.1 picograms.

Ethanol is used to precipitate the DNA from the solution. It reduces solubility and allows the DNA to come out of solution, making it possible to collect and visualize the DNA.

1. The amount of fruit used in extraction should be the same for both strawberry and kiwi to ensure a fair comparison.
2. The temperature at which the DNA extraction process is performed should be consistent to avoid discrepancies in the yield of DNA.

One reason the student may have repeated the experiment is to verify results and ensure reliability. Repeating experiments can help confirm that the findings are consistent and not due to experimental error.

Mitosis results in two genetically identical daughter cells, maintaining the same chromosome number as the parent cell, facilitating growth and repair. In contrast, meiosis produces four genetically diverse daughter cells, each with half the chromosome number, which is essential for sexual reproduction and genetic variation.