Complete Table 7 with ticks (✓) to show which elements are found in the following biological molecules.

After Watson and Crick proposed the model of DNA structure, scientists investigated the possible mechanisms for DNA replication.

Two scientists grew a bacterial population, providing them with a nitrogen source containing only the heavy isotope of nitrogen, 15N. As soon as all the DNA in this population contained 15N, the scientists changed the nitrogen source to one containing only the lighter isotope of nitrogen, 14N. They changed the nitrogen source at 0 hours.

During the investigation, the scientists measured the size of the population of bacterial cells.

Figure 11 shows the scientists' results.

The generation time for a population of bacteria is the time taken for all the bacteria to divide once by binary fission.

Use Figure 11 and the following equation to calculate the generation time for this population of bacteria. Give your answer in hours.

Number of generations = log10(size of population at time +4 hours / size of population at time – 4 hours) / log10 2

Generation time ______________ hours

At intervals during this investigation, the scientists removed samples of the bacterial population, isolated the DNA and measured the density of the DNA.

DNA made using 15N has a higher density than DNA made using 14N.

Figure 12 shows the scientists' results.

There are three possible models of DNA replication.

These models are shown in Figure 13.

Which of these models, P, Q or R, is supported by the results shown in Figure 12?

Give the letter and name of the model supported and explain why the results do not support the other models.

Model

Name

Explanation for first unsupported model

Explanation for second unsupported model