DNA replication

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,¹⁵ N. As soon as all the DNA in this population contained¹⁵ N, the scientists changed the nitrogen source to one containing only the lighter isotope of nitrogen,¹⁴ N. 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.

size of population at time 4 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 using¹⁵ N has a higher density than DNA made using¹⁴ N.

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.

Explanation for first unsupported model

Explanation for second

Describe the role of DNA polymerase in the semi-conservative replication of DNA.

Figure 5 shows the percentage of rat cells undergoing DNA replication. Some cells contained a protein called cyclin D and some cells did not contain cyclin D. All cells were in early interphase at time 0

It took less time for 25% of cells with cyclin D to be undergoing DNA replication than for 25% of cells without cyclin D.

Use Figure 5 to calculate this time difference as a percentage decrease.

Show your working.

Cyclin D stimulates the phosphorylation of DNA polymerase, which activates the DNA polymerase.

Describe how an enzyme can be phosphorylated.

Some tumour cells contain higher than normal concentrations of cyclin D.

Use Figure 5 to suggest why higher than normal concentrations of cyclin D could result in a tumour.

Scientists investigated the cell cycle in heart cells taken from mice 6 days before their birth and then at 4, 14 and 21 days after their birth.

Their results are shown in Table 4. Age 0 days = day of birth.

Describe and explain the data in Table 4.

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.

Cells with BrdU in their DNA are detected using an anti-BrdU antibody with an enzyme attached.

Use your knowledge of the ELISA test to suggest and explain how the scientists identified the cells that have BrdU in their DNA.

Describe the role of two named enzymes in the process of semi-conservative replication of DNA.

Scientists investigated the function of a eukaryotic cell protein called cyclin A. This protein is thought to be involved with the binding of one of the enzymes required at the start of DNA replication.

The scientists treated cultures of cells in the following ways. C – Control cells, untreated D – Added antibody that binds specifically to cyclin A E – Added RNA that prevents translation of cyclin A F – Added RNA that prevents translation of cyclin A and added cyclin A protein

They then determined the percentage of cells in each culture in which DNA was replicating.

Their results are shown in Table 5.

Suggest explanations for the results in Table 5.

Name two enzymes involved in the semi-conservative replication of DNA.

Sometimes, damage occurs during DNA replication. One enzyme involved in repairing damage to DNA is called ATR.

ATR works as follows.

ATR phosphorylates other enzymes involved in repairing DNA.

ATR also phosphorylates substrates required to repair DNA.

When ATR phosphorylates other enzymes, these enzymes become able to bind to their substrates.

Use your knowledge of enzyme structure to suggest why.

The enzyme-catalysed reactions activated by ATR only occur if the substrates have been phosphorylated.

Use your knowledge of energy changes in enzyme-catalysed reactions to suggest why.

Sometimes, a mutagenic agent causes DNA to break. A different enzyme called ATM binds to the broken DNA. This leads to the activation of a protein coded for by a tumour suppressor gene. The effect of ATM binding is to stop cell division until DNA is repaired.

A mutation could result in a person having non-functional forms of the gene that produces ATM.

What can you predict about the possible effects of having a non-functional form of ATM?