DNA structure

Complete Table 1 by adding a tick (✓) if the feature is present in the biological molecules.

Lactase catalyses the hydrolysis of lactose.

A student investigated the effect of galactose on lactase activity.

The student used the following method.

1. Mix together (in tube X) solutions of lactose, galactose and lactase in a 15: 3: 1 volume ratio so that the total volume is 90 cm³

2. Leave the mixture at 30 °C for 10 minutes.

3. Measure the glucose concentration in the mixture.

4. Repeat steps 1 to 3 with two different mixtures.

Calculate the volume of galactose solution used in tube X.

Table 2 shows the student’s results.

Galactose is a competitive inhibitor of lactase.

Use this information and your knowledge of enzymes to explain the results obtained in Table 2.

Give:

• one change the student could make in tube X to confirm that galactose is a competitive inhibitor

• the result you would expect.

Write an essay on one of the topics below.

Either

Phosphorus-containing substances and their importance in biological systems.

Or

The mechanisms and importance of transport within organisms.

Describe how a quaternary protein is formed from its monomers.

Do not include the process of translation in your answer.

Describe the structure of DNA and the structure of a chromosome.

Mutation can result in an increase in genetic variation within a species.

Describe and explain the other processes that result in increases in genetic variation within a species.

Complete Table 3 to give three differences between DNA molecules and tRNA molecules.

Scientists investigated ribosomal RNA in liver cells.

Figure 10 shows the method they used to isolate the ribosomes from the liver cells. The detergent dissolves lipids.

The scientists broke open the cells to produce a suspension of cell contents.

Describe how the scientists would remove large organelles from this suspension of cell contents.

Explain the position of the bands of ribosomes in tubes A and B in Figure 10.

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 structure of DNA.

Name and describe five ways substances can move across the cell-surface membrane into a cell.

Figure 14 shows transmission electron micrographs of two cells, one animal cell and one prokaryotic cell.

Contrast the structure of the two cells visible in the electron micrographs shown in Figure 14.

Write an essay on one of the topics below.

EITHER

The importance of DNA as an information-carrying molecule and its use in gene technologies.

OR

The importance of bonds and bonding in organisms.

Give the two types of molecule from which a ribosome is made.

Describe the role of a ribosome in the production of a polypeptide. Do not include transcription in your answer.

Table 1 shows the base sequence of part of a pre-mRNA molecule from a eukaryotic cell.

Complete the table with the base sequence of the DNA strand from which this pre-mRNA was transcribed.

In a eukaryotic cell, the base sequence of the mRNA might be different from the sequence of the pre-mRNA.

Explain why.

Write an essay on one of the topics below.

EITHER

The importance of nitrogen-containing substances in biological systems.

OR

The importance of diffusion in organisms.