DNA & Gene Expression (Edexcel International A Level Biology)

The polynucleotide DNA is composed of mononucleotides linked together.

Two polynucleotides form a DNA molecule.

The diagram shows part of a DNA molecule.

Draw a circle around one mononucleotide that includes the base labelled R.

(1)

(ii)

Which row of the table identifies the bonds labelled S, T and U?

(1)

The base labelled P is adenine.

Which is the base labelled Q?

(1)

Meselson and Stahl carried out experiments that provided evidence for the semi‐conservative replication of DNA.

Heavy nitrogen (N¹⁵) and light nitrogen (N¹⁴) were used in these experiments.

The flow chart summarises part of one experiment performed by Meselson and Stahl.

After each stage, a sample of DNA was taken from the bacteria and the DNA molecules separated using a density gradient in a tube.

The heavier DNA molecules form bands lower down the gradient than the lighter DNA molecules.

The height of each band is proportional to the percentage of DNA molecules in the sample.

(i)

Complete the diagram to show the results of this experiment.

Use the dotted lines to help you to position the bands on the diagram.

The first one has been done for you.

(5)

The experiments of Meselson and Stahl disproved the dispersive theory of DNA replication.

The diagram shows the expected results if the dispersive theory was correct.

Which diagram would show the bands of DNA molecules on the density gradient at stage 2, if the dispersive theory was correct?

(1)

An average‐sized human chromosome contains a DNA molecule with about 300 million nucleotides, arranged in pairs.

Describe the structure of a nucleotide pair.

Before a cell can divide, the DNA molecule has to replicate. This takes place in the S phase of the cell cycle.

Human DNA replicates at a rate of 50 nucleotides per second.

Calculate how long it would take, to the nearest hour, for 150 million nucleotides to replicate.

Assume that replication starts at one end of the molecule and continues to the other end.

In human cells, replication of DNA occurs at several sites along the molecule.

These sites are called replication bubbles.

The image shows a replication bubble.

The diagram shows how the DNA is replicated in one replication bubble.

Explain the role of DNA polymerase in a replication bubble.

Use the information in the diagram and your own knowledge to support your answer.

(2)

In human cells, S phase lasts about 10 hours.

Suggest why each DNA molecule is replicated using many replication bubbles

A gene contains the genetic code for the sequence of amino acids in a polypeptide chain.

The table shows the genetic codes found in DNA.

The genetic codes TAA, TAG and TGA are stop codons, that do not code for amino acids.

Explain the nature of the genetic code.

Use information in the table to support your answer.

The diagram shows the sequence of nucleotide bases in part of a DNA template (antisense) strand.

Give the sequence of amino acids that is coded for by these bases.

(1)

(ii)

Explain the possible effects on a protein if there is a substitution mutation in the 9^th base in this DNA strand.

Use information in the table to support your answer.

(5)