The control of gene expression

RNA and DNA are polynucleotides.

Two types of RNA are messenger RNA and transfer RNA.

The table shows some properties of these two types of RNA.

Place ticks (✓) in the correct boxes to indicate which properties are true for each type of RNA.

Outline how gene expression is regulated within cells at the post‑transcriptional level.

A student writes this method for extracting DNA from animal cells:

1. Grind the sample of cells using a mortar and pestle.

2. Mix the sample with detergent.

3. Add ethanol to precipitate the DNA from the solution.

4. Spool the DNA onto a glass rod.

Another student suggests that step 1, grinding the sample of cells, is unnecessary.

Explain why step 1 is unnecessary.

State one other step that the student should include in the method and explain why this step is necessary.

Electrophoresis is used to separate fragments of DNA so that the separated DNA fragments can be compared and analysed.

The diagram shows an electrophoresis box. Solutions of DNA samples have been added to three of the wells in the box.

Solutions containing

State two other solutions that should be added to the box before performing electrophoresis and explain the purpose of each solution.

Electrophoresis can be used to detect disease‑causing alleles, such as the mutant HBB allele that causes sickle cell disease.

Restriction enzymes are used to produce fragments of HBB alleles for analysis by electrophoresis.

These restriction enzymes:

• produce one fragment when the sickle cell HBB allele is present

• produce two smaller fragments when the normal HBB allele is present.

The diagram shows the results of electrophoresis of DNA samples from five people. It only shows DNA fragments from the HBB gene.

State the lane numbers that indicate:

• a genotype that is homozygous for the sickle cell HBB allele

• a genotype that is homozygous for the normal HBB allele

• a heterozygous genotype for the HBB gene.

Homozygous sickle cell........

Homozygous normal........

Bacterial cells can be used for genetic engineering.

A donor gene is placed into a plasmid, which is taken up by a bacterial cell.

This process is inefficient. Not all plasmids form recombinant DNA with the donor gene, and only a small percentage of bacterial cells take up plasmids.

A scientist estimates that 40% of their plasmids join to the donor gene to form recombinant DNA.

One in 2500 bacterial cells takes up a plasmid to form a transformed bacterial cell.

The scientist uses 1 × 10⁹ bacterial cells.

Calculate the number of cells that take up a plasmid containing the donor gene.

The percentage of plasmids that are taken up by bacterial cells is known as the transformation efficiency.

Suggest one factor that scientists could change to improve transformation efficiency.

Which of the statements describe(s) control of gene expression at the post-transcriptional level?

1 binding of a repressor protein to an operator sequence 2 the removal of introns to form mature RNA 3 editing of primary RNA

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Lactose metabolism in E. coli is controlled by the lac operon.

Which of the structures associated with the lac operon contain nucleic acids?

1: the operator 2: the regulator gene 3: the structural gene product

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

The onion plant, Allium cepa, is grown as a food crop around the world.

The table below contains statements about the root cells of an onion.

Place ticks (✓) in the boxes in the table to indicate whether the statements are true or false.

Fig. 1 shows a cross section of the root of an onion plant.

Identify the tissues shown at M and N.

The colour of onion bulbs is determined by two genes, A/a and B/b.

• A is a dominant allele and codes for the production of a red pigment.

• Onion bulbs that are homozygous for the recessive allele, a, produce no pigment and are white.

• B is a dominant allele that inhibits the expression of allele A.

• The recessive allele, b, allows the production of the red pigment.

A white onion plant was cross-pollinated with a red onion plant. All 15 offspring had the genotype AaBb.

Identify the following:

The genotype of the white onion plant........

The genotype of the red onion plant........

The phenotype of the offspring........

State the type of gene interaction shown by the genes A/a and B/b.

Suggest how allele B inhibits the expression of allele A.

Fig. 20 shows the disaccharide lactose, which is found in milk.

Another disaccharide is maltose. Maltose and lactose both contain the same number of atoms of each element, C, H and O.

State two other structural similarities between lactose and maltose.

Complete the table below to identify differences between the structures of lactose and maltose. The first one has been completed for you.

One of the monomers of lactose is galactose.

The bacterium E. coli usually uses glucose as a respiratory substrate.

Under certain circumstances, E. coli is able to use galactose as a respiratory substrate by breaking down lactose into glucose and galactose and then using both glucose and galactose as respiratory substrates.

Explain how the structure of galactose allows it to be used as a respiratory substrate.

E. coli usually grows in conditions where the extracellular concentration of lactose is low. In such conditions lactose does not easily cross the bacterial cell surface membrane.

Suggest and explain why lactose is unable to cross membranes.

In order for lactose to enter the cytoplasm of E. coli a protein is required.

The E. coli living in the digestive system of young mammals are more likely to contain this protein than E. coli living in the digestive system of old mammals.

Suggest an explanation for this observation.

Lactose is a reducing sugar.

Benedict’s reagent can be used to detect the presence of lactose in a solution.

A colorimeter can be used to measure the concentration of lactose.

The colorimeter first needs to be calibrated.

Describe how a method that uses Benedict’s reagent and a colorimeter could be calibrated to measure the concentration of lactose in an unknown sample.

Tigers, Panthera tigris, are predatory mammals. They have evolved striped patterns on their fur (as shown in Fig. 3.1a on the insert), which provide camouflage in their habitats.

Adaptations can be divided into three types.

State the type of adaptation represented by the tiger’s stripes.

* Describe and explain how a tiger with striped fur may have evolved from a non-striped ancestor.

In your answer you should discuss the different types of genes that might be involved in the creation of the striped pattern in the tiger’s fur.

One subspecies of tiger is the Bengal tiger. One in 10 000 Bengal tiger births results in a white Bengal tiger.

White Bengal tigers (shown in Fig. 3.1b on the insert) have black stripes but lack orange fur.

The allele that causes white fur is recessive and is a result of a mutation to a gene called SLC45A2.

According to the Hardy-Weinberg principle, the following equations can be used to estimate allele frequency within a population:

p² + 2pq + q² = 1

p + q = 1

Use the Hardy-Weinberg equations to calculate the percentage of Bengal tigers that are heterozygous for the SLC45A2 gene.

Give your answer to one significant figure.

Show your working.