DNA structure

Figure 8 shows part of a DNA molecule.

The letters A, C, G and T in Figure 8 represent four different compounds.

What are A, C, G and T called?

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The section of DNA shown in Figure 8 contains the following sequence of these compounds:

A G T A A G T A C

A sequence of three compounds codes for one amino acid.

How many amino acids would this section of DNA code for?

Sometimes a change occurs in the sequence of the compounds A, C, G and T in the DNA.

What name is given to a change in DNA?

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Changes in the DNA of a bacterial pathogen can produce new strains of the pathogen.

Many people have been infected with a bacterial pathogen called MRSA.

Figure 9 shows how many cases of MRSA occurred in one country from 1993 to 2011.

Calculate the mean rate of increase in MRSA cases per year in the 12 years from 1993 to 2005.

Most antibiotics will not kill MRSA bacteria.

Which word describes bacteria that are not killed by antibiotics?

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Figure 9 shows that the number of cases of MRSA decreased after 2006.

Suggest one reason for this decrease.

Some people keep rabbits as pets.

Figure 8 shows two types of rabbit.

The Himalayan fur pattern is controlled by a dominant allele.

A heterozygous male Himalayan rabbit was crossed many times with each of two female Himalayan rabbits, A and B.

The results of the two types of cross were different.

• Crosses with female A: all of the 210 offspring were Himalayan rabbits.

• Crosses with female B: some of the 240 offspring were Himalayan rabbits and some were white rabbits.

In Questions 08.1 and 08.2, use the following symbols to represent alleles: H = dominant allele for Himalayan fur h = recessive allele for white fur.

What is the genotype of female Himalayan rabbit A ?

Give the reason for your answer.

Determine the probability that one of the offspring from a cross with female Himalayan rabbit B will be white.

You should:

• draw a Punnett square diagram

• use the symbols H and h

• identify the phenotype of each offspring genotype.

Probability of offspring being white =

Adult Himalayan rabbits have:

• white fur over most of the body

• black fur on the nose, ears and feet.

The black colour is caused by production of a substance called melanin.

Figure 9 shows how melanin is made from another substance called tyrosine.

Figure 9

White rabbits have one base change in the gene that codes for the enzyme tyrosinase.

Explain how a change of one base in the DNA of the gene coding for tyrosinase could stop the tyrosinase working.

For the first four weeks of life, Himalayan rabbits produce white fur over the whole body.

The black areas of fur start to develop after four weeks.

In the first four weeks, young rabbits stay in the nest with their mother.

A rabbit’s body temperature is 39 °C.

The tyrosinase in Himalayan rabbits is:

• most active between 15 °C and 25 °C

• not active above 35 °C.

The fur on the nose, ears and feet of Himalayan rabbits is white for the first four weeks of life.

Explain what causes the fur to be white in the first four weeks of life.

From the age of four weeks, young rabbits move from the nest to the outside environment.

Explain why a Himalayan rabbit develops black nose, ears and feet from the age of four weeks.

Melanin is also produced in human skin.

Melanin absorbs ultraviolet (UV) radiation from sunlight and prevents the radiation reaching deep into the skin.

UV radiation is a type of ionising radiation.

Explain the advantage of producing melanin in human skin when exposed to bright sunlight.

Figure 13 shows six stages in the process of meiosis.

In Figure 13, A is the first stage and F is the final stage. Stages B to E are not in the correct order. Give the correct order of stages A to F.

At the end of meiosis the number of chromosomes is different from the number of chromosomes at the start of meiosis.

Give the number of chromosomes in one cell in Figure 13:

• at the start of meiosis

• at the end of meiosis.

Explain why the change in the number of chromosomes is important.

Meiosis produces cells that are genetically different. Describe how meiosis produces cells that are genetically different.

Chromosomes contain DNA. Figure 14 shows part of a DNA molecule.

What type of substances are labelled A, C, G and T in Figure 14 ?

DNA is made of nucleotides. How many nucleotides are shown in Figure 14 ?

The nucleus of a cell contains DNA.

Name the structures inside the cell nucleus that contain DNA.

Figure 13 shows part of a DNA molecule.

Name the part of the DNA molecule labelled X.

What type of substances are labelled A, C, G and T in Figure 13 ?

Figure 14 shows another section of a DNA molecule.

Four of the substances you named in Question 08.3 are not labelled in part Y of Figure 14. Label each of these substances with the correct letter, A, C, G or T. Use information from other parts of Figure 14 to help you.

What is happening to the DNA in part Z of Figure 14 ?

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A gene is a length of DNA. What type of substance does a gene code for?

Most human body cells contain 6 10^–12 grams of DNA. What mass of DNA will a human sperm cell contain?

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What is the name of the type of cell division that produces sperm cells?

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The nucleus of a cell contains DNA.

Name the structures inside the cell nucleus that contain DNA.

Figure 1 shows part of a DNA molecule.

Name the part of the DNA molecule labelled X.

What type of substances are labelled A, C, G and T in Figure 1 ?

Figure 2 shows another section of a DNA molecule.

Four of the substances you named in Question 01.3 are not labelled in part Y of Figure 2. Label each of these substances with the correct letter, A, C, G or T. Use information from other parts of Figure 2 to help you.

What is happening to the DNA in part Z of Figure 2 ? Tick (✓) one box.

A gene is a length of DNA. What type of substance does a gene code for?

Most human body cells contain 6 10^–12 grams of DNA. What mass of DNA will a human sperm cell contain? Tick (✓) one box.

What is the name of the type of cell division that produces sperm cells? Tick (✓) one box.

This question is about DNA.

Describe the shape of a DNA molecule.

Figure 10 shows part of a DNA molecule.

DNA codes for a sequence of amino acids.

Which part of DNA forms the code for a particular amino acid?

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Which substance is produced when amino acids are joined together?

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DNA is made of repeating units. One of the units is labelled A in Figure 10.

What is the name of the repeating unit labelled A ?

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The DNA in one human body cell is the length of 6 000 million repeating units (part A

Each repeating unit is 0.34 nanometres (nm) long.

Calculate the length of the DNA in the cell in millions of nanometres.

Give your answer to Question 07.5 in metres.

DNA analysis can show people which alleles they have.

Patients who have certain types of cancer can be offered DNA analysis.

The family of the patient can also be offered DNA analysis.

Suggest one advantage of having DNA analysis.

DNA is a polymer of nucleotides.

Why is DNA described as a polymer?

Figure 5 shows part of a DNA molecule.

Describe the structure of a nucleotide.

The length of a DNA double helix increases by 0.34 nm for every pair of nucleotides. The total number of nucleotides in a human body cell is 1.2 10¹⁰. Calculate the total length of double helix in a human body cell. Give your answer in metres. Use information from Figure 5.

Some parts of DNA do not code for proteins. Describe how non-coding parts of DNA can affect the expression of genes.

Genetic material is made of DNA.

Which structures in the nucleus of a human cell contain DNA?

Figure 16 shows part of one strand of a DNA molecule.

Label parts X, Y and Z on Figure 16.

Choose answers from the box.

A complete DNA molecule is made of two strands twisted around each other.

What scientific term describes this structure?

DNA codes for the production of proteins.

A protein molecule is a long chain of amino acids.

How many amino acids could be coded for by the piece of DNA shown in Figure 16 ?

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Scientists have now studied the whole human genome.

Give two benefits of understanding the human genome.

Genetic material is made of DNA.

Which structures in the nucleus of a human cell contain DNA?

Figure 3 shows part of one strand of a DNA molecule.

Label parts X, Y and Z on Figure 3.

Choose answers from the box.

A complete DNA molecule is made of two strands twisted around each other.

What scientific term describes this structure?

DNA codes for the production of proteins.

A protein molecule is a long chain of amino acids.

How many amino acids could be coded for by the piece of DNA shown in Figure 3 ?

Tick (✓) one box.

Scientists have now studied the whole human genome.

Give two benefits of understanding the human genome.

Scientists want to breed cows that produce milk with a low concentration of fat.

Figure 10 shows information about the milk in one group of cows.

The cows were all the same type.

In Figure 10 the mean percentage of fat in the milk is equal to the modal value.

Give the mean percentage of fat in the milk of these cows.

A student suggested:

‘The percentage of fat in milk is controlled by one dominant allele and one recessive allele.’

How many different phenotypes would this produce?

Tick one box.

Give the evidence from Figure 10 which shows the percentage of fat in the milk is controlled by several genes.

One of the genes codes for an enzyme used in fat metabolism.

A mutation in this gene causes a reduction in milk fat.

The mutation changes one amino acid in the enzyme molecule.

Explain how a change in one amino acid in an enzyme molecule could stop the enzyme working.

The scientists found one cow with a mutation.

The cow’s milk contained only 2.9% fat.

Figure 11 shows the percentage of fat in the milk of cattle related to the cow with the mutation.

The values for male cattle are the mean values of their female offspring.

Animal 8 is homozygous.

The mutation in animal 7 produced a dominant allele for making low-fat milk.

Give evidence from Figure 11 that animal 7 is heterozygous.

Animals 7 and 8 produced 11 offspring. These offspring were produced by in vitro fertilisation (IVF).

The embryos from IVF were transferred into 11 other cows.

Suggest why IVF and embryo transfer were used rather than allowing animals 7 and 8 to mate naturally.

Draw a Punnett square diagram to show a cross between animals 7 and 8 .

Identify which offspring produce low-fat milk and which offspring produce high-fat milk.

Use the following symbols: D = dominant allele for making low-fat milk d = recessive allele for making high-fat milk

low-fat milk.

The scientists will selectively breed some of the animals shown in Figure 11.

Describe how the scientists would do this.