DNA, Genes & Chromosomes (AQA A Level Biology)

The synthesis of proteins is determined by the genetic code. The genetic code is provided by the sequence of nucleotides in DNA. As the code is the same for almost all organisms it is described as universal. 

Name and describe the other features of the genetic code.

Identify the structure labelled X in Figure 1.

Figure 1

Papain is an enzyme. It is 345 amino acids long.

State the minimum number of DNA bases needed to code for papain?

Figure 2 shows the sequence of DNA bases coding for ten amino acids in the enzyme papain.

Figure 2

C  A  A  T  T  T  C  A  A  A  G  T  T  G  C  T  T  T  T  T  G  T  T  G  C  A  A  T  A  T

The number of each type of amino acid coded for by this sequence of DNA bases is shown in Table 1.

Table 1

Use Table 1 and Figure 2 to determine the sequence of amino acids in this part of the enzyme. Write your answer in Figure 3.

Figure 3.

Lactase is an enzyme that is used to produce lactose-free milk. A student was given the RNA sequence of the enzyme lactase in Figure 1 and was asked to use Table 1 to identify the first four amino acids of lactase.  

Figure 1

A U G G A A C U C U C U

Table 1 shows the genetic code (mRNA codons).

Table 1

Describe and explain how the mRNA nucleotide sequence worked out to correspond to the first four amino acids using Table 1 may not be the same as the mRNA nucleotide sequence for those amino acids present in a typical bacterial cell.

In an electron micrograph, a DNA molecule appears 1.5 mm wide, however the actual width of a DNA molecule is 2 nm. What is the magnification?

Figure 2 shows the stages that occur to produce a part of a polypeptide (found in a human).

Figure 2

Explain the features of the genetic code that the diagram illustrates.

The genetic code is described as degenerate. 

Describe what is meant by this, using an example from Table 1 to illustrate the answer.

Table 1

State where the DNA molecule in Figure 1 would be found and explain why?

Figure 1

Annotate Figure 2 to illustrate the following terms: 

i) Locus        ii) Gene        iii) Centromere 

Figure 2

Figure 3 illustrates a small section of a DNA molecule from the nucleus of a eukaryotic cell.

Name the structures labelled X and Y.

Figure 3

Distinguish between mitochondrial DNA and nuclear DNA of eukaryotic cells.

On Figure 1 circle the hydrogen bond between the guanine and cytosine bases?

Figure 1

Compare the sizes and structures of prokaryotic and eukaryotic chromosomes. 

Myoglobin is a eukaryotic  protein consisting of a single polypeptide chain of 153 amino acids.

State the minimum number of DNA bases needed to code for Myoglobin?

The haemoglobin molecule consists of four polypeptide chains: two ? and two ? chains. Table 1 shows some of the first seven amino acids of the ? chain of haemoglobin and the bases in the sequence of DNA that code for these seven amino acids.

Table 1

Table 2 shows the codons that code for seven amino acids.

Table 2

Using Table 2 determine the missing amino acids and complete Table 1.

Figure 1 shows part of the mRNA and its corresponding amino acid sequence fo cytochrome c, a protein involved in aerobic respiration, for Mus musculus (house mouse)and Loxodonta africana (African elephant). 

Figure 1

Identify the DNA triplet that codes for the amino acid serine (Ser).

Figure 2 shows part of the mRNA and its corresponding amino acid sequence for cytochrome c, a protein involved in aerobic respiration, for Gorilla gorilla (gorilla)and Loxodonta africana (African elephant).

Figure 2

Explain how Figure 2 illustrates the genetic code.

Compare the DNA in prokaryotic cells with the DNA in the nucleus of eukaryotic cells.

State the molecules that make up a ribosome. 

Table 1 shows a sequence of DNA. Complete Table 1. 

Table 1

Table 2 shows a sequence of DNA bases and the mutated version of the same sequence. Table 3 shows the DNA triplets that code for some amino acids.

Table 2

 Table 3

Two single base mutations have occurred. Use the information provided in Table 2 and Table 3 to state and explain the mutations and their effect on the amino acids that will be produced from the mutated DNA sequence.

Suggest the effect of the two single mutations from part c) could have on the protein structure coded by this mutated sequence of DNA. 

Some species of organisms can have the same percentage of bases as other organisms yet they can still be very different animals. Explain why. 

In all living organisms the ratio (C + A) ÷ (G + T) is constant but the ratio of   (A + T) ÷ (C + G) can vary between organisms. Using your knowledge of the structure  and function of DNA, explain this information.  

Explain why the DNA which codes for the human protein, alpha globin, has 850 base pairs but there are only 141 amino acids in this protein.

Tumour suppressor genes code for proteins that control cell division in cells. Describe and explain how a mutation in a tumour suppressor gene could lead to a malignant tumour.

Proteins are produced in two stages. Different types of RNA molecules are required within these stages. Table 1 shows the various types of RNA and the total percentage of each within the cell of a desert rat.  

Table 1

Use the information in Table 1 and explain which organelle would contain the greatest percentage of RNA. 

A typical cell from a desert rat contains approximately 43 000 000 000 RNA nucleotides.Using Table 1 and the information given calculate the approximate number of tRNA nucleotides in the cell of a desert rat. Give your answer in standard form and to two significant figures. 

Explain why desert rat cells contain a large number of different mRNA nucleotides but only 64 different tRNA molecules.  

Some mutations can result in a frameshift. Describe and explain what is meant by a  frameshift and how this may affect the protein made.

Table 1 shows the amino acids coded for by each of the mRNA codons. It also shows the properties of some of the amino acids. 

Table 1

Key

+ Represents amino acids with a positive charged R group

- Represents amino acids with a negative charge R group

Use information from Table 1 to explain, using an example, why the DNA code is said to be degenerate. 

A scientist investigated the structure of an enzyme containing 647 amino acids. She looked at mutations that commonly occurred and their effects on the protein structure.The mutations were all single base substitutions. 

One of the changes she discovered was at position 389 where Asp was substituted for Glu. 

Use Table 1 to suggest the likely effect of this mutation on the protein structure. 

Table 2 shows the positions of three different single base substitutions.  

Table 2

The mutation at position 531 did not affect the enzymes structure but the mutation at position 109 did. Using your knowledge of protein structure and information from Table 2  to explain this. 

Use information from Table 1 to tick one box that shows which single base mutation below would result in the amino acid Leu being replaced with the amino acid Arg. 

Table 1

There are several different types of gene mutation that can occur within a sequence of DNA. Figure 1 shows a section of a gene with two different substitutions. 

Figure 1

Explain why a substitution at point A will not lead to a change in protein structure but a substitution at point B can.

Further along in the DNA strand, beyond point B in the coding region, there was a further substitution of a base. This also did not lead to any changes in the protein structure and function. Suggest and explain two reasons why. 

Scientists have discovered that within species genomes regions of introns do not change in frequency at the same rate as exons. Explain why.  

Cytosine and guanine can be bonded together in two ways, in a dinucleotide or in a base pair. Explain how these bonds differ.