14. Genetic Control of Protein Structure and Function

Question 1

(2 marks)

Explain why there has been an increase in skin cancer in the UK

Answer 1

• Increased exposure to UV light
• Caused by decline in ozone/ more foreign holidays/ sunbathing
• Damages DNA/ increases rate of mutation

Question 2

(2 marks)

Explain the role of tRNA.

Answer 2

• Each type carries specific amino acid/ has anti codon
• 3 bases on tRNA bind to 3 bases on mRNA/ codon and anti codon interaction
• tRNA attaches to ribosomes/ holds amino acids in place

Question 3

What is meant by saying that DNA is a degenerate code?

Answer 3

That most amino acids have more than one codon

Question 4

What is meant by saying that DNA is a universal code?

Answer 4

The same codons code for the same amino acids in all organisms

Question 5

What is meant by saying that DNA is non overlapping?

Answer 5

Each base in the sequence is only read once

Question 6

What is the structure of an RNA nucleotide? (ie features)

Answer 6

• One of the organic bases- AGCU
• Pentose sugar (ribose)
• A phosphate group

Question 7

How is the structure of mRNA suited to its function?

Answer 7

• Possesses the correct sequences of the many triplets of organic bases that code for specific polypeptides
• Easily broken down so only exists when is needed to manufacture a given protein.

Question 8

What is the purpose of the anticodon in tRNA?

Answer 8

• Complementary shape to the mRNA codon so in the ribosome the tRNA can attach to the mRNA during translation to deposit its amino acid to form a polypeptide chain.
• Anticodon on tRNA is the same as the DNA codon but with uracil instead of thymine

Question 9

Describe the process of transcription.

Answer 9

• DNA helicase breaks hydrogen bonds at a certain section of DNA
• RNA polymerase joins free nucelotides to the template strand (however using U instead of T for the new RNA stand)
• As the RNA polymerase builds the pre mRNA the DNA strands rejoin behind it so only about 12 base pairs are exposed at any one time
• When the stop codon is reached, RNA polymerase detaches and the production of pre mRNA is complete

Question 10

What is splicing?

Answer 10

Removing introns from the pre mRNA to form mRNA with just exons

Question 11

How can many different proteins be formed from the same gene?

Answer 11

After splicing, the exons can be recombined in many different orders, therefore allowing the production of many different proteins

Question 12

How is a polypeptide synthesised from mRNA? (Steps)

Answer 12

• Ribosome attaches to starting codon
• tRNA with complementary anticodon sequence attaches, carrying an amino acid
• TRNA with a complementary anticodon moves to the ribosome and pairs with the next codon on the mRNA, again, carrying an amino acid
• Ribosome moves along mRNA, bringing together 2 tRNA molecules at any one time
• 2 amino acids are joines by a peptide bond using an enzyme and ATP
• Ribosome moves onto the third codon and a third complementary tRNA attaches. As this happens the first tRNA is released and picks up its specific amino acid again from the amino acid pool in the cell
• Polypeptide synthesised by ribosome continuing to move along like this until a stop codon is reached

Question 13

What usually happens in the assembling of a protein? (eg what happens to the polypeptide chain to make it into a functional protein)? (3 ways)

Answer 13

• Polypeptide folded or coiled into a secondary structure
• Secondary structure is folded to form a tertiary structure
• Different polypeptide chains and non protein groups are linked to form a quaternary structure

Question 14

What is a mutation?

Answer 14

Any change to the quantity or structure of DNA of an orgnism

Question 15

Which type of mutation is hereditary?

Answer 15

Germ line mutations are passed on to the offspring whereas somatic cell mutations are not.

Question 16

What is a substitution nonsense mutation?

Answer 16

If a base change results in the formation of one of the three stop codons, making the protein (probably) much shorter and non functional

Question 17

What is a substitution mis-sense mutation?

Answer 17

Base change results in a different amino acid being coded for. The significance of this will depend on the precice role of the amino acid. If its important in forming bonds that determine the tertiary structure then the mutation means the tertiary structure will be different and the protein will be non functional. However if the mutation is in an intron then it will make no difference

Question 18

What is a substitution silent mutation?

Answer 18

When the substituted base still codes for the same amino acid as before

Question 19

What is the significance of a deletion mutation?

Answer 19

• Causes a frame shift
• All subsequent triplets are read differently
• Non functional protein as the entire amino acid sequence produced could be different if the mutation is at the start of the sequence

Question 20

Give examples of the causes of mutations.

Answer 20

-Can arise spontaneously, without outside influence. Although mutations are random, they occur at a set frequency which is about 1-2 mutations per 100,000 genes per generation

• High energy radiation that can disrupt the DNA molecule
• Chemicals that alter the DNA structure or interfere with transcription

Question 21

What are the pros and cons of mutations?

Answer 21

Pros:

• Produce genetic diversity which is essential for natural selection and speciation
• Allows evolution to occur

Cons:

• Produce an organism less well suited for its environment
• Mutations that occur in body cells rather than gametes can disrupt normal cellular activities such as cell division

Question 22

How is cell division controlled by genes? (2 types)

Answer 22

• Proto-oncogenes STIMULATE cell division

- Tumour suppressor genes SLOW cell division

Question 23

How is normal cell division stimulated/ maintained?

Answer 23

• Growth factors attach to protein receptors on the cell surface membrane
• Via relay proteins in the cytoplasm, the genes necessary for cell division are ‘switched on’

Question 24

How can ONCOGENES affect cell division?

Answer 24

• Receptor protein on membrane can be permanently activated so cell division is switched on even in the absence of growth factors
• Oncogene may code for a growth factor that is produced in excessive amounts, thus stimulating excessive cell division

Question 25

What is the significance of a mutation in a tumour suppressor gene?

Answer 25

• If mutated, it becomes inactivated (stops inhibiting cell division)
• Cell division increases
• Mutant cells so formed are usually structurally and functionally different from normal cells
• Most mutated cells die however any that survive are capable of making clones of themselves and forming tumours.

Question 26

(1 mark)

Name two substances in the backbone of a pre-mRNA molecule

Answer 26

• Phosphate

- Ribose (ignore pentose sugar)

Question 27

(1 mark)
Give one way in which the structure of an mRNA molecule is different from the
structure of a tRNA molecule.

Answer 27

Does not contain hydrogen bonds/base pairs
/contains codons / does not contain anticodon /
straight/not folded / no amino acid binding
site/longer;

Question 28

(2 marks)
Explain why the percentages of bases from the middle part of the chromosome and
the end part are different

Answer 28

1. Different genes;
2. Have different (base) sequences / combinations of
   (bases);
3. (Pre-mRNA) transcribed from different DNA/codes
   for different proteins;