Cellular Control

Question 1

What is a mutation?

Answer 1

A mutation is a change in the sequence of bases in DNA.

Question 2

What three things can cause a change in sequence within a gene?

Answer 2

The substitution, deletion or insertion of one or more nucleotides (or base pairs) within a gene.

Question 3

What does a substitution of a single nucleotide do?

Answer 3

• Changes the codon in which it occurs.
• If the new codon codes for a different amino acid this will lead to a change in the primary structure of the protein.

Question 4

Due to the degenerate nature of the genetic code, what may happen?

Answer 4

May mean that the new codon still codes for the same amino acid leading to no change in the protein synthesised.

Question 5

What type of mutation does the insertion or deletion of a nucleotide cause?

Answer 5

A frameshift mutation.
- Triplet code means that sequences of bases are transcribed consecutively in non-overlapping groups of three. This is the reading frame of a sequence of bases.

Question 6

What does the addition or deletion of a nucleotide do to a reading frame?

Answer 6

The addition or deletion of a nucleotide moves or shifts the reading frame of the sequence of bases. This will change every successive codon from the point of mutation.

Question 7

What are the different effects of different mutations?

Answer 7

No effect - there is no effect on the phenotype of an organism because normally functioning proteins are still synthesised.
Damaging - the phenotype of an organism is affected in a negative way because proteins are no longer synthesised or proteins synthesised are non-functional. This can interfere with one, or more essential processes.
Beneficial - Very rarely a protein is synthesised that results in a new and useful characteristic in the phenotype. For example, a mutation in a protein present in the cell surface membrane of human cells means that HIV cannot blind and enter those cells. People with this mutation are immune to infection from HIV.

Question 8

When does a mutation occur?

Answer 8

Often during DNA replication.

Question 9

What increases the rate of a mutation.

Answer 9

Mutagens. A mutagen is a chemical, physical, or biological agent which causes mutations.

Question 10

What can the absence of a base lead to?

Answer 10

Can lead to insertion of an incorrect base through complementary base pairing during DNA replication.

Question 11

Example of physical mutagens and what it does?

Answer 11

• Ionising radiations such as X-rays.
• Break one or both DNA strands - some breaks can be repaired but mutations can occur in the process.

Question 12

Example of chemical mutagens and what it does?

Answer 12

• Deaminating agents.
• Chemically alter bases in DNA such as converting cytosine to uracil, changing the bases sequence.

Question 13

Examples of biological agents and what they do?

Answer 13

Alkylating agents - Methyl or ethyl groups are attached to bases resulting in the incorrect pairing of bases during replication.
Base analogs - Incorporated into DNA in place of the usual base during replication, changing the base sequence.
Viruses - Viral DNA may insert itself into a genome, changing the base sequence.

Question 14

Effect on silent mutation.

Answer 14

• Have no effect on the phenotype of an organism, as they do not change any proteins, or the activity of any proteins synthesised.
• They may also result in changes to the primary structure but do not change the overall structure or function of the protein synthesised.

Question 15

Where do silent mutations occur?

Answer 15

• They can occur in the non-coding regions of DNA (introns) or code for the same amino acid due to the degenerate nature of genetic code.

Question 16

Effect of nonsense mutations and what does it do.

Answer 16

• Nonsense mutations result in a codon becoming stop codon instead of coding for an amino acid.
• This result is a shortened protein being synthesised which is normally non-functional. These mutations normally have negative or harmful effects on phenotypes.

Question 17

Effects of missense mutations.

Answer 17

• Missense mutations result in the incorporation of an incorrect amino acid into the primary structure when the protein is synthesised.
• The result depends on the role the amino acid plays in the structure and therefore function of the protein synthesised.

Question 18

What is a conservative mutation?

Answer 18

A conservative mutation occurs when the amino acid change leads to an amino acid being coded for which has similar properties to the original, this means the effect of the mutation is less severe.

Question 19

What is a non-conservative mutation?

Answer 19

• When the new amino acid coded for has different properties to the original, this is more likely to have an effect on protein structure, and may cause disease.

Question 20

What is a beneficial mutation?

Answer 20

• An alteration to a polypeptide that results in an altered characteristic in an organism that causes beneficial effects for the organism.

Question 21

What are the benefits of being heterozygous for sickle cell anaemia?

Answer 21

-Heterozygous individuals only get mild symptoms of the condition, but are resistant to mania. This is beneficial as hundreds of of million of people get malaria each year with over a million fatal cases.
- They have the ability to withstand malaria infection without having sickle cell disease.

Question 22

What are the different changes in chromosome structure include?

Answer 22

Deletion - a section of chromosome breaks off and is lost within the cell.
Translocation - a section of of one chromosome breaks off and joins another non-homologous chromosome.
Inversion - a section of chromosome breaks off, is reversed, and then joins back onto the chromosome.

Question 23

What happens at the different levels that the genes are regulated at?

Answer 23

Transcriptional - genes can be turned on or off.
Post-transcriptional - mRNA can be modified which regulates translation and the types of proteins produced .
Translational - translation can be stopped or started.
Post-translational - proteins can be modified after synthesis which changes their functions.

Question 24

Distinguish between the terms “regulatory gene” and “structural gene”.

Answer 24

A regulatory gene produces the transcription factor/repressor protein, whereas the structural protein produces the enzymes.

Question 25

Describe how genes Z and Y are switched on in bacteria that are moved to a nutrient medium that contains lactose.

Answer 25

• When lactose is present it binds to the transcription factor/repressor protein.
• This changes the shape of the repressor protein meaning it can’t join to the operator region and block the RNA polymerase from joining to the promoter region so RNA polymerase does join to the promoter region and can move along so transcription of the structural genes can occur and be expressed.

Question 26

Describe how genes Z and Y are switched on in bacteria that are moved to a nutrient medium that contains lactose.

Answer 26

• When lactose is present it binds to the transcription factor/repressor protein.
• This changes the shape of the repressor protein meaning it can’t join to the operator region and block the RNA polymerase from joining to the promoter region so RNA polymerase does join to the promoter region and can move along so transcription of the structural genes can occur and be expressed.