Biology - Chapter 19

Question 1

What is a mutation?

Answer 1

A change in the sequence of nucleotides/bases in DNA molecules

Question 2

What are the 5 types of mutations?

Answer 2

1) Insertion/deletion
2) Point mutation/substitution
3) A nonsense mutation
4) A missense mutation
5) A silent mutation

Question 3

What are insertion/deletion mutations?

Answer 3

Where one or more nucleotide pairs are inserted or deleted from the sequence.
This alters the sequence of nucleotides as every successive codon from the point of mutation is changed known as a ‘frameshift’

Question 4

What are point mutation/substitutions?

Answer 4

Occurs when one base pair is replaced by another

Question 5

What are nonsense mutations?

Answer 5

A codon becomes a stop codon so the protein is shorter and often non-functional.
Translation is stopped early which gives rise to a truncated polypeptide due to premature introduction of a stop codon.

Question 6

What are missense mutations?

Answer 6

A codon change which results in the production of an incorrect amino acid in the primary structure which alters the tertiary structure of a protein.
- Can be silent, beneficial, or harmful
- Conservative: Amino acid change codes for a similar amino acid
- Non-conservative: New amino acid has different properties to the original

Question 7

What are silent mutations?

Answer 7

• A codon change which does not always affect the amino acid sequence/primary structure
• Possibly occurs due to degenerate nature of the genetic code (code for the same amino acid)
• Does not affect the phenotype
• Occur in non-coding regions of DNA (Introns)

Question 8

What are three keywords to describe a mutated protein?

Answer 8

1) Amorph - Loss of protein function
2) Hypomorph - Reduction of protein function
3) Hypermorph - Increase in function

Question 9

Give an example of a harmful mutation and how it occurs.

Answer 9

A mutation in the CFTR protein causes cystic fibrosis

Question 10

What is a mutagen?

Answer 10

Any agent (physical or environmental) that can induce a genetic mutation or can increase the rate of mutation

Question 11

How does the mutagen ‘X-rays’ affect mutation?

Answer 11

• Physical mutagen
• Causes DNA damage by breaking the DNA strands
• Can lead to the formation of pyrimidine dimers where two adjacent pyrimidines (cytosine and thymine) chemically bond together

Question 12

How does the mutagen ‘Nitrous acid/ affect mutation?

Answer 12

• Chemical mutagen
• Causes base deamination: the removal of an amino group (NH2) from a molecule

Question 13

How does the mutagen ‘alkylating agents’ affect mutation?

Answer 13

• Biological mutagen
• Alkylating agents transfer their alkyl group to nitrogenous bases of DNA and to the phosphate group (alkylation)

Question 14

How does the mutagen ‘Base analogues’ affect mutation?

Answer 14

• Biological mutagen
• Chemicals that resemble DNA bases and can be incorporated into DNA during replication (i.e. insertion of incorrect nucleotide), causing mutations

Question 15

How does the mutagen ‘viruses’ affect mutation?

Answer 15

• Biological mutagen
• The viruses can insert their own genetic material into the host’s DNA
• Causing insertions or deletions of base pairs

Question 16

When can mutations have neutral effects?

Answer 16

• Mutation causes no change in the organism
  e.g. when the mutation occurs in a non-coding region of DNA or is a silent mutation
• When a change in tertiary structure of the protein has no effect on the organism

Question 17

What determines whether a mutation is beneficial or harmful?

Answer 17

Depends on the environment of an organism.

Question 18

Give an example of a beneficial mutation.

Answer 18

A mutation may lead to humans developing trichromatic vision

Question 19

What 3 levels can gene expression be controlled in?

Answer 19

1) Transcriptional
2) Post-transcriptional
3) Post-translational

Question 20

Give an example of transcriptional control

Answer 20

The lac operon

Question 21

What is the ‘lac operon’?

Answer 21

A section of DNA composed of structural genes and control sites.

Control sites: here, they control the expression of beta-galactosidase

Question 22

What is the function of beta-galactosidase?

Answer 22

Hydrolyses lactose in E.coli

Question 23

What is the structure of lac operon?
(Promoter region, operator region, and structural genes)

Answer 23

A promoter region for structural genes: binding site for RNA polymerase (initiates transcription)

An operator region: binding site for inhibitor (repressor) to inhibit the transcription of a gene

Structural genes: Gives rise to 3 products -
1) Beta galactosidase
2) Lactose permease
3) Another enzyme

Question 24

What are the 3 structural genes in lac operon and what do they each code for?

Answer 24

lacZ: codes for lactase
lacY: Codes for permease (allows lactose into the cell)
lacA: codes for transacetylase

Question 25

Name the two structures located to the left (upstream) of the lac operon on the bacterium's DNA.

Answer 25

1)Promoter for regulatory gene 2)Regulatory gene lacL

Question 26

Describe the function of regulatory gene 'lacL'

Answer 26

Codes for the lac repressor protein

Question 27

Describe the function of the lac repressor protein.

Answer 27

It contains 2 binding sites that allow it to bind to the operator (in the lac operon) and to lactose (effector molecule).

Question 28

What happens when the lac repressor protein binds to the operator?

Answer 28

- Prevents the transcription of the structural genes - So, RNA polymerase cannot attach to the promoter

Question 29

What happens when the lac repressor protein binds to lactose?

Answer 29

The shape of the repressor protein distorts and so it cannot bind to the operator (thus, transcription is not inhibited)

Question 30

Describe what happens when lactose is absent (in a medium that the bacterium is growing in).

Answer 30

1) Regulatory gene (lacL) codes for/ is transcribed and translated to produce lac repressor protein 2)The lac repressor protein binds to the operator region upstream to lacZ 3) RNA polymerase is unable to bind to the promoter region 4)Transcription of structural genes does not take place 5) No lactase enzyme is synthesised

Question 31

Describe what happens when lactose is present (in a medium that the bacterium is growing in).

Answer 31

1) Bacterium uptakes lactose 2) Lactose binds to the second binding site of the lac repressor protein 3) This causes conformational change of the DNA binding site as its shape is distorted 4) Lac repressor protein is now unable to bind to the operator region 5) Now, RNA polymerase can successfully bind to the promoter region and transcription takes place 6) The mRNA from all three structural genes is translated 7) Enzyme lactase is produced 8) So, lactose can be broken down and used for energy by the bacterium

Question 32

What are transcription factors?

Answer 32

Also control gene expression by having the ability to switch genes on and off. They do this by interacting with the promoter region/sequence of DNA to either initiate or inhibit transcription.

Question 33

How is gene expression controlled at the post-transcriptional level?

Answer 33

Introns are removed from the primary mRNA transcript which, in turn, creates a mature transcript only consisting of protein-producing/coding regions known as exons.

Question 34

What are introns vs exons?

Answer 34

Introns: Non-coding regions Exons: Coding/protein-producing regions

Question 35

How is gene expression controlled at the post-translational level? Use adrenaline as an example.

Answer 35

For example, adrenaline (a protein) can be activated with the help of cyclic AMP. 1) Adrenaline binds to a complementary receptor 2) Enzyme 'adenylate cyclase' is activated 3) The enzyme converts ATP to cyclic AMP 4) Starts an enzyme-reaction cascade within the cell 5) So, adrenaline is now activated

Question 36

What is meant by the term 'homeobox'?

Answer 36

A DNA sequence that codes for a protein transcription factor.

Question 37

What is the role of a protein transcription factor that is coded by a homeobox sequence?

Answer 37

Attaches to a specific binding site on DNA and regulate the transcription of genes by turning various genes on and off in the correct order.

Question 38

What is a homeobox gene?

Answer 38

Any gene that contains a homeobox sequence

Question 39

Name two properties of homeobox gene sequences in plants, animals, and fungi.

Answer 39

They are highly conserved and are similar as they have been maintained by natural selection i.e. they remain relatively unchanged when travelling back in evolutionary time.

Question 40

Why are the homeobox sequences in plants, animals, and fungi so similar?