P2 Regulation of Gene Expression

Question 1

What is a gene mutation and when does it occur?

Answer 1

• A gene mutation is a change in the DNA’s base sequence.
• It can occur spontaneously during DNA replication - however mutagenic agents increase the rate of mutations.

Question 2

What are the three types of gene mutation?

Answer 2

1. Deletion
2. Substitution
3. Insertion

Question 3

When does deletion occur? (gene mutation)

Answer 3

• When a nucleotide is removed from a DNA sequence.
• This causes a frameshift (a change to the sequence of triplets), changing the sequence of amino acids that are coded for and results in a large change to the primary structure of the resulting protein.

Question 4

When does substitution occur? (gene mutation)

Answer 4

• When a base is swapped out for a different one.
• This only affects a single triplet in the DNA sequence - it can result in a different amino acid (changing the primary structure) or the same amino acid (doesn’t change the primary structure), this occurs because the genetic code is degenerate.

Question 5

When does insertion occur? (gene mutation)

Answer 5

• When one or more nucleotides are added to a DNA sequence.
• This either results in a frameshift (a change to the sequence of triplets), or the addition of an amino acid.

Question 6

What affect does a frameshift have?

Answer 6

• Causes a different primary structure, therefore changing the bonds in the secondary structure and creating either a non-functional protein, or a different protein.

Question 7

What is gene expression?

Answer 7

• When an active gene is transcribed and translated, resulting in a polypeptide.
• The product of gene expression is a protein (not a physical trait).
• Some genes are always expressed because they are always needed, eg. enzymes in respiration.
• Other genes are only expressed when they are needed (this control can occur during transcription, translation and post-translation).

Question 8

What is the difference between gene expression and protein synthesis?

Answer 8

• Protein synthesis refers to the translation of mRNA to tRNA to form a polypeptide.
• Whereas gene expression refers to the entire process of transcription and translation.

Question 9

What are the two types of genes involved in transcriptional control?

Answer 9

Structural genes and regulatory genes

Question 10

What is a structural gene?

Answer 10

Codes for a protein that has a function within the cell.

Question 11

What is a regulatory gene?

Answer 11

• Codes for a protein that controls structural genes.
• One regulatory gene usually controls lots of structural genes.

Question 12

What is a promoter, where is it located and how is it involved in transcriptional control?

Answer 12

• A promoter is a DNA sequence that sits between a regulatory gene and a structural gene that enables the transcription of structural genes.
• This is because it is the binding site for RNA polymerase (the enzyme that carries out transcription).
• Promoters exist in prokaryotes and eukaryotes, but act differently in both.

Question 13

How do promoters act in prokaryotes?

Answer 13

• In prokaryotes, RNA polymerase binds straight away to the promoter and transcribes the structural gene.
• But, in order to control gene expression at the transcriptional level, prokaryotes have another DNA sequence that sits between the promoter and the first structural gene called the operator.

Question 14

What is a transcription factor?

Answer 14

• The protein that the regulatory gene codes for, that controls the transcription of the structural genes.

Question 15

What is the transcription factor in prokaryotes?

Answer 15

A repressor protein - produced by a regulatory gene and binds to the operator. As a result RNA polymerase is unable to bind to the promoter, so the structural gene cannot be transcribed.

Question 16

What is an operon?

Answer 16

• A cluster of structural genes that is under the control of one promoter (it contains structural genes, an operator and a promoter, the promoter is the binding site of RNA polymerase that controls the transcription of the cluster of structural genes).

Question 17

What is lac operon?

Answer 17

• An operon that contains the genes needed to digest lactose.
• It is found in bacteria, where lactose is not usually present, therefore lac operon often needs to inhibit the structural genes that code for lactose (become inactive).

Question 18

How does lac operon work when no lactose is present?

Answer 18

1. The regulatory gene (lacI) produces a repressor protein, which binds to the operator (lacO).
2. This means RNA polymerase is unable to bind to the promoter, so the structural genes cannot be transcribed.

Question 19

How does lac operon work when lactose is present?

Answer 19

1. The regulatory gene (lacI) produces a repressor protein.
2. Lactose in the cell binds to this repressor protein, causing it to change it’s tertiary structure.
3. Therefore the repressor protein is unable to bind to the operator and RNA polymerase binds to the promoter, meaning that the structural genes can be transcribed.

Question 20

Why isn’t lac operon always on/active?

Answer 20

• Lactose is a sugar that can be broken down during respiration (it is a source of ATP). However, gene expression requires ATP.
• Therefore lac operon is only active when lactose is present to ensure ATP is not wasted on expressing the genes coding for lactose digestion when lactose isn’t present.

Question 21

How do DNA sequences in eukaryotic cells differ from prokaryotic cells?

Answer 21

DNA sequences in eukaryotic cells don’t contain an operator.

Question 22

Describe transcriptional control in eukaryotes.

Answer 22

• The regulatory gene produces a transcription factor which binds to the promoter.
• This enables RNA polymerase to also bind to the promoter, and RNA polymerase transcribes the structural genes.

Question 23

How do transcription factors differ in eukaryotes and prokaryotes?

Answer 23

Transcription factors in eukaryotes always stimulate gene expression, whereas in prokaryotes they repress/inhibit gene expression.

Question 24

How are transcription factors in eukaryotes activated?

Answer 24

• When transcription factors in eukaryotes are first produced by regulatory genes, they are inactive (cannot bind to the promoter).
• A hormone enters the cell, diffuses into the nucleus and binds to the inactive transcription factor, causing a change in its tertiary structure.
• This activates the transcription factor, allowing it to bind to the promoter (then RNA polymerase can also bind to the promoter and transcribe the structural genes).

Question 25

What are the differences in transcriptional control in prokaryotes and eukaryotes?

Answer 25

1. RNA POLYMERASE: in prokaryotes it can bind to the promoter without a transcription factor, but in eukaryotes it can only bind to the promoter with a transcription factor.
2. BINDING SITE OF TRANSCRIPTION FACTOR: in prokaryotes it binds to the operator, but in eukaryotes it binds to the promoter.
3. EFFECT OF TRANSCRIPTION FACTORS: in prokaryotes they inhibit transcription, but in eukaryotes they stimulate transcription.
4. NAME OF TRANSCRIPTION FACTOR: in prokaryotes it is called a repressor protein, and in eukaryotes it is called a transcription factor.

Question 26

What is post-transcriptional control of gene expression?

Answer 26

The changes mRNA goes through after transcription to ensure the mRNA is ready for translation.

Question 27

Describe the processes of post-transcriptional control:

Answer 27

• Transcription in eukaryotes produces a primary mRNA (not ready for translation).
  1. During RNA splicing, the non-coding introns are removed.
  2. During RNA editing, some RNA bases are substituted, deleted or added in.
• This results in a mature mRNA (ready for translation).

Question 28

What is post-translational control of gene expression?

Answer 28

The changes (activation) a protein goes through to ensure the protein is ready to perform a specific function in the cell.

Question 29

Describe the process of post-translational control:

Answer 29

Proteins are activated by phosphorylation:
1. Cyclic AMP (cAMP) binds to/activates the enzyme protein kinase.
2. Protein kinase then phosphorylates the protein, changing it’s tertiary structure (activating it).

Question 30

What is a body plan?

Answer 30

The observable, spatial arrangement of an organism’s body parts.

Question 31

What are homeobox genes?

Answer 31

• Genes that control the initial development of an organism’s body plan:
  they exist in plants, animals and fungi
  they are active during the embryonic stage
• Homeobox genes code for transcription factors (which control the expression of structural genes), so they are a type of regulatory gene.

Question 32

What is the homeodomain?

Answer 32

The DNA binding site of a transcription factor.

Question 33

What is the homeobox?

Answer 33

• The specific DNA sequence within the homeobox gene that codes for the homeodomain.
• All human homeobox genes contain identical homeobox’s - the same sequence of 180 base pairs.

Question 34

Why do intraspecific organisms have identical homeobox’s, interspecific organisms have very similar homeobox’s?

Answer 34

• Any mutation in the homeobox genes means the organism is unlikely to survive, so mutations are selected against by natural selection.
• Therefore homeobox genes are highly conserved by natural selection.

Question 35

What is a hox gene?

Answer 35

A type of homeobox gene that controls the symmetry of organisms body plans - only found in animals.

Question 36

What cellular processes control the development of a body plan?

Answer 36

1. Mitosis (produces new cells for growth, repair and reproduction).
2. Apoptosis (removes old cells, or cells that are no longer needed).
   - Mitosis and apoptosis are controlled by hox genes - which respond to internal stimuli, such as DNA damage, and external stimuli, such as stress. The hox genes then initiate either mitosis or apoptosis.