P2 Regulation of Gene Expression Flashcards
What is a gene mutation and when does it occur?
- 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.
What are the three types of gene mutation?
- Deletion
- Substitution
- Insertion
When does deletion occur? (gene mutation)
- 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.
When does substitution occur? (gene mutation)
- 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.
When does insertion occur? (gene mutation)
- 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.
What affect does a frameshift have?
- Causes a different primary structure, therefore changing the bonds in the secondary structure and creating either a non-functional protein, or a different protein.
What is gene expression?
- 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).
What is the difference between gene expression and protein synthesis?
- 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.
What are the two types of genes involved in transcriptional control?
Structural genes and regulatory genes
What is a structural gene?
Codes for a protein that has a function within the cell.
What is a regulatory gene?
- Codes for a protein that controls structural genes.
- One regulatory gene usually controls lots of structural genes.
What is a promoter, where is it located and how is it involved in transcriptional control?
- 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.
How do promoters act in prokaryotes?
- 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.
What is a transcription factor?
- The protein that the regulatory gene codes for, that controls the transcription of the structural genes.
What is the transcription factor in prokaryotes?
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.
What is an operon?
- 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).
What is lac operon?
- 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).
How does lac operon work when no lactose is present?
- The regulatory gene (lacI) produces a repressor protein, which binds to the operator (lacO).
- This means RNA polymerase is unable to bind to the promoter, so the structural genes cannot be transcribed.
How does lac operon work when lactose is present?
- The regulatory gene (lacI) produces a repressor protein.
- Lactose in the cell binds to this repressor protein, causing it to change it’s tertiary structure.
- 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.
Why isn’t lac operon always on/active?
- 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.
How do DNA sequences in eukaryotic cells differ from prokaryotic cells?
DNA sequences in eukaryotic cells don’t contain an operator.
Describe transcriptional control in eukaryotes.
- 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.
How do transcription factors differ in eukaryotes and prokaryotes?
Transcription factors in eukaryotes always stimulate gene expression, whereas in prokaryotes they repress/inhibit gene expression.
How are transcription factors in eukaryotes activated?
- 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).
What are the differences in transcriptional control in prokaryotes and eukaryotes?
- 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.
- BINDING SITE OF TRANSCRIPTION FACTOR: in prokaryotes it binds to the operator, but in eukaryotes it binds to the promoter.
- EFFECT OF TRANSCRIPTION FACTORS: in prokaryotes they inhibit transcription, but in eukaryotes they stimulate transcription.
- NAME OF TRANSCRIPTION FACTOR: in prokaryotes it is called a repressor protein, and in eukaryotes it is called a transcription factor.
What is post-transcriptional control of gene expression?
The changes mRNA goes through after transcription to ensure the mRNA is ready for translation.
Describe the processes of post-transcriptional control:
- 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).
What is post-translational control of gene expression?
The changes (activation) a protein goes through to ensure the protein is ready to perform a specific function in the cell.
Describe the process of post-translational control:
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).
What is a body plan?
The observable, spatial arrangement of an organism’s body parts.
What are homeobox genes?
- 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.
What is the homeodomain?
The DNA binding site of a transcription factor.
What is the homeobox?
- 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.
Why do intraspecific organisms have identical homeobox’s, interspecific organisms have very similar homeobox’s?
- 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.
What is a hox gene?
A type of homeobox gene that controls the symmetry of organisms body plans - only found in animals.
What cellular processes control the development of a body plan?
- Mitosis (produces new cells for growth, repair and reproduction).
- 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.
