6.1.1 Cellular control Flashcards
Define mutation
A change in a DNA base sequence
What can cause mutations?
- Mutations can occur spontaneously if DNA is misread during replication.
- Mutagens can increase the rate of mutation (e.g. ultraviolet/ionising radiation, chemicals and viruses).
What are the three types of mutation?
Substitution
Deletion
Insertion
(Of one or more nucleotides)
Describe substitution
One base is replaced by another
Describe deletion
One base is removed from the sequence
Describe insertion
One base is added to the sequence
Effect of mutations
May change the amino acid sequence coded for by the gene. This results in a different polypeptide and a different tertiary structure.
Why don’t all mutations result in a change to the amino acid sequence of a protein?
The genetic code is degenerate. Many amino acids are coded for by more than one triplet.
Why are deletions and insertions (=indel mutations) much more likely to cause a change in the amino acid sequence of a protein?
They cause a frameshift.
Describe frameshift
The entire sequence shifts by one base. This changes all triplets after deletion/insertion so changes the amino acid sequence.
What are transcription factors?
Proteins that bind to specific DNA sequences to initiate the transcription of genes into mRNA.
What happens when a gene is ‘switched off’?
Transcription factors cannot bind to DNA. This prevents the transcription process.
What modifications can promote transcription in eukaryotes? Why?
- Acetylation and phosphorylation
- Reduce the positive charge on histones DNA more loosely coiled.
What modifications can inhibit transcription in eukaryotes? Why?
- Methylation
- Increases hydrophobic interactions of histones making DNA more tightly coiled.
What two forms can chromatin exist in?
Heterochromatin and Euchromatin
Describe heterochromatin
Densely packed making it difficult for RNA polymerase to access genes, reducing transcription
Describe euchromatin
Loosely packed, facilitating RNA polymerase access to genes increasing transcription
What is chromatin?
Negatively charged DNA wraps around positively charged histones to produce chromatin.
(Can be remodelled for gene expression)
What is an operon?
A cluster of genes controlled by a single promoter allowing for coordinated expression
Key components of operons
Regulatory gene(s)
Promoter region
Operator region
Structural gene(s)
Describe regulatory gene(s)
Code for proteins that regulate the expression of structural genes.
Describe promoter region
Site where RNA polymerase binds to initiate transcription
Describe operator region
Sequence where regulatory proteins can bind
Describe structural genes
Genes that code for proteins (e.g. enzymes)
Name and describe the structural genes in lac operon
lacZ - codes for B-galactosidase, breaks down lactose to glucose and galactose
lacY - codes for lactose permease, transports lactose into the cell
lacA - codes for transacetylase, modifies lactose/its by products
Name and describe the regulatory gene in lac operons
lacI
Codes for a repressor protein which can inhibit and control lac operons activity
Describe the steps for how lac operons functions when lactose is absent
- Repressor protein binds to operator region
- RNA polymerase blocked from the promoter region
- RNA polymerase cannot transcribe structural genes
- Enzymes for lactose metabolism aren’t produced
Describe how lac operons functions when lactose is present
- Lactose binds to the repressor protein
- Repressor protein changes shape and is released from the operator region
- RNA polymerase can bind to the promoter region and initiate transcription
- RNA polymerase transcribes structural genes leading to production of enzymes necessary for lactose metabolism
How does the presence of glucose indirectly inhibit the lac operon?
Via a signalling molecule, cyclic AMP (cAMP)
Describe what happens when only lactose is present
- cAMP levels increase and cAMP binds to the cAMP receptor protein (CRP)
- The CRP-cAMP complex upregulates the transcription of the lac operon
- Lactose metabolism is optimised.
Describe what happens when both glucose and lactose are present
1.Glucose reduces cAMP levels
2. The CRP-cAMP complex cannot form
3. lac operon transcription is downregulated
4. Lactose metabolism enzymes not produced
What are introns and exons?
Introns - non coding regions of DNA
Exons - Coding regions of DNA
How can pre-mRNA be processed (post transcriptional control)?
- Addition of a 5’ cap: stabilises mRNA, delays degradation, assists in ribosomes binding
- Addition of a 3’ poly-A tail: stabilises mRNA, delays degradation
- Splicing: Introns removed and exons joined together providing protein sequence
How is gene expression controlled at the post translational level?
Proteins are activated by cAMP.
e.g. protein kinase (adds phosphate groups to proteins by phosphorylation)
Define body plan
Basic structured arrangement of an organism’s parts (determined by genetic and developmental factors)
Define homeobox genes
A group of regulatory genes with a conserved DNA sequence that guides development of body plans
Define Homeobox sequence
Highly conserved and similar DNA sequence (in plants, animals and fungi) crucial for development of body plan
Define Hox genes
Subset of homeobox genes (in animals) containing homeobox sequence essential for correct positioning of body parts.
How do hox genes control development?
- Homeobox sequence codes for the homeodomain (part of a protein that binds to DNA)
- Homeodomain acts as a transcription factor so It binds to DNA, switching genes on/off
- This modifies transcription of proteins necessary for development of body plans
Why are homeobox genes so conserved?
- A mutation would have large effects by altering body plan
- Many other genes would also be affected by a mutation in a homeobox gene
- Mutations are likely to be lethal and selected against
What can genes that regulate mitosis/ the cell cycle and apoptosis respond to?
Internal and external stimuli
Importance of mitosis and apoptosis in body plan development
Refine body morphology (alongside cell differentiation)
