6 - ICH - Cellular control Flashcards
State the different forms of gene mutation that can occur
Substitution
Addition
Deletion
Define:
Gene mutation by:
- Substition
- Deletion
- Addition
Substition = A mutation where ≥1 nucleotides are substituted for another in a DNA strand
Deletion = A mutation where ≥1 nucleotides are deleted and lost from the DNA strand
Addition = A mutation where ≥1 nucleotides are added into the DNA strand
Define Mutation
Mutation = Spontaneous change to the sequence of nucleotides
What can a gene mutation result in?
The change in the DNA base sequence may result in a difference in the amino acid sequence of the subsequent polypeptide.
- This could result in a Non-functional protein
What are mutagenic agents? Give another name for them?
Examples (2)
Mutagenic agents (mutagens) = External factors that increase the basic mutation rate
E.g
- High neergy radiation that can disrupt the DNA molecule
- Chemicals that alter DNA structure or interfere with transcription
Are mutations passed onto the next generation?
- Mutation in body cells are NOT passed onto the next generation
- Mutations occuring during the formation of gametes may be inherited, often producing sudden and distinct differences between indiviuals
Effect of substitution? + Example
May change a single amino acid in the polypeptide chain.
- May have no effect because DNA code is degenerate - more than one trip codes for each amino acid
- May have a dramatic effect e.g. a substitution of one base in the gene coding for ß chains in haemoglobin molecules causes sickle cell anaemia
Effect of deletion and addition?
Have a much more dramatic effect than substitution
- Gain or loss of a base will affect every triplet after the effected base in the sequence = Frame-shift
- Results in the alteration of the entire amino acid sequence following mutation. Tertiary structure is likely ro change and thus results in a non-functional protein
Effect of mutations on the way a protein functions
- Neutral (2)
- Harmful (2)
- Beneficial (4)
Neutral:
- Might’ve occured in non-coding part of DNA
- Degenerate nature of DNA code may mean the new codon still codes for the same amino acid as more than one triplet codes for each amino acid ∴ no effect on protein shape or function
Harmful:
- Mutated gene may produce new vresion of allele of a gene
- Many mutated genes give rise to genetic diseases e.g. sickle cell anaemia
Beneficial:
- Some mutations produce beneficial alleles
- e.g. Decrease in melanin production in the skin of early humans migrating temperature climates, results in paler skin allowing more Vitamin D to be made in the skin in climates with less sunshine
- People with dark skin cannot make as much vitamin D and suffer from rickets
- Paler skinned people would have a survival advantage over dark skinned people in areas of less sunshine
What is a Point mutation?
What are the 3 types of point mutation?
Point mutation
- Mutations where only one base is affected
- 3 Types
- Silent mutation - No change in amino acid sequence of polypeptide
- Missense mutation - Mutation changes code for 1 amino acid ∴ 1 amino acid in sequence changes
- Nonsense mutation - Mutation changes the code turning the triplet into a stop codon ∴ polypeptide is shorter than expected
What is gene regulation?
Gene regulation = Controlling whether a gene is being expressed or not
Why is gene regulation needed and who is it needed for?
Gene regulation is required for eukaryotic and prokaryotic cells to specialise and work in a coordinated way
State the 4 mechanisms used to regulate gene expression (give a brief decription of what each does)
- Transcriptional - Genes an be turned on/off
- Post-transcriptional modification - mRNA can be modified which regulated translation and the types of polypetide/ protein produced
- Translational - Translation can be stopped/started
- Post translational modification - Proteins can be modified after synthesis which changes their functions
What is an operon?
Operon = Section of DNA that contains a cluster of structural genes that are all transcribes together
State an example of using transcription to regulate genes
Lac operon
What is the Lac Operon - background
Lac Operon is a DNA base sequence that controls the production of enzymes needed for lactose metabolism in E.Coli bacteria.
These enzymes include:
- ß Galactoside Permease (lactose permease) - Enables the cell to take up lactose
- Lactase (ß Galactosidase) - hydrolyses lactose → glucose + galactose
Lac Operon:
What happens if the E.Coli is grown on:
- Medium containing only glucose
- Medium containingonly lactose
-
Medium containing only glucose
- Doesn’t produce lactose permease or lactase
- Genes that code for them are not expressed
-
Medium containingonly lactose
- Genes are switched on and both enzymes are produced
Lac Operon:
What makes up the lac operon? (3)
- Structure genes - Comprises the length of DNA with base sequences needed to produce the mRNA that codes for the polyptptides that make up the 2 enzymes lactase and lactose permease
- Protomer - Portion of DNA to which the enzyme RNA polymerase attaches to in order to begin the process of transcription of mRNA
- Operator - Allows promoter to work if nothing is bound to it. A repressor switches off the operator
What is repressor and is it a part of the lac operon?
What is it’s role in the lac operon?
A repressor is a protein moelecule coded for by a regulatory gene that is NOT a part of the lac operon
Role:
The repressort has 2 binding sites:
- One is complementary to operator DNA so it binds with it preventing RNA polymerase from binding to the promoter ∴ structural genes are not being expressed ∴ mRNA can’t be transcribed ∴ no enzymes produced
- One is complementary to the lactose and when it’s bound to it, the repressor protein changes shape and can’t bind to the operator ∴ meaning that RNA polymerase can bind to promoter and transcription can take place. Structural genes are expressed ad enzymes are produced
List 3 advantages of being able to switch off genes
- Save energy
- Amino acids can be used to make vital proteins
- No space wasted for extra enzymes
What happens are the lac operon?
- Lactose is NOT present
- Lactose is present
LACTOSE NOT PRESENT:
- Regulatory gene is expressed and the lac repressor, which is a transcription factor that binds to the operator site when there’s no lactose present
- This blocks transcription as RNA polymerase can’t bind to promoter
LACTOSE PRESENT:
- Lactose binds to repressor gene, changing the repressor’s shape ∴ no longer complementary to operater site
- RNA polymerase binds to promoter, transcription of structural genes begin
Transcriptional level control:
- Eukaryotes
- Prokaryotes
Eukaryotes:
- Transcription factors bind to specific DNA sites near the start of their target genes
- Transcription factor binds to a specific DNA site near the start of the taget gene
- RNA polymerase binds to complex
- Transcription starts
Prokaryotes:
- Transcription factors binding to operons e.g. lac operon
What is chromatin?
What is chromatin subsivided into + what is different about the 2 types?
Chromatin = Uncondensed DNA in a complex with histones
- Heterochromatin = Tightly would DNA causing chromosomes to be visible during cell division
- Euchromatin = Loosely wound DNA present during interphase
Without any modification why can euchromatin be transcribed but not heterochromatin?
- Euchromatin is loosely wound DNA so can be transcribed
- Heterochromatin is tightly wound DNA so RNA polymerase can’t access the genes ∴ heterochromatin must be modified in order for their respective genes to be transcribed
What type of gene modification is histone modification?
Purpose?
- Histone modification = transcriptional control
- DNA coils around histones because they’re positively charged and DNA is negatively charged
- Histones are modified to increase or decrease the degree of packing (condensation)
-
Adding acetyl groups (acetylation) or phosphate groups (phosphorylation) makes it more negative
- Causes DNA to coil less tightly ∴ allows genes to be transcribed
-
Adding methyl groups (methylation) makes histones more hydrophobic
- Bind more tightly to each other causing DNA to coil more tightly ∴ prevent gene transcription
Describe the role of cyclic AMP (cAMP) in post transcriptional. pre translational control
- Binding of RNA polymerase only results in a relatively slow rate of transciption which needs to be increased to produce the required amount of enzyme to metabolise lactose efficiently
- Achieved by the binding of cAMP receptor protein (CRP)
- CRP can only bind when it in turn is bound to cAMP
Relationship between the concentrations of cAMP, lactose and glucose?
- Transport of glucose into an E.Coli decreases the levels of cAMP
- Reducing level of transcription of genes responsible for metabolism of lactose
- ∴ means that if both lactose and glucose are present it will favour metabolising glucose over lactose
State and explain the 2 methods of post transcription/ pre translational control:
Method 1 (4)
Method 2 (2)
RNA Processing:
- Transcription results in the production of the precursor molecule pre-mRNA (contains introns) which needs to be modified into mature-mRNA (no introns)
- A Cap = modified nucleotide, is added onto 5’ end and a string of adenine bases onto the 3’ end in order to stabalise it and prevent degradation in cytoplasm
- Splicing occurs where introns are removed to form mature-mRNA
- Occurs in nucleus
RNA Editing:
- Nucleotide sequences of some mRNA molecules can be changed through addition, deletion and substitution
- Increases the range of proteins produced, which may have different functions that can be produced from a single mRNA molecule or gene
Name the 3 main mechanisms involved with regulating the process of protein synthesis in translational control
Degradation of mRNA
- The more resistant the molecule the longer it will last in the cytoplasm and the greater the quantity pf protein will be synthesised
Binding proteins to mRNA
- Prevents it binding to ribosomes and the synthesis of proteins
Activation of initiation factors
- May aid the binding of mRNA to ribosomes
- The eggs of many organisms produce large quantities of mRNA which is not required until after fertilisation, at which point initiation factors are activated
4 examples of post trasnplational control
- Addition of non-protein groups e.g. carbohydrate chains, lipids or phosphates…
- Modifying amino acids and formation of disulphide bridges
- Folding or shortening of proteins
- Modification by cAMP - cAMP is required for protein activation providing the energy required for a protein to form its tertiary structure
