6.1.1 Cellular Control Flashcards
What is a mutation?
A random change in the nucleotide base sequence within the DNA molecule
What 3 things could happen to the base sequence/
- Substitution
- Deletion
- Insertion
Why might a mutation in a gene not lead to a different primary structure?
The genetic code is degenerate (many amino acids coded for by more than one codon)
What increases the rate at which mutations occur?
Mutagens (chemical, physical, biological agents)
Give examples and effects of physical mutagens
Ionizing radiations (e.g. X-rays/gamma rays/UV)
Break one/both strands of DNA causing mutations
Give examples and effects of chemical mutagens
Deanimating agents
Alter bases in DNA, altering the base sequence
Give examples of biological agents
Alkylating agents - Methyl/ethyl groups attached to bases, leading to incorrect pairing of bases during replication
Base analogs - incorporated into DNA instead of usual base during replication, changing the base sequence
Viruses - viral DNA inserts itself into the genome, changing the base sequence
What is a point mutation?
When a single base pair is changed
What is a substitution mutation?
One base pair replaced for another (usually leads to a single, different amin acid being inserted during translation)
What is a deletion mutation?
A base pair is lost from the DNA sequence, causing a frame shift (Every codon ‘down stream’ is different)
What is an insertion/addition mutation?
An extra base pair is inserted into the DNA sequence, causing a frame shift (Every codon ‘down stream’ is different)
What is a mis-sense effect on an amino acid sequence?
Where one nucleotide base is substituted with another meaning one amino acid in the sequence is incorrect
What is a nonsense effect on an amino acid sequence?
Where one base is substituted with another causing a stop codon to be created
What is a silent effect on an amino acid sequence?
Subsitution occues but has no effect on the amino acid due to the degenerate nature of the code (usually the 3rd base)
Why can mutations have no effect on phenotype as a normally functioning protein is made?
- Degenerate nature of code
- Amino acid replaced with a similar amino acid
- Amino acid is in a part of the polypeptide that doesn’t affect it’s folding or function
- If a mutation occurs in a non-coding region (intron)
What damaging effects can mutations have on proteins?
- Not functional
- Not made at all
What beneficial effect can mutations have on proteins?
A protein has new and useful characteristics (this drives natural selection)
What is a chromosome mutation?
A mutation affecting the whole chromosome or a number of chromosomes within a cell (casued by mutagens)
Name and describe the 4 types of chromosome mutation
Deletion - A section of chromosome breaks off and is lost
Duplication - Sections on a chromosome are duplicated
Translocation - A section of one chromsome breaks off and joins another chromosome
Inversion - A section of a chromosome breaks off, is reversed and joins back onto the chromosome
What are housekeeping genes?
Genes that code for enzymes needed for reactions in metabolic pathways (e.g. respiration) and are needed constantly
What are tissue specific genes?
Genes that code for protein based hormones that are only required by certain cels at certain times
What is gene regulation?
When genes are turned on/off and the rate of protein synthesis is changed according to demand
What is transcriptional gene regulation?
Genes being turned on/off
What is post-transcriptional gene regulation?
mRNA modified which regulates translation + the types of protein produced
What is translational gene regulation?
Translation stopped or started
What is post-translational gene regulation?
Proteins modified after synthesis which changes their function
What is an operon?
A group of genes controlled by the same regulatory mechanism and are expressed at the same time
Why are operons used?
Very efficient way of saving resources - when gene products not needed, al lthe genes involved in their production can be switched off
What type of organisms are operons most commonly used in?
Prokaryotes
What is the Lac operon?
A region of DNA in E. coli that codes for 3 enzymes that allow use of lactose sugars
What 3 enzymes does E. coli produce to use the disaccharide lactose as a respiratory substrate?
- Lactose permease - transports lactose into the cell
- Beta Galactosidase - hydrolyses lactose into glucose + galactose
- Transacetylase
What is the role of the regulatory gene in the Lac Operon?
Codes for repressor proteins which regulates transcription of structural genes
What is the role of the operator region in the Lac Operon?
Can switch structural genes on/off
What is the role of the promote region in the Lac Operon?
Where RNA polymerase binds for transcription of structural genes
What is the role of structural genes in the Lac Operon?
DNA that can be transcribed to make one long piece of mRNA that is translated into Beta Galactosidsae, Lactose permease, Transacetylase
Explain how the Lac Operon functions without the presence of lactose
- RNA polymerase transcribes the regulatory gene
- The regulatory gene is translated to form the repressor protein
- The repressor protein binds to the operator region, also covering the promotor region, preventing RNA polymerase from doing transcription
- Therefore no mRNA is formed - no trasnslation - no enzymes made
Explain how the Lac Operon Functions in the presence of lactose
- RNA polymerase transcribes the regulatory gene
- The regulatory gene is transcribed to form the repressor protein
- The repressor protein binds to the operator region, also covering the promoter region
- Lactose binds to a site on the other side of the repressor protein, causing it to change shape and leave the operator region (acts as an inducer molecule)
- Promoter region is now open so RNA polymerase can transcribe structural genes
- mRNA transcribed at ribosomes to form Beta Galactosidase, Transacetylase, Lactose permease
How does cAMP increase transcription in the Lac Operon?
cAMP binds to CRP which binds to DNA in the promote region to increase transcription
How does a lack of glucose influence the rate of transcription of strctural genes in the Lac Operon?
- Less glucose in E.coli
- Increased levels of cAMP
- More cAMP binds to CRp
- CRP binds to promoter region
- More transcription of Lac genes by RNA polymerase as RNA can now bind to DNA
How does an increase of glucose influence the rate of transcription of strctural genes in the Lac Operon?
- Transport of glucose into E.coli
- Decreases levels of cAMP
- CRP doesn’t bind to promoter region
- Less transcription of lac genes as less binding of RNA polymerase to DNA
What is Heterochromatin?
Tightly wound DNA causing chromosomes to be visible during mitosis
GEnes can’t be transcribed
What is Euchromatin?
Loosely wound DNA present during interphase
Genes can be transcribed
What can be used to loosen DNA coiled around histone proteins?
Acetyl or phosphate groups to reduce histone’s positive charge, allowing certain genes to be transcribed
What can be used to tighten DNA coiled around histones?
Methly groups make histones more hydrophobic, preventing transcription of genes
What is the exact product of transcription?
pre-mRNA
How is pre-mRNA modified to produce mature mRNA?
- Cap added to 5’
- Tail added to 3’ end
- Both gelp stabalise mRNA + delay degradation in cytoplasm
- Splicing (Non coding section cut out (introns) coding sections (exons) joined together
- Nucleotide sequence of mRNA can be altered through base addition, deletion, substitution
How can gene expression be controlled at a translational level?
- More resistant mRNA lasts longer in cytoplasm so more protein synthesised
- Inhibitory proteins stop mRNA binding to ribosomes + prevents protein synthesis
- Initiation factors helps mRNA bind to ribosomes
How can gene expression be controlled at a post-translational level?
- Addition of non-protein groups (e.g. protein kinases add phosphate groups)
- Modifying amino acids + formation of disulphide bridges
- Folding/shortening proteins
- Modification by cAMP
What organisms are homeobox genes found in?
Plants, animals, fungi
What do homeobox genes control?
Body plan, polarity, positioning of organs
Do homeobox genes differ between organisms?
Not really - similar + highly conserved
What do homeobox genes code for?
Transcription factors that bind upstream of other homeobox genes to turn them on (start transcription)
What are transcription factors?
Proteins that regulate gene expression coded by the homeobox genes
What is the homeobox sequence?
A section of DNA in the homeobox gene 180 base pairs long
What is the difference between the homeobox gene and the homeobox sequence?
The homeobox gene is the entire gene and is much longer than 180 base pairs
The homeobox sequence is only 180 pairs long and codes for the homeodomain in proteins
What is the homeodomain?
A section of a protein coded for by the homeobox sequence that binds to DNA and switches on/off other genes
What are homeobox genes essential for?
Control of development + body plans
What are hox genes?
A group of homeobox genes only present in animals
What are hox genes resonsible for?
Positioning of body plans
How do number + arrangement of hox genes vary between different animals
Increase in the number of Hox genes may lead to greater complexity in body structure
What is a cluster of hox genes called and what are they responsible for?
A hox cluster
Responsible for correct positioning of body parts
What are the 3 types of hox genes and what do they do?
Maternal effect genes - determine polarity (where head + rear will be)
Segmentation genes - Where body segments are formed
Homeotic selector genes - Direct development of all segments in the body plan
What can hox gene mutations lead to?
Body parts being in the wrong position
Why are Drosophilia melangaster (fruit flys) used to study genetics + gene regulation?
- Low cost
- Fewer public ethics concerns
- Rapid rate of reproduction
- Fruit fly genetics + development well understood
- Simple body plan
- Structures observable with a light microscope
At what stage of development are segments identified in a body plan?
The embryo
How are hox genes arranged on a chromosome?
In sequence, according to where they’re expressed (from anterior to posterior/head to tail)
What do hox genes in head control development of?
Mouthparts
What do hox genes in the thorax control development of?
Wings, limbs or ribs
Where does the homeodomain bind to?
The groove in the DNA double helix
What is a morphogen and give an example
Reinoic acid (Vitmain A derivative)
Involved in activating homeobox genes for tissue development, activating homeobox genes in the correct order
How does too much Vitmain A affect early pregnancy?
Interfers with gene expression and results in birth defects (particularly in the head)
How can excess vitamin A be prevented in early pregnancy?
- Special pregnancy multivitamins with no Vitamin A
- Not eating liver (contains high Vit A conc.)
What is apoptosis?
Programmed cell death
Explain the process of apoptosis
- Enzymes break down cytoskeleton
- Cytoplasm dense + organelles tightly packed
- Plasma membrane forms blebs
- Chromatin condense + nuclear envelope breaks down
- Cell breaks up into vesicles + phagocytes digest conents safely with lysosomes
- No hydrolytic enzymes released/ no damage to tissues
- Hydrolytic enzymes enclosed in vesicles that phagocytes ingest
Explain the negatives of Necorsis
Not safe for other cells
Content leak from cell due to trauma
Including hydrolytic enzymes that damage other cells + tissues
When is apoptosis used in our development?
Used to remove webbing between fingers to form individual digits when defining a developing embryo
Getting rid of harmful T lymphocytes (Those with complementary receptors to our own cells + ineffective T lymphocytes (No receptors at all)
How is apoptosis controlled?
Cytokines, hormones, growth factors, nitric oxide
How does nitric oxide control apoptosis?
Makes the iner mitochondrial membrane more permeable to hydrogen ions - proteins released into cytoplasm that bind to apoptosis inhibitor proteins
Explain the exact process od apoptosis being controlled, starting with death ligands
- Death ligands (proteins) bind to death receptor
- Capase 8 enzyme activated
- Capase 8 activates Capase 3
- Causing apoptosis
How are cells killed if DNA is greatly damaged>
- Tumour supressor gene coding for p53 protein activated
- Causes cytochrome C to be released
- Activates caspase 3
- Apoptosis caused
