6.1 - Cellular Control Flashcards
Mutation def
A change in the base sequence of DNA
Three ways mutations can occur
- Insertion
- Deletion
- Substitution
How can mutations affect the proteins that are made in cells?
- Different DNA base sequence can alter the primary structure of the protein
- This can then cause a difference in the tertiary structure of the protein
- This can affect its function
- You can also have silent mutations, where the mutation has no effect on the protein made
3 possible effects of mutations
- Silent mutations - no effect on protein
- Missense mutation - usually has a small effect on protein
- Nonsense mutation - usually has a large effect in protein
How do silent mutations occur?
- Most amino acids are coded for by more than once DNA codon
- The genetic code is degenerate
- The mutation/change in base sequence will still code for the same amino acids
- Same protein = no effect
Non-overlapping genetic code def
No codon codes for more than one amino acid
Redundant/degenerate code def
More than one codon codes for the same amino acid
How do missense mutations occur?
- Change in base pair causes a change in amino acid
- Changes primary and tertiary structure of protein
- Changes shape and function of protein
How nonsense mutations occur
- Change in base pair causes it to become a stop codon
- Causes early termination of the polypeptide chain
- Change in shape/function of protein
- e.g. cystic fibrosis
How do insertion and deletion mutations occur?
- Extra base pairs are inserted into base sequence
- Some base pairs may be deleted in base sequence
Effect of insertion and deletion mutations (indel mutations)
- If mutations result in number of base pairs not being a multiple of three (codon)
- This causes a frame shift
- Frameshifts alter all of the subsequent DNA codons
- This causes a large change in primary and tertiary structure of protein
- Large change in shape/function
- e.g. Tay-Sachs
What happens if 3 bases are inserted/deleted in base sequence?
- No frame shift occurs
- 1 extra/less amino acid in polypeptide chain
Effect of beneficial mutations
- Lead to evolution - new allele gives advantage against environmental pressures
- More likely to be passed onto offspring and pass down generations
Neutral mutations effect
- Not beneficial or harmful
- e.g. attached/detached ear lobes
What are STRs?
- Short Tandem Repeats
- Same repeating triplets of bases
- Different people had different numbers of the same STRs
- These numbers can increases from generation to generation
- Huntington’s disease is associated with STRs
Why can’t cancers be inherited
- They are somatic mutations
- They don’t affect gametes and so can’t be passed to offspring
Monosaccharides of lactose
Alpha glucose and beta-galactose
What is lac operon?
A prokaryotic operon required for the metabolism of lactose in E.coli
What proteins/enzymes are produced by lac operon?
- Lactose permease
- Beta-galactosidase
Lac operon info
- E. coli. normally metabolises glucose
- If glucose is absent, but lactose is present
- Lactose causes 2 enzymes to be made:
- Lactose permease
- Beta-galactosidase
Info and function of two proteins produced by lac operon
Lactose permease - acts as a carrier protein for lactose to enter the cell
Beta-galactosidase - breaks the lactose disaccharide into a-glucose and beta-galactose via hydrolysis reaction
Repressor protein on lac operon mechanism
- A small distance away from the operon is the regulatory gene, I
- Codes for a repressor protein (LacI)
- When this regulatory gene is expressed
- Repressor protein produced binds to the operator
- Prevents RNA polymerase form binding to promoter region
- Repressor protein prevents genes LacZ and lacY from being transcribed (expressed)
- So enzymes for metabolism are not made
- The genes are ‘off’
Lactose effect on lac operon mechanism - lactose present glucose absent
- When lactose is added to culture medium
- Once all glucose has been used
- Lactose binds to LacI Repressor protein molecules
- This alters shape of LacI Repressor protein
- Prevents it from binding to operator
- RNA polymerase enzyme can bind to promoter region
- And begin transcribing structural genes into mRNA
- That will then be translated into the two enzymes
- Lactose induces the enzymes needed to break it down
Site at which genes bind to on lac operon to transcribe and translate proteins/enzymes
Operator region
Operon def
length of DNA made out of structural and control genes (P and lacO) that function together
Structural genes def
Code for proteins
Regulatory genes
controls the expression of structural genes by switching them on/off- makes repressor protein/transcription factors (I) - not part of the operon
Operator region
Region next to structural genes that Repressor binds to (lacO)
Promoter region
Binding site for RNA polymerase
Repressor protein info
Binds to operator region preventing RNA polymerase from binding to promoter region preventing transcription
What type of cells is Lac Operon present in?
Prokaryotic
The bacterium uses glucose as a respiratory substrate, in the absence of glucose, it can use lactose.
Describe the effect of lactose on the bacterium.
(6 Marks)
- LacI gene on Lac Operon transcribes and translates repressor protein
- this normally binds to the Lac Operon at the operator region
- prevents RNA polymerase binding at promoter region - prevents transcription
- presence of lactose changes mechanism
- lactose shape is complementary to shape of repressor protein
- lactose binds to the repressor protein
- repressor undergoes a conformational shape change
- no longer able to bind to promoter region
- more on photos
Control of gene expression at transcriptional level in eukaryotes
- all somatic cells contain all chromosomes but certain genes are only expressed in certain cells - can be controlled by transcription factors
E.g. mucus production in goblet cells
What are transcription factors
They control which genes are switched on/off or which are expressed
What do transcription factors bind to and what do they do
- They bind to specific promoter regions of DNA for the gene they control
- they can help or prevent RNA polymerase from binding and transcribing the gene
Control of gene expression at post-transcriptional level in eukaryotes
(Introns and exons, endonuclease enzymes)
- within a gene there are (non-coding) introns and exons (expressed/coding regions)
- both introns and exons are transcribed to produce primary mRNA which is then spliced by an endonuclease enzyme to remove the introns
- this leaves behind the exons, now joined together to produce mature mRNA which will leave the nucleus to be translated
Why have introns in DNA?
- Some genes can be spliced in different ways
- allows genes to code for more than one protein
Do prokaryotes have introns?
No
Control of gene expression at post-translational level
- once a protein has been made
- may be activated by cAMP
- this can involve adding functional groups, e.g. many enzymes are phosphorylated
More detail:
- signalling molecule (first messenger) binds to receptor on cell surface membrane
- this causes a G-protein to activate adenyl cyclase
- this converts ATP to cAMP (second messenger)
Signalling with 1st and 2nd messengers at post-transcriptional level mechanism
- A signalling molecule, such as glucagon, binds to a receptor on plasma membrane of target cell
- This activates a transmembrane protein which then activates Adenyl cyclase
- The activated G protein activates adenyl cyclase enzymes
- Activated adenyl cyclase enzymes catalyse the formation of many molecules of cAMP from ATP
- cAMP activates PKA (protein kinase A - a protein)
- Activated PKA catalyses the phosphorylation of various proteins
- This hydrolyses ATP in the process
- This activates many enzymes in the cytoplasm, e.g. those that convert glycogen to glucose
- PKA may phosphorylate another protein
- This then enters the nucleus and acts as a transcription factor - to regulate transcription
- This is a cascade of processes caused by the second messenger, cAMP
Homeotic genes def
They control morphogenesis (anatomical development) of organisms
Homeobox gene info
- is a homeotic gene that contains a 180 base pair homeobox sequence that codes for a 60 amino acid sequence called a homeodomain sequence within a protein
- these proteins are transciption factors
- they are master genes - they switch on/off many other genes
Homeodomain sequence function
- its shape is specific to part of the enhancer region on DNA so it binds to the DNA to initiate/stop transcription to switch genes on or off
- this controls the development of the body plan
Conserved def
Has remained in all descendent evolutionary history
Apoptosis def
Programmed cell death
Homeobox gene function
- they are highly conserved (found in all common ancestors or organism)
- animals,plants and fungi use homeobox genes to control the development of body plans (anatomical development)
Why are homeobox gene sequences similar in most organisms?
- there are very few mutations in the gene base sequences
- as they are very important
- and mutations would have a very large effect on the body plan
- these mutations would likely have been selected against as they would have killed organisms
What are hox genes?
A type of homeotic genes only found in animals
- they control the formation of anatomical features in the correct locations of the body plan
How are hox genes expressed?
- expressed one by one along the anterior-posterior axis
- this causes the development of particular body parts in this particular order
Hox genes info
- they are similar across different classes of animals
- hox genes are switched on in segments causing development in segments
- (these segments are very obvious in worms and insects)
Variety of Hox genes info
- the number and arrangement of Hox genes varies among different types of animals
- at some point in evolution, Hox clusters have been duplicated, leading to a greater complexity on body structure
- Tetrapods (including humans) have 4 similar Hox gene clusters
Model organisms used to learn about how gene expression is controlled
- Fruit flies
- Mice
Charcteristics scientists look for in animals they use in their experiments
- cheap to buy and keep
- reproduce quickly
- small
- large cells - easy to view under microscope
- readily available
Why can information learnt from these model organisms be applied to humans?
- all in same kingdom
- have shared ancestors
- similar cells
- have shared genes and similar embryonic development/similar homeobox/similar Hox genes
Apoptosis def
Programmed cell death.
- Hox genes can switch on other genes that promote apoptosis and mitosis
Necrosis def
Cell death from trauma which involves hydrolytic enzymes
How does apoptosis happen mechanism
- Cytoskeleton broken down by enzymes
- The cell shrinks, the cell surface membrane forms blebs (small protrusions) and chromatin condenses
- DNA breaks up and the nuclear envelope breaks down into fragments. Bleb forms vesicles containing organelles
- Vesicles are engulfed and digested by phagocytes so the old cell and its contents can cause no damage to other cells
Controlling apoptosis and the cell cycle (including mitosis)
- genes which regulate the cell cycle and apoptosis are able to respond to internal and external stimuli e.g. stress
- as a result, cell signalling molecules are released e.g. cytokines, hormones, nitric oxide
- nitric oxide can induce apoptosis by damaging DNA in certain cells or disrupting metabolic processes in some cells
Importance of mitosis and apoptosis in control of development of body form
- mitosis causes growth from zygote to adult
- when cells reach Hayflick limit
- they are destroyed by apoptosis
- so no hydrolytic enzymes released - could destroy neighbouring cells
- rate of mitosis and apoptosis should be equal
- otherwise tumours could form (caused by mutations to transcription factors - more mitosis occurs than apoptosis)
- or tissues will degenarate (more apoptosis than mitosis), e.g. Alzheimers
- apoptosis vital in body development
- e.g. separation if digits in body development, loss of vestigial tail before birth etc.
Describe how the information coded on genes is used to synthesise polypeptides and how these popleptides control the physical development of an organism.
(6 Marks)
Synthesis:
- DNA transcribed into mRNA
- intron splicing occurs to produce mature mRNA
- Only one strand of DNA copied
- complemntary base pairing occurs during transcription
- codon is read - made of 3 bases
- translation of mature mRNA occurs
- ribosome binds to mRNA - tRNA binds to active sites - correct tRNA molecule to binds for the codon being read
- role of tRNA described
Roles of polypeptides:
- structural protein
- enzymes - catalyse reactions/control metabolism
- proteins can be hormones
- recpetor proteins
- adenyl Cyclase
- role of adenyl cyclase etc.
- could synthesise Transcription factors
- switch genes on/off
- homeotic/homeobox genes
- apoptosis, etc.
Homeobox genes show ‘astonishing similarity across widely different species of animal’. Explain why there has been very little change by mutation in these genes.
(2 Marks)
These genes are very important
Mutation would have a very big effect on body plan/anatomical development
Many other genes would be affected/knock on effect
Mutation likely to be lethal - selected against in natural selection/evolution (context of survival)
IAA is known to bind to transcription factors.
Suggest how IAA can stimulate cells to synthesise proteins.
(4 Marks)
IAA enters the cell
Ref to movement within cell - IAA in cytoplasm to nucleus
Effect when binds to transcription factor - IAA joins to promoter region or activates transcription factor
Ref to switching on/off genes
Activity at promoter region - e.g. RNA polymerase transcribes gene
Formation of mRNA - mature mRNA in transcription
Translation produces protein - mRNA binds to ribosome etc.
State what is meant by a homeobox gene.
2 Marks
Homeotic/regulatory gene
contains, 180 bp / homeobox, sequence ;
that codes for homeodomain (on protein) ;
(gene product) binds to DNA ;
initiates transcription / switch genes, on / off
; control of, development / body plan ;
