cellular control Flashcards
GENE MUTATION-
what is a gene mutation?
what are the 3 main ways that a mutation in the DNA base sequence can occur?
-a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide protein
•insertion of one or more nucleotides
•deletion of one or more nucleotides
•substitution of one or more nucleotides
what is insertion of nucleotides?
what does an insertion mutation do?
what may an insertion mutation also do?
so?
-when a nucleotide with a new base is randomly inserted into the DNA sequence
-changes the amino acid that would have been coded for by the original base triplet, creating a new, different triplet of bases
-have a knock on effect by changing the triplets further on in the DNA base sequence , known as a frame shift mutation
-may change the amino acid sequence dramatically, and therefore its protein function
what is deletion of nucleotides?
what does this do?
also has?
so?
-when a nucleotide and therefore its base is randomly deleted from the DNA sequence
-changes the amino acid that would have been coded for
-a knock on effect by changing the groups of 3 bases further on in the DNA sequence, known as a frameshift mutation
-changes amino acid sequence produced and therefore the ability of the protein function
what is substitution of nucleotides?
what does this do?
what 3 forms can substitution mutations take?
-when a base in the DNA sequence is randomly swapped for a different base
-a substitution mutation will ONLY change the amino acid for the triplet in which the mutation occurs = no knock on effect
•silent mutations = mutation does not alter amino acid sequence of polypeptide (as codons may code for same amino acids= degenerate)
•missense mutations = mutation alters a single amino acid in the polypeptide chain
•nonsense mutations = mutation creates a premature stop codon (stops translation of the mRNA molecule into an amino acid sequence) so polypeptide chain produced incomplete and therefore function of protein
the effects of gene mutations on proteins:
what are the 3 categories?
•beneficial mutations
•harmful mutations
•neutral mutations
what are beneficial mutations?
what is an example?
so?
-a mutation in the polypeptide chain results in an altered characteristic that causes beneficial effects
-the production of the pigment melanin:
•early humans living in Africa had dark skin as they produced high concentrations of melanin
•this provides protection from the harmful UV radiation from the sun
•though, in cooler climates, mutations led to decrease production of melanin and meant they could synthesise more vitamin D
-a decrease in production of melanin was a beneficial mutation
what are harmful mutations?
example?
-a mutation that led to an altered characteristic in an organism that causes harmful effects for the organism
-cystic fibrosis caused by a deletion mutation of three nucleotides in the gene coding for the protein CFTR
•this loss of function of the CFTR protein causes extremely thickened mucus = lung problems
what are neutral mutations?
how can this occur?
example?
-no selective advantage or disadvantage to the organism
•the mutation does not alter the polypeptide
•mutation only alters polypeptide slightly so that protein structure and function is not changed
•mutation alters the structure or function of the polypeptide but the resulting difference in characteristic provides no advantage or disadvantage
-ability to taste a bitter-tasting chemical in Brussels sprouts as a result of the mutated allele of the TAS2R38 gene, no effect as it is not toxic to us
GENE CONTROL-
there are several mechanisms that exist within cells to make sure that the correct genes are expressed in the correct cell and the correct time, what are these called?
what are the 3 main types of regulatory mechanisms?
what are all these controlled by?
what is a structural gene?
-regulatory mechanisms
•regulation at the transcriptional level
•regulation at the post-transcriptional level
•regulation at the post translational level
-different regulatory genes
-controlled by regulatory genes and they code for proteins that have a function within a cell, eg enzymes
regulatory mechanism at the TRANSCRIPTIONAL LEVEL-
what is lac operon?
what is an operon?
in a bacterial cell?
-an example of a regulatory mechanism at the transcriptional level (occurs during transcription)
-a group or a cluster of genes that are controlled by the same promoter
•the lac operon controls the production of the enzyme lactase and two other structural proteins
•lactase breaks down the substrate lactose so that it can be used as an every source in the bacterial cell
•this is known as an inducible enzyme (only synthesised when lactose is present)
•helps prevent bacteria from wasting energy and materials
structure of lac operon-
what are the components of the lac operon, in the right order?
what is located to the left of the lac operon on the bacterium’s DNA?
•promotor for structural genes
•operator
•structural gene lacZ that codes for lactase
•structural gene lacY that codes for permease (allows lactose into the cell)
•structural gene lacA that codes for transacetylase
•promotor for regulatory gene
•regulatory gene lacl that codes for the lac repressor protein
what does the lac repressor protein have?
what happens when it binds to the operator?
what happens when it binds to lactose?
draw the structure of lac operon-
-two binding sites that allow it to bind to the operator in the lac operon and to lactose (effector molecule)
-it prevents the transcription of the structural genes because RNA polymerase cannot attach to the promoter
-the shape of the repressor protein distorts and it can no longer bind to the operator
what happens when lactose is absent? (in the medium that bacteria is growing in)
draw-
-the regulatory gene is transcribed and translated to produce lac repressor protein
-the lac repressor protein binds to the operator region upstream of lacZ
-due to the presence of the promotor repressor protein, RNA polymerase is unable to bind to the promotor region
-transcription of the structural DNA does not take place
-no lactase is synthesised
what happens when lactose is present in the medium the bacterium is growing in?
draw-
-there is an uptake of lactose by the bacterium
-the lactose binds to the second binding site on the repressor protein, distorting its shape so that it cannot bind to the operator site
-RNA polymerase is then able to bind to the promotor region and transcription takes place
-the mRNA from all 3 structural genes is translated
-enzyme lactase is produced and lactose can be broken down and used for energy by the bacterium
what are transcription factors?
what do transcription factors allow?
-eukaryotes use transcription factors to control gene expression:
they are proteins that bind to specific regions of DNA to control the transcription of genes
•allow organisms to respond to their environment
•some hormones achieve their effect
how do transcription factors work?
draw general idea-
-transcription factors bind to the promotor region of a gene (the region of DNA ‘upstream’ of the gene that controls the expression of a gene)
•it will control the rate of transcription
-this binding can either allow or prevent the transcription of the gene from taking place
-the presence of a transcription factor will either increase or decrease the rate of transcription of genes into mRNA
in mammals, what is oestrogen involved in?
describe gene control of oestrogen-
draw this process-
•controlling the oestrus cycle
•sperm production
-oestrogen is a lipid soluble molecule so can diffuse through plasma membrane of cells
-it then moves to the nucleus and binds to an oestrogen receptor
-these receptors are transcription factors that are able to initiate transcription for many different genes by binding to their promotor regions
-once bound, oestrogen causes a change in the shape of the receptor
-causing the receptor to move away from the protein complex its normally attached to and binds to the promotor region of one of its target genes
-allows RNA polymerase to bind and begin transcribing that gene
gene control: gibberellin
what does gibberellin control?
what is increased?
-seed germination by stimulating the synthesis of amylase by influencing transcription of the amylase gene
-when gibberellin is applied to a germinating seed there is an increased amount of the mRNA for amylase present
describe the mechanism of this process?
draw-
-the breakdown of DELLA protein by gibberellin is necessary for the synthesis of amylase
-DELLA protein (repressor protein) is bound to the transcription factor, preventing it from binding to the promotor of the amylase gene so no transcription can occur
-gibberellin binds to a gibberellin receptor and enzyme which starts the breakdown of DELLA
-the transcription factor is no longer bound to DELLA protein and so it binds to the promotor of the amylase gene
-transcription of amylase begins
-amylase is produced
POST-TRANSCRIPTIONAL MODIFICATION-
within eukaryotes there are coding and non-coding sequences of DNA, what do these do?
-the coding sequences are called exons
•these are the sequences that will be translated into the amino acids that will form the final polypeptide
-the non-coding sequences are called introns
•these are not translated (do not code for amino acids)
when transcription of a gene occurs, both the exons and the introns are transcribed, what does this mean?
what is this RNA molecule referred to as?
-this means the mRNA molecule formed also contains exons and introns
-primary mRNA or pre-mRNA
describe post-transcriptional modification?
draw-
-as the introns are not translated, they must be removed from the pre-mRNA molecule formed also
-the exons are then all fused together to form a continuous mRNA molecule called mature mRNA that is ready to be translated
-this process is known as ‘splicing’ (the modification of the RNA molecule after transcription but before translation occurs)
-splicing ensures that only the coding sections of mRNA are used to form proteins by translation
-if any introns were included in the mature mRNA, the protein would not be formed properly so may not function properly
CONTROL AT THE POST-TRANSLATIONAL LEVEL-
after the polypeptides are formed by translation, what happens?
what do some polypeptides then require?
what does cAMP do?
-they undergo modifications in the Golgi apparatus or in the cytosol
-proteins may need to be activated by cyclic AMP (cAMP)
•cAMP is derived from ATP and is formed by the action of the enzyme adenyl cyclase
-activates protein kinases
-(in eukaryotic cells) cAMP activates protein kinase A (PKA)
-PKA is an inactive precursor enzyme
-once it’s activated, it can activate other proteins eg proteins
BODY PLANS-
what is a homeobox?
then?
example?
how?
-a homeobox is a DNA sequence that codes for a protein transcription factor
•these then attach to DNA at specific locations and regulate the transcription of genes
•eg genes that control early development of eukaryotic organisms
•by turning different genes on and off in the CORRECT ORDER
