cellular control Flashcards
what is a mutation
Change in sequence of base pairs
may results in altered polypeptide
3 types of factors that increase the risk of mutations
physical
chemical
biological
what are physical mutagens and how do they work
ionising radiation - X rays
break DNA strands
what are chemical mutagens and how do they work
deaminating agents
chemically alter bases in DNA
e.g - converting cytosine to uracil
what are biological mutagens and how do they work
alkylating agents
(methyl / ethyl groups attaching to bases - incorrect pairing)
base analogs
(inserted into DNA in stead of usual base)
viruses
(viral DNA insert into genome - changing sequence)
how might mutations not effect the phenotype
masking
degenerate – many different triplets code for same amino acid = no effect
occur in non-coding sections
what are mutations that dont effect phenotype called
silent mutations
how do mutations occur
insertion of nucleotides
deletion of nucleotides
substitution of nucleotides
effect of insertion of nucleotides
changes amino acid
also has knock on effect further along DNA sequence – frameshift mutation
dramatically changes sequence
effect of deletion of nucleotides
changes triplet
knock on effect – frameshift mutation
effect of substitution of nucleotides
only change amino acid for the triplet – no knock on effect
types of substitution mutations
silent
missence
nonsense
what is a missence mutation
alters single amino acids in chain – sickle cell anaemia
what is nonsense mutation
creates a premature stop codon – polypeptide chain incomplete – cystic fibrosis
what is a silence mutation
does not alter amino acid sequence = degenerate
generally changes in 2nd / 3rd base
give an example of a beneficial mutation to humans
early humans Africa – dark skin due to high conc of melanin
provided protection from harmful UV radiation – still allowing vitamin D to be synthesised
as humans moved into cooler – mutations caused decrease in melanin
paler skin – selective advantage – synthesis more vitamin D
= lighter skin absorbs less UVB
= cooler climates – already low levels
= need as much as can get
example of harmful mutation
genetic diseases – cystic fibrosis
loss of function of protein
example of neutral mutations
ability to taste bitter tasting chemicals in brussel sprouts
what is a chromosome mutation vs gene mutation
gene mutations – occur in single genes
chromosome mutations – affect whole chromosome / number of chromosomes
how would chromosome mutations occur
deletion – section of chromosome breaks off
duplication – sections copied on chromosome
translocation – section of 1 chromosome breaks off + joins non-homologous chromosome
inversion – section of chromosome breaks off + is reversed + joins back on
what is the purpose of regulatory mechanisms
ensure correct genes expressed in correct cells at correct time
allows for specialisation of cells
what are the levels of regulation
transcriptional level
post-transcriptional level
translational level
post translational level
examples of regulation at transcriptional level
lac operon / transcription factors / chromatin remodelling / histone modification
examples of regulation at post-transcriptional level
editing of primary mRNA + removal of introns
examples of regulation at translational level
degradation of mRNA
examples of regulation at post - translational level
activation of proteins by cyclic AMP
what is a structural gene
codes for proteins that function within a cell
what is a regulatory gene
codes for proteins / RNA that control expression of structural genes
what is heterochromatin
tightly wound DNA
what is euchromatin
loosely wound DNA in interphase
what is chromatin remodelling
Simple form of regulation that ensures proteins needed for cell division are made in time
how does chromatin remodelling act as a regulatory mechanism
transcription of genes can not occur for heterochromatin
RNA polymerase cant access genes
Protein synthesis only occurs during interphase
benefits of chromatin remodelling
prevents energy-consuming protein synthesis happening in cell division
why does DNA coil around histone
Histone – positively charged
DNA – negatively charged
so coils around
how does histone modification act as a regulatory mechanism
Add acetyl groups / phosphate groups – reduces positive charge on histones + DNA coils less tightly
(Allows certain genes to be transcribed)
Add methyl groups – histones more hydrophobic – bind more tightly to each other + DNA coils more tight
(Prevent transcription of genes)
what reduces positive charge on histone + its effect
acetyl groups / phosphate groups – DNA coils less tightly
what increases positive charge on histone + its effect
methyl groups – bind more tightly to each other + DNA coils more tight
what is an operon
group of genes that are under the control of the same regulatory mechanism / promoter
are expressed at same time
benefits of an operon
allow for smaller / simpler genome structure
efficient at saving resources – if certain gene products not needed – all genes switched off
what does the lac operon control
controls production of enzyme lactase + 2 structural proteins
what is an inducible enzyme
enzyme that is made under certain conditions // dependant on presence of their substrate
how is lactase an inducible enzyme
only synthesised when lactose is present = not when glucose is present or no point
describe the structure of a lac operon
promoter for structural genes
operator
structural gene lacZ for lactase
structural gene lacY for permease
structural gene lacA for transacetylase
promotor for regulatory gene
regulatory gene lacl that codes for lac repressor protein
what is an operator
segment of DNA that a repressor binds to – inhibiting transcription of genes
what happens if lactose is absent
transcription of lac genes repressed
regulatory gene always switched on
lac repressor protein made
protein binds to operator region upstream of lacZ
now – RNA polymerase unable to bind to promoter region
transcription of structural genes cant happen
no lactase enzyme is made
what happens if lactose is present
uptake of lactose by bacteria
lactose binds to second binding site on repressor protein
distorts shape – conformational change
can not bind to operator
RNA polymerase can now bind to promoter
Transcription happens
Lactase produced
what is a transcription factor
Protein that controls transcription of genes by binding to specific region of DNA
Bind to promoter region – either allow or prevent transcription
what type of hormone is oestrogen
lipid soluble
how does oestrogen act as a transcription factor
Oestrogen diffuses through the cell surface membrane into the cytoplasm + nucleus
attaches to an ERα oestrogen receptor that is held within a protein complex
causes the ERα oestrogen receptor to undergo a conformational change
new shape of the ERα oestrogen receptor allows it to detach from the protein complex
diffuses towards the gene to be expressed
ERα oestrogen receptor binds to a cofactor
enables it to bind to the promoter region of the gene
stimulates RNA polymerase binding and gene transcription
when to use DNA / RNA
all cells = same DNA
express diff genes = analyse RNA
what are exons
coding sequences
eventually be translated into amino acid = polypeptide
EXPRESSED
what are introns
non coding base sequences
what happens in terms of exons / introns in transcription
both exons + introns transcribed on premRNA
what happens to modify the premRNA into mRNA
splicing
what is splicing
primary / pre-mRNA has exons + introns
introns removed from molecule
exons fused together – to form continuous mRNA = mature mRNA
cap + tail added
how are introns removed
5’ splice site at beginning of intron
3’ splice site at end of intron
what is a cap + where is it added
modified nucleotide added to 5’ end
what is a tail + where is it added
long chain of adenine nucleotides added to 3’ end
purpose of cap + tail
tail - help stabilise mRNA + delay degradation in cytoplasm
cap – aid binding mRNA to ribosomes
describe the control at post-translational level
polypeptides undergo modifications in Golgi / apparatus + cytosol
some require activation from cAMP
cAMP – activates protein kinase A
once activates – can activate other proteins
example of control at post-translational level
when muscle cells require energy enzyme = glycogen phosphorylase releases glucose from glycogen
This enzyme is activated by cAMP, which changes the shape of the enzyme to expose its active site
what do homeotic genes control
polarity – head and tail
segmentation – distinct body parts
what is morphogenesis + what is it controlled by
Process that causes organisms to form shape
homeotic genes
what are homeobox genes
Subset of homeotic genes
any gene that contains a homeobox
what is a homeobox
DNA sequence that codes for a protein transcription factor – 180 base pairs
what are the key characteristics of a homeobox
Highly conserved – maintained by natural selection / remain unchanged through evolution
Sequences are similar in animals / plants – all code for amino acid sequences that will form transcription factors
DNA-binding region of them all must have same shape
what is the homeodomain
Sequence of 60 amino acids that the homeobox codes for
what shape is a homeodomain
folds into shape with three alpha helices
what is a protein with a homeodomain called
transcription factor
what is a hox gene
Subset of homeobox genes
Involved in correct positioning of body parts
where are hox genes found
Only found in bilaterian animals
Have half way symmetry
Exist in all common ancestors
what happens when hox genes are mutated
body parts develop in wrong place
whats special about hox genes
show spatial linearity
show temporal order
what is spatial linearity
Order of genes along chromosome matches expression patterns
Those that code for head – at the top end
what is temporal order
Starts with expression of anterior hox genes
Head ones expressed first – head gets made first
how is apoptosis expressed in cells
DNA of cell becoming denser + more tightly packed
Nuclear envelope breaking down + chromosomes condensing
Vesicles forming with hydrolytic enzymes
Phagocytes engulfing + digesting cell = phagocytosis
why is apoptosis needed in body plans
some cells made by mitosis earlier may no longer be needed
example of apoptosis in body plans
Fingers + toes first develop as combined unit
Separated later by apoptosis of cells between digits
how is mitosis controlled
by two types genes
Proto-oncogenes – stimulate cell division
Tumour-suppressor genes – reduce cell division
how does cyclins regulate mitosis
Cyclins – regulators
CDK’s – catalysts
Cyclins bind to CDKs = conformational change // activating the CDK
Now CDK capable of phosphorylating target proteins – transfer phosphate from ATP to protein
Activate / inactivate proteins – control progression of cell cycle
Proteins may release transcription factors
what are the internal stimuli that effect mitosis / apoptosis
Irreparable genetic damage
Release of hormones
RNA decay
Internal biochemical changes that lead to cell changes or cellular injury (e.g. oxidative reactions)
overall how does apoptosis get activated
stress // hormones
activate a protein
acts as transcription factor
activates genes involved in apoptosis
what are the external stimuli that effect mitosis / apoptosis
presence of cell-signalling molecules = cytokines / hormones / growth factors
viruses + bacteria
change in light intensity
lack of nutrients
drugs
