Gene Regulation and Protein Synthesis Flashcards
structure of RNA
single stranded
local stretches of intramolecular base-pairing (sums and loops)
3 main RNA classes
rRNA - combines with proteins to form ribosomes where protein synthesis takes place, stable
tRNA - carries amino acids to be incorporated into protein, stable
mRNA - carries genetic info for protein synthesis
what is the most common type of RNA in cells
rRNA - 80%
tRNA - 15%
mRNA - 5%
describe the structure and function of tRNA
adapters between nucleic acid code and amino acid code
anti-codon consists of 3 nucleotides
3D structure
where is the attachment site for specific amino acid
anticodon sequence codes for specific amino acid
added to free 3’ end OH attachment site
how is RNA made
use one DNA strand to copy nucleotide sequence
how many types of RNA polymerase do eukaryotic cells have
3 - Pol I, Pol II, Pol III
Pol II synthesises all mRNA
5 steps of transcription
RNA polymerase II binding - detection of initiation sites (promotors) on DNA, transcription factors required (NO PRIMER)
DNA chain separation - local unwinding of DNA to gain access to nucleotide sequence
Transcription initiation - selection of first nucleotide of growing RNA
Elongation - addition of further nucleotides to RNA chain
Termination - release of finished RNA
role of RNA polymerase
bind promotors (DNA sequence), specific to RNA pol II
Transcription starts at nucleotide +1
TATA box is present about 25 nucleotides upstream before transcriptional start
what is TBP
TATA box binding protein
recognises TATA box
introduces kink into DNA (determines transcriptional start and direction)
Provides platform for further transcription factors & RNA polymerase
what is TFIID
general transcription after
required for all pol II transcribed genes
how is transcription initiated
general transcription factors
Pol II & TFIIF extend transcript on their own
TFIID remains at promotor, a new initiation complex can assemble allowing transcription at low basal rate
what happens during transcription elongation
transcription bubble moves in one direction along DNA
DNA is unwound infant of DNA polymerase then rewound behind it
in what direction is RNA chain synthesised
in a 5’ to 3’ direction
new RNA is complementary to template strand
identical to the coding strand (U replaces T)
how Is transcription terminated
new RNA strand makes stem loop structure followed by stretch of Us
A specific enzyme cleaves finished RNA, releasing it
how is transcription regulated
requires specific transcription factors (DNA binding proteins, DNA binding domain and transcriptional activation which causes other protein/factors to increase rate of transcription)
Bind to specific DNA sequences near promotor enhance/ repress transcription (loop back on itself)
how are genes expressed
in response to specific stimuli eg hormones, cell stress
transcription factor activated by modification eg by extracellular factors phosphorylation/cleavage
describe steroid receptors and how they are activated
transcription factors (nucelar hormone receptors) DNA-binding and ligand-binding domain highly conserved in cell cytoplasm (inactive) when ligand (steroid) binds move to nucleus and bind to DNA at steroid response elements
describe the activation of the glucocorticoid receptor
steroids transported in blood (by albumin binding)
enter large cell by diffusion
bind to inactive steroid receptor in cytoplasm causing receptor activation
translocates to nucleus
binds to response elements (as homodimer)
coordinated regulation of genes
are all genes expressed
no
some at basal rate
some never expressed DNA will be tightly coiled around histone, packaged away - never transcribed
genes are regulated by transcription factors
what are coding regions
exons, disturbed by non coding regions - introns
all transcribed into pre-mRNA
what is splicing
removal of introns before translation forming mRNA, only exons in mature mRNA
works in 5’ to 3’ direction
this is how one gene can make different proteins depending on whats classed as an intron and exon
how is the end of mRNA processed
addition of poly(A)tail
capping - addition of 5’ cap
what are codon anti-codon base pairs
during translation anticodons of tRNA molecules form base pairs with codons on mRNA
3 nucleotides
genetic code based on triplets
how many combinations code for 20 amino acids
64 amino acids
what is the start codon
AUG - methionine
what does degenerate mean
many codons make the same amino acid
define unambiguous
each codon codes for only one amino acid - stop
what is the stop codon
UAA
UAG
UGA
what determines where translation takes place
reading frames 3 different types
coding frame will give longest stretch without a stop codon in between
components of translation
amino acids tRNAs Aminoacyl-tRNA synthetases ribosomes mRNA ATP & GTP as sources of energy a specific set of protein factors for each of (initiation of protein synthesis, elongation of polypeptide chain/translocation, termination)
what is the role of aminoacyl-tRNA
synthetases bind amino acids to their corresponding tRNA molecule (highly specific)
at least one for each amino acid
ATP provides energy for formation of covalent bond
catalysis ATP to ADP
how many molecules of rRNA are in a ribosome
4
also hae protein components
what are the 3 binding sites on a ribosome
E = exit P = peptidyl A = Aminoacyl
how is translation initiated
initiation factors
where does the energy for translation come from
hydrolysis of GTP supplies energy for initiation
what happens when mRNA comes in contact with rRNA
small ribosomal subunit binds to 5’ end of mRNA
moves along mRNA until AUG start codon found (ATP dependant)
describe initiation of translation process
special initiator tRNA with UAC anticodon base pairs with start codon (carries methionine)
large subunit joins assembly and initiator tRNA is located at site P
role of elongation factor EF-1a
brings next aminoayl-tRNA to A site
GTP is hydrolysed
EF released from tRNA
second EF regenerates EF 1 to pick up next amino-acyl-tRNA
how do peptide bonds form between adjacent amino acids at ribosome
peptidyl transferase catalyses peptide bond formation between amino acid in P and A sites so peptide is in A site
What moves ribosome along mRNA
EF-2
what does the tRNA do once it has dropped its amino acid off at site P
empty tRNA enters E site so can exit and reload
tRNA with growing peptide moves from A to P site, A site empty for next amino acid
right > left
E P A
what bond is between amino acids
peptide
when does termination of translation occur
when A site of ribosome encounters stop codon
no aminoacyl-tRNA base-pairs with stop codon
what happens during termination
release factorRF binds to stop codon
GTP hydrolysis
finished protein cleaved off tRNA
components dislocate
what is the polysome
many ribosomes acting on one mRNA strand to synthesise polypeptide at same time working
ribosomes added in 5’ to 3’ direction
what is a point mutation
change in single base in DNA
what is a missense mutation
results in change of amino acid sequence
can change protein function eg haemoglobin in sickle cell
what are the 3 types of point mutation
missence
nonsense
silent
what is a nonsense mutation
creates termination codon
changes length of protein due to premature stop of translation
what is a silent mutation
no change of amino acid sequence
due to degeneracy of genetic code
no effect on protein function
what is a frameshift mutation
addition/deletion of single base
changes reading frame of translation
what are chromosomal mutations
affect larger portions of genome
deletion
duplication
translocation inversion
what happens to finished protein
targeting - moving protein to final cellular destination, many possible locations, depends on amino acids in sequence
modification - addition functional groups
degradation - unwanted/damaged proteins removed
what do free ribosomes in cystol make proteins destined for
cystol
nucleus
mitochondria
translocated (post-translationally)
what doe bound ribosomes on rough ER make proteins destined for
plasma membrane ER Golgi-apparatus secretion translocated (co-translocationally)
what is gycosylation
addition and processing of carbohydrates in ER and golgi
what happens during post-translational modifications
formation of disulphide bonds in ER
folding/assembly multisubuni proteins in ER
specific proteolytic cleavage in ER/gogli/secretory vesicles
hereditary for of emphysema - misfiling of protein antitrypsi in ER
