What is a gene?
specific seq of DNA nucleotides, codes for polypeptide/RNA
Define transcription and translation.
transcription - process where DNA used as template to direct synthesis of RNA molecule w/ base seq comp to gene, occurs in nucleus
translation - process where mRNA used as template to direct synthesis of polypeptide; mRNA seq converted into specific no, type, seq of a.a in polypeptide, occurs in ribosome
describe transcription process
Initiation
RNA r&b to core promoter, forms TIC w/ GTFs; RNA pol unwinds, unzips DNA molecule over gene region >
breaks H bonds b/w comp bases, starts transcribing RNA strand
Elongation
free RNA NT align opposite template strand via CBP, U in mRNA pairs w/ A in DNA, A>T, C>G, G>C; forms H bonds w/ comp bases >
RNA pol joins RNA NT tgt, catalyse formation of PDE bonds b/w RNA NT, results in synthesis of RNA >
RNA pol catalyses addition of RNA NT to 3'-OH end of RNA strand >
RNA strand synthesised 5'>3', RNA strand peels away from DNA template, allows 2 separated DNA strands to rewind in transcribed regions
Termination
RNA pol reaches specific seq of a.a.(terminator), RNA pol dissociates from RNA molecule & gene >
newly synthesised RNA molecule released
what are the 3 types of post-transcriptional modification
1. 5' capping
2. 3' polyadenylation
3. RNA splicing
describe post-transcriptional modification
5' capping
7-methylguanosine cap added to 5' of pre-mRNA
3' polyadenylation
poly(A) tail added to 3' end of pre-mRNA >
protects mRNA from degradation by ribonucleases, facilitates export of mature mRNA out of nucleus into cytoplasm
RNA splicing
in pre-mRNA, introns excised, exons spliced tgt, forms mature mRNA molecule w/ continuous coding seq
describe structure of mRNA
single-stranded molecule, synthesised in nucleus during transcription >
transcribed from gene in DNA via CBP, base seq of mRNA comp to DNA template strand, varies in length according to length of gene >
mirror copy of non-template strand, has limited lifespan
state roles of mRNA
template for polypeptide synthesis
mature mRNA exported out of nucleus via nuclear pores, enters cytoplasm & binds to free ribosomes/RER-bound ribosomes >
translation; ribosomes use genetic info encoded in mRNA to synthesise polypeptides, triplets of bases(codon) on mRNA codes for specific a.a, mRNA strand codes for a.a seq of specific polypeptide
describe structure of rRNA
large molecule, consists segments of ssDNA, segments of dsDNA helices >
rRNA transcribed from rRNA genes, synthesised at nucleolus
state roles of rRNA
rRNA combines w/ proteins in nucleolus, form SRS & LRS >
rRNA in SRS binds to mRNA during translation >
rRNA in LRS binds to tRNA @ P(peptidyl-tRNA) site, A(aminoacyl-tRNA) site >
peptidyl transferase in LRS catalyses formation of peptide bonds b/w adjacent a.a
describe structure of tRNA
smallest of all RNAs, each tRNA ss polyribonucleotide of ~80 RNA NT, folds back on itself form clover-leaf shape w/ 3 loops & a stem >
structure held by H bonds b/w comp bases @ diff regions >
CCA stem @ 3' end
site tRNA attaches covalently to specific a.a coded for by anticodon of tRNA
anticodon loop
triplet NT seq(anticodon), CBPs w/ specific codon on mRNA
activating enzyme site
site for aminoacyl-tRNA synthetase, catalyses attachment of tRNA w/ specific a.a
ribosome recognition site
make specific bp w/ rRNA in ribosomes
state roles of tRNA
intermediate b/w codon of MRNA & a.a seq of polypeptide chain >
carries correct a.a from cytoplasm to polypeptide chain being synthesised @ ribosome
state the main features of genetic code
Punctuated
'start', 'stop' signals; AUG is start codon, initiates transcription, codes for Met a.a; UAA/UGA/UAG are stop codons, terminates transcription
Universal
same triplet of nucleotides codes for same a.a in all organisms
Non-overlapping
codons don't overlap
Degenerate
due to degeneracy of genetic code, > than 1 codon codes for same a.a
Triplet
3 bases in mRNA code for 1 a.a(codon)
describe structure of ribosome
ribosome consists of SRS & LRS, made up of proteins, rRNA >
SRS has binding site for rRNA >
LRS has 3 tRNA binding sites;A(aminoacyl-tRNA) site, P(peptidyl-tRNA) site, E(exit) site >
A site holds aminoacyl-tRNA carrying next a.a to be added to polypeptide chain, P site holds tRNA w/ growing polypeptide, E site is where tRNA leaves ribosome
describe translation process
A - a.a. attaches to tRNA @ 3' end, forms aminoacyl-tRNA(AT) catalysed by AT synthetase; AC determines a.a. tRNA binds to
I - tRNA w/ Met a.a, AC UAC, binds to SRS w/ aid of IF. SRS r&b 5' end mRNA, moves until reaches AUG. AC UAC CBPs w/ AUG on mRNA; initiator tRNA+SRS+mRNA followed by attachment of LRS, forms TIC; GTP brings components together. initiator tRNA fits into P site of LRS, A site ready for next AT.
E - AC of next AT w/ specific a.a. CBPs & forms H bonds w/ mRNA codon @ A site of ribosome. PT bond formed b/w N-end of a.a. @ A site & C-end of ppt @ P site, catalysed by PT. ribosome moves 1 codon down mRNA 5'->3', moves tRNA carrying ppt from A->P site, tRNA in P site->E site, leaves ribosome, A site free for next AT
T - stop codon reaches A site, PRF r&b to stop codon, H2O added to ppt, hydrolyses ppt from tRNA in P site, releases ppt from tRNA & ribosome. TIC disassembles
how are proteins synthesised in prokaryotes?
nuclear envelope absent, translation of mRNA begins as soon as 5'-OH end of MRNA peels away from DNA template, no PTM carried out >
RNA pol, ribosomes both founds in cytoplasm, transcription & translation occur simultaneously