H2 Biology 8 - DNA Replication(ii)

specific seq of DNA nucleotides, codes for polypeptide/RNA

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

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

1. 5' capping

2. 3' polyadenylation

3. RNA splicing

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

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

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

large molecule, consists segments of ssDNA, segments of dsDNA helices >

rRNA transcribed from rRNA genes, synthesised at nucleolus

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

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

intermediate b/w codon of MRNA & a.a seq of polypeptide chain >

carries correct a.a from cytoplasm to polypeptide chain being synthesised @ ribosome

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)

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

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

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