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DNA & Genomics (transcription), structure - Coggle Diagram
DNA & Genomics (transcription)
synthesis of a RNA molecule using 1 of the DNA strands as a template
product
synthesised RNA is similar to the non-template DNA strand & complementary to the template DNA strand
STAGE 1 : Initiation
prokaryotes
sigma factor of RNA polym. recognises & binds to the double stranded DNA at both the -35 & -10 sequences of promoter
-10 sequence is a consensus sequence of 5'-TATAAT-3' known as a Pribnow box
sigma factor is released from the core enzyme
RNA polym. unwinds & separates the 2 strands of DNA by breaking hydrogen bonds btw bases
eukaryotes
TATA binding protein recognises & binds to the TATA box (-25 position) of promoter
general transcription factors & RNA polym. are recruited to form the transcription initiation complex
RNA polym. unwinds & separates the 2 strands of DNA
STAGE 2 : Elongation
RNA polym. adds ribon. to the free 3'OH end of the growing RNA chain
RNA polym. catalyses the synthesis of a new strand of RNA in the 5' to 3' direction via complementary base pairing with the template strand
RNA polym. catalyses the formation of phosphodiester bond btw 3'OH end of RNA & 5'phosphate grp of the NTP added
energy for the process comes from the 2 phosphate grps which are removed as the nucleotide joins the groing end
as RNA polym. moves, it separates the DNA helix -> expose a new segment of template strand
a short RNA-DNA hybrid is formed in the unwound region -> behind the unwound region, the DNA template strand pairs with it non-template strand to reform the double helix -> RNA emerges as a free single strand
STAGE 4 : Termination
prokaryotes
termination occurs after the terminator sequence found on the DNA template strand is transcribed
short RNA-DNA hybrid is separated -> newly synthesised RNA transcript & RNA polym. are released
no further modification required before translation
lack nucleus & translation occur simultaneously while transcription is still taking place
eukaryotes
termination occurs after the polyadenylation signal sequence (AAUAAA) found on DNA template strand is transcribed
pre-mRNA transcript is cleaved -> releasing pre-mRNA & RNA polym.
post-transcriptional modification occurs before mature mRNA is transported to ribosomes in cytoplasm via nuclear pore
post-transcription modification
occur in nucleus -> produces a mature RNA for translation
5' cap
methyl guanosine nucleoside triphosphate is added to 1st nucleotide by a 5'-5' triphosphate linkage, catalysed by guanylyl transferase
5' methylguanosine cap
added after transcription of the 1st 20-40 nucleotides
function
facilitate export of mature mRNA from nucleus into cytoplasm
protects mRNA from 5' exonucleases -> confers stability to mRNA
3' poly-A tail
structure
about 200 adenine residues are added to the 3' end of pre-mRNA called 3' poly-A tail, catalysed by poly-A polym.
function
facilitate export of mature mRNA from nucleus into cytoplasm
slow down degradation by 3' exonucleases -> longer the poly-A tail, longer the half-life of mRNA
RNA splicing
removal of introns (non-coding sequences) & joining of exons (coding sequences)
snRPs are located in nucleus & comprise RNA and proteins
specific snRPs recognises & binds to 5' splice site & 3' splice site
additional proteins interact with snRPs to form spliceosome
spliceosome excise introns & ligate exons -> releasing introns in a lariat structure
alternative splicing
pre-mRNAs are processed by joining exons in diff. combinations
diff. mature mRNAs are produced from the same pre-mRNA
a single gene can code for >1 kind of polyp.
increases no. & variety of proteins in cell without increasing genome size
no. of proteins produced can be > no. of genes
genetic code
mapping btw triplets of bases in genetic material & a.a in a polyp.
characteristics
triplet
3 consecutive nucleotide bases specify for 1 a.a
degenerate / redundant
1 codon can code for the same a.a
codons that code for the same a.a usually vary in 3rd base
unambiguous
no codon specifies for >1 a.a
non-overlapping
successive triplets are read in order with no overlapping of nucleotide bases
unpunctuated
no gaps btw adj. codons
universal
genetic code is common to almost all organisms
start codon : AUG (codes for methionine)
stop codons : UGA / UAA / UAG
do not code for a.a
structure