Medical Genetics-Transcription and
Translation
Robert F. Waters, PhD
DNA
 hRNA
 mRNA
 rRNA
 tRNA

DNA
Helical Structure
 Base Pairing
 Chromatin/Chromosomes
 Operon

Protein Synthesis-Overview

Major classes of RNA

mRNA
• Prokaryotic and Eukaryotic
• Eukaryotic (hnRNA – mRNA)
• Leader (Header) region
• 5’ untranslated region (5’ UTR)
• Eukaryotic cap region attached 7-methylguanylate
• Trailer region
• 3’ untranslated region (3’ UTR)
• Amount of mRNA
• About 5% in prokaryotes
• Stability
• mRNA lasts for just a few minutes
RNA

Ribosomal RNA (rRNA)
• Prokaryotic
• Three kinds
• 23s rRNA (2904 nucleotides)—component of
50s rRNA subunit
• 16s rRNA (1541 nucleotides)—part of small
30s rRNA subunit
• 5s rRNA (120 nucleotides)—part of 50s rRNA
subunit
• Abundance – about 80% of all RNA
RNA Continued:

Ribosomal RNA
• Eukaryotic
•
•
•
•
28s (60s subunit)
18s (40s subunit)
5.8s (60s subunit)
5s (60s subunit)
• Transcription product of separate gene
• Abundance
• 4% -- 45s
• 71% -- full rRNA
RNA Continued:

tRNA (Transfer RNA)

Prokaryotic
• Average about 80 nucleotides
• Some common structure so may function
with rRNA
• tRNAs derived from larger precursor tRNAs
• Abundance (about 15% of total RNA in
prokaryotes)
RNA Continued:

tRNA (Transfer RNA)

Eukaryotic (Very similar to prokaryotic)
• Average about 80 nucleotides
• Some common structure so may function
with rRNA
• tRNAs derived from larger precursor tRNAs
• Abundance (about 15% of total RNA in
prokaryotes)
Other RNAs

Small RNAs

Eukaryotes
• Small Cytoplasmic RNAs (scRNAs)
• Signal recognition particles
• Small Nuclear RNAs (snRNAs)
• Snurps
• Spliceosome
• Separation of introns and exons
Transcription

Formation of RNA from DNA
Nucleus
 Single strand of DNA acts as template
 Substrates

• ATP
• GTP
• CTP
• UTP
• Note cancer chemotherapy
Direction of RNA Synthesis

RNA chain grown proceeds from 5’
to 3’
Codon sequence
 Therefore amino acid sequence
 Not understood as to which DNA strand

Elongation Enzymes in
Transcription

Prokaryotes

DNA Dependent RNA Polymerase
• Core enzyme is trimeric
• Necessary for RNA elongation
• Holoenzyme has core enzyme with another subunit
required for initiation

Eukaryotes


Many large enzymes usually greater than
500000 MW
Called Type I, II, III
Promoters of Transcription

Prokaryotic Promoter sequences
Initiation site (startpoint)
 Pribnow Box (similar to TATA box)

• TATAAT
• 9-18 BP upstream from initiator

-35 sequence
• 15 BP upstream from Pribnow Box
• TTGACA
Promoters of Transcription

Eukaryotic promoters

May be a purine or pyrimidine
• Usually purine in prokaryotes

Multiple promoter sequence
• Order of boxes
• TATA Box (25 BP from Initiator)
• CCAAT (CAAT BOX)
• GGGCG (GC BOX)
• NOTE: RNA Polymerase III promoters occur
downstream of the startpoint!
• Between +8-+30 and +50-+70
Initiation of Transcription

Prokaryotes

 (sigma) factor
• Allows RNA polymerase holoenzyme to attach to
promotor sequences

Process
•  (sigma) factor facilitates opening of DNA
• Enzyme (holoenzyme) forms phosphodiester bond
between first two bases
• Elongation begins and after 10 nucelotides have
been added,  factor is released
• Released  factor can bind with another holoenzyme
and initiate transcription at some other location
Initiation of Transcription

Eukaryotic Initiation Factors

Need four (4) factors to initiate transcription
from a TATA Box region
• TFIID
• Binds directly to the TATA Box promoter region
• TFIIA
• Binds with TFIID that is already bound to TATA Box
• TFIIB
• Facilitates RNA Polymerase II binding to aggregate
• TFIIE
• Binds to preinitiation complex and triggers beginning
of transcription at initiation point (start point)
Termination of Transcription

Prokaryotes

Factor-independent termination
• Inverted repeat causing a loop
• Loop area rich in GC pairs
• Stability for slow down or stop
• Stretch of Uracils (poly U) after loop region

Factor-dependent termination
• Certain sequences act as termination sequences in
presence of Rho-factor ().

Eukaryotes


Not much known
Transcription does continue several BP beyond
pseudo-termination with Poly A (Tail region)
Post-transcriptional
Processing in Eukaryotes

Formation of 5’ Cap
Cap 0 – methyl group
 Cap 1 – two BP
 Cap 2 – three BP


Poly adenylation (Tail)

Sliceosome
• SNURPS
• Intronic excision
• Contiguous exons
Protein Synthesis
(Translation)
Sequence of triplet code form AA
sequence on RER
 Genetic Code

mRNA (CODON)
 Redundancy
 64 possible codes

• 20 Aas

Stop Codons (UAA, UAG, UGA)
Activation of tRNAs

Coupling of AAs to tRNA

Aminoacyl-tRNA synthetases
• Activate Amino Acids and facilitate
attachment to 3’ end of tRNA
Associated with anti-codon
 Wobble-base
 Repair of improper associated tRNA

Eukaryotic Ribosomes
80s from a 60s and 40s subunits
 60s subunit
 A-site (Aminoacyl Site)
 P-site (peptidyl transfer site)
 T-site (Termination site)
