Transcript 7_dna_protein_times

RNA
Coding strand = sense
Three types of RNA
mRNA: messenger RNA
Template strand = antisense
tRNA: transfer RNA-- amino
acid transfer
rRNA: ribosomal RNA-components of ribosomes, the protein
assemble apparatus.
Transfer RNA Forms a Cloverleaf
Covalent bond
dihydrouridine
pseudouridine
Loop
Stem
CLOVERLEAF
Class I tRNAs: small extra arm, 3-5 bp, ~75%,
Class II tRNAs: large extra arm, 13-21 bp, ~25%,
The acceptor stem and anticodon are
at ends of tertiary structure of tRNAs
The TC arm and the
acceptor arm form
continuous helix and so
are the D arm and the
anticodon arm.
The tertiary structure of
tRNA is like the letter L
The Meaning of a tRNA Is Determined by Its Anticodon
Aminoacyl-tRNA synthetase
Cysteinyl-tRNACys
Reduced
Alanyl-tRNACys
Messenger RNA Is Translated by Ribosomes
S: Svedburgs: sedimentation rate.
Bigger particle: higher S.
Most Important use of S:
Characterize ribosomes and
source organism
•Bacteria ribosome:
~70S (50S + 30S)
•Higher eukaryotic cytoplasm
ribosome: ~80S (60S + 40S)
Ribonucleoprotein particle:
rRNAs:
The assembly Bacteria:
of the
23S, 16S, 5S
ribosomal two Mammalian:
subunits
28S, 18S, 5.8S, and 5S
2+
needs Mg
Proteins
62
82
Polyribosomes
bacteria: 10 ribosomes per mRNA;
eukaryotes: ~8
30S subunit associates with mRNA;
50S S subunit carries the newly synthesized
protein;
tRNA spans both subunits.
The Cycle of Ribosomes
The Life Cycle of Messenger RNA
In bacteria: transcription and
translation occur in the single cellular
compartment and are closely related.
•mRNA life span: short, several
minutes.
•Transcription speed: ~40 nt/sec;
•Translation speed: ~15 aa/sec;
•Degradation speed: half of the
speed of transcription & translation.
In eukaryotic cells:
•transcription and maturation occur
at nucleus; translation happens in the
cytoplasm.
•mRNA life span: longer, several
hours.
Ribosomes bind to mRNA
as soon as its 5’ was
transcribed.
For each cistron in the
polycistronic mRNA, a
new ribosome is used
sequentially.
Wait!
Eukaryotic mRNA is modified
during or after transcription
“Cap (a nucleotide (usually G) in reverse 3’->5’
orientation) & Shoe (polyA)”,
Transcription, modification, processing,
nucleocytoplasmic transport, and
translation (animal cells)
5’ - 5’
Cleavage
Cleavage
Splicing?
Transport
Translation
The 5’ End of Eukaryotic mRNA Is Capped
1 methyl group, Cap0
(guanine-7-methyl-transferase)
Cap1 (2 methyl groups)
7
(2’-O-methyl-transferase)
GTP
5’
Gppp +
5’
pppApNpNpNp...
Guanylyl transferase
5’ -5’
GpppApNpNpNp… + pp + p
Cap2
Maturation
(3 metyl
groups)
The starting nt of an mRNA:
5’ ppp(A/G) pNpNpNp...
The 3’ Terminus Is Polyadenylated
Poly(A) polymerase
Poly(A) binding protein (PABP)
(One PABP monomer of ~70 kDa, binding polyA
every 10-20 nts)
Functions of PolyA and PABP:
1. Stablizing mRNA
2. Removal (both polyA or
PABP) will inhibit translation
The histone mRNAs do not
have PolyA!
Prokaryote mRNA Degradation Involves multiple Enzymes
Endonucleases and exonucleases
Endonucleases degrading direction: 5’ -> 3’;
Exonucleases degrading direction: 3’ -> 5’;
Bacterial polyA may play a role in mRNA degradation by
proving a binding site for nucleases
Yeast
Eukaryote mRNA
Degradation Pathways
Deadenylation (exonuclease?)
Decapping
5’ – 3’ degradation by
XRN1 (an exonuclease)
The Destabilization Elements on the Eukaryotic mRNAs
ARE (AU-Rich Element, 50 bps, at 3’ trailer)
initiates degradation in unstable mRNAs
Block of IRE (Iron Response Element)
sequences prevents degradation.
Deadenylation
The general model for the stabilization of mRNA:
Stability is conferred by inhibiting the function of destabilizing sequences.
Nonsense mutations trigger a surveillance system
Nonsense mutations may
cause mRNA to be degraded.
A surveillance system could have two types of
components. Protein(s) must bind in the nucleus to mark
the result of a splicing event. Other proteins could bind to
the mark either in the nucleus or cytoplasm. They are
triggered to act to degrade the mRNA when ribosomes
terminate prematurely.
Eukaryotic RNAs are transported
mRNA can be specifically localized
Ash1 mRNA forms a
ribonucleoprotein containing a
myosin motor that moves it along an
actin filament.
Ash1 mRNA is exported from the nucleus into the
cytoplasm where it is assembled into a complex with the
She proteins. The complex transports it along actin
filaments to the bud.