Download AP Protein synthesis

Transcription/ Translation
From Gene to Protein
(protein synthesis)
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Transcription
Protein is made outside of the nucleus at the
ribosome, but DNA can’t leave the nucleus.
• Transcription - is the making of mRNA from DNA.
• messenger RNA (Ribo Nucleic Acid) is a copy of DNA
that can leave the
nucleus.
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mRNA
mRNA is single stranded
mRNA contains no T’s (thymine)
It replaces that T’s with U’s (uracil)
It can leave the nucleus.
It’s base pairs are read it sets of 3 called
codons.
• A-T-G
• U-A-C
C-G-G
G-C-C
T-T-A
A-A-U
DNA
mRNA
Transcription in Three stages
• The three stages of transcription:
– Initiation – starting transcription
– Elongation – laying down the mRNA nucleotides
– Termination – finalizing the mRNA
Promoter
• Promoter – the beginning of the DNA sequence to be
copied. (RNA polymerase II attaches at the promoter)
• TATA Box – the promoter begins with a sequence of
DNA that includes the base pairs T and A.
Transcription Factors
• Transcription Factors – Proteins that allow the
RNA Polymerase II to bind to the promoter.
Transcription Initiation Complex
• Transcription initiation complex – the promoter,
RNA Polymerase II, and Transcription factors
all combined and ready to start transcription.
RNA Polymerase
• RNA Polymerase – enzyme that unwinds DNA
and lays down RNA nucleotides.
Termination
• A termination code caused the mRNA to be cut
free from the DNA. (ex. AAUAAA)**
• At this point the mRNA is pre-mRNA is needs a
few modifications
Alteration of mRNA
• The 5 end receives a modified 5 cap – a modified
guanine nucleotide with 3 phosphate groups
• The 3 end gets a poly-A tail – several adenine
nucleotides in a row (50+)
• These modifications have several functions:
– They allow mRNA to leave the nucleus
– They protect mRNA from hydrolytic enzymes
– They help ribosomes attach to the 5 end
Eukaryotic RNA Splicing
• Finally, the sections of the pre-mRNA that will not be
used to code for the protein are cut out.
• introns – section that are cut out of the pre-mRNA
• exons –sections that are left in the finalized mRNA
Fig. 17-10
5 Exon Intron
Exon
Exon
Intron
3
Pre-mRNA 5 Cap
Poly-A tail
1
30
31
Coding
segment
mRNA 5 Cap
1
5 UTR
104
105
146
Introns cut out and
exons spliced together
Poly-A tail
146
3 UTR
Spliceosomes
• Spliceosomes – carry
out RNA splicing
• Made of
ribonucleoproteins (snRNPs)
that recognize the splice sites
Ribozymes
• Ribozymes -are catalytic RNA molecules that
function as enzymes and can splice RNA
(present in snRNP’s)
(ribozyme)
Alternative RNA splicing
• alternative RNA splicing – choosing different
regions of introns or exons from the same premRNA sequence
• So one gene can code for more than one
protein.
tRNA
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t-RNA – Transfer RNA (single stranded)
Each tRNA has an
anti-codon that matches
up with a codon on the
messenger RNA
Each tRNA also has a
particular amino acid
attached to it.
Amino Acids
• Each tRNA also has a particular amino acid
attached to it.
• aminoacyl-tRNA synthetase – enzyme that
attaches amino acids to tRNA
Ribosomes
• Ribosomes match the tRNA anticodons with mRNA
codons in protein synthesis
• The two ribosomal subunits are made of proteins and
ribosomal RNA (rRNA)
• small subunit holds mRNA
• Large subunit holds the tRNA
Building a Polypeptide
• A ribosome has three binding sites for tRNA:
– A site- holds the tRNA that carries the next amino
acid to be added to the chain
– P site- holds the tRNA that carries the growing
polypeptide chain
– E site- is the exit site, where discharged tRNAs
leave the ribosome.**
P A
Fig. 17-17
3 U A C 5
5 A U G 3
Initiator
tRNA
Large
ribosomal
subunit
P site
GTP GDP
E
mRNA
5
Start codon
mRNA binding site
3
Small
ribosomal
subunit
5
A
3
Translation initiation complex
Fig. 17-18-4
Amino end
of polypeptide
E
3
mRNA
Ribosome ready for
next aminoacyl tRNA
P A
site site
5
GTP
GDP
E
E
P A
P A
GDP
GTP
E
P A
Polyribosomes
• polyribosome (or polysome) -several ribosomes
translate a single mRNA simultaneously,
allowing a cell to make many copies of a protein
very quickly
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Completing the Protein
• signal peptide – a short peptide added to the
finished protein.**
• signal-recognition particle (SRP) – attaches to
the signal peptide that that it may enter the ER.
Predicting Proteins
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Predict the protein
DNA = ATG CGG
RNA = UAC GCC
Prot. = Tyr Ala