Protein Synthesis
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Transcript of Protein Synthesis
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Protein Synthesis
The Building of Proteins from a Nucleic Acid Template
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DNA
• Provides the Template.• Information contained in the SEQUENCE
of N-bases found along the DNA molecule.
• Transferring that information into an Amino Acid SEQUENCE is the trick.
A G C C T A G G G A T A GT C G G A T C C C T A T C
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A 2-step Process
1. Transcription
DNA RNA
2. Translation
RNA Protein
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Transcription• Production of RNA.
• RNA = Ribonucleic Acid.
– Ribose Sugar.– Single Stranded Molecule.– Uracil instead of Thymine.
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Transcription• RNA Polymerase
• Attaches at a Promoter – a sequence of the DNA that indicates where a gene starts.
• With the help of Transcription Factors.
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• A “window” in the DNA is opened – ultimately, a gene.
• RNA Nucleotides are added (from 5’ to 3’) in accordance with the DNA parent template (which is read from 3’ to 5’).
Transcription
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Transcription
• Complimentary Base Pairing.
• DNA RNA
G - C
C - G
T - A
A - U
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Transcription
• Elongation of a single stranded RNA molecule.
• The DNA window closes behind.
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Transcription• Termination – a sequence of bases is
reached signaling RNA Polymerase to release.
• The “gene” closes
• The RNA transcript is released.
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RNA Processing
In Eukaryotic cells, things get a little more complex:
1. Several types of RNA polymerase are present – each with a slightly different function – I, II, and III.
2. A larger mRNA is transcribed…
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RNA Processing
• During Transcription, RNA polymerase II transcribes MORE than just the protein-encoding part of the gene.
• Untranslated Regions (UTRs) are transcribed at the 5’ and 3’ ends.
– The 3’ UTR is called a Polyadenylation signal.
A A U A A A
5’ UTR 3’ UTR
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RNA Processing• AFTER Transcription is terminated,
additional alterations are made to the ends of this “pre-mRNA” strand.
• A 5’ cap at the leading (5’) end (composed of a Guanine-like nucleotide).
• A poly-A tail at the trailing (3’) end (composed of many Adenine nucleotides).
A A U A A A5’cap Poly-A tail
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5’ Cap and Poly-A TailThese END pieces serve several functions:
1. Help “ferry” the eventual mRNA transcript out of the nucleus.
2. Protect the protein-encoding area from degradation.
3. Help attach the proper end (5’) of the transcript to the place where Translation will take place – the Ribosome.
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Further RNA Processing
• RNA splicing – removes portions of the transcript.
• Removed segments are called INTRONS.
• The remaining coding segments are called EXONS.
A A U A A A5’cap Poly-A tail
Introns
Exons
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Further RNA Processing
• INTRONS are recognized by snRNPs…– “small nuclear
RiboNucleoProteins”.– snRNPs cut out
INTRONS and join the adjacent EXONS.
– This occurs in the nucleus.
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Further RNA Processing
• The resulting molecule is the RNA transcript proper (no longer the pre-mRNA).
A A U A A A5’cap Poly-A tail
A A U A A A5’cap Poly-A tail
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Why?
• Introns may be vestigial.
• Introns may have regulatory roles.
• Alternative RNA splicing – more than one polypeptide from a single gene.
• More chances of unique Exon combinations (taking into account “crossing over” during meiosis).– (Cross-overs at introns won’t disrupt exons).
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3 Types of RNA
• mRNA = Messenger RNA– The “blueprint”
• rRNA = Ribosomal RNA– The “workbench”
• tRNA = Transfer RNA– The “truck”
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mRNA – messenger RNA
• The “message” – the blueprint for the production of a polypeptide – a protein.
• But there’s only 4 N-bases, and 20 amino acids!
• What’s the code?
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The Genetic Code• Triplet Base Code.
• Every 3-letter word in the RNA Transcript is a CODON.
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The Genetic Code
• Each CODON codes for a single Amino Acid.
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The Genetic Code
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rRNA – Ribosomal RNA
• The location for protein synthesis – the workbench on which a polypeptide is built.
• rRNA makes up a RIBOSOME.
• Ribosomes have 2 sub-units.
Large subunit
Small subunit
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rRNA – Ribosomal RNA
• The Large subunit has several binding sites:
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tRNA – Transfer RNA
• The “transfer-er” – the truck that brings individual Amino Acids to the workbench for the production of a polypeptide – a protein.
• With a driver – the Anticodon.
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tRNA – Transfer RNA
• Each tRNA carries a specific Amino Acid to the Ribosome.
• It “knows” when to drop this Amino Acid off by…
• COMPLIMENTARY BASE PAIRING of its Anticodon with the corresponding Codon found on the mRNA.
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tRNA – Transfer RNA
• Aminoacyl-tRNA synthetase = the enzyme responsible for hooking Amino Acids to tRNAs.
• Aminoacyl tRNA = truck with cargo.
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Translation• Translating the RNA
sequence data into Protein sequence data.
• N-bases Amino Acids
• In the cytoplasm of Eukaryotic cells.
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Translation• Initiation - All 3 types of RNA come together.
• This requires some energy (provided by Guanosine Triphosphate -- GTP).
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Translation• Elongation – Amino Acids are added with
the help of elongation factors (proteins).
1. Codon meets Anticodon at the ‘A’ binding site.
2. A Peptide Bond Forms between adjacent Amino Acids
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Translation
• Then, a shift in the Ribosome (a “Translocation”) moves the empty tRNA to the exit (E-site), opening the A-site for the next tRNA.
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Translation
• Termination…
• Continued elongation until a “STOP” codon is reached (UAG, UAA, or UGA).
• A release factor (protein) binds to this codon, effectively releasing the polypeptide.
• The mRNA, tRNA, and rRNA separate.
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Protein Synthesis
Primary StructureOf the Protein
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Protein Synthesis• In response to a need for a particular
protein – need it NOW.
• The same gene can be transcribed by several RNA polymerase molecules simultaneously…allowing faster protein production.
• Polyribosomes – strings of Ribosomes translate a single mRNA simultaneously – allowing faster protein production.
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Polyribosomes
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Bound and Free Ribosomes• Free-floating Ribosomes are the sites for
free-floating protein production.
• Ribosomes bound to membranes are the sites for membrane-bound and secretory proteins.
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Review – Fig. 17.26
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Point Mutations• Point Mutations occur at single nucleotides
along DNA’s template strand.
• Substitutions – the replacement of a pair of nucleotides at a particular location.
• Silent Mutations.• Not-So-Silent Mutations.
– Missense– Nonsense
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Substitution Mutations
• Original Sequence:
• Silent Mutation:
• Missense Mutation:
• Nonsense Mutation:
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Point Mutations
• Insertions & Deletions – adding or removing bases along the sequence.
– Results in a FRAME SHIFT because all other codons “downstream” are affected.
– Can reverse if there are 3 insertions or 3 deletions.
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Insertions & Deletions• Original Sequence:
• Insertion/nonsense:
• Deletion/missense:
• 3-Nucleotide Insertion/Deletion (missense):
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Assignment:
• Review chapters 16 & 17!Transcription:http://www.ncc.gmu.edu/dna/mRNAanim.htm
http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html
Translation:http://www.ncc.gmu.edu/dna/ANIMPROT.htm
http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3.html
Overall: http://www.johnkyrk.com/DNAtranslation.html