Chapter 17 Protein Synthesis

I. Big Picture DNA →mRNA→protein Transcription – synthesis of RNA using DNA Translation – actual production of protein Figure 17.3

II. Transcription Occurs – In nucleus Need – DNA, RNA polymerase, RNA nucleotides, transcription factors

II. Transcription - Initiation Transcription factors attach to the promoter (TATA Box) which causes RNA polymerase II to attach creating a transcription initiation complex.

II. Transcription - Initiation

II. Transcription - Elongation 2. RNA polymerase unwinds the DNA and begins to match free-floating RNA nucleotides to the template strand in a 3’→5’ direction. 3. RNA polymerase continues down the template strand. As it does the RNA is kicked out and the DNA zips back up.

II. Transcription - Termination 4. This continues passing a terminator (TTATTT) and then it goes 10-35 nucleotides past and cuts off the mRNA.

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III. RNA Processing 5’ cap added - Guanine (modified) added to 5’ end - Attach here sign for ribosome - Protects end Poly (A) tail - 50-250 adenines - Keeps from breaking down - Facilitates export of mRNA out of the nucleus

III. RNA Processing RNA Splicing - average pre mRNA 8,000 bp / average processed mRNA 1,200 / average protein 400 A.A. - removal of part of the RNA molecule - Introns – non coding segments - Exons – coded sections (expressed) - snRNP – small nuclear ribonucleoproteins / recognize splice sites at ends of introns / made of snRNA and proteins

III. RNA Processing Spliceosome – several snRNP and other proteins / cut out introns and put exons together Ribozymes – RNA molecules that function as enzymes Why have introns? Regulate passage of RNA from the nucleus / alternative RNA splicing, different polypeptides based on exons

IV. Translation Occurs: In the cytoplasm Need: mRNA, tRNA, Ribosome, aminoacyl-tRNA synthetase, ATP, GTP, release factor, initiation factor, A.A.

IV. Translation - tRNA Transfer RNA / moves AA to ribosome 45 different tRNA molecules Has the anti codon that matches up w/ the codon on the mRNA 3rd base in the anti-codon is flexible / U in the codon can match with A or G from the anti-codon / I (inosine - like A) can match with U,C,or A / wobble Aminoacyl-tRNA synthetase – adds AA to tRNA / uses ATP / 20 different ones

IV. Translation - Ribosome Allows tRNA to match to mRNA codons Larger in eukaryotic cells than in prokaryotic cells Large and small subunits / 2/3 rRNA, 1/3 protein / made in nucleolus P site – holds tRNA carrying the growing polypeptide A site – holds tRNA with next AA E site – Exit of tRNA

IV. Translation - Initiation Small ribosome unit attaches after the 5’ cap and the initiator tRNA matches to the start codon AUG. The large ribosome unit attaches forming the translation initiation complex. AUG is found in the P site. Initiation factors and GTP (energy) are used to do this.

IV. Translation - Elongation A tRNA w/ matching anti-codon matches up in the A site. A elongation factor moves the tRNA into the A site. rRNA molecule in the large ribosome unit catalyzes (ribozyme) a bond between the AA and the whole polypeptide is attached to the tRNA in the A site. The A site moves to the P site and the P site exits from the E site. This takes GTP. mRNA moves 5’→3’ through the ribosome.

IV. Translation - Termination 7. A stop codon reaches the A site and a release factor binds to the codon which causes water to bond to the end of the polypeptide. Everything releases.

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V. Where do Proteins Go? Ribosomes are found in the cytosol, some free some bound to the outside of the ER. Signal Peptide – sequence of AA targeting polypeptide for ER Signal-recognition Particle (SRP) – identifies the signal peptide and moves to a translocation complex (hole in the ER).

VI. Prokaryotic vs. Eukaryotic Eukaryotic RNA polymerase depends on transcription factors Transcription is terminated differently Ribosomes are different Pro can transcribe and translate at the same time Eu process their RNA Eu have ways to target proteins to organelles

VII. Point Mutations Mutation – change in DNA Point Mutation – change in one base pair / can be passed down if gamete is effected 1. Substitution - base pair sub – replacement of a base pair / can have no effect or major effects - missense mutations – mutation still codes for an AA just not the right one - nonsense mutation – change AA to stop

VII. Point Mutations Insertion and Deletion - addition or loss of base pair - disastrous - Frameshift mutation – lead to nonsense

VII. Point Mutations Mutagens - Errors during replication, repair, or recombination - Spontaneous mutations - UV radiation / thymine dimers - Base Analogues / look like nucleotides - change bases - insert during replication

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