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20 Cards in this Set
- Front
- Back
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What are the components of a prokaryal ribosome?
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-composed of 2 sub-units
-70S complex -50S- 34 proteins, 2 RNA's, 23S, and 5S -30S complex- 21 proteins, 1 RNA, 16S |
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What are the components of a eukaryal ribosome?
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-composed of 2 sub-units
-80S complex -60S - 50 proteins, 3 RNA'S, 28S, 5.8S, and 5S -40S - 30 proteins, 1 RNA's, 18S |
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What are the 3 stages of protein synthesis?
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Initation, Elongation, and Termination
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Initiation in Eukaryal cells
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1. Complex forms that recognizes the 5' cap structure of the mRNA and the 3'poly-A tail
2. tRNA (met) joins the complex and the complex moves 5' to 3' until the first AUG (met) codon is reached -tRNA (met) is used, but methionine is not formylated -A RBS is not used in the initiation of translation in the eukaryal system. |
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Initiation in Prokaryal cells
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- Initiation uses a special tRNA (met) initiator tRNA (tRNA iMet) that is charged with formylmethionine.
- indentification of the correct meth codon involves a base pairing interaction between the 5' end of the mRNA and the 3'end of the 16S rRNA. This sequence is the mRNA is called the Shine Dalgarno (SD) or the ribosome binding site (RBS) |
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Polysomes
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Several ribosomes moving on the mRNA.
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Ribosomes
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Are ribonucleoprotein complexes composed of two subunits.
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Initiation in prokaryotes uses a special tRNA called
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tRNA (IMet) that is charged with formylmethionine
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IF2
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Is specific for fmet-tRNA (iMet)this is bound to the P site.
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Shine-Dalgarno (SD) or RBS
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-Sequence is 5-8 nt and can vary in its pairing position with the 16S rRNA
-(Polycistronic mRNA's have multiple RBS) |
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RBS is not used in the initiation of translation in what system?
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Eukaryal
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Elongation is similar in prokaryotes and eukaryotes
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-Incoming AA-tRNA binds to the anticodon in position A
-The AA attached to the tRNA in position P is transferred to the AA of the tRNA in position A of the ribosome forming a peptide bond. -The ribosome move 3 nt in the 3' direction placing the n-1 tRNA in the E site and the tRNA with the new AA into the P position. The cycle is repeated until a stop codon appears in the A site of the ribosome. |
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Termination
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When a stop codon appears in the A position a release factor protein binds to the ribosome in the A site. Release factors have a 3D structure that is similar to a tRNA and specifically recognize the stop codon
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Stop codons
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UAG, UGA, UAA
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How is the correct reading frame set by bacterial cells?`
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Watson-Crick pairing interactions between a sequence 5' of the start codon on the mRNA and a complementary sequence located at the 3' end of the 16S rRNA in the 30S subunit.
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In what direction does the ribosome move on the mRNA during translation? What is the polarity of this system?
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-Moves 5' to 3'
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What proteins aid in the folding of newly synthesized proteins? Are there other circumstances where these proteins participate in protein folding?
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DNA-K and DNA-J bind to newly synthesized proteins and facilitate the slow folding of the protein (dependant on ATP hydrolysis)
-These complexes also work on heat or chemically denatured proteins as well |
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How are secreted and membrane proteins held in an unfolded state before they reach their final location in the cell?
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-proteins destined for secretion from the cytoplasm or insertion into membranes may need to be held in an unfolded state before they arrive at their final destination
- This is the case for secreted and periplasmic protease or nuclease, which have activites that are deleterious to the cell. - Membrane proteins may also be held unfolded to allow translocation and to insure that folding occurs in the correct environment. -Sec B binds to the nacent unfolded protein and directs it to the translocation machinary. |
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If protein is only partially folded it will be passed to the...
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GroEL-GroES complex. This dependant on ATP hydrolysis.
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Sec-dependant translocation system
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Is found in bacteria. A similar system is present in eukaryal cells for transport into the ER and organelles.
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