Unit 2: Translation Flashcards
ribosomes (2)
- complex structure of RNA and protein, bound to the cytosolic face of the rough ER in the cytoplasm, on which proteins are synthesized
- consist of a small subunit and a large subunit, each composed of 1-3 types of ribosomal RNA and 20-50 types of ribosomal protein
Compare the sizes of eukaryotic and prokaryotic ribosomes
- eukaryotic ribosomes are larges than prokaryotic ribosomes
How many binding sites are on the large subunit of a ribosome?
- 3 binding sites: A site, P site, and E site
What is the major role of ribosomes? (2)
- when the mRNA is placed on the ribosome, to ensure that the sequence in the mRNA coding for amino acids is read in successive, non-overlapping groups of 3 nucleotides
- establishing the correct reading frame for the codons
codon (2)
- group of 3 adjacent nucleotides in RNA that specifies an amino acid in a protein that terminates polypeptide synthesis
- each codon in the mRNA codes for a single amino acid in the polypeptide chain
reading frames
- following a start codon, a consecutive sequence of codons for amino acids
anticodon
- the sequence of 3 nucleotides in a tRNA molecule that base pairs with the corresponding codon in an mRNA molecule
aminoacyl tRNA synthetases (4)
- an enzyme that very accurately attaches a specific amino acid to a specific tRNA molecule
- directly responsible for actually translating the codon sequence in a nucleic acid to a specific amino acid in a polypeptide chain
- most organisms have 1 enzyme for each amino acid
- enzyme binds to multiple sites on any tRNA that has an anticodon corresponding to the amino acid and tRNA
charged tRNA and uncharged tRNA
- charged: a tRNA with its amino acid attached
- uncharged: tRNA with no amino acid attached
What does the specificity of DNA-RNA and codon-anticodon interactions result from?
- base pairing
genetic code
- correspondence between codons and amino acids, in which 20 amino acids are specified by 64 codons
initiation codon (4)
- codon which translation begins
- coded by AUG, which specifies Met
- polypeptide is synthesized from the amino end to the carboxyl end: Met forms the amino end of any polypeptide being synthesized
- in many cases, the Met is cleaved off by an enzyme after synthesis is complete
What does the position of the initiator AUG codon establish?
- the reading frame that determines how the downstream codons are to be read
stop codon
- UAA, UAG, or UGA where the polypeptide is finished and is released into the cytosol
many amino acids are specified by more than one codon, what does this say about the genetic code?
- it is universal (because all known organisms use the same code) redundant and degenerate (because more than one codon can code for the same amino acid)
what are 3 characteristics of the redundancy in the genetic code?
- redundancy results almost exclusively from the third codon position
- when an amino acid is specified by 2 codons, they differ either in whether the 3rd position is a U or a C (pyrimidine bases), or in whether the third position is an A or G (purine bases)
- when an amino acid is specified by four codons, the identity of the 3rd codon position does not matter
wobble effect (2)
- the 5’ base that pairs with the 3’ base in the codon is chemically modified into a form that can pair with 2 or bases at the 3rd position in the codon
- in the ribosome, there is less than perfect alignment between the third position of the codon and the base that pairs with it in the anticodon so the requirements for base pairing are somewhat relaxed
initiation of translation
- first stage of translation where methionine is established as the first amino acid in the new polypeptide chain
elongation of translation
- process in protein translation where successive amino acids are added one by one to the growing polypeptide chain
termination in translation
- the time where the addition of amino acids stops and the completed polypeptide chain is released from the ribosome
initiation factors
- a protein that binds to mRNA to initiate translation
initiation factors in eukaryotes
- one group of initiation factors binds to the 5’ cap that is added to the mRNA during processing
- factors recruit a small subunit of ribosome and other initiation factors bring up a transfer RNA charged with Met
- initiation complex then moves along the mRNA until it encounters the first AUG triplet
what happens when the first AUG codon is encountered?
- a large ribosomal subunit joins the complex, the initiation factors are released, and the next tRNA is ready to join the ribosome
Where does the first tRNA bind with the ribosome?
- it binds with the P site
Where does the 2nd tRNA bind to the ribosome?
- A site
Once the second tRNA is in place, what happens?
- a reaction where the bond connecting the Met to its tRNA is transferred to the amino group of the next amino acid in line, forming a peptide bond between the 2 amino acids
What happens after another amino acid is added to the polypeptide?
- the polypeptide gets attached the the A site, and the ribosome shifts on codon to the right so that the uncharged tRNA shifts to the E site and is released into the cytoplasm and the peptide-bearing tRNA shifts to the P site so that another charged tRNA can move into the A site
elongation factors
- protein that breaks the high-energy bonds of the molecule GTP to provide energy for ribosome movement and elongation of a growing polypeptide
release factor
- a protein that causes a finished polypeptide chain to be freed from the ribosome
What occurs after termination?
- small and large ribosomal subunits disassociate both from the mRNA and from each other
How does translation initiation occur in eukaryotes?
- initiation complex forms at the 5’ cap and scans along the mRNA until the first AUG is encountered
How does translation initiation occur in prokaryotes?
- initiation complex is formed at one or more internal sequences present in the mRNA known as the Shine-Dalgarno sequence (RBS)
- this ability allows prokaryotic mRNAs to code for more than one protein
polycistronic mRNA
- a single molecule of mRNA codes for multiple different polypeptide chains, each with its own AUG initiation codon proceeded by 8 nucleotides upstream bu its own Shine-Dalgarno sequence (RBS)
- results from transcription of a group of functionally related genes located in tandem along the DNA and transcribed as a single unit from one promoter
What is true about the RBS?
- it is only found in bacteria
If 30 bases were added between the RBS or the 5’-cap and the AUG start codon for an equivalent mRNA, what would be the effect on translation?
- no bacterial translation, eukaryotic unaffected
Correctly describe an open reading frame
- codes for the correct protein and is defined by the start codon
What is the dominant interaction involved in the anticodon-codon interaction
- hydrogen bonding
What direction are codons and anticodons stated in?
- codons: 5’ to 3’ direction
- anticodons 3’ to 5’ direction
