Protein Synthesis Flashcards
tRNA
This is a small carrier RNA that facilitates protein synthesis by bringing amino acids to the ribosome (site of translation in protein synthesis). A tRNA has a three base sequence at one end of the molecule called an ANTICODON and an amino acid at the other end.
rRNA
Ribosomal RNA (rRNA): It along with some proteins make up the structure of the ribosome.
Prokaryotic Genes:
- Generally organized in a continuous, uninterrupted manner, lacking introns (non-coding sequences).
- Because they are already compact and efficient, there is no need for introns to be removed.
- Simplified structure allows for rapid transcription and translation - prokaryotes respond quickly to environmental changes.
Initiation (transcription)
A section of DNA upstream from the gene called a PROMOTOR allows an enzyme called RNA polymerase to attach to the DNA at the “transcription bubble” (where the DNA has puffed open exposing the region to be transcribed). The start DNA triplet TAC is converted to AUG - the start codon - Transcription has begun.
Elongation (transcription)
Free RNA nucleotides are added by RNA polymerase that are complimentary to the DNA bases (on the template strand) and this continues until the termination code is reached.
Termination (transcription)
When the last DNA triplet of the gene has been copied, a sequence that interferes with the ability of the RNA polymerase to remain attached so it disassociates, completing transcription.
Transcription can only occur in the ____ direction
5’ - 3’ direction (this is how RNA polymerase adds nucleotides) therefore we use the 3’ - 5’ side of the DNA double strand to make mRNA. As DNA is antiparallel, mRNA will always copy from the 3’ - 5’ side.
TRANSLATION
Occurs in the cytoplasm
m-RNA must have a 5’ cap to exit the nucleus
Nucleoli - region of dense RNA - creates ribosomes and t-RNA
* Template side is the antisense (3’-5’) side.
* 4 steps (initiation, elongation, translocation and termination)
Ribosome moves - not mRNA, meaning that multiple ribosomes can move along the same strand - called a polysome.
Ribosomes
- Facilitates codon and anticodon binding.
- Components of ribosomes are produced in the nucleus, and transported to the cytoplasm.
- A site, P site, E site
Catalyse the formation of peptide bonds between amino acids
Translation (initiation):
- Brings together mRNA, tRNA (with first amino acid) and ribosomal subunits.
- Small ribosomal subunit binds to mRNA and an initiator, tRNA.
- Start codon = AUG, Start anticodon = UAC
Small ribosomal subunit attaches to 5’ end of mRNA
Translation (translocation)
- The tRNA with the polypeptide chain in the A site is relocated to the P site
- tRNA at the P site moves to the exit site and leaves the ribosome.
The ribosome moves down the mRNA in the 5’-3’ direction
- tRNA at the P site moves to the exit site and leaves the ribosome.
A site
A (Aminoacyl) Site – Holds the incoming tRNA carrying the next amino acid to be added to the polypeptide chain.
P site
P (Peptidyl) Site – Holds the tRNA with the growing polypeptide chain.
E site
E (Exit) Site – Where the empty tRNA exits the ribosome after transferring its amino acid.
RNA Splicing (eukaryotes):
- Non-coding sections are dispersed between coding sections
- Introns are excised (cut out), and the remaining exons are joined together to form mature mRNA
Translation (elongation)
Within the ribosome are three codon regions designated as “E” (exit), “P” (peptidyl tRNA binding site) and “A” (aminoacyl tRNA binding site).
Elongation involves tRNA’s bringing in the correct amino acid (determined by the anticodon/codon matching) to the “A” site where the ribosome will connect it to the amino acid in the “P” site, using ATP (energy) to facilitate the formation of a peptide bond between these two amino acids. As this is occurring, the ribosome moves on one codon, creating a new “A” site for the new amino acid to enter and connect to the growing polypeptide chain.
TERMINATION Translation
A stop codon on the mRNA (UAA, UAG or UGA) indicates that the synthesis process is concluded. When a ribosome matches a stop codon it releases from the mRNA, the polypeptide is also released and we have successfully gone from DNA to a protein.
Translation
Process C is translation (½). Each of three sequential bases of the mRNA (codon) (½) is
matched to a corresponding sequence (anticodon) (½) on tRNA (½) molecules, each of which
carries a specific amino acid (½). The tRNA molecules place the amino acids in order so they
can then form peptide bonds/links (½) between them. Once the peptide bonds have formed
the tRNA molecules are released from the ribosome and the growing polypeptide chain is
produced (½).
