Nucleic Acids 7.3 Translation. Recap video 4UUVHPQM 4UUVHPQM.
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Transcript of Nucleic Acids 7.3 Translation. Recap video 4UUVHPQM 4UUVHPQM.
Nucleic Acids7.3 Translation
Recap video http://www.youtube.com/watch?v=nHM4
UUVHPQM
The Structure of the Ribosome
Proteins and RNA molecules (rRNA)
2 sub-units (large and small) 3 binding sites for tRNA on the
surface: E = exit site P = peptidyl site A = aminoacyl site
2 tRNA molecules can bind at the same time to the ribosome
Binding site for the mRNA on the surface
The Structure of the Ribosome
Protein database http://www.rcsb.org/pdb/home/home.do
Structure of tRNA
Loops: sections that become double stranded by base pairing
Base sequence CCA forms a site for attaching amino acid
Anti-codon: triplet of bases Part of a loop of 7
unpaired bases
tRNA activating enzymes http://www.youtube.com/watch?v=KThCr1XdUGw
Initiation of TranslationTo begin translation: An mRNA molecule binds to the small ribosomal subunit at
the mRNA binding site An initiator tRNA carrying Methionine (MET) binds to the
start codon “AUG” in the “P-site” The large ribosomal subunit binds to the small one
Initiation of Translation The next codon signals another tRNA to bind in
the “A-site” A peptide bond is formed between the amino
acids in the P and A sites
ElongationFollowing initiation The ribosome
translocates 3 bases along the mRNA Moving the tRNA in the
“P-site” to the “E-site” This tRNA is set free
A new tRNA with the correct anticodon binds to the next codon in the “A-site”
This series of steps repeats
Termination of Translation The process continues until a stop
codon is reached The polypeptide is released The components disassemble
Note: direction of movement from 5’ to 3’
What does 5’ to 3’mean?
Free Ribosomes Free ribosomes synthesize proteins for
use primarily within the cell
Bound Ribosomes Bound ribosomes synthesize proteins
primarily for secretion or for use in lysosomes
Free vs Bound Ribosomes Proteins perform specific
functions. They must be sorted to go to the correct location.
Whether a ribosome is free in the cytosol or bound to the ER depends on a signal sequence in the polypeptide
The signal sequence is at the beginning of the polypeptide
Free vs Bound Ribosomes As the signal
sequence is made it becomes bound to a signal recognition protein
Translation stops until the ribosome attaches to a receptor on the ER
Translation begins again, with the polypeptide moving into the lumen of the ER.
Translation and Transcription in Eukaryotes vs Prokaryotes
Cellular function is compartmentalized
Nocompartmentalization
Modifications of mRNA after transcription before exiting nucleus
No modifications
Delay between transcription and translation
No delay
Polysomes Polysomes are multiple ribosomes
attached to a single mRNA molecule.
Strings of polysomes attached to DNA molecule in a prokaryote
Polysomes In prokaryotes, multiple
polysomes are visible associated with one gene.
In eukaryotes, polysomes occur on the cytoplasm and next to the ER.
Multiple ribosomes translating a single mRNA molecule with the cytoplasm. The polypeptides are longer the closer to the end of the mRNA.