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.