Nucleic Acids 7.3 Translation. Recap video 4UUVHPQM 4UUVHPQM.

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Nucleic Acids 7.3 Translation

Transcript of Nucleic Acids 7.3 Translation. Recap video 4UUVHPQM 4UUVHPQM.

Page 1: Nucleic Acids 7.3 Translation. Recap video   4UUVHPQM  4UUVHPQM.

Nucleic Acids7.3 Translation

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Recap video http://www.youtube.com/watch?v=nHM4

UUVHPQM

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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

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The Structure of the Ribosome

Protein database http://www.rcsb.org/pdb/home/home.do

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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

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tRNA activating enzymes http://www.youtube.com/watch?v=KThCr1XdUGw

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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

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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

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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

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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’

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What does 5’ to 3’mean?

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Free Ribosomes Free ribosomes synthesize proteins for

use primarily within the cell

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Bound Ribosomes Bound ribosomes synthesize proteins

primarily for secretion or for use in lysosomes

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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

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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.

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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

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Polysomes Polysomes are multiple ribosomes

attached to a single mRNA molecule.

Strings of polysomes attached to DNA molecule in a prokaryote

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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.