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Chapter 2

BIOLOGY 1A03 Chapter Notes - Chapter 2: Aminoacyl Trna Synthetase, Aminoacyl-Trna, Eukaryotic Translation


Department
Biology
Course Code
BIOLOGY 1A03
Professor
Dr.Da Silva
Chapter
2

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Theme 2: From Gene to Protein
Module 3: Translation
1) Molecular Components Contribute to Translation:-
Translation requires many Components:
The information contained in our genome is encoded in our genes and this genetic
information is transcribed from the sequences of nucleotides along strands of DNA into
RNA
It is then in the process of translation that cellular components are able to read the
genetic message in the messenger RNA sequence, and translate the message into the
specific primary amino acid sequence of a protein
This process requires many cellular components that include initiation, elongation and
release factor proteins, transfer RNA, the correct matching of transfer RNA and amino
acids (which is facilitated by synthetase enzymes) along with ribosomal RNA and
ribosomal proteins which make up the assembled ribosome
tRNA structure allows for specificity in translation:
transfer RNA plays a critical role in enabling the translation of the information in the
mRNA genetic message to a polypeptide
This process occurs because tRNA molecules are able to transfer amino acids, from a
pool of cytoplasmically situated amino acids, to a growing polypeptide strand in a
ribosome
Since each type of tRNA molecule is not identical, and as a result can translate a specific
mRNA codon into a specific amino acid
Each tRNA molecule is made up of a single RNA strand ranging between 70-90
nucleotides in length and there is a large degree of complementarity along many
stretches of a tRNA molecule, and this often results in many stretches of hydrogen
bonding between complementary nucleotide bases
This allows for the formation of four double-helical segments and three characteristic
loops seen in all tRNA molecule
DHU loop
Anticodon loop
TpsiC loop
The whole tRNA molecule can also fold upon itself into a roughly L-shaped three-
dimensional structure
An anticodon region of the tRNA molecule is a specific nucleotide triplet that forms
complementary base-pairs with a specific mRNA codon that codes for a specific
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amino acid and These anticodons are conventionally written in the 3’->5’ direction
and align properly with mRNA codons in the 5’->3’ direction
At the 3’ end of the tRNA molecule, there is a protruding amino acid attachment site
that is made up of a single stranded CCA nucleotide sequence. The terminal A (or
adenine) is the actual point of attachment for an amino acid during an important process
referred to as tRNA molecule activation
Aminoacyl tRNA synthetase activates tRNA molecules:
The correct translation of a genetic message requires the correct matching of tRNA and
specific amino acids
This activation of a tRNA molecule with a specific amino acid is carried out by a family
of enzymes called aminoacyl tRNA synthetases
Each aminoacyl tRNA synthetase is specific to the type of tRNA and corresponding
amino acid that it will bind
In particular, the active site of these enzymes recognizes the anticodon end of the tRNA
and the region of the amino acid attachment site
This leads to the existence of 20 aminoacyl tRNA synthetases, one for each amino acid.
Once an uncharged tRNA and one corresponding amino acid is bound to the active
site of these enzymes can then the enzyme can catalyze the covalent attachment of a
tRNA molecule to its amino acid using the energy from ATP hydrolysis
The enzyme attaches the Amino Acid to the 3' end of the tRNA
This leads to a charged tRNA molecule, or an aminoacyl tRNA being released from the
enzyme, which can now deliver its specific amino acid to a growing polypeptide chain on
a ribosome
Translation involves codon-anticodon pairing:
the correct translation of a genetic message requires the correct matching of tRNA and
specific amino acids
there is also the requirement that there be the correct pairing of the tRNA anticodon with
the appropriate mRNA codon
matching the correct base-pairing between a codon in mRNA (which codes for a specific
amino acid) and an anticodon in tRNA largely contributes to the primary sequence of
amino acids in the translated polypeptide
mRNA can code for a possible 64 codons and 20 amino acids with a great degree of
redundancy
the AUG codon in mRNA codes for the amino acid methionine which signals to the
protein translation machinery to begin translating the mRNA at that location
There are only approximately 45 tRNA molecules meaning that some tRNA molecules
may be able to bind to more than one codon and this is largely due to the chemical
nature of the codon-anticodon pairing interactions
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