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'compare Formation Of The Initiation Complex For Protein Synthesis In Bacterial And Animal Cells.

An essay I wrote as part of a biochemistry module.

Date : 27/10/2011

Author Information

James

Uploaded by : James
Uploaded on : 27/10/2011
Subject : Biology

In the cells, strands of messenger RNA (mRNA) are translated to form a chain of amino acids, which can then fold to form a 3D protein. Translation can be divided into 3 critical stages, initiation, elongation and termination. This essay focuses on the differences in the formation of the prokaryotic and eukaryotic initiation complex.

Ribosomes are the structures integral in the translation of the genetic code and Ribosomes differ in prokaryotes and eukaryotes, prokaryotes have 70s ribosomes consisting of a 30s unit and a 50s unit while eukaryotes have 80s ribosomes consisting of a 30s and a 50s unit. These units combine with mRNA and initiation factors to make up what we call an initiation complex; this is the first stage of translation. In prokaryotes the small 30s unit of the ribosome recognisees and aided by proteins called initiation factors (IF3, IF1) which control the process binds to a section of the mRNA called the Shine-Delgarno sequence, the Shine-Delgarno sequence is a sequence of bases which corresponds to a complementary or anti-Shine-Delgarno sequence on a section of the ribosomal rRNA. The Shine-Delgarno sequence is only found in prokaryotes and is needed to bring the mRNA strand and ribosome into alignment with a 3 base sequence of the strand called the start codon.

Methionine is coded solely by the sequence AUG and is almost always the first amino acid in the polypeptide chain, hence we say that AUG is the initiation sequence or start codon for polypeptide synthesis. Prokaryotic initiation begins with a modified version of Methionine being added called N-Formyl Methionine. Once the 30s ribosomal unit, mRNA and initiation factors are arranged in the correct position the tRNA molecule responsible for transporting N-Formyl Methionine - tRNAfmet is guided to the complex by a further initiation factor (IF3-GTP) where it base pairs with the start codon, the 50s ribosomal unit then binds and the initiation complex is complete. Initiation is different in Eukaryotes, initially tRNAi binds to the 40s ribosomal subunit, which then binds mRNA via the 5'cap. The 40s subunit then moves along the mRNA strand until it recognises the initiating AUG codon. AUG is recognised by the tRNA anti codon which base pairs with AUG in a site on the ribosome called the P site. Once this occurs the 60s unit binds, the initiation complex is now complete and elongation can begin.

A reading frame is a means of breaking down mRNA into 3 base sections or codons. There are 3 reading frames in mRNA, depending on where translation begins along the sequence. Reading frames organise the mRNA into the 3 base codons, which correspond to an amino acid. Reading frames are important for determining initiation codons such as AUG and stop codons that trigger termination of translation.

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