Discuss the roles of the following in protein synthesis in mammalian cells:- the large and small subunits of the ribosome, mRNA, tRNA, release factors, SRP

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Discuss the roles of the following in protein synthesis in mammalian cells:- the large and small subunits of the ribosome, mRNA, tRNA, release factors, SRP.

Genes drive metabolism, growth and differentiation in living cells.  They do this by promoting the synthesis of proteins which in turn catalyse many biological reactions.  Therefore the synthesis of proteins is one of the most central and basic events in the life of every cell. The process of proteins synthesis is divided into two different parts; transcription and translation.  Transcription is performed by RNA polymerase which uses one strand of the DNA helix as a template to synthesis messenger RNA (mRNA).  This mRNA then migrates from the nucleus to the cytoplasm.  Translation then occurs and according to Lodish et al (1995) this process is when the base sequence of the mRNA is used to order and to join the amino acids in the protein.  To explain the roles of ribosomes, mRNA, transfer RNA (tRNA), release factors and signal recognition particles (SRP) the process of translation needs to be explained in more detail.  The pathway of translation is divided into three main steps; initiation, elongation and termination.  

The mRNA has a protein coding region which is composed of a non-overlapping string of codons called an open reading frame.  At the beginning of the open reading frame is a start codon and at the end there is a stop codon.  In eukaryotes the start codon is always 5’-AUG-3’ and specifies the first amino acid and defines the reading frame for the rest of the codons.  Near all eurkaryotic mRNA only contains a single open reading frame and therefore are monocistronic mRNA.  Also the mRNAs have a 5’cap end that bind ribosomes with the help of initiating proteins.  

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The initiation process involves the large and small units of ribosomes.  Ribosomes are megadalton molecules that are composed of ribosomal RNA (rRNA) and ribosomal proteins.  The ribosomes direct protein synthesis by translating mRNA into a polypeptide chains. At physiological magnesium concentrations the ribosomes dissociate spontaneously into small 40s subunits and large 60s subunits of ribosomes.  The large subunits contain the peptidyl transferase centre (catalyses the formation of the peptide bond), and the binding factor centre. The small subunits contain the decoding centre which the mRNA enters and exits through.  Ribosomes contain three tRNA binding sites; ‘A site’ is the ...

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