Protein Synthesis

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Introduction

Protein synthesis involves two complex processes called transcription and translation which involve a variety of different components and structures to function correctly.

Key Components

Before we begin, here are some terms which you should familiarise yourself with.

  • Introns: non-coding regions of DNA
  • Exons: coding regions of DNA
  • A group of three nucleotides is called:
    • A triplet in DNA
    • A codon in mRNA
    • An anti-Codon in tRNA

Transcription

Transcription occurs in the nucleus and is the process in which DNA is transcribed into mRNA.

Firstly, a specific section of DNA unwinds and unzips, exposing the nucleotides. Then, RNA polymerase, an enzyme responsible for initiating and catalysing this process, binds to the promoter region located on the template strand. By using free floating RNA nucleotides, RNA polymerase synthesises a strand of primary mRNA (pre-mRNA) via complementary base pairing. Once the nucleotide sequence at the end of the gene signals to stop transcription, the pre-mRNA sequence is released. Then, the mature mRNA molecule exits the nucleus via nuclear pores and binds to a ribosome. (Ribosomes are composed of rRNA, ribosomal RNA, and proteins)


Transcription

Post Transcriptional Factors

Post transcriptional factors facilitate the conversion of pre-mRNA into mature mRNA and only occur in eukaryotic organisms.

  • At the 3’ end, a poly-a-tail is added to contribute to the stability of the mRNA
  • At the 5’ end, a methyl-g-cap is added to protect the mRNA from enzyme attack
  • Spliceosomes cut the introns out and splice the exons together. It is important to note that a single pre-mRNA strand can become many different mature mRNA strands via a process called alternative splicing which involved exon juggling. This is the process where exons are rearranged or spliced out to produce different mRNA strands

Translation

Translation occurs at a ribosome and is the process in which mRNA is translated into a protein. Proteins which are synthesised at free-floating ribosomes in the cytosol are used within the cell, while those produced on the rough endoplasmic reticulum are secreted for use outside of the cell.

Once the mRNA strand exits the nucleus it binds to a ribosome, it begins to synthesise the polypeptide chain once the start codon is recognised. Active tRNA molecules bring specific amino acids to the mRNA-ribosome complex. The anticodon on the tRNA molecule temporarily binds with the complementary codon, releasing the amino acid which forms a peptide bond with an adjacent amino acid. This continues until a stop codon appears and the polypeptide chain is released.


Translation

Features of the Genetic Code

  • Non-Overlapping: bases are read three at a time
  • Unambiguous: one codon only codes for one amino acid
  • Redundant: more than one triplet codes for the same amino acid
  • Universal: the genetic code is the same in all organisms, except for a few protozoa and bacteria