GENE EXPRESSION © 2007 Paul Billiet ODWSODWS

Two steps are required 1. Transcription The synthesis of mRNA use the gene on the DNA molecule as a template This happens in the nucleus of eukaryotes 2. Translation The synthesis of a polypeptide chain using the genetic code on the mRNA molecule as its guide © 2007 Paul Billiet ODWSODWS

RIBONUCLEIC ACID (RNA) Found all over the cell (nucleus, mitochondria, chloroplasts, ribosomes and the soluble part of the cytoplasm) © 2007 Paul Billiet ODWSODWS

Types Messenger RNA (mRNA) <5% Ribosomal RNA (rRNA) Up to 80% Transfer RNA (tRNA) About 15% In eukaryotes small nuclear ribonucleoproteins (snRNP) © 2007 Paul Billiet ODWSODWS

Structural characteristics of RNA molecules Single polynucleotide strand which may be looped or coiled (not a double helix). Sugar Ribose (not deoxyribose). Bases used: Adenine, Guanine, Cytosine and Uracil (not Thymine). © 2007 Paul Billiet ODWSODWS

mRNA A long molecule 1 million Daltons Ephemeral Difficult to isolate mRNA provides the plan for the polypeptide chain © 2007 Paul Billiet ODWSODWS

rRNA Coiled Two subunits: a long molecule 1 million Daltons a short molecule Daltons Fairly stable Found in ribosomes Made as subunits in the nucleolus rRNA provides the platform from protein synthesis © 2007 Paul Billiet ODWSODWS

tRNA Short molecule about Daltons Soluble At least 61 different forms each has a specific anticodon as part of its structure. tRNA “translates” the message on the mRNA into a polypeptide chain © 2007 Paul Billiet ODWSODWS

Transcription: The synthesis of a strand of mRNA (and other RNAs) Uses an enzyme RNA polymerase Proceeds in the same direction as replication (5’ to 3’) Forms a complementary strand of mRNA It begins at a promotor site which signals the beginning of gene is not much further down the molecule (about 20 to 30 nucleotides) After the end of the gene is reached there is a terminator sequence that tells RNA polymerase to stop transcribing NB Terminator sequence ≠ terminator codon © 2007 Paul Billiet ODWSODWS

Editing the mRNA In prokaryotes the transcribed mRNA goes straight to the ribosomes in the cytoplasm In eukaryotes the freshly transcribed mRNA in the nucleus is about 5000 nucleotides long When the same mRNA is used for translation at the ribosome it is only 1000 nucleotides long The mRNA has been edited The parts which are kept for gene expression are called EXONS (exons = expressed) The parts which are edited out (by snRNP molecules) are called INTRONS © 2007 Paul Billiet ODWSODWS

Transcription plan Transcription DNA messenger RNA Gene Nucleus © 2007 Paul Billiet ODWSODWS

Translation plan TRANSLATION Complete protein Polypeptide chain Ribosomes Stop codon Start codon © 2007 Paul Billiet ODWSODWS

Translation Location: The ribosomes in the cytoplasm that provide the environment for translation The genetic code is brought by the mRNA molecule © 2007 Paul Billiet ODWSODWS

What is the genetic code? The genetic code consists of the sequence of bases found along the mRNA molecule There are only four letters to this code (A, G, C and U) The code needs to be complex enough to represent 20 different amino acids used to build proteins © 2007 Paul Billiet ODWSODWS

How many combinations? If one base represented one amino acid this would only be able to produce 4 different combinations. (A, C, G and U) If pairs of bases represented each amino acid this would only be able to produce 4 x 4 = 16 combinations. (AA, AC, AG, AU, CA, CC, CG, CU etc) If triplets of bases represented each amino acid, this would be able to produce 4 x 4 x 4 = 64 combinations This is enough combinations to code for the 20 amino acids but is the code actually made of triplets? © 2007 Paul Billiet ODWSODWS

Nature is logical! Over 10 years biochemists synthesised bits of mRNA with different combinations Then they used them to synthesise polypeptides The results proved the logical answer was correct The genetic code is made of triplets of bases called codons © 2007 Paul Billiet ODWSODWS