Bellwork: How does a cell interpret the genetic code Bellwork: How does a cell interpret the genetic code? How does the genetic code affect the expressions of specific traits in an organism?

Ribosomes and protein synthesis Section 13.2

The genetic code Step one - copy DNA to produce RNA RNA contains instructions on how to make proteins Proteins are chains of amino acids called polypeptides Up to 20 different amino acids are commonly found in polypeptides Specific amino acids and their arrangement determine properties of different proteins Amino acid arrangement influences shape, which determines proteins function

The genetic code continued… RNA contains four different bases Essentially a language with 4 letters Genetic code is read three letters at a time Each ’word’ is three bases long and corresponds to an amino acid In mRNA, each ‘three letter’ word is a codon Three consecutive bases that specify a single amino acid

Reading codons There are 64 possible three base combinations in RNA Due to the 4 different bases Most amino acids can be specified by more than one codon Leucine is coded by 6 different codons.. Genetic code table makes decoding codons easy

Start and Stop codons These are essentially punctuation marks in the genetic code Methionine codon serves as the initiation or start codon for protein synthesis After this point, the mRNA is read three bases at a time There are 3 different stop codons which mark the end of polypeptide formation

Translation Why are ribosomes so important? mRNA = instruction manual Ribosome reads the instruction manual and assembles the parts together The assembly process performed by ribosomes is called translation

Translation overview

Steps of translation Step 1: Ribosome attaches to mRNA molecule in the cytoplasm Step 2: As each codon passes through the ribosome, tRNA brings the proper amino acids into the ribosome. The ribosome attaches amino acids together into a growing chain Each tRNA molecule is three unpaired bases, called the anticodon Each tRNA molecule anticodon is complementary to one mRNA codon Step 3: Ribosomes help to form peptide bonds between amino acids and build the protein, like a production line Step 4: Polypeptide chain grows until it reaches a stop codon Here the newly formed polypeptide and mRNA molecule are released.

Roles of tRNA and rRNA in translation All three types of RNA come together during translation mRNA carries coded message tRNA molecules carry the amino acids called for by each codon Ribosomes are made of about 80 proteins and 3 or 4 different rRNA molecules rRNA helps keep ribosomal proteins in place, and help locate start of mRNA message They may also help in formation of peptide bonds

Molecular basis of heredity Most genes simply contain instructions for making proteins Proteins are key in controlling what traits are exhibited Many proteins are enzymes, which catalyze and regulate chemical reactions Skin colour can be controlled by a gene that codes for an enzyme to prdouce a specific pigment Proteins act as microscopic tools, each specifically designed to build or operate a component of a cell

Central Dogma Information is transferred from DNA to RNA to protein There are however many exceptions – e.g. viruses transfer information in the opposite direction, from RNA to DNA Acts as a useful generalization to show how genes work Genetic code operates virtually the same way in all living organisms Some organisms may vary which amino acid applies to specific codons Code is always read three bases at a time Code is always read in the same direction There is remarkable unity between all living organism with the molecular biology of the gene.