Ribosomes and Protein Synthesis (Ch 13.2)

The genetic code Converting DNA to protein is a two step process Transcription Translation Protein Long chains of amino acids called polypeptide chains (macromolecule) 20 different amino acids can make polypeptide chains Configuration of amino acids = properties of the protein (shape, function, etc.) Genetic code: the language composed of the four bases in RNA (adenine, cytosine, guanine, uracil) The genetic code is read three bases (letters)at a time – Each protein in three bases long (word) – each code corresponds to a specific protein Codon: three consecutive bases (word) that specifies a single amino acid

Reading Codons Combinations How many combinations can we have from 4 bases? Amino Acids One or many codons can specify an amino acids Example: UUA / UUG / CUU / CUC / CUA / cug = leucine Example: UGG = tryptophan You have to use a genetic code table to understand the polypeptide chain that is being made (all the amino acids forming together) Start /Stop codons Polypeptide chains need to be told when to start or stop Example: AUG = start codon

Translation Dna  Protein Transcription: mrna copying the dna strand to leave the nucleus Translation: mrna is translated into protein using rrna in ribosomes and trna Ribosomes Made up of rRna Ribosomes use the codons in mrna to assemble amino acids and create polypeptide chains Trna had an anticodon that base pairs with each codon  codes for an amino acid Amino acids = polypeptide chains = proteins

Steps in Translation Step 1 A ribosomes attaches to an mrna strand in the cytoplasm Codons pass through the ribosome  trna places anticodons on the codons Anticodons: complementary code or base pair to the mrna codon Trna anticodons code for amino acids that will continually attach to the growing chain (polypeptide chain)

Steps in Translation Step 2 Ribosome forms a peptide bond between the first two amino acids The bond between the trna and the amino acid is broken The trna leaves the ribosome and the next trna attaches to the next codon on the mrna strand

Steps in Translation Step 3 The process will continue until a stop codon is reached Stop codon = polypeptide chain and mrna strand are released from the ribosome Process completed!

Roles of rna mrna rrna trna Mrna carries the coded message from the dna molecule in the nucleus to the cytoplasm outside of the nucleus Transcription: Dna codes for the mrna strand that will be taken out of the nucleus and used to make proteins Ribosomes are composed of protein and rrna molecules They hold the ribosomes in place for translation and find the start of the mrna They help in the chemical reaction that holds the amino acids together Trna delivers the exact anticodon to the codons on the mrna strand – anticodons code for amino acids It helps “read” the coded mrna strand and turn it into protein Translation: turning mrna code into protein

Molecular basis for heredity What does protein have to do with genes? Proteins are enzymes that catalyze reactions The proteins stimulate color in a flower, patterns of growth, etc. Proteins build or operate components of a cell Central dogma Information is transferred from Dna  rna  protein Helps to discuss gene expression Exceptions: viruses work differently