Chapter 13: RNA and Protein Synthesis Section 13-2: Ribosomes and Protein Synthesis
The Genetic Code The first step in decoding genetic messages is transcribing DNA into RNA The RNA contains the code for making a protein Proteins are made by joining together amino acids into long chains called polypeptides There are 20 different amino acids commonly found in polypeptides
The Genetic Code The specific amino acid sequence in a polypeptide determines a protein’s properties – including its shape and function RNA contains 4 bases – adenine, cytosine, guanine, and uracil These bases are the letters of the special language known as the genetic code
The Genetic Code The code is “read” in groups of three letters called codons Each codon in mRNA is like a word in the code made of 3 consecutive bases specifying a single amino acid to be added to the polypeptide chain
How to Read Codons 4 bases in RNA means 64 possible codons To read, start at middle and work out Most amino acids have more than one codon Methionine codon AUG is the start codon There are 3 stop codons – UAA, UAG, UGA
Translation Sequence of bases in mRNA = instructions for the order of amino acids in a polypeptide More folding required to make a protein Ribosomes read codon sequence in mRNA to assemble amino acids into polypeptides The decoding of an mRNA message into a protein is a process known as translation
Steps in Translation mRNA is transcribed in nucleus, then enters cytoplasm
Steps in Translation Starts when a ribosome attaches to mRNA and begins reading each codon, directing tRNA to bring the correct amino acid Each tRNA carries one amino acid – it gets matched with the codon through an anticodon complementary to the mRNA Ribosome helps form a peptide bond between the each amino acid Polypeptide chain continues to grow until the ribosome reaches a “stop” codon It releases both the newly formed polypeptide and the mRNA molecule, completing the process of translation
Steps in Translation
The Roles of tRNA and rRNA in Translation Ribosomes are composed of ~ 80 proteins and three or four different rRNA molecules rRNA molecules hold ribosomal proteins in place and locate the beginning of the mRNA message May carry out chemical reaction that joins amino acids together
The Molecular Basis of Heredity Most genes contain instructions for making proteins, many of which are enzymes Proteins are microscopic tools, each specifically designed to build or operate a component of a living cell Molecular biologists study life at the molecular level The central dogma of molecular biology is that information is transferred from DNA to RNA to protein There are many exceptions to this “dogma”
The Molecular Basis of Heredity Gene expression is putting genetic information into action in living cells, and involves DNA, RNA, and proteins DNA carries information for traits and is used to make mRNA, which is decoded into an amino acid sequence to make a protein, which produces a trait Genetic code is near universal Some organisms show variations, but code is read the same way