Ribosomes and Protein Synthesis

Ribosomes and Protein Synthesis All organisms use the same genetic code - Suggests Genetic code: - Arose very early in history of life - Was passed on over time to all living organisms - Genes can be transferred between species - Human gene can be inserted into bacterial genome → bacteria will make a human protein The Genetic Code • What is the genetic code, and how is it read? • The genetic code is read three “letters” at a time, so that each “word” (codon) is three bases long and corresponds to a single amino acid. RNA contains four different bases: adenine, cytosine, guanine, and uracil, which form the genetic code, with just four “letters”: A, C, G, and U. The first step in decoding genetic messages is to transcribe a nucleotide base sequence from DNA to RNA. This transcribed information contains the code that is translated into a protein. Proteins are made by joining amino acids together into long chains, called polypeptides. [20 different amino acids are commonly found in polypeptides] How to Read Codons Because there are four different bases in RNA, there are 64 possible three-base codons (4 × 4 × 4 = 64) in the genetic code. The genetic code The sequence of nucleotides on the DNA (gene) determines the sequence of nucleotides on the mRNA (transcription). The sequence of nucleotides on the mRNA determines the sequence of amino acids in the polypeptide (translation). The polypeptide folds and combines with others into a particular protein. The shape of the protein determines its function. Translation • What role does the ribosome play in assembling proteins? • Ribosomes use the sequence of codons in mRNA to assemble amino acids into polypeptide chains. Main Players: Ribosomes (Factory) • 2 subunits • Ribosomal RNA • Protein Transfer RNA = tRNA (Tools) Messenger RNA = mRNA (Blueprint) Amino Acids (Raw materials) Protein (product – workhorses of life) Ribosomes and Protein Synthesis Transfer RNA (tRNA) -Anticodon binds to the mRNA -Each Anticodon corresponds to a specific Amino Acid -The body is a long RNA sequence Steps in Translation Messenger RNA is transcribed in the nucleus and then enters the cytoplasm for translation. Initiation: mRNA joins with and is threaded through the ribosome. Functional ribosome = 2 subunits, mRNA , and 1st tRNA (Met) Ribosomes are composed of roughly 80 proteins and three or four different rRNA molecules that help hold ribosomal proteins in place and help locate the beginning of the mRNA message. Elongation cycle (Formation of Peptide Bonds): As the ribosome reads each codon of mRNA, it directs tRNA to bring the specified amino acid into the ribosome. One at a time, the ribosome then attaches each amino acid to the growing chain. - tRNA carrying Methionine binds first to the “start sequence” AUG- the next tRNA binds to the next codon in the other binding site- “peptide bond” forms between the two amino acids- tRNA for methionine vacates the P site leaving methionine- remaining tRNA shifts to the P site leaving the A site for the next tRNA to read the next codon - As the ribosome moves along mRNA, peptide bonds form between amino acids within the ribosome. Termination: The chain of amino acids grows as each codon is read until a “stop” sequence is reached. Translation (summary) The sequence of nucleotide bases in an mRNA molecule is a set of instructions (gene) that gives the order in which amino acids should be joined to produce a polypeptide. Ribosomes use the sequence of codons in mRNA to assemble amino acids into polypeptide chains. The forming of a protein requires the folding of one or more polypeptide chains. The decoding of an mRNA message into a protein is a process known as translation. The Molecular Basis of Heredity • What is the “central dogma” of molecular biology? • The central dogma of molecular biology is that information is transferred from DNA to RNA to protein. The Molecular Basis of Heredity Gene expression is the way in which DNA, RNA, and proteins are involved in putting genetic information into action in living cells. - DNA carries information for specifying the traits of an organism. - The cell uses the DNA as a template for making mRNA. - The codons of mRNA specify the sequence of amino acids in a protein. - Proteins, in turn, play a key role in producing an organism’s traits. The Molecular Basis of Heredity One of the most interesting discoveries of molecular biology is the near-universal nature of the genetic code. Although some organisms show slight variations in the amino acids assigned to particular codons, the code is always read three bases at a time and in the same direction. Despite their enormous diversity in form and function, living organisms display remarkable unity at life’s most basic level, the molecular biology of the gene.