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Lesson Overview

Lesson Overview. 13.2 Ribosomes and Protein Synthesis. The Genetic Code. Proteins are polypeptides chains of amino acids. 20 different amino acids. Amino acid sequence influences the shape of the protein, which in turn determines its function. The Genetic Code.

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Lesson Overview

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  1. Lesson Overview 13.2 Ribosomes and Protein Synthesis

  2. The Genetic Code • Proteins are polypeptides chains of amino acids. • 20 different amino acids. • Amino acid sequence influences the shape of the protein, which in turn determines its function.

  3. The Genetic Code • RNA contains four different bases, A, C, G, and U, which form a “language,” or genetic code. • The genetic code is read three “letters” at a time, known as a codon, that corresponds to a single amino acid.

  4. How to Read Codons • There are 64 possible three-base codons (4 × 4 × 4 = 64) in the genetic code. • To read a codon, start at the middle of the circle and move outward.

  5. How to Read Codons • Most amino acids can be specified by more than one codon. • For example, six different codons—UUA, UUG, CUU, CUC, CUA, and CUG—specify leucine. But only one codon—UGG—specifies the amino acid tryptophan.

  6. Start and Stop Codons • The methionine codon AUG serves as the “start,” codon for protein synthesis. • There are three different “stop” codons, which end translation.

  7. Steps in Translation • 1. Ribosome attaches to mRNA. • 2. tRNA attaches anticodon (UAC) to mRNA’s start codon (AUG). • 3. Ribosome has a second binding site, so a 2nd tRNA molecule attaches to the next codon. • 4. The 2 amino acids on the tRNAs attach to each other. • 5. The tRNA, minus the methionine, is ejected from the ribosome

  8. Steps in Translation • The polypeptide chain continues to grow until the ribosome reaches a “stop” codon on the mRNA molecule. • When the ribosome reaches a stop codon, it releases both the newly formed polypeptide and the mRNA molecule, completing the process of translation.

  9. The Molecular Basis of Heredity • The central dogma of molecular biology is that information is transferred from DNA to RNA to protein, in a process called gene expression. • 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.

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