Ribosomes and Protein Synthesis

13.2 Ribosomes and Protein Synthesis

The Genetic Code

• The first step to decoding genetic messages is transcription
• The transcribed information contains a code for making proteins
• Proteins are made from joining long chains of amino acids together to form polypeptides
• The specific amino acids in a polypeptide determine the properties of different proteins
• The sequence of amino acids influences the shape of the protein, which in turn determines the protein’s function
• RNA contains four different bases: adenosine, cytosine, guanine and uracil

• A, C, G, U – these four bases are a type of language called the genetic code
• The genetic code is read three “letters at a time, so that each “word” is three bases long and corresponds to a single amino acid
• Each three-letter “word” in mRNA is known as a codon
• A codon consists of three consecutive bases that specify a specific amino acid to be added to the polypeptide chain

• Because there are four bases in RNA, there are 64 possible three-base codons in the genetic code
• Most amino acids can be specified by more than one codon
• Decoding codons is a task made simple by using a genetic code table (one will be given you)
• The genetic code of a protein begins with a “start” codon: AUG
• Following the start codon, mRNA is read, three bases at a time, until it reaches one of three different stop codons that end translation
• At that point, the polypeptide is complete

Translation

• Ribosomes use the sequence of codons in mRNA to assemble amino acids into polypeptide chains
• The decoding of an mRNA message into a protein is called translation
• Transcription and translation are two different processes, but transcribed mRNA directs the translation process

1. Translation begins when a ribosome attaches to an mRNA molecule in the cytoplasm. As each codon passes through the ribosome, tRNAs bring the proper amino acids into the ribosome. One at a time, the ribosome then attaches these amino acids to the growing chain.

1. Each tRNA molecule carries just one kind of amino acid
2. Each tRNA molecule has three unpaired bases, collectively called the anticodon

1. The ribosome helps form a peptide bond between the first and second amino acids. The tRNA floats away from the ribosome, allowing the ribosome to bind to another tRNA. The ribosome moves along the mRNA from right to left, binding new tRNA molecules and amino acids.
2. The process continues until the ribosome reaches one of the three stop codons. Once the polypeptide is complete, it and the mRNA are released from the ribosome.

• All three forms of RNA – mRNA, tRNA, and rRNA – come together in the ribosome during translation
• The mRNA molecule carries the coded message
• The tRNA molecules deliver exactly the right amino acid called for by each codon
• The rRNA molecules help hold ribosomal proteins in place and help locate the beginning of the mRNA message.

Molecular Basis of Heredity

• The central dogma of molecular biology is that information is transferred from DNA to RNA to protein
• Gene expression is the way in which DNA, RNA, and proteins are involved in putting genetic information into action in living cells