Genetic Code: Codons and RNA bases

The Genetic Code and Codons

• Genetic code is read in three-letter groupings, these groupings are called codons
• A codon consists of three nucleotide bases in messenger RNA (mRNA)
• Each codon specifies a particular amino acid

Codon Composition

• A codon consists of three consecutive bases
• Codons specify a single amino acid to be added to a polypeptide chain
• All living organisms read the genetic code in sets of three bases at a time
• Examples of codons are: AUG, AAC, and UCU

Genetic Code Table

• There are 64 possible three-base codons in the genetic code
• The genetic code table simplifies reading of codons

RNA Bases and Codons

• There are four different bases in RNA
• This results in 64 possible three-base codons (4 x 4 x 4 = 64)
• Most amino acids can be specified by multiple codons
• Six codons (UUA, UUG, CUU, CUC, CUA, CUG) all specify leucine as an example
• Only one codon (UGG) specifies tryptophan
• Each codon represents one amino acid
• A single amino acid can be coded for by between one and six codons

Reading Codons

• Start at the middle of the circle with the first letter of the codon and move outward on a codon table

Start and Stop Codons

• Methionine codon AUG serves as the "start" codon for protein synthesis
• There are three "stop" codons: UAA, UAG, and UGA

The Role of Methionine (AUG)

• The methionine codon AUG serves as the initiation (start) codon for protein synthesis

mRNA and Translation

• mRNA is read three bases at a time until one of the three different stop codons is reached
• Stop codons signals the end of translation

Translation Overview

• Transcribed mRNA directs the translation process
• Translation produces proteins by decoding the sequence of mRNA codons
• In eukaryotic cells, transcription occurs in the cell's nucleus
• Translation is carried out by ribosomes after transcribed mRNA enters the cell's cytoplasm

Translation and tRNA

• Translation begins when a ribosome attaches to an mRNA molecule
• tRNA molecules carry amino acids, which then bind to mRNA codons

Steps of Translation

• Translation starts when a ribosome attaches to an mRNA molecule in the cytoplasm
• Translation begins with the start codon AUG
• tRNA molecules carry amino acids coded by each codon
• AUG always codes for methionine, thus the first amino acid in translation is always methionine
• tRNAs bring proper amino acids into the ribosome, as each codon passes through
• Each tRNA has an anticodon
• tRNA anticodon bases are complementary to the bases of a codon on the mRNA strand
• tRNA attaches its anticodon to its appropriate mRNA codon
• The anticodon for methionine (start codon AUG) is UAC

Polypeptide Assembly

• The ribosome forms a peptide bond: It also breaks the bond holding the initial tRNA molecule to its amino acid

Products of Transcription and Translation

• The ribosome facilitates the formation of a peptide bond between first and second amino acids
  • These amino acids are methionine and phenylalanine by default
• The product of transcription is mRNA
• The product of translation is a protein
• Amino acids come from with in the cell, transported via tRNA molecules

Completion of Translation

• Translation stops when the ribosome reaches one of the three stop codons
• Upon reaching a stop codon, the ribosome releases both new synthesized polypeptide and the mRNA molecule
• Translation will bypass a stop codon mistakenly made into a regular codon
  • The result will be the addition of another amino acid
  • Potential error in the protein product

RNA roles in Translation

• mRNA, tRNA, and rRNA are involved in translation:
• mRNA carries instructions for polypeptide synthesis, from the nucleus to ribosomes in the cytoplasm
• tRNA carries amino acids to ribosomes and matches them to the coded mRNA message
• rRNA provides an important structural part of the ribosome

Molecular Biology's Central Dogma

• The central dogma: information transfers from DNA to RNA to protein
• DNA → RNA → Protein

Gene Expression

• A gene has been expressed when a gene (DNA code segment) is used to build a protein

Gene Expression Details

• The ultimate use/expression of code is the protein produced at the end of transcription and translation
• Scientists will say that region of DNA has been expressed when its code was used to build a protein
• DNA carries traits information for organism
• The cell uses the DNA base sequences as a template to make mRNA
• mRNA codons specify protein amino acid sequence
• Proteins play a key role in producing organism's traits

Additional Information

• Many RNA molecules not translated into proteins still play key roles in gene expression
• One of the most interesting discoveries of molecular biology is the near-universal nature of the genetic code
  • Although minor variations occur in organisms regarding amino acids and particular codons
  • The code is always read three bases at a time and in the same direction