The Genetic Code

• Gene-a section of DNA that codes for an amino acid sequence (on RNA), which is a building block for proteins.

• There are 20 different amino acids made up of three letter words (four letters to choose from-nitrogenous bases of RNA).

• These three letter words are called codons– Example: Codon AUG, codes for methionine.

Amino Acids and Codons

Gene ExpressionProtein Synthesis

Simple Summary of Gene Expression/Protein Synthesis

• DNA produces an RNA strand in the nucleus• RNA single strand leaves the nucleus through

a nuclear pore• RNA enters cytoplasm• RNA attaches to a ribosome• Ribosome and RNA molecule are

decoded/”read” and produce a protein

DNA(Deoxyribonucleic Acid)

RNA(Ribonucleic Acid)

Double strand Single strand

Helix (spiral) Straight

Sugar present: deoxyribose

Sugar present: ribose

Nitrogen bases:Adenine, Thymine, Guanine, Cytosine

Nitrogen bases:Adenine, Uracil, Guanine, Cytosine(Uracil replaces thymine)

Found in the nucleus Found in the nucleus and the cytoplasm (not confined in the nucleus)

Three Classes of RNA

1. Messenger RNA (mRNA):Takes a message made from DNA in the nucleus

to a ribosome in the cytoplasm

2. Ribosomal RNA (rRNA):A part of/embedded in the ribosome

and the site in which polypeptides are made. (two subunits: small and large ribosome)

3. Transfer RNA (tRNA):Transfers amino acids to the ribosome (helps

translate the message)

From DNA RNA Protein

1. Step One: Transcription-RNA molecule is made that is complimentary to

DNA moleculeDNA sequence: AAGCTRNA complimentary sequence: UUCGA-RNA polymerase is the enzyme used to split apart

the DNA and join the RNA nucleotides together-messengerR NA (mRNA ) is released into the

cytoplasm and reaches a ribosome

• transcription

2. Step Two: Translation:-The information stored on the mRNA is

read/translated and codes for a specific amino acid (needed to make the polypeptide protein)

• Transfer RNA (tRNA)-bring amino acids to the ribosome (containing rRNA) and is where mRNA gets read/translated

Steps of Translation

I. Initiation: • A start codon on mRNA, AUG methionine, allows

translation to begin• mRNA codon pairs with the complimentary

tRNA anticodon• Example codon on mRNA: ACC• Anticodon on tRNA: UGG

• The tRNA codes for/attaches to an amino acid• The tRNA synthetase (enzyme) recognizes

which amino acid belongs to which tRNA molecule

Amino Acid

tRNA

Anticodon

tRNA synthetase helps bind the amino acid to the tRNA molecule

mRNA strand codon

II. Elongation-polypeptide gets longer one amino acid at a time

III. Termination-synthesis stops due to a stop codon

The three stop codons on the mRNA strand are: ___, ____, ____

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