Protein Synthesis

The DNA Code

• It is a universal code.

• The order of bases along the DNA strand codes for the order in which amino acids are chemically joined together to form a polypeptide

From DNA to Protein

• Protein synthesis involves two types of nucleic acids:

DNA (deoxyribonucleic acid)

RNA (ribonucleic acid)

RNA• RNA, like DNA, is a polymer formed by a

sequence of nucleotides

• Three Types of RNA:

messenger RNA (mRNA)

transfer RNA (tRNA)

ribosomal RNA (rRNA)

Differences Between DNA and RNA

DNA RNAdouble-stranded single-stranded

sugar = deoxyribose sugar = ribose

bases = A,T,C,G bases = A,U,C,G(uracil takes the place of thymine)

Protein Synthesis

involves two processes:

1. Transcription: the copying of the genetic message (DNA) into a molecule of mRNA

2. Translation: mRNA is used to assemble an amino acid sequence into a polypeptide

A sequence of 3 bases called a codon codes for one amino acid

Transcription• occurs in the nucleus of the cell

• Steps in transcription

1)Initiation: DNA strand separates and serves as a template (pattern) for mRNA assembly. mRNA polymerase attaches to the template Strand of DNA.

2) Elongation: RNA Polymerase attaches to the template strand of DNA. Free mRNA nucleotides match up to the exposed nucleotides on the DNA strand.

3) Termination: mRNA strand separates from the DNA strand when a “stop codon” is reached and the entire gene has been transcribed.

4) the mRNA strand carries the code for the production of one polypeptide

Translation• occurs in the cytoplasm of the cell, at the

ribosome• Always begins with the mRNA codon AUG• 1) Initiation: mRNA moves out of the nucleus

and into the cytoplasm to a ribosome

2) Elongation: mRNA is “read” by the ribosome as the anticodons of the tRNA molecules attach to their complementary condons on the mRNA molecules creating a chain of amino acids.

3) Termination: • mRNA molecule moves across the ribosome• tRNA temporarily attaches to the mRNA strand.• The amino acids are joined by a peptide bond by

enzymes until a “stop” codon is reached

tRNA Molecule

4. a polypeptide is produced

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