Molecular Genetics

Protein Synthesis Overview

The Central Dogma

• DNA contains the blueprint for protein synthesis, but proteins are synthesized outside of the nucleus

• Does DNA need to exit and re-enter the nucleus? If so, how does it avoid being damaged?

• How can proteins be synthesized at different rates if there is only 1 complete set of DNA in every cell?

The Central Dogma

• The central dogma of molecular genetics is that protein synthesis occurs as a two-step process:– transcription - copying of information in

DNA into mRNA (messenger RNA)– translation - using the mRNA blueprint to

create a chain of amino acids (involves ribosomes)

The Central Dogma

Ribonucleic Acid (RNA)

DNA RNA

deoxyribose sugar ribose sugar

double stranded single stranded

A-T, C-G A-U, C-G

in nucleus in nucleus and in cytoplasm

RNA

• Types of RNA:– messenger RNA (mRNA) - information

from DNA is transcribed onto mRNA; length depends on length of gene transcribed

– transfer RNA (tRNA) - transfers amino acids to ribosome to build polypeptide

– ribosomal RNA (rRNA) - structural component of ribosome

Overview of Transcription

• Initiation– RNA polymerase binds to DNA at the promoter

site (near the beginning of the gene)

• Elongation– using DNA template, RNA polymerase adds

ribonucleotides to build mRNA transcript

• Termination– RNA polymerase recognizes signal to stop

transcription (termination sequence)

Overview of Translation

• Initiation– ribosome recognizes specific sequence on mRNA

and binds to that site

• Elongation– ribosome moves along mRNA 3 nucleotides at a

time; tRNA delivers amino acids to elongate polypeptide chain

• Termination– 3-base “stop” signal reached and ribosome falls off

the mRNA, releasing polypeptide chain

Genetic Code

• codon - 3-base sequence on mRNA that codes for a specific amino acid

• start codon - does not code for an amino acid; signals initiation

• stop codon - does not code for an amino acid; signals termination

Genetic Code