Protein Synthesis

Let’s make some protein!

Protein Synthesis: An Overview

• Genetic information is contained within the nucleus of a cell

• DNA in the nucleus directs protein synthesis

• Protein synthesis occurs in ribosomes located in the cytoplasm

• How does a ribosome synthesize the protein required if it does not have access to DNA?

THE CENTRAL DOGMA OF PROTEIN SYNTHESIS

Protein Synthesis: An Overview

• The answer lies in an intermediate substance known as mRNA (m stands for ‘messenger’)

• Information is copied from DNA into mRNA, this is called transcription

• mRNA leaves the nucleus and enters the cytoplasm of the cell

• Ribosomes use the mRNA as a blueprint to synthesize proteins composed of amino acids, this is translation

Remember RNA…?

• Ribose sugar• Single stranded• A pairs with U• G pairs with C• Resides in nucleus

and cytoplasm

RNA

• There are three types of RNA:• mRNA is the “blueprint” for construction of

a protein• rRNA is the “construction site” where the

proteins are made• tRNA is the “truck” delivering the proper

amino acids to the site of protein synthesis

Genes and Proteins

• Genes are a sequence of nucleotides in DNA that code for a particular protein, such as a chemical (adrenalin), enzyme (lactase), or hormone (dopamine)

• Proteins drive cellular processes, determine physical characteristics, and manifest genetic disorders by their absence or presence (ex. Hemophilia)

Transcription

• Double-stranded DNA is unzipped and copied onto a strand of messenger RNA (mRNA)

• Recall that on RNA, the A nucleotide binds with U, instead of with T

Translation• After transcription the mRNA exits the

nucleus via nuclear pores• Ribosomes bind to the mRNA• Ribosomes read the sequence on mRNA• The mRNA is read in triplets (called

codons)• Each triplet is the codon for an amino acid

(protein building block)

Match Game

• Consider the following mRNA sequence:

A U G A A U G A G C U G A A C

How would translation match this section?

Transfer RNA

• The ribosome alone cannot synthesize the protein chain

• The correct amino acids must be delivered to the polypeptide building site by tRNA

Ribosomes• Ribosomes are the site

of protein synthesis.

• The ribosome has binding sites for the mRNA transcript and the tRNA molecules.