Protein Synthesis -Translation

Recall that after a gene of DNA has been used to make mRNA through transcription, the mRNA is then processed by Ribozymes to form mature mRNA made out of the exons.

Before we can go any further, we must get familiar with three different forms of RNA. RNA is still RNA, but some strands of RNA do not act like mRNA. Instead they fold over and perform different jobs.

THREE TYPES Of RNA mRNA – Messenger RNA – one we are already familiar with. Function to carry a revised message of the DNA out of the nucleus to a Ribosome (protein factory).

Yellow tRNA’s delivering proper amino acids 2. tRNA – Transfer Ribonucleic Acid Function – A type of RNA molecule that coils up on itself heads out to the cytoplasm and grabs a particular Amino Acid and delivers it to a Ribosome (protein factory) Simplified Model Yellow tRNA’s delivering proper amino acids

Fig. 25.10 This tRNA will only grab and deliver the amino acid “Arginine” it also has a very specific triplet of bases called an anticodon. This tRNA has the specific anticodon – GCU. GCU can only attach onto the mRNA codon - CGA

3. rRNA- Ribosomal Ribonucleic Acid Function: A type of RNA that forms off of DNA found in the Nucleolus of the cell. This RNA then associates with proteins to build 3-D globular subunits. These subunits then migrate out to the cytoplasm and unite to form a Ribosome (protein factory)

NUCLEOLUS

Reminder – Every three bases of mRNA represents a triplet called a CODON. Each codon codes for a specific Amino Acid to make sure that the protein gets its proper primary structure. Amino Acid Order

In a real piece of mRNA, the first codon will typically be AUG, the last codon to be read will be either UAA, UGA, or UAG. Notice that there are only 20 Amino Acids, but there are 64 possible codons = DEGENERACY

The first part of translation is called INITIATION The first part of translation is called INITIATION. During initiation, the ribosomal subunits (small then large) hop onto the mRNA and then the appropriate tRNA bonds (H-bonding) its anticodon onto the proper mRNA codon

The second phase of translation is called ELONGATION The second phase of translation is called ELONGATION. During elongation a second tRNA comes and specifically binds onto the second mRNA codon. The Ribosome slides over and continues by creating a site for a third tRNA to deliver a third Amino Acid. As this is happening, enzymes are running dehydration synthesis reactions between each amino acid to create peptide bond. So a polypeptide chain is being built.

The final step of translation is called TERMINATION The final step of translation is called TERMINATION. During termination, the ribosome hits a “STOP” codon of mRNA. This prevents a tRNA from coming into that site, but instead a release factor comes in to the site. The release factor causes the ribosome to split and release the newly formed polypeptide.

Often more than one ribosome can simultaneously hop on and translate the same mRNA. When this happens, the chain of ribosomes is known as a “POLYSOME” or “POLYRIBOSOME” Polysome

Complete #5 on Applying What You Know – W.S.

Fig. 25.13