By Erika Ann Baca
In order for a cell to be able to take genetic material, DNA, and turn it into a functional protein, transcription and translation must occur. So what’s the difference? Let’s start with transcription.
Transcription is the process of converting DNA into RNA. This process is completed by RNA polymerases that read the DNA template strand in the 3’ to 5’ direction in order to synthesize a RNA strand in the 5’ to 3’ direction. RNA polymerase binds to the template strand of DNA and uses this strand to produce the RNA sequence by base pairing.
Remember! In RNA uracil is used in place of thymine. Another important note about transcription is that in prokaryotes, only one RNA polymerase is used and transcription occurs in the cytoplasm. In eukaryotes however, three RNA polymerases are used and occurs in the nucleus. RNA pol I transcribed rRNA, RNA pol II transcribes mRNA, and RNA pol III transcribed tRNA – all which will be used in the next step, translation.
Translation is the process of taking our newly synthesized mRNA strand and using it to create a polypeptide chain. This is made possible by the use of a ribosome. A ribosome is composed of both rRNA and proteins and has three different important sites for translation. The Aminoacyl site (A site) is where incoming amino-acyl tRNAs will enter. The peptidyl site (P site) holds the growing peptide chain and the exit site (E site) is where the tRNAs, now without an amino acid, will exit the ribosome. Do you see how all three types of RNA came into play?
Before protein synthesis can occur, the ribosome must bind to the mRNA strand. Actual binding varies in prokaryotes and eukaryotes. Prokaryotes find a specific sequence called the Shine-Delgarno sequence while eukaryotes scan the mRNA for the first start codon. One the first start codon is found, the “initiator” tRNA with the correct anti-codon and amino acid can enter the ribosome at the p-site. Only the initiator tRNA will start here, all other tRNAs will enter at the A site.
After the start codon is found, protein synthesis happened in a pretty common pattern. An amino-acyl tRNA with the correct anticodon to mRNA’s codon will enter the A-site, a peptide bond will form between the amino acids in the A site and P site, causing the bond holding the amino acid to the tRNA in the P site to break. The ribosome will translocate to the next codon on the mRNA and the A site tRNA moves to the P site, the P-site tRNA moves to the E-site, and the E-site tRNA exits the ribosome because it no longer holds an amino acid.
This process continues until a stop codon is reached (UAG, UGA, UAA). When a stop codon is reached, a release factor binds rather than a Trna. When this happens, the polypeptide chain is released and the ribosome comes off of mRNA.
In prokaryotes and eukaryotes, translation occurs in the cytoplasm. In prokaryotes, transcription and translation can occur at the same time.
Knowledge Check: Try this informal quiz to check for understanding.
1. What are the three sites of the ribosome?
a. E, T, P
b. U, A, G
c. E, P, A
2. What does RNA pol II create?
3. Transcription and translation can happen at the same time in eukaryotes.
4. Thymine is 1 of 4 bases used in transcription
5. What is an amino-acyl Trna?
a. tRNA that causes release of ribosome
b. tRNA with an amino acid bound
c. tRNA used only in prokaryotic translation
Answers: 1c, 2a, 3F, 4F, 5b
Erika is a junior majoring in Biology and minoring in Health Medicine and Human Values at UNM. She hopes to one day become a physician. Erika spends all her free time spoiling her puppy and thinks everyday should be national chocolate day.