The Synthesis of Proteins: The Review Francis Saud Amir

What you need to know: We all have DNA, it’s the universal genetic code Our DNA codes for proteins, but not directly. That’s why we need RNA DNA must be translated into mRNA and transcribed in ribosomes to code for proteins transcription and translation both have 3 parts: Initiation, elongation, and termination Protein synthesis requires different proteins Ribosomes are required. There are 3 RNA polymerases in the eukaryotic process The processes are slightly different in eukaryotes and prokaryotes

DNA Information: DNA is a double-stranded helix, like a twisting ladder DNA runs in the 5’ -> 3’ direction DNA is antiparallel, so one strand’s 5’ -> 3’ runs in a different direction than the other DNA is composed of nucleotides, Adenine, Thymine, Guanine, and Cytosine DNA can’t leave the chromosome in nucleus in eukaryotic cells In a prokaryotic cell, the DNA is already in the cytoplasm

Transcription: Initiation This is the process by which DNA is copied into mature RNA (mRNA) A transcription factor (protein that binds to promoter region, regulate transcription) is required. When a factor attaches to the promoter region (TATA box), initiation has begun. Transcription factor proteins are needed to recruit the correct polymerases RNA polymerase II comes when the transcription factors have binded to the TATA box Then we can move into the elongation phase P.S: In an inducible gene, there is always an operon blocking the TATA box region & in a repressible gene, there is only an operon blocking when there are sufficient amounts of the product.

Transcription: Elongation DNA strand will temporarily be single stranded RNA polymerase II is the polymerase that adds nucleotides for the growing RNA strand at the 3’ end The RNA polymerase II adds complementary nucleotides T -> A A-> U G<->C As polymerase moves down the base strand of DNA, more nucleotides are added It’s literally just called elongation because the RNA strand is getting larger

Transcription: Termination RNA polymerase II has the ability to recognize the sequence of DNA that tell it to stop coding. RNA polymerase will enter a ‘hairpin loop’, a small loop of DNA where polymerase stops pausing long enough for DNA to wind up again. This pushes the polymerase off, ending the elongation of the RNA strand. A hairpin loop in DNA is mostly adenines, which are weak when transcribed into uracils. With the release of the RNA, a 5’ cap will be added to the 5’ end and a poly-a-tail on the 3’ In prokaryotes, the mRNA can go into the translation phase somewhat quickly. In eukaryotes, the premature mRNA stand has to have the “junk” genes taken off (introns, they don’t code for anything) snRNA comes and finds the introns on the pre-mRNA strand. After the introns have been snipped off, the mRNA is matured the mRNA then leaves the nucleus (eukaryotes only) and enters the cytoplasm

The Ribosome will Ribo-all ;) The ribosome is the majestic machine that will transform a simple strand of mRNA into a functional protein!This is enabled by numerous enzymes that are in the ribosome. mRNA strand goes through the large subunit and small subunit.

So what does the ribosome do???

Ribosomes construct mRNA into protein which will later be discussed in more detail. The ribosome itself is made of different parts to accomplish this goal. The ribosome is viewed as a protein synthesis factory. It’s here that the polypeptide chain is create and it’s here that mRNA is read for the correct amino acid. The amino acid sequence will determine the protein it produces along with other modifications. What do ribosomes do??

The Parts of the Ribosome Thing about each site as yesterday, today and tomorrow. The E site is yesterday. The tRNA site is held there and is about to exit the ribosome. It has already been processed and made. The P site is the current codon of mRNA being processed. It is today, the tRNA that is presently growing the polypeptide chain.

Finally tomorrow is the site Finally tomorrow is the site. It is the future and it is the codon that will come next and depending on that codon the tRNA will enlarge the peptide chain.

Introduction to What Occurs in Translation The mRNA attaches to the ribosome The sequence of codons that are on the mRNA determine the sequence of amino acids in the polypeptide to be synthesized The tRNA brings an amino acid to the ribosome causing the anticodon of the tRNA base-pairs with the codon of the mRNA The newly arrived amino acid is attached with a peptide bond to other amino acids already present A tRNA is released from the ribosome This process repeats until the stop codon is reached on the mRNA, and the polypeptide is completed.

Translation: Initiation After transcription has occurred, the information required to build proteins depends on the DNA. This DNA comes in codons, which are three letter codons. The mRNA, tRNA, and ribosomal subunits are transported across the nuclear envelope, and to the cytoplasm. Initiation is initiated by a start codon on the DNA which is read by the ribosome. The start codon is always AUG, when the ribosome reads this, it will add the amino acid to begin creating a polypeptide chain. tRNA brings an MET, which is the initiator factor to translation. As in transcription translation also occurs in three steps, the initiation, elongation, and termination. Energy for translation is provided by GTP, like ATP, GTP also supplies energy. A ribosome has three binding sites for tRNA molecules. The A site accept the amino acid coming from the tRNA. The P site contains the tRNA while it's making a chain of amino acids( polypeptide) The third position is the E site, like an exit. It hold the tRNA after it gives up its amino acid.

The codon on the tRNA is the anti-codon

Step by Step Process ( No puns here, yw) Initiation: where the small ribosomal subunit attaches near the end of the mRNA. The tRNA signal the AUG codon to start the process. The large ribosomal subunit then attaches to the mRNA with the tRNA, which then occupies the three binding sites. Elongation: occurs as the additional tRNA’s drop their codons off. Translocation occurs as the ribosomes move over and over again over the binding sites. All the tRNA’s shift as a new tRNA come into place. Termination: occurs when the ribosome encounters a stop codon, and now it knows it’s finally done. However the ribosomes now attach the the mRNA again ( same one or different) and restart the process. ( Back to square pun (one)) ;D

Elongation As you can see the tRNA is dropping off its codon, which has a three letter code, till the stop codon appears letting him know to stop.

Termination This image shows how a stop codon is placed letting the mRNA know to stop. This process is called termination. Stop codons are UAA, UAG,UGA.

The Final Product The final product is a protein! We did it! The protein can then go through post translational changes. Theses changes include, phosphorylation, cutting/snipping/cleaving, glycosylation, ubiquitination, methylation, acetylation, etc. All theses changes change the shape of the protein and occur after it has been released from the ribosome.

Questions {Just answer it you fools! we’re giving out chocolate} What are transcription factors? What is their function? Where do they bind? Where does termination of the mRNA strand occur? What must pre-mRNA go through to become mature? What does it use? How many nucleotides per codon. What is the start codon? What are the functions of the A, P, and E site? What is the anti-codon? Where is it found? Which Comes first: Transcription or Translation? How many subunits compose a ribosome? Where in the cell is the ribosome located? How does termination of translation occur? What is one stop codon?

The End. Thank you!