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Protein Synthesis

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Protein synthesis begins with a process called translation which occurs in the ribosomes, located in the endoplasmic reticulum. mRNA copies genetic information from the nucleus in a process called transcription. Then the mRNA brings that information to the ribosome and the tRNA brings amino acids which are arranged according to the information on the mRNA.

At the molecular level one of the most important processes to understand Biology is the process known as protein synthesis. This is how you take the instructions that are written in the DNA of your nucleus, copy those instructions into a temporary form then send them out to the rest of the cell for the ribosomes to construct the proteins the carrier of the commands of the nucleus. It's by understanding this process scientists are delving into some fascinating research where they're learning how to do things like have bacteria build medicines for us this is amazing stuff so it's pretty cool to learn that. It's actually not that hard it's a two step process.

Like I said, you have to copy the instructions and that's a process known as transcription. This is how you open up a section of your DNA and build a messenger RNA copy of that one gene. It's very easy sometimes to confuse transcription and replication which is the copying of the entire molecule of DNA. DNA replication is only done once in a cells life, that shortly before cell division while transcription is being done all the time in order to carry out all the functions that the cell needs to do and I think of it as making a transcript which is the written copy of say a TV show that you watches where they write down everything that somebody said.

That's different form translation which also begins with a t but it means a very different thing if I watch a translation of a show of I watch if I ask for a translation of a show I'm not understanding the same language. Remember transcription you're going from DNA which is Deoxyribose Nucleic Acid to RNA Ribonucleic Acid it's in the same you can think of it as language in chemical form. Here I'm going from one language the RNA transcript that just came out of the nucleus and I'm using it to build an amino acid sequence which is a completely different kind of chemical so translation is where the ribosomes use the instructions that were copied into that messenger RNA and they guide ribosome and how to or which amino acids to joins one after the other to build an entire protein so let's take a quick look at this youtube video and we're going to take a look at how this process works.

I'll go ahead and maximize it and what we're going to see here is inside the nucleus we have the DNA and we can see an enzyme going along the DNA hunting for the gene using a promoter which is the sequence of a's, t's, c's and g's on the DNA to guide the enzyme and tell it where the gene is and which strand to copy. It's found the promoter now it knows where the gene is unwinds the double helix and now start copying just one side that's again different from DNA replication and it's binding RNA nucleotides wherever there's an a it puts u because it's RNA wherever there's a c it puts a g wherever there's a g it puts a c wherever there's a t it puts an a and so and so forth, so it's going along and it's copying one strand of the DNA in RNA form. Now this video is going to have the RNA start to leave and go straight to the ribosome in eukaryotes creatures like you and me with a nucleus when the RNA leaves, it often gets edited just to get rid of some information that's not needed and to makes some modification that are necessary but the RNA leaves goes out into cytoplasm where one counter of the two parts that make up a ribosome.

First it's going to encounter the smaller half or small subunit of a ribosome and that's what we see here and this smaller half of the ribosome helps hold the RNA so it can easily be read. Now that it's been found we need a couple of other things we need the large subunit of the ribosome which is larger and we need tRNA this is transfer RNA that carries the amino acids to the ribosome and helps using it anticodon matches up to the 3 nucleotide groups that are in RNA called codons. Now that we have the first tRNA and then what's known as start codon the large subunit connects. Now we have a fully assembled ribosome with the r messenger RNA and tRNA and we're ready for the next tRNA to come in and that's what we see it enters in through a location called the a site and it ducks it's anticodon to the codon of the messenger RNA now this weird little geometric shapes are some graphic artist knowledge idea of what the amino acid is. That bizarre alien thing that came in, that was an enzyme that join those two amino acids together breaking it from this tRNA once that tRNA is done delivering it's amino acid it goes away to some other parts of the cell to pick up a new amino acid so it can deliver the next one if needed.

Whatever is in x codon the appropriate tRNA comes in bring with it it's amino acid it matches up it's codon following the same basic base pairing rules it allows the enzyme to attach amino acids together and that's what's going on tRNAs drop off amino acids, ribosomes puts them together and the growing amino acid chain or protein starts to come out of the ribosome. When ultimately it reaches what's known as the stop codon the ribosome stops the two halves fall apart, the protein chain is released and the messenger RNA floats away perhaps to be led by some other ribosome and here you can see your cells are full of ribosomes doing this protein synthesis process all the time. That's protein synthesis.