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Biology Transcription

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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.

Transcription is the process in biology in which a cell forms messenger RNA (mRNA) to guide protein synthesis, also known as translation. The mRNA copies a single gene from the DNA in the form of a series of codons. Codons are groups of three bases which indicate which amino acid the ribosome should use to form proteins.

Transcription is the first step in the process known as protein synthesis, the way that a cell takes a set of instructions in its DNA known as a gene and uses that set of instructions to build a protein so that it can carry out a particular function or make a structure for the cell. Now again, transcription is how do you copy that one gene into a form known as m or messenger RNA so that the ribosome can then build up without damaging the DNA in the nucleus.

Well the process begins using this concept known as codons, now DNA is written in bases a's, t's, c's and g's but amino acids there's 20 different versions of them so you can't just use a to represent adenine, t to represent thymine and so on because you'd just run out of bases so instead you use them in groups of three.

Let's take a quick look at this codon chart over here and here we can see the various different kinds of codons and so you can see that say for example ucu stands for serine while ccu stands for proline and this is how the ribosomes take the messenger RNA but it's just a long sequence of a's t's oh sorry a's, u's, g's and c's and uses them to determine what amino acids to use.

Now part of the important process here is something known as a promoter that's the indicator to a to the enzymes that are going to do transcription. Where does a gene begin? Kind of like when you're flipping through a recipe book you'll see a picture of the food and of a title and that tells you oh there's a recipe starting here or when you're looking through your Biology textbook you're flipping you see ooh the big picture and the big words that indicates a beginning of a chapter I should taking my notes here, so the first step is when the DNA helix gets opened then the RNA polymerase an enzyme that builds RNA will come along and it follows the standard base pairing rules of wherever you see a guanine you put a cytosine wherever you see a cytosine you put a guanine. It does get a little bit tricky when you're talking about a's and t's because RNA does not use t thymine it instead uses uracil so if I was an RNA polymerase now it's coming on here, and I see the DNA sequence here where I suppose say see an a I'll put u uracil, where I see a g I put a c, where I see a t I put an a, c I put a g, c I put a g, a I put a u, a I put a u, t I put an a, c I put a g, g I put a c see pretty simple.

Now the last thing that will happen is once the RNA sequence is been put together, before it leaves the nucleus if you're talking about eukaryote, it needs to undergo some editing in eukaryotic DNA, yours and mine and plants and fungi you'll have these things called introns or intervening regions of DNA that does not get used to build the protein instead that's get edited out leaving just the sequences that will be expressed, the expressed regions otherwise known as exons so you clip out the introns express together the exons and then finally you put on something called the 5 prime cap, don't worry about that it just protects it from enzymes, and then on the tail end of it you pull it on a long sequence of adenines otherwise known as a poly-A-tail.

Now let me go over this whole process again by taking a look at this youtube video so I'll go ahead and we'll go to youtube and we'll go ahead and make this nice and big, so here we see the DNA and the enzyme is going along, it's landed on the promoter and saying aha! 20 to 30 odd bases along I'll sure find the gene and then it finds the gene and it start to unwind it open up the helix. Now we only need the code be of one side of the DNA double helix that's called the coding strand and so here's a raw RNA nucleotide shtoonk shtoonk shtoonk the RNA polymerase is now joining all these RNA nucleotides together to form a messenger RNA molecule and it keeps on going along shtoonk shtoonk shtoonk shtoonk shtoonk adding more and more and more and you can see in the background a few others being made. This is the non co- coding strand so it doesn't get copied so we're just making up for a messenger RNA strand here using the RNA polymerase and the RNA polymerase just keeps going until it reaches a sequence of a's, t's, c's and g's that indicate you should end sometimes called a termination sequence and we're done so we'll go ahead and stop it here and we can see that's a messenger RNA it can now leave the cell and go after the ribosomes to end protein synthesis through a process known as translation.