YouTube Video — Transcript

Now that we've covered DNA replication, let's talk about transcription.

The first thing you need to know is that transcription has nothing to do with cell replication processes such as DNA replication, mitosis, or cell division.

So, what is transcription and why is it necessary?

Well, transcription is the first step in the process of using the genetic code in DNA to synthesize or build all the different proteins in your body.

One problem with synthesizing these proteins is that the instructions for making them are in the DNA, which is located inside the nucleus.

But, the place proteins are always made is outside the nucleus, either in the ribosomes floating in the cytoplasm or in the ribosomes embedded in the rough endoplasmic reticulum.

So, how does the genetic code for synthesizing proteins get from the DNA to the ribosome?

DNA uses a messenger called messenger RNA or mRNA to carry the genetic code from the nucleus to the ribosome.

The process of building this messenger RNA is called transcription.

Now, let's see how transcription happens.

Transcription begins when an enzyme called RNA polymerase attaches to a segment of DNA called a gene. A gene contains the code to build a specific protein, which is a macromolecule made up of a sequence of amino acids in a specific order.

And within a gene, the specific order of nitrogenous bases dictates the order of amino acids that will make up the protein. Each group of three consecutive bases in the gene is actually a code for a particular amino acid.

As a result, each group is referred to as a codon.

RNA polymerase causes a particular area of the DNA helix to unwind and separate into two strands. One of the strands, often called the template strand, is the side of DNA that is read or transcribed by the messenger RNA. The other strand of DNA, often called the non-template strand, isn't transcribed by the messenger RNA.

So, how are DNA instructions transcribed into messenger RNA? Well, using the template strand as a guide, RNA polymerase uses the base pair rule to assemble free nucleotides in the nucleus into a complementary strand of RNA.

For example, RNA polymerase reads the DNA base thymine on the template strand,

then binds it to a free nucleotide containing adenine.

This process continues with cytosine binding to guanine and guanine binding with cytosine.

Remember though, RNA will never contain thymine.

So, whenever RNA polymerase sees adenine on the DNA template strand, it pairs adenine with uracil.

By using the template strand of DNA as a guide, the genetic code from the non-template strand of DNA has actually been transcribed into messenger RNA.

When transcription is complete, the messenger RNA, which is small enough to fit through a nuclear pore, takes the genetic code out of the nucleus to the ribosome, the site of protein synthesis.

The process of actually building the protein at the ribosome is called translation, which we'll cover in a separate video.

To summarize, transcription is the process of transcribing or copying the genetic code for building a protein into messenger RNA.

A gene is a segment of DNA containing the instructions or code for building a protein.

A codon is a group of three consecutive nitrogenous bases in a gene containing the code for a specific amino acid in a protein.

RNA polymerase unwinds the strands of DNA in a gene.

The template DNA strand contains the complementary bases that need to be read to generate messenger RNA.

The base pair rule is followed when assembling messenger RNA.

Messenger RNA is actually a copy of the DNA non-template strand with uracil substituted for thymine.