What Is Transcription? (Biology) — Definition & Process

What Is Transcription?

The word transcribe means to copy or write down information. Transcription is the process of literally transcribing the genetic information stored in DNA into RNA. It's the first step in protein synthesis. After DNA gets transcribed into messenger RNA (mRNA), the RNA gets translated into protein.

Here's an analogy: Imagine you're studying by writing down notes from a textbook. You don't copy the information word for word (because that would be boring). Instead, you take down a version of the information in your own words.

In this example, the textbook would be DNA, your notes would be RNA, and you would be RNA polymerase. The overall process of taking notes would be like transcription.

What Happens When DNA Is Transcribed?

During this process, a strand of RNA that is complementary to one of the original DNA strands is created. This strand of mRNA goes on to get translated into a protein.

DNA and RNA are mostly similar; both are nucleic acids, but DNA is double-stranded while RNA is single-stranded. The other major difference is that DNA uses cytosine, guanine, adenine, and thymine. But RNA uses uracil instead of thymine.

Think of RNA as a disposable copy of instructions. On the other hand, DNA is the original copy that needs to be kept in good condition.

The Start of Gene Expression

DNA holds the information that our cells need to make us who we are. But DNA stores information in a code that the cells cannot use right away. First, the DNA has to be transcribed and then translated.

To express a certain trait, the cell has to start with transcription. A special enzyme has to move along a segment of DNA within the nucleus and "read" the genetic code. It then makes a version of the code that can leave the nucleus to be translated.

That special enzyme is RNA polymerase. Before the real work can begin though, the DNA helix comes undone and exposes the gene that's going to be transcribed. At the start of the gene, there is a sequence of base pairs called the promoter. The RNA polymerase and a bunch of transcription factors bind to the promoter on one of the DNA strands.

As RNA polymerase moves down the strand of DNA, it assembles a complementary strand of RNA. Remember that DNA and RNA are very similar and can store information in nearly identical ways. When the RNA polymerase is done transcribing, the final product is called messenger RNA, or mRNA. This molecule leaves the nucleus and carries the genetic information to the cytoplasm. There it can be translated into proteins.

This is a general overview of the process. In reality, it is much more complex.

The Start of Protein Synthesis

Transcription is how complementary RNA molecules are produced by copying part of the DNA sequence. We're going to need some background information before we're able to really understand this.

RNA is very similar to DNA. It consists of a long chain of nucleotides, but instead of two strands, RNA is made of a single strand. Also, DNA has adenine, cytosine, guanine, and thymine nucleobases. But RNA has adenine, cytosine, guanine and uracil. The sugar deoxyribose is replaced by the sugar ribose. RNA is like a copy of DNA that can be thrown away; it's a blueprint.

There are three types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). Each type serves a different purpose.

• Messenger RNA (mRNA) is used as, well, a messenger. It carries the genetic instructions from DNA to the rest of the cell.
• Ribosomal RNA (rRNA) exists in ribosomes along with amino acids.
• Transfer RNA (tRNA) transfers each amino acid to the ribosome as instructed by the mRNA.

So, when DNA is transcribed, an enzyme called RNA polymerase separates the two strands of DNA. It then uses one of these strands as a blueprint for a new strand of RNA.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Transcription}}$ : How complementary RNA molecules are produced by copying part of the DNA sequence

$\color{#8925AE}{\text{RNA Polymerase}}$ : The enzyme that separates the strands of DNA during transcription and makes a complementary RNA strand

$\color{#8925AE}{\text{Introns}}$ : Parts of the DNA sequence that are not used to carry genetic information

$\color{#8925AE}{\text{Exons}}$ : The DNA sequences that code for proteins

The Steps of Transcription

Transcription is the process of creating a strand of mRNA (messenger RNA) from a DNA template. Like many other processes in biology, it can be broken down into three simple steps:

Initiation:

The first step is for an enzyme to bind to the promoter region of DNA. Next, RNA polymerase II will begin to separate the strands and add RNA nucleotides complementary to the template strand, creating a 5' to 3' mRNA strand.

Elongation:

In the elongation phase, RNA polymerase II moves down the DNA strands while still synthesizing the new mRNA strand. At this time the enzyme may leave the promoter, but RNA polymerase continues to move down the DNA.

Termination:

Once a certain termination sequence has been transcribed into the mRNA strand, the whole process is halted through a few different methods. Some of these methods include the formation of a hairpin loop or the binding of another enzyme necessary for termination.

Once the DNA has been transcribed, we are left with a complementary mRNA strand that is necessary for other biological processes.

(Video) DNA, Hot Pockets, and the Longest Word Ever

by CrashCourse

In protein synthesis, we go from DNA to RNA, and then RNA to protein. The first step of this process is called transcription. This is when the genetic information found in DNA is copied over into another nucleic acid, RNA.

Check out this video by Crash Course to see how RNA polymerase moves along a DNA template strand to create mRNA. The mRNA molecule will then be used in the process of translation.