Transcription Lesson : Definition, Stages & Role of RNA Polymerase

Lesson Overview

• What Is Transcription in Biology?
• The Stages of Transcription
• Components in Transcription: RNA Polymerase
• Transcription vs. Translation

Our body is constantly producing RNA through a process called transcription. Understanding the stages of transcription and the role of RNA polymerase is fundamental to comprehending how genetic information is utilized by the cell to produce proteins.

What Is Transcription in Biology?

Transcription is the process by which the information encoded in DNA is copied into a complementary RNA molecule. Specifically, it's the synthesis of an RNA transcript from a DNA template. This process is catalyzed by the enzyme RNA polymerase, which reads the DNA sequence and uses it to build a new RNA strand.

For Instance:

If a gene in DNA has the sequence "TAC," RNA polymerase will create a corresponding "AUG" sequence in the mRNA. This mRNA molecule then carries the genetic information from the DNA in the nucleus to the ribosomes in the cytoplasm, where it will be translated into a protein.

The Stages of Transcription

Transcription proceeds through three main steps of transcription: initiation, elongation, and termination. Each step involves specific molecular events and is tightly regulated.

1. Initiation: 

This is the starting point of transcription. It begins with the binding of RNA polymerase to a specific DNA sequence called the promoter. The promoter region signals to RNA polymerase where to begin transcription and which strand of DNA to use as a template. Promoters contain specific sequences, like the TATA box (in eukaryotes) or the Pribnow box (in prokaryotes), that are recognized by RNA polymerase or associated transcription factors.

Fig 1. It shows the initiation stage of transcription, showing RNA polymerase binding to the promoter on DNA.

2. Elongation: 

This is the stage where the RNA transcript is synthesized. RNA polymerase moves along the DNA template strand, unwinding the double helix ahead of it and rewinding it behind. As it moves, it uses the template strand to synthesize a complementary RNA molecule. This process is called processive, meaning that RNA polymerase remains bound to the DNA strand throughout elongation, ensuring continuous RNA synthesis.

Fig. 2 RNA polymerase moves along the DNA strand, synthesizing RNA by adding ribonucleotides complementary to the DNA template.

Free ribonucleotides (ATP, GTP, CTP, and UTP) are used as building blocks. RNA polymerase adds these nucleotides to the 3' end of the growing RNA chain, creating a phosphodiester bond. The RNA transcript grows in the 5' to 3' direction, antiparallel to the DNA template strand.

3. Termination: 

This is the final stage of transcription, where the RNA transcript is released from the DNA template. The termination signal varies depending on the organism and the specific gene being transcribed.

Fig 3. The image shows a terminator sequence on the DNA, which signals the end of transcription.

In eukaryotes, termination is often coupled with RNA processing events. These include:

• Capping: The addition of a modified guanine nucleotide to the 5' end of the mRNA.
• Splicing: The removal of non-coding regions (introns) from the pre-mRNA, leaving only the protein-coding regions (exons).
• Polyadenylation: The addition of a string of adenine nucleotides (poly(A) tail) to the 3' end of the mRNA.

These three stages – initiation, elongation, and termination – along with RNA processing in eukaryotes, ensure that the correct genetic information is accurately transcribed from DNA into RNA, setting the stage for protein synthesis.

Components in Transcription: RNA Polymerase

RNA polymerase is the central enzyme responsible for catalyzing the synthesis of RNA from a DNA template during transcription. It's a complex molecular machine with multiple subunits, each playing a specific role. The structure and function of RNA polymerase vary somewhat between bacteria and eukaryotes, but the core catalytic activity is conserved.  

Fig. 4 It shows the process of transcription, where RNA polymerase uses a DNA template to create a complementary RNA molecule.

Bacterial RNA Polymerase:

In bacteria like E. coli, RNA polymerase is composed of a core enzyme and a sigma factor.  

• Core Enzyme: This consists of several subunits (α₂, β, β', ω) and is responsible for the actual polymerization of RNA nucleotides. It binds to the DNA and carries out the catalytic activity of transcription.

• Sigma Factor: This protein is crucial for promoter recognition. It helps the core enzyme bind specifically to promoter regions on the DNA, ensuring that transcription starts at the correct location. Different sigma factors can recognize different promoter sequences, allowing bacteria to regulate the transcription of specific genes under different conditions. 
  
  

Eukaryotic RNA Polymerases:

Eukaryotic cells have three main types of RNA polymerases, each responsible for transcribing different types of RNA:  

Fig. 5 It shows the structures and components of the three main RNA polymerases and the types of RNA they produce.

• RNA Polymerase I: Located in the nucleolus, it transcribes most ribosomal RNA (rRNA) genes, which are essential components of ribosomes. 
  
• RNA Polymerase II: Found in the nucleoplasm, it transcribes messenger RNA (mRNA) precursors, which encode proteins. It also transcribes some small nuclear RNAs (snRNAs) involved in RNA splicing. This is the most extensively studied RNA polymerase.
  
• RNA Polymerase III: Also located in the nucleoplasm, it transcribes transfer RNA (tRNA) genes, which are involved in protein synthesis, as well as some other small RNAs.
  
  

Mechanism of Action:

Regardless of the specific type, all RNA polymerases share a common mechanism of action. They: 

1. Bind to DNA: RNA polymerase recognizes and binds to the promoter region on the DNA template. 
   
2. Unwind DNA: It unwinds a short stretch of the DNA double helix, forming a transcription bubble.
   
3. Initiate RNA Synthesis: Using the DNA template strand, it begins synthesizing a complementary RNA molecule, using ribonucleotide triphosphates (NTPs) as building blocks. 
   
4. Elongate RNA Chain: It moves along the DNA template, continuously adding nucleotides to the 3' end of the growing RNA chain, creating phosphodiester bonds. 
   
5. Terminate Transcription: It encounters a termination signal, stops RNA synthesis, and releases the RNA transcript from the DNA template.
   
   

Transcription vs. Translation

Take This Quiz :

Transcription Quiz: Biology Trivia!

Transcription and Translation Quiz with Answers