Transcription: Microbiology Genetics Pre-Nursing, Pre-Med & Health Field Careerd


Hi, I’m Kathy with Level Up RN! In this article, we’re going to break down the process of transcription—an essential topic in microbiology. At the end of this lesson, I’ll quiz you on key points to test your understanding, so be sure to stick around for that!


If you have our Level Up RN Microbiology Flashcards, pull them out now to follow along. Focus on the bold red text on the back of the cards—these are the high-yield facts you’re most likely to see on your exams.

What is Transcription?

Transcription is the process by which messenger RNA (mRNA) is synthesized from a DNA template. Think of DNA as the original blueprint and mRNA as a photocopy of a specific section of that blueprint. This "photocopy" carries the genetic instructions needed to make a protein.

Here’s a simplified breakdown of the process:

  1. DNA provides the instructions.
  2. mRNA is created by copying these instructions.
  3. The mRNA is then used to build proteins through a process called translation (which we’ll cover in a separate video).

But before we jump into translation, let’s take a deep dive into the steps of transcription, focusing first on prokaryotic cells and then comparing it to eukaryotic cells.

Steps of Transcription in Prokaryotic Cells

Transcription in prokaryotic cells happens in three main stages: Initiation, Elongation, and Termination.

1. Initiation: Starting the Process

  • Transcription begins at a specific region on the DNA strand called the promoter site.
  • The enzyme RNA polymerase recognizes and binds to the promoter. This is like turning on the engine of a car—you can’t go anywhere until the enzyme latches on to the correct starting point.
  • Once attached, RNA polymerase unwinds the DNA strand slightly, exposing the template strand for transcription.

2. Elongation: Building the mRNA Strand

  • After initiation, RNA polymerase moves along the DNA strand from the 5' to 3' direction (pronounced "five prime to three prime").
  • As it moves, it adds nucleotides to build a complementary mRNA strand. The nucleotides are matched based on the base-pairing rules:
    • Guanine (G) pairs with Cytosine (C)
    • Adenine (A) pairs with Uracil (U) in RNA (remember, in DNA, Adenine pairs with Thymine, but Thymine is replaced by Uracil in RNA).

Here’s an example to make this clearer:

  • If the DNA sequence is G-C-A-T, the mRNA strand will be C-G-U-A.

It’s important to note that RNA polymerase can only add nucleotides in one direction—the 5' to 3' direction. This is a key detail to remember for your exams!

3. Termination: Ending the Process

  • Transcription ends when RNA polymerase reaches a specific sequence on the DNA called the terminator site.
  • Once the terminator is reached, RNA polymerase detaches from the DNA, and the newly formed mRNA molecule is released.
  • This mRNA strand is now ready to be used in the next step of protein synthesis: translation.

Key Differences Between Transcription in Prokaryotic and Eukaryotic Cells

Now that we’ve covered the basics of transcription in prokaryotic cells, let’s explore how this process differs in eukaryotic cells.

1. Location of Transcription

  • In prokaryotic cells, transcription happens directly in the cytoplasm because these cells don’t have a nucleus.
  • In eukaryotic cells, transcription takes place inside the nucleus. Once the mRNA is synthesized, it must travel from the nucleus to the cytoplasm to undergo translation.

2. mRNA Processing in Eukaryotic Cells

Unlike prokaryotic cells, eukaryotic cells require the mRNA to go through several processing steps before it’s ready to be translated. Let’s break down these steps:

mRNA Processing Steps in Eukaryotic Cells

A. Addition of the 5' Cap

  • As soon as transcription begins, a special nucleotide called the 5' cap is added to the 5' end of the mRNA strand.
  • Purpose of the 5' Cap:
    • Protects the mRNA from being degraded by enzymes.
    • Helps the ribosome recognize and bind to the mRNA during translation.

B. Addition of the Poly-A Tail

  • After transcription ends, a string of adenine nucleotides, known as the poly-A tail, is added to the 3' end of the mRNA.
  • Purpose of the Poly-A Tail:
    • Protects the mRNA from degradation.
    • Assists in the export of the mRNA from the nucleus to the cytoplasm.
    • Signals that the mRNA is ready for translation.

C. RNA Splicing: Removing Introns

  • The initial mRNA strand, called the primary transcript, contains both exons and introns:

    • Exons are the coding sequences that will be used to make proteins.
    • Introns are non-coding sequences that need to be removed.

  • During RNA splicing, a complex called the spliceosome removes the introns and joins the exons together.

    • Hint: Remember that exons are expressed (they stay in the final mRNA), while introns are excised (they are cut out).

D. Alternative Splicing: One Gene, Many Proteins

  • In some cases, the exons can be joined together in different combinations through a process called alternative splicing.
  • Why is this important?
    • It allows a single gene to produce multiple different proteins, increasing the diversity of proteins that a cell can make without needing extra genes.

Quiz Time! Test Your Knowledge

Let’s see how much you’ve learned! Here are four quick questions to reinforce key concepts:

  1. Where does transcription begin on the DNA strand?
    Answer: At the promoter site.

  2. In which direction are new nucleotides added to the mRNA strand?
    Answer: In the 5' to 3' direction.

  3. What is the name of the site on the DNA where transcription stops?
    Answer: The terminator site.

  4. During RNA splicing, which sequences are removed from the mRNA: introns or exons?
    Answer: Introns are removed.

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