Transcription
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of DNA and RNA structure. You can test your knowledge on these below.
What bases are found in DNA?
Adenine, thymine, guanine and cytosine.
What bases are found in RNA?
Adenine, uracil, guanine and cytosine.
How is the structure of DNA different to mRNA?
- DNA is double stranded. mRNA is single stranded.
- DNA forms a double helix structure. mRNA is linear.
- DNA contains deoxyribose sugar. mRNA contains ribose sugar
- DNA contains thymine. mRNA contains uracil instead.
- DNA has hydrogen bonds between complementary base pairs. mRNA does not have complementary base pairs or hydrogen bonds.
- DNA contains triplets. mRNA contains codons.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains transcription or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Transcription
What is Transcription?
Transcription is the first stage of protein synthesis, where the genetic code from DNA is copied into mRNA so it can be used for translation in the ribosome.
Transcription occurs in the nucleus.
Step-by-Step Process of Transcription
DNA Unwinding
- The enzyme DNA helicase breaks hydrogen bonds between complementary DNA bases, separating the strands.
- This exposes the template strand which is used to make mRNA.
- Note, only 1 DNA strand acts as the template strand.
Free RNA Nucleotides Bind to the Template Strand
- Free RNA nucleotides align with complementary bases on the template strand using base pairing rules:
- A → U (uracil, instead of thymine).
- T → A.
- C → G.
- G → C.
RNA Polymerase Joins Nucleotides Together
- The enzyme RNA polymerase moves along the template strand, joining adjacent RNA nucleotides on the new strand via phosphodiester bonds, forming pre-mRNA (in eukaryotes) or mRNA (in prokaryotes).
- The formation of phosphodiester bonds is an example of a condensation reaction and requires ATP.
Splicing (in Eukaryotes Only)
- In eukaryotes, pre-mRNA contains introns (non-coding regions) and exons (coding regions).
- Splicing removes introns, leaving only exons to form mature mRNA.
- This step does not occur in prokaryotes, as their mRNA does not contain introns.
mRNA Leaves the Nucleus
- The mature mRNA exits the nucleus via a nuclear pore and travels to a ribosome in the cytoplasm for translation.
Key Differences Between Prokaryotic & Eukaryotic Transcription
| Feature | Prokaryotes | Eukaryotes |
| Location | Cytoplasm. | Nucleus. |
| Splicing | No splicing needed. | Splicing removes introns. |
| Type of mRNA | mRNA is ready immediately. | Pre-mRNA is formed first. |
Key Terms
- Transcription: The process of making mRNA from DNA in the nucleus.
- RNA Polymeras: The enzyme that joins RNA nucleotides together with phosphodiester bonds to form mRNA.
- Template Strand: The DNA strand that mRNA is complementary to.
- mRNA: A single-stranded RNA molecule that carries the genetic code from DNA to the ribosome.
- Introns: Non-coding sequences in eukaryotic pre-mRNA that are removed by splicing. They do not code for amino acids in the polypeptide.
- Exons: Coding sequences in mRNA that are joined together after splicing. They code for amino acids in the polypeptide.
- Splicing: The process of removing introns and joining exons in eukaryotic mRNA.
No answer provided.
Exam Tips
In transcription questions, always mention:
- The role of RNA polymerase.
- Base pairing rules (A pairs with U, not T!).
- Splicing only happens in eukaryotic cells.
You may also be asked to compare the process of transcription to the process of DNA replication. Look back at these study notes from Module 1 and make sure you can give differences and similarities.
For example, in DNA replication, both strands act as templates and DNA polymerase is used. In transcription only one strand acts as a template and RNA polymerase is used.
No answer provided.
Describe the process of transcription in a eukaryotic cell. (6 marks)
- DNA helicase breaks hydrogen bonds between complementary bases, causing the DNA to unwind. (1)
- One strand acts as a template. (1)
- Free RNA nucleotides align with complementary bases on the template strand, using base pairing rules (A-U, T-A, C-G, G-C). (1)
- RNA polymerase joins the nucleotides together. (1)
- Via phosphodiester bonds (requiring ATP). (1)
- Splicing removes introns, leaving only exons in the final mRNA. (1)
- The mRNA leaves the nucleus via a nuclear pore and moves to a ribosome for translation. (1)
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!