4.2.1Additional lessons
Transcription in Protein Synthesis — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 102 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Stages of Transcription
Transcription is the first stage of gene expression, making an RNA copy of a gene.
1
In the nucleus, the DNA double helix unwinds at the gene and the hydrogen bonds between base pairs break.
2
One strand, the template strand, is exposed and free RNA nucleotides pair with its bases by hydrogen bonds.
3
RNA polymerase joins the RNA nucleotides by covalent bonds, building the sugar-phosphate backbone of the mRNA.
4
When the gene has been copied, the mRNA separates from the DNA and the DNA double helix re-forms.
5
The mRNA leaves the nucleus through a nuclear pore and travels to a ribosome in the cytoplasm.
| DNA template | A | T | G | C |
|---|---|---|---|---|
| mRNA base | U | A | C | G |
Part 2 of 5Key Features
Several terms describe the molecules and strands involved and are often confused.
Template strand
- Template strand The DNA strand that is read by RNA polymerase and is complementary to the mRNA that is made.
Coding strand
- Coding strand The other strand, whose base sequence matches the mRNA apart from thymine being replaced by uracil.
RNA polymerase
- RNA polymerase The enzyme that links RNA nucleotides together;
- DNA polymerase, in contrast, is used in DNA replication.
mRNA
- mRNA A single-stranded molecule that carries the genetic message from the nucleus to the ribosome.
Part 3 of 5Transcription and Gene Expression
Transcription and Gene Expression
Selective expression
- Selective expression Cells transcribe only the genes they need, so not all of the roughly 20,000 protein-coding genes are active in every cell.
Switching genes
- Switching genes Controlling whether a gene is transcribed is a key way to switch it on or off.
DNA stability
- DNA stability Hydrogen bonds and strong phosphodiester bonds make DNA stable, so it remains a reliable template over many cell divisions.
Why mRNA
- Why mRNA DNA is too large to pass through nuclear pores, so a smaller mRNA copy carries the message out.
Part 4 of 5Worked Example
Question
- The template strand of a gene reads TAC GGA AGA CTT GGG.
- Write the mRNA sequence.
- Pair each template baseT with A, A with U, C with G, G with C.
- TAC becomes AUG and GGA becomes CCU.
- AGA becomes UCU, CTT becomes GAA and GGG becomes CCC.
Answer The mRNA is AUG CCU UCU GAA CCC.
Part 5 of 5Exam Checklist & Tips
4.2.1 – Key Points for the Exam
- Transcription The synthesis of mRNA using a DNA template strand.
- RNA polymerase The enzyme that builds an mRNA strand from RNA nucleotides.
- Template strand The DNA strand that is transcribed.
- mRNA Messenger RNA, carrying the code for a polypeptide to the ribosome.
- Gene expression The use of a gene's information to make a product such as a protein.
Common Mistakes
- Mixing up RNA polymerase with DNA polymerase.
- Writing thymine in mRNA; RNA contains uracil, which pairs with adenine.
- Saying the mRNA copies the coding strand base by base; it is complementary to the template strand.
- Placing transcription in the cytoplasm of eukaryotes; it takes place in the nucleus.
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