4.2.5Additional lessons
Mechanism of Transcription — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 34 of 101
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 5Key Players
Transcription has distinct phases that are controlled at specific base sequences.
Promoter
- Promoter A DNA sequence upstream of the gene to which RNA polymerase binds so that transcription starts at the right place.
Template strand
- Template strand This strand of DNA is read by RNA polymerase and is also called the antisense strand.
Coding strand
- Coding strand This strand has the same sequence as the mRNA, except that T is replaced by U, and is also called the sense strand.
Terminator
- Terminator A sequence that signals the end of the gene so that the polymerase releases the new RNA.
| Strand | Direction | Relationship to mRNA |
|---|---|---|
| Template (antisense) | Read 3' to 5' | Complementary to mRNA |
| Coding (sense) | 5' to 3' | Same sequence, T for U |
Part 2 of 5Phases of Transcription
1
Initiation: RNA polymerase, helped by transcription factors in eukaryotes, binds to the promoter and unwinds a short length of DNA.
2
The enzyme reads the template strand in the 3' to 5' direction and selects free RNA nucleotides that pair with exposed bases.
3
Elongation: the RNA polymerase joins nucleotides to the 3' end of the growing RNA, so the mRNA is built in the 5' to 3' direction.
4
Behind the enzyme the DNA double helix re-forms, and the mRNA peels away from the template.
5
Termination: on reaching the terminator sequence, RNA polymerase and the new mRNA detach from the DNA.
Part 3 of 5Regulation and Context
Regulation and Context
Transcription factors
- Transcription factors These proteins bind to promoter regions and help or block the binding of RNA polymerase, so they control gene activity.
Energy source
- Energy source The incoming RNA nucleotides are triphosphates, and cleaving two phosphates provides the energy for the bond formed.
Bubble size
- Bubble size Only a short stretch of about ten to twenty base pairs is unwound at any moment, not the whole molecule.
Prokaryote contrast
- Prokaryote contrast In bacteria transcription and translation can occur at the same time because there is no nucleus separating them.
Part 4 of 5Worked Example
Question
- The coding strand of a gene is 5' ATGGCATTC 3'.
- Give the template strand and the mRNA.
- The mRNA matches the coding strand with U for T: 5' AUGGCAUUC 3'.
- The template is complementary and antiparallel to the coding strand: 3' TACCGTAAG 5'.
- Checkpairing template with mRNA gives A-U, T-A, C-G consistently.
Answer
- mRNA 5' AUGGCAUUC 3';
- template 3' TACCGTAAG 5'.
Part 5 of 5Exam Checklist & Tips
4.2.5 – Key Points for the Exam
- Promoter DNA sequence where RNA polymerase attaches to begin transcription.
- Template strand Strand of DNA copied during transcription.
- Coding strand Strand whose base sequence matches the mRNA.
- Terminator Sequence that signals the end of transcription.
- Transcription factor Protein that regulates binding of RNA polymerase to a promoter.
Common Mistakes
- Mixing up the coding and template strands; the mRNA matches the coding strand.
- Stating that RNA polymerase moves 5' to 3' along the template; it reads the template 3' to 5'.
- Saying that helicase is needed in transcription; RNA polymerase itself unwinds the DNA.
- Thinking the whole DNA molecule is unwound, rather than a small region.
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