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Gene transcription, RNA processing in eukaryotes, and gene expression
Learn step-by-step with practice exercises built right in.
DNA → RNA → Protein
Transcription: DNA → RNA (this topic) Translation: RNA → Protein (next topic)
Purpose: Synthesize RNA from DNA template
Key enzyme: RNA polymerase
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| RNA polymerase | One type | Three types (I, II, III) |
| Promoter | -10, -35 boxes | TATA box, others |
| Processing | None | Extensive |
| Location | Cytoplasm | Nucleus |
| Coupling | Transcription + translation | Separated |
Primary transcript (pre-mRNA) must be processed before translation
Alternative splicing:
Gene organization:
A gene has the following DNA template strand: 3'-TACGCAATGCGA-5'. (a) Write the mRNA sequence transcribed from this template, (b) identify the start and stop codons, and (c) write the amino acid sequence that would be translated (use the genetic code).
Given: Template strand: 3'-TACGCAATGCGA-5'
(a) mRNA sequence:
Transcription rules:
Step-by-step:
Template (3'→5'): 3'- T A C G C A A T G C G A -5'
↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
mRNA (5'→3'): 5'- A U G C G U U A C G C U -3'
(b) Start and stop codons:
Start codon: AUG
Looking for stop codons:
Note: This appears to be partial gene sequence. A real gene would have:
(c) Amino acid sequence:
Translation using genetic code:
Divide mRNA into codons (3-nucleotide groups):
mRNA: 5'- AUG CGU UAC GCU -3'
Codons: ↓ ↓ ↓ ↓
Using genetic code table:
| Codon | Amino Acid | Abbreviation |
|---|---|---|
| AUG | Methionine | Met (M) |
| CGU | Arginine | Arg (R) |
| UAC | Tyrosine | Tyr (Y) |
| GCU | Alanine | Ala (A) |
Polypeptide:
Or using single-letter code: MRYA
Complete picture:
DNA coding strand: 5'-ATGCGTTACGCT-3' (not given, but complementary to template)
DNA template strand: 3'-TACGCAATGCGA-5' (given)
↓ Transcription
mRNA: 5'-AUGCGUUACGCU-3'
↓ Translation
Polypeptide: Met-Arg-Tyr-Ala (N-terminus → C-terminus)
Key Concepts:
Genetic Code Properties:
Reading frame:
Example if we shift by +1:
Why AUG is special:
| Section | Format | Questions | Time | Weight | Calculator |
|---|---|---|---|---|---|
| Multiple Choice | MCQ | 60 | 90 min | 50% | 🚫 |
| Free Response (Long) | FRQ | 2 | 50 min | 30% | 🚫 |
| Free Response (Short) | FRQ | 4 | 40 min | 20% | 🚫 |
Avoid these 3 frequent errors
See how this math is used in the real world