AP Biology 6.3 Transcription and RNA Processing Study Notes - New Syllabus Effective 2025
AP Biology 6.3 Transcription and RNA Processing Study Notes- New syllabus
AP Biology 6.3 Transcription and RNA Processing Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the mechanisms by which genetic information flows from DNA to RNA to protein.
Key Concepts:
- Transcription and RNA Processing
6.3.A How Genetic Information Flows from DNA → RNA → Protein
🧠 Big Idea:
Genetic information stored in DNA is converted into RNA through transcription and then translated into proteins during translation. This flow is called the Central Dogma of Molecular Biology.
1. The Central Dogma of Biology
This is the basic pathway of gene expression.
- DNA (deoxyribonucleic acid) stores genetic instructions.
- RNA (ribonucleic acid) acts as the messenger that carries the instructions.
- Proteins are built from those instructions and do the actual work inside the cell (enzymes, hormones, structures, etc.).
2. Transcription – Making RNA from DNA
🧬 Occurs in the nucleus (in eukaryotes)
Steps:
- Helicase unzips the DNA double helix.
- RNA Polymerase binds to a gene’s promoter and starts building a complementary RNA strand from one of the DNA strands (called the template strand).
- mRNA (messenger RNA) is synthesized in the 5′ to 3′ direction using base-pair rules:
- A ↔ U (uracil replaces thymine in RNA)
- C ↔ G
📝 Result: A single-stranded mRNA molecule that carries the code for making a protein.
3. Translation – Making Protein from mRNA
🌍 Happens in the cytoplasm at the ribosome
It involved:
- mRNA: carries the genetic code (codons)
- tRNA (transfer RNA): brings the correct amino acids
- Ribosome: reads the mRNA and builds the protein
Steps:
- Ribosome reads mRNA three bases at a time (these triplets are called codons).
- Each codon codes for one amino acid.
- tRNA has anticodons that match mRNA codons and carry specific amino acids.
- Ribosome links the amino acids together in the correct order → forming a polypeptide chain.
🎯 Result: A protein with a unique shape and function.
🔍 Why This Matters:
- The sequence of bases in DNA decides the order of amino acids in proteins.
- Mutations in DNA can change the mRNA → this may change the protein → possibly change the function!
6.3.A.1 How RNA Structure & Sequence Determine Its Function
📌 RNA is not just a messenger it comes in different forms, and each has its own shape, structure, and job. The order of bases (A, U, C, G) and the folding/shape of the RNA strand decide how it functions inside a cell.
1. mRNA (Messenger RNA) – The Genetic Courier
📍 Function: Delivers the genetic code from the DNA in the nucleus to the ribosomes in the cytoplasm, where proteins are made.
🧾 Key Points:
- mRNA is a linear, single-stranded molecule.
- Its sequence of bases is organized into codons (3-base sequences).
- Each codon = one amino acid.
- Acts like a “recipe” for building proteins.
2. tRNA (Transfer RNA) – The Amino Acid Deliverer
📍 Function: Matches the correct amino acid to each mRNA codon during protein synthesis (translation).
🧾 Structure and Features:
- tRNA has a cloverleaf shape due to internal base-pairing.
- On one end: anticodon – 3 bases that pair with the codon on mRNA.
- On the other end: an amino acid attachment site.
🎯 Each type of tRNA carries only one specific amino acid.
🔁 During translation:
- tRNA binds to mRNA by matching codon ↔ anticodon.
- Brings its amino acid to the ribosome to build the growing polypeptide chain.
3. rRNA (Ribosomal RNA) – The Protein Factory Base
📍 Function: Makes up the structure of the ribosome, where proteins are built.
🧾 Key Features:
- Combines with proteins to form the large and small subunits of the ribosome.
- rRNA ensures that mRNA and tRNA are properly aligned.
- Also catalyzes the formation of peptide bonds between amino acids (acts like an enzyme!).
🧠 Summary Chart:
| RNA Type | Structure | Function |
|---|---|---|
| mRNA | Linear | Carries genetic code from DNA to ribosome |
| tRNA | Clover-shaped | Delivers amino acids based on mRNA codons |
| rRNA | Complex folded shape | Builds the ribosome and catalyzes protein synthesis |
📌 Takeaway:
RNA is not just a messenger—its sequence + shape = function. Different RNAs work together to read the DNA code and turn it into functional proteins.
6.3.A.2 Transcription: How RNA is Made from DNA
📌 What is Transcription?
Transcription is the process where a single strand of DNA is used as a template to build a complementary RNA strand. It’s like copying the instructions from a DNA “blueprint” into an RNA “message” that can be used outside the nucleus.
🛠️ Key Player: RNA Polymerase
- RNA polymerase is the enzyme that makes it all happen.
- It reads the DNA template strand (also called the antisense strand).
- It then adds complementary RNA nucleotides to build the RNA strand.
- A pairs with U (RNA uses uracil instead of thymine)
- T pairs with A
- C pairs with G
- G pairs with C
⚙️ How Transcription Happens – Step by Step:
Initiation
- RNA polymerase binds to a specific region on DNA called the promoter.
- This tells it where to start making RNA.
Elongation
- RNA polymerase moves along the DNA strand.
- It builds a single-stranded RNA molecule by adding RNA bases in the 5′ → 3′ direction (meaning new nucleotides are added to the 3’ end of the RNA).
Termination
- Once RNA polymerase reaches a termination signal on the DNA, it stops.
- The completed RNA strand is released and ready for its next role (mRNA → translation, or rRNA/tRNA → function).
🔍 Important Notes:
- Only one DNA strand is used during transcription—the template strand.
- The other strand (coding strand or sense strand) is not used, but its sequence matches the RNA (except that T is replaced by U).
- The new RNA strand is complementary to the template DNA strand.
📘 Conclusion:
Transcription is the first step in gene expression—it’s how the instructions in DNA are turned into RNA, setting the stage for protein synthesis or other cellular functions.
6.3.A.3 Direction of Transcription by RNA Polymerase
📌 What Does RNA Polymerase Do?
RNA polymerase is the key enzyme that builds the mRNA strand during transcription. It knows exactly which direction to read and which direction to build, which is crucial for making the correct RNA copy of a gene.
🔁 Direction Matters
| 🧬 Process | 🔍 Direction |
|---|---|
| Reading DNA (template strand) | 3′ → 5′ |
| Synthesizing mRNA | 5′ → 3′ |
RNA polymerase reads the DNA template strand from the 3′ (three-prime) end to the 5′ (five-prime) end.
It then adds RNA nucleotides in the 5′ to 3′ direction, meaning the new RNA strand grows at the 3′ end.
🧠 Why This Direction?
This directionality ensures that:
- The correct RNA sequence is made,
- The mRNA is properly oriented for translation later on,
- And it matches (almost exactly) the non-template (coding) DNA strand, except:
- Uracil (U) is used instead of Thymine (T).
🧬 Key Terminology:
| Term | Meaning |
|---|---|
| Template Strand | The DNA strand RNA polymerase reads (3′ → 5′) |
| Coding Strand | The unused DNA strand; matches the RNA (except U vs T) |
| mRNA | The RNA transcript created (5′ → 3′ direction) |
| RNA Polymerase | The enzyme that does all the work |
📘 Summary:
- RNA polymerase reads the DNA template strand in the 3′ to 5′ direction
- It builds the new RNA strand in the 5′ to 3′ direction
- This direction is essential for accurate gene expression and for RNA to be used correctly in translation later.
6.3.A.4 mRNA Processing in Eukaryotic Cells
🧾 What Happens After Transcription?
In eukaryotic cells, once mRNA is transcribed, it isn’t ready to be translated just yet! It first goes through important modifications to become a mature mRNA that can leave the nucleus and be translated into protein.
🛠️ Key mRNA Modifications (Post-Transcriptional Processing):
| Step | What Happens | Why It’s Important |
|---|---|---|
| GTP Cap (5′ Cap) | A modified guanine (GTP) is added to the 5′ end of the mRNA | Helps ribosomes recognize and bind to the mRNA for translation |
| Poly-A Tail | A chain of adenines (A’s) is added to the 3′ end of the mRNA | Increases mRNA stability and prevents breakdown in the cytoplasm |
| RNA Splicing | Non-coding regions (introns) are cut out, and coding regions (exons) are joined together | Removes “junk” sequences and creates a functional mRNA for protein synthesis |
🔄 Alternative Splicing: One Gene, Many Proteins!
- Exons can be included or skipped in different combinations.
- This process allows one gene to produce multiple protein versions.
- Helps increase protein diversity without needing more genes!
🧪 Example:
One gene in humans can lead to dozens of different proteins depending on how the mRNA is spliced!
💡 Summary:
- mRNA is not ready immediately after transcription.
- It gets a GTP cap (for ribosome binding), a poly-A tail (for stability), and splicing (to remove introns).
- Alternative splicing allows cells to make different proteins from the same gene – key to complexity in multicellular organisms!