AP Biology 6.4 Translation Study Notes - New Syllabus Effective 2025
AP Biology 6.4 Translation Study Notes- New syllabus
AP Biology 6.4 Translation Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Explain how the phenotype of an organism is determined by its genotype.
Key Concepts:
- Translation
6.4.A How Genotype Determines Phenotype
🧠 What Is Genotype vs. Phenotype?
| Term | Definition | Example |
|---|---|---|
| Genotype | The genetic code or sequence of DNA in an organism (alleles) | Ex: TT, Tt, or tt for plant height |
| Phenotype | The observable traits or characteristics that result from gene expression | Ex: Tall or short plant |
🧬 DNA → RNA → Protein → Trait
This process is called the Central Dogma of molecular biology:
DNA (gene) → mRNA (transcription) → Protein (translation) → Trait (phenotype)
🧩 How Does Genotype Affect Phenotype?
- Genes are blueprints for proteins.
- Proteins control structure and function in the body — like enzymes, pigments, and structural components.
- Different alleles = different proteins = different traits.
🔁 Types of Alleles and Their Effects:
| Allele Type | Effect on Phenotype |
|---|---|
| Dominant (A) | Shows up in the phenotype if present (even one copy is enough) |
| Recessive (a) | Only shows when both alleles are recessive (aa) |
| Codominant | Both alleles fully show (e.g. AB blood type) |
| Incomplete Dominance | Blended traits (e.g. red + white flower = pink) |
🧬 Example:
Gene for flower color:
- RR → Red flower (homozygous dominant)
- Rr → Red flower (heterozygous)
- rr → White flower (homozygous recessive)
➡ The genotype (RR, Rr, rr) determines which protein (pigment) is made, and that creates the phenotype (color).
🧪 Summary:
- Your DNA (genotype) holds instructions to make proteins.
- These proteins are responsible for traits (phenotypes).
- So, phenotype = the physical expression of what your genes say!
6.4.A.1 – Where Does Translation Happen?
📌 Key Idea: The process of translation turning mRNA into a chain of amino acids (a polypeptide) takes place on ribosomes, the cell’s protein factories! 🧪✨
🧫 Where Exactly Does Translation Occur?
🔹 In Prokaryotes:
- Ribosomes float freely in the cytoplasm.
- No nucleus → transcription and translation happen almost at the same time (coupled!).
🔹 In Eukaryotes:
- Ribosomes are found in two main places:
- Free ribosomes in the cytoplasm → make proteins that stay in the cell.
- Ribosomes on the rough ER (RER) → make proteins for export, membranes, or lysosomes.
🧬 What’s Special About Ribosomes?
- Made of rRNA + proteins
- Have 2 subunits: Large (joins amino acids) and Small (binds mRNA)
- Read mRNA codons 🧾 and match them to tRNA anticodons to build proteins
🌟 Important Facts:
- Translation always starts in the cytoplasm, even for proteins going to organelles!
- Rough ER = “docked” ribosomes translating proteins for secretion
- Cytoplasmic ribosomes = make housekeeping proteins for internal use
- Prokaryotes don’t have ER → translation occurs right after transcription
🧠 Memory Tip:
Free ribosomes = local delivery 🚚, Rough ER ribosomes = global shipping ✈️
6.4.A.2 – Simultaneous Transcription & Translation in Prokaryotes
📌 Key Idea: In prokaryotic cells, the processes of making RNA (transcription) and building proteins (translation) happen at the same time!⏱️
🧫 Why Can They Happen Together?
- Prokaryotes lack a nucleus → No barrier between DNA & ribosomes
- As soon as mRNA is being made by RNA polymerase, ribosomes jump on it and start making protein right away!
🔄 What’s This Process Called?
Coupled Transcription and Translation → It’s super-efficient and helps bacteria respond quickly to their environment
🧬 What Does It Look Like?
Imagine:
- RNA polymerase is making mRNA from DNA
- Ribosomes attach to the growing mRNA strand and begin translating it into protein right away
You’ll often see multiple ribosomes on the same mRNA = a polyribosome or polysome
🌟 Quick Facts:
- Happens only in prokaryotes, not eukaryotes
- Speeds up gene expression
- Useful for rapid protein production in changing environments
- Eukaryotes can’t do this because their transcription happens in the nucleus, and translation happens in the cytoplasm
🧠 Memory Tip:
No nucleus? No waiting! Transcribe & translate at the same station!
6.4.A.3 – The Process of Translation (Protein Synthesis)
📌 Big Idea: Translation is the process where the cell reads the mRNA and builds a protein. It’s like turning a recipe into a dish!
3 Main Steps of Translation:
1️⃣ Initiation
- Begins when the ribosome binds to mRNA at the start codon: AUG
- AUG codes for methionine (Met) = first amino acid
2️⃣ Elongation
- Ribosome reads mRNA in triplets (called codons)
- Each codon tells the ribosome which amino acid to add next
- tRNA molecules bring the correct amino acids using their anticodons
- Amino acids are joined by peptide bonds to form a growing chain
3️⃣ Termination
- When the ribosome reaches a stop codon (UAA, UAG, UGA), translation ends
- The completed polypeptide (protein) is released and ready to fold/work
🧠 Key Translation Features:
- Codons = groups of 3 nucleotides on mRNA
- Each codon codes for one amino acid
- Some amino acids are coded by multiple codons = genetic code is redundant
- The code is universal = used by almost all organisms → Proof of common ancestry
- tRNA is the “translator” → carries amino acids and matches them to mRNA codons
- Ribosomes = site of translation → made of rRNA + proteins
- No need to memorise all codons — just remember AUG = start
🧬 Summary Table:
| Feature | Description |
|---|---|
| Codon | 3-letter sequence on mRNA |
| Start Codon | AUG (Methionine) → begins translation |
| tRNA | Carries amino acid; has anticodon |
| Peptide Bond | Joins amino acids together |
| Stop Codons | UAA, UAG, UGA → end translation |
| Universal Code | Same code in nearly all organisms |
🧠 Memory Tip:
AUG is GO! UGA, UAA, UAG are NO! (Start & Stop codons 🟢🛑)
🧬 6.4.A.4 – Retroviruses: When RNA → DNA
📌 Big Idea: Retroviruses break the usual DNA → RNA → Protein rule! Instead, they go RNA → DNA using a special enzyme called reverse transcriptase.
🧫 What Are Retroviruses?
- A type of virus that carries its genetic material as RNA, not DNA
- Example: HIV (Human Immunodeficiency Virus)
🔄 Alternate Flow of Information
Normal cells:
DNA → RNA → Protein
Retroviruses:
RNA → DNA → RNA → Protein
⚙️ Key Steps in Retrovirus Infection:
1. Entry: Virus injects RNA genome into the host cell
2. Reverse Transcription:
- Viral enzyme reverse transcriptase makes DNA from RNA
- This breaks the central dogma of molecular biology!
3. Integration:
- The newly made viral DNA is inserted into the host’s genome
- Now it becomes a permanent part of the host’s DNA
4. Transcription & Translation:
- Host cell machinery transcribes and translates viral genes
- Makes viral proteins and RNA genomes for new viruses
5. Assembly:
New virus particles are put together and exit the cell to infect others
🌍 Why Is This Important?
- Retroviruses can hide inside the genome for a long time (latent infection)
- They can disrupt host genes, sometimes causing disease (e.g. cancer)
- Reverse transcriptase is a major drug target in HIV treatment
🧠 Memory Tip:
Retro = Rewind: RNA back to DNA!