AP Biology 6.1 DNA and RNA Structure Study Notes - New Syllabus Effective 2025
AP Biology 5.5 Environmental Effects on Phenotype Study Notes- New syllabus
AP Biology 5.5 Environmental Effects on Phenotype Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the structures involved in passing hereditary information from one generation to the next.
Key Concepts:
- Genetic Information: DNA & RNA
- DNA Replication
- Transcription & RNA Processing
- Translation
- Genetic Information In Retroviruses
6.1.A – Describe the Structures Involved in Passing Hereditary Information from One Generation to the Next
🧬 What Does Heredity Mean?
Heredity is the passing of genetic information (DNA) from parents to offspring. This info determines traits like eye color, blood type, or the ability to roll your tongue.
🧠 Key Structures Involved in Heredity
1. DNA (Deoxyribonucleic Acid)
- Main molecule that carries genetic information.
- Double-helix structure (looks like a twisted ladder).
- Made of repeating units called nucleotides, each with:
- A phosphate group
- A sugar (deoxyribose)
- A nitrogenous base (A, T, G, C)
- Bases pair specifically:
- A-T (Adenine–Thymine)
- G-C (Guanine–Cytosine)
- DNA is like a biological instruction manual for making proteins.
2. Genes
- Segments of DNA that code for specific proteins.
- Each gene has the info for one trait.
- Located at specific spots (loci) on a chromosome.
🧠 Example: A gene for eye color may tell the body to make a pigment-producing protein.
3. Chromosomes
- Long, tightly coiled strands of DNA + proteins (histones).
- Found in the nucleus of eukaryotic cells.
- Humans have 46 chromosomes (23 pairs) in each body cell.
- One set from mother, one from father
🔑 During reproduction, only gametes (egg & sperm) carry 23 chromosomes, so the offspring gets the full 46 again.
4. Nucleus
- The control center of the cell.
- Houses the DNA.
- Ensures accurate copying and distribution of genetic info during cell division.
5. RNA (Ribonucleic Acid)
- Temporary copy of DNA used to make proteins.
- DNA → RNA → Protein = Central Dogma of Biology
- RNA is single-stranded, with Uracil (U) replacing Thymine (T).
Types of RNA:
- mRNA (messenger RNA) – Carries gene instructions out of nucleus
- tRNA (transfer RNA) – Brings amino acids during protein building
- rRNA (ribosomal RNA) – Part of ribosome, helps make proteins
6. Gametes (Sex Cells)
- Eggs (♀️) and sperm (♂️)
- Created through meiosis, a special kind of cell division
- Carry half the DNA (haploid) so the zygote gets full DNA after fertilization
7. Zygote
- Formed when egg and sperm fuse.
- Now diploid (full 46 chromosomes).
- This cell divides and grows into a full organism using the inherited DNA
🧠 Recap – Who’s Involved in Heredity?
| Structure | Role |
|---|---|
| DNA | Stores genetic info |
| Genes | Code for traits/proteins |
| Chromosomes | DNA-packed structures |
| Nucleus | Cell’s DNA hub |
| RNA | Helps use DNA to make proteins |
| Gametes | Pass DNA to offspring |
| Zygote | First cell of a new organism |
6.1.A.1 – How Genetic Information Is Stored & Passed Through DNA (and RNA)
📌 Big Idea:
Genetic information that determines all traits of living organisms is stored in DNA and in some viruses, it’s even stored in RNA. This info is what gets passed from generation to generation during reproduction.
🔁 DNA: The Molecule of Heredity
- DNA (deoxyribonucleic acid) is the main molecule that carries heritable information.
- It contains the instructions for building proteins, which control traits and cellular functions.
- In some viruses, RNA takes over this role instead of DNA.
🧫 Types of Organisms & Their Chromosomes:
1. Prokaryotic Organisms (like bacteria)
- Have circular chromosomes (imagine a rubber band shape).
- Usually, one single circular DNA molecule floating in the cytoplasm (since they don’t have a nucleus).
- May also have plasmids — small, extra DNA circles that carry special traits (like antibiotic resistance).
2. Eukaryotic Organisms (like plants, animals, fungi)
- Have multiple linear chromosomes (like straight rods).
- These chromosomes are found in the nucleus.
- Made of DNA + proteins (mainly histones).
- DNA wraps around histones like thread around a spool → this helps organize & compact the long DNA.
- These DNA-histone bundles are called chromatin when loosely packed and chromosomes when fully condensed (during cell division).
📚 Summary Chart:
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Chromosome Shape | Circular | Linear |
| Number of Chromosomes | Usually 1 | Multiple |
| Location | Cytoplasm | Nucleus |
| DNA Packing Proteins | None | Uses histones |
| Extra DNA? | Yes, plasmids | Sometimes (e.g., mitochondria/chloroplasts) |
💡 Bonus Tip:
Histones are like storage organizers. They keep DNA from getting tangled and help control which genes are active or silent.
6.1.A.2 – Plasmids: Extra DNA Circles in Cells
📌 What Are Plasmids?
Plasmids are small, circular pieces of DNA that exist outside the main chromosomes in a cell.
Think of them as bonus DNA packets separate from the cell’s main genetic material.
🧫 Where Do We Find Plasmids?
| 🌍 Organism Type | 🔍 Plasmids Present? | 📍 Location |
|---|---|---|
| Prokaryotes (like bacteria) | Very common | In the cytoplasm |
| Eukaryotes (like yeast) | Can be present | In the nucleus or organelles (rare) |
🔄 Key Features of Plasmids:
- Circular DNA molecules (just like a loop of string).
- Carry a small number of genes usually non-essential, but helpful!
- Commonly found in bacteria (prokaryotes).
- Independent replication plasmids can copy themselves separately from chromosomes.
- Often used in biotechnology to transfer genes (e.g., in genetic engineering).
💡 Why Are Plasmids Important?
- Gene sharing: Bacteria can pass plasmids to each other in a process called conjugation, helping them spread traits quickly (like antibiotic resistance).
- Survival advantage: Plasmids often carry genes that help bacteria survive tough environments (like toxins or antibiotics).
- In biotech labs, scientists use plasmids to insert foreign genes into organisms (e.g., making insulin-producing bacteria).
🧠 Example:
A bacterium might have:
- 1 circular chromosome = main DNA (basic survival info)
- 2–3 plasmids = extra features (like resistance to antibiotics)
📌 Summary:
- Plasmids = extra circular DNA, separate from chromosomes.
- Found in both prokaryotes (commonly) and eukaryotes (rarely).
- Carry non-essential but helpful genes.
- Key players in horizontal gene transfer and biotechnology.
6.1.B – What Makes DNA the Perfect Hereditary Material?
🔑 What Is Hereditary Material?
DNA is the instruction manual of life – it carries the genetic code passed from parents to offspring. But why is DNA so good at this job? Let’s break it down 👇
🧩 1. Stable Structure
- DNA is a double helix – two strands wound around each other like a twisted ladder.
- This shape protects the genetic code inside 🧬.
- The sugar-phosphate backbone gives it strength and stability — so the message doesn’t degrade easily over time.
🧬 2. Base Pairing Rules = Accurate Copying
- The bases follow specific pairing:
- A (adenine) pairs with T (thymine)
- C (cytosine) pairs with G (guanine)
- This ensures that when DNA is copied, it makes almost perfect duplicates, which is critical for heredity!
🔁 3. DNA Can Be Replicated
- One of the most amazing things about DNA: it can make copies of itself!
- Each strand serves as a template to build the other — like a zipper being unzipped and re-zipped.
- This allows genetic information to be passed on to new cells and new generations.
✨ 4. Carries a Huge Amount of Info
- The order of bases (A, T, G, C) is like letters in a language.
- These sequences code for genes, which build proteins and control everything in the body.
- Even with just 4 types of bases, DNA stores billions of instructions!
🛠 5. Can Be Modified, but Not Easily Broken
- DNA can mutate (change), which allows evolution and adaptation over time.
- But it’s also protected by repair enzymes and its tightly packed form (like wrapping it in bubble wrap).
6.1.B.1 – Base Pairing in Nucleic Acids: A Key to Genetic Accuracy
🧠 What Are Nucleic Acids?
Nucleic acids (DNA & RNA) are long chains made up of nucleotides, and each nucleotide has three parts:
- A sugar (deoxyribose in DNA, ribose in RNA)
- A phosphate group
- A nitrogenous base (the “letter” of the genetic code: A, T, G, C, or U)
🧬 The Nitrogenous Bases:
There are two main types of bases in nucleic acids:
1. Purines
- Double-ring structures (bigger molecules)
- Includes:
- Adenine (A)
- Guanine (G)
2. Pyrimidines
- Single-ring structures (smaller molecules)
- Includes:
- Cytosine (C)
- Thymine (T) – in DNA
- Uracil (U) – replaces T in RNA
🔗 Base Pairing Rules (Chargaff’s Rules)
| Molecule | Base Pairing |
|---|---|
| DNA | A 🧬 T G 🧬 C |
| RNA | A 🧬 U G 🧬 C |
- A pairs with T (or U in RNA) → 2 hydrogen bonds
- G pairs with C → 3 hydrogen bonds (stronger pair)
⚖️ Why purines always pair with pyrimidines?
To keep the DNA double helix uniform in width – one big base with one small base = perfect fit.
🌍 Conserved Through Evolution
This base-pairing system is conserved across all living organisms – meaning it’s stayed the same through billions of years of evolution 🔁
Why? Because it’s efficient, accurate, and reliable for:
- Copying DNA during cell division
- Passing down genetic info
- Building RNA & proteins correctly