AP Biology 4.5 Cell Cycle Study Notes - New Syllabus Effective 2025
AP Biology 4.5 Cell Cycle Study Notes- New syllabus
AP Biology 4.5 Cell Cycle Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the events that occur in the cell cycle.
Key Concepts:
- The Cell Cycle
4.5.A – Describe the events that occur in the cell cycle
What to Remember:
- The cell cycle is a continuous, ordered series of events in eukaryotic cells.
- It ensures cell growth, DNA duplication, and accurate cell division.
- Occurs in two main phases:
- Interphase (G₁, S, G₂)
- Mitotic Phase (Mitosis + Cytokinesis)
Key Events:
- Growth of cell and organelles (G₁)
- DNA replication (S phase)
- Preparation for division (G₂)
- Mitosis: Nuclear division into two identical nuclei
- Cytokinesis: Cytoplasm splits → 2 daughter cells
- Optional G₀: Resting phase (no division)
Summary Line:
The cell cycle = grow → copy → divide — all under tight regulation to keep cells healthy and functional.
4.5.A.1 – Cell Cycle: Events & Stages
What’s the Cell Cycle, anyway?
The cell cycle is like your cell’s daily planner—a carefully regulated routine that controls how cells grow, copy their DNA, and divide.
Goal: Create two healthy, identical daughter cells from one parent cell.
Super strict control, because any mistake = 🚨 uncontrolled cell division → cancer.
Main Phases of the Cell Cycle
The whole cycle is split into 2 key parts:
| 💡 Part | 📌 What Happens |
|---|---|
| Interphase | Prep work (cell grows, copies DNA, makes proteins) |
| Mitotic Phase | Actual splitting (mitosis + cytokinesis) |
INTERPHASE – Chill but Busy Phase
(~95% of the cell’s time is spent here! Not lazy at all!)
It has 3 stages:
1️⃣ G₁ (Gap 1)
- Cell grows in size
- Makes organelles and cytoplasmic stuff
- Fully metabolically active
Think: “GET READY” stage.
2️⃣ S (Synthesis)
- DNA replication begins!
- Chromatin duplicates → makes sister chromatids connected at a centromere.
Think: “COPY THAT!”
3️⃣ G₂ (Gap 2)
- Final prep: protein synthesis + energy (ATP)
- Centrosomes duplicate
- Cell is geared up for division!
Think: “GET SET!”
G₀ PHASE – The Exit Door
Sometimes cells take a break and exit the cycle into G₀.
- Not dividing (like going into sleep mode 😴)
- Can re-enter cycle if needed (e.g., wound healing)
- Some stay permanently in G₀ (like brain & muscle cells)
Examples: Neurons, cardiac muscle cells
MITOTIC PHASE – Let’s Split!
Now comes the real action:
| ⚙️ Part | 📌 What Happens |
|---|---|
| Mitosis | Nucleus divides (PMAT magic) |
| Cytokinesis | Cytoplasm splits = 2 new daughter cells 👯♀️ |
WHY IS THIS IMPORTANT?
- Helps with growth, healing, and replacement of dead/damaged cells
- Controlled by checkpoints
- If errors happen → could lead to diseases like cancer
4.5.B – How does mitosis pass on chromosomes from one generation of cells to the next?
Simple logic:
When one cell divides by mitosis, it creates two new baby cells — and both babies get an exact copy of all the DNA their parent had!
What really happens?
- DNA is copied before mitosis (in S phase).
- Then, during mitosis:
- The sister chromatids (twins) are pulled apart
- One goes to each side of the cell
- 💥 2 new cells, each with same DNA, same chromosomes, no mess!
Why it matters?
- Helps in growth, healing wounds, and replacing dead cells.
- Ensures all your cells have the exact same genetic info.
- No drama = No cancer or mutation chaos
Memory trick:
Mitosis = Mirror copy ➡️ 2 identical twin’s cells!
4.5.B.1 – How mitosis helps in chromosome transmission from one generation of cells to the next?
Mitosis – What’s Going On?
Mitosis = Cell division of somatic (body) cells
Purpose?
- Growth
- Repair
- Asexual reproduction
Mitosis makes sure each new cell gets the same set of chromosomes as the parent cell.
That means: No DNA lost, no DNA doubled → Just right!
So basically:
2n → 2n
✔️ Chromosome number stays same
✔️ Cells are genetically identical
Mitosis Comes After Interphase
(DNA has already doubled in S-phase)
Interphase = 95% of the cell cycle!
S-phase = DNA & histone synthesis
✔️ Centrioles duplicate
✔️ ATP & proteins made
✔️ Cell becomes metabolically
💡 Remember:
N = Number of chromosomes
C = DNA content
e.g. N = 4 → C = 8 after S-phase
Now Mitosis Begins – Main Stages (PMAT )
1️⃣Prophase
Setup mode!
- Chromatin condenses → Chromosomes visible
- Sister chromatids joined at centromere
- Spindle fibres begin to form (from centrioles)
- Centrosomes move to opposite poles
- Nuclear membrane, Golgi, ER, nucleolus all disappears
💡 Pro tip: Tubulin protein needed here! ATP requirement ↑↑
At the end of prophase:
→ N = 4, C = 8
→ Spindle fully ready
2️⃣ Metaphase
Align & get ready!
- Chromosomes align on metaphase plate
- Spindle attaches at kinetochore
- Cell is checked at M checkpoint here!
N = 4, C = 8
Colchicine blocks this stage by stopping spindle formation
3️⃣ Anaphase
Pull apart!
- Sister chromatids separate (now become individual chromosomes)
- Move to opposite poles of the cell
- Segregation happens via spindle fibres
N = 4 → 8
C = 8 → 4 + 4
(Each side now has full copy!)
4️⃣ Telophase
Pack it up!
- Spindle breaks down
- Nuclear membrane, Golgi, ER, nucleolus reappear
- Chromosomes start relaxing back into chromatin
N = 4, C = 4 in each nucleus now
Cytokinesis (Not part of mitosis but follows it)
Division of cytoplasm to make 2 cells:
| 🌱 Animal Cell | 🌿 Plant Cell |
|---|---|
| Centripetal | Centrifugal |
| Cleavage furrow formation | Cell plate forms |
| Involves aster fibres | No aster fibres |
| Has centrioles | No centrioles |
Result?
2 daughter cells
- Genetically identical
- Same N and same C
- Same size as mother cell after G1 growth
- Used in growth, repair, and asexual reproduction only!
NO change in chromosome number!
Somatic → Somatic
2n → 2n ✅
Memory Shortcuts
- PMAT = Prophase, Metaphase, Anaphase, Telophase
- Colchicine = Blocks metaphase
- Cytokinesis = Not part of mitosis but important end step
- M checkpoint = Ensures chromosomes aligned & attached properly
✅ So finally…
Mitosis = Perfect DNA copying machine
One cell → makes two with exact genetic copy 💯