Question
A 42-year-old laboratory scientist is revising basic cell biology. She asks: At which point in cell division does loosely arranged nuclear DNA coil tightly to become visible chromosomes?
Options:
A. Telophase
B. Anaphase
C. Metaphase
D. Prophase
E. Interphase
Answer: Prophase
Detailed discussion for MRCP
The Cell Cycle: The Framework for Understanding Cell Division
The cell cycle describes the entire life of a cell—from one division to the next. It is divided into two major parts:
- Interphase – preparation phase
- M phase – division phase
Importantly, most of a cell’s life is spent in interphase, not dividing.
Interphase (NOT part of mitosis)
Interphase is when the cell:
- Performs its normal functions
- Grows
- Copies its DNA
- Prepares for division
Interphase consists of three phases: G1, S, and G2.
G1 Phase (Gap 1)
This is the phase of normal cellular activity.
- Cell grows in size
- Organelles and proteins are synthesised
- The cell decides whether to divide or not
DNA status in G1:
- DNA is in a loose form called chromatin
- 46 chromosomes
- 46 chromatids
📌 This is the decision point of the cell cycle.
G0 Phase (Side Branch – NOT part of the main cycle)
From G1, a cell may:
- Proceed to S phase and divide, or
- Exit the cycle into G0
G0 characteristics:
- Cell is alive but not dividing
- Examples:
- Neurons
- Cardiac muscle cells
📌 G0 is not between G1 and S
📌 G0 branches off from G1
S Phase (Synthesis Phase) ⭐ HIGH YIELD
This is where DNA replication occurs.
- Each chromosome copies itself
- An identical copy is produced
- The two identical copies remain attached at the centromere
- These identical copies are called sister chromatids
DNA status after S phase:
- DNA is still chromatin (loose, not condensed)
- 46 chromosomes
- 92 chromatids
📌 Chromosome number is counted by centromeres, not chromatids
📌 No condensation occurs in S phase
G2 Phase (Gap 2)
This is the final preparation stage.
- Cell checks DNA for errors
- Proteins needed for mitosis are synthesised
DNA status in G2:
- Chromatin
- 46 chromosomes
- 92 chromatids
M Phase (Division Phase)
The M phase consists of:
- Mitosis – nuclear division
- Cytokinesis – cytoplasmic division
Mitosis: Division of the Nucleus
Mitosis is the process by which a somatic (body) cell divides to produce two genetically identical diploid daughter cells. It is essential for growth, tissue repair, and cell turnover .
Mitosis occurs in five stages:
Prophase → Metaphase → Anaphase → Telophase → Cytokinesis
Prophase ⭐ DEFINING EVENT
Prophase is the first stage of mitosis and is defined by chromatin condensation.
What happens:
- Loose chromatin condenses into short, thick, visible chromosomes
- Each chromosome consists of two sister chromatids
- The nucleolus disappears
- The mitotic spindle begins to form from centrosomes
📌 Nothing new is copied here
📌 Only packing/tightening of DNA occurs
💡 Why condensation matters:
- Loose DNA would tangle or break
- Condensed chromosomes can be pulled apart accurately
📌 Chromatin condenses into chromosomes ONLY in prophase
Prometaphase (often grouped with prophase)
- Nuclear envelope breaks down
- Spindle microtubules attach to kinetochores
Metaphase
- Chromosomes align at the metaphase plate (cell equator)
- Chromosomes are maximally condensed and organised
Still:
- 46 chromosomes
- 92 chromatids
Anaphase ⭐ EXAM TRAP
- Sister chromatids separate at the centromere
- Each chromatid becomes an independent chromosome
At this moment:
- 92 chromosomes
- 92 chromatids
- All within the same cell
Telophase
- Nuclear membranes reform
- Chromosomes decondense back into chromatin
Cytokinesis
- Cytoplasm divides
- Two separate daughter cells are formed
Final result (each daughter cell):
- 46 chromosomes
- 46 chromatids
- DNA in chromatin form
Chromosome and Chromatid Numbers: One-Glance Logic
| Stage | Chromosomes | Chromatids |
|---|---|---|
| G1 (before S) | 46 | 46 |
| After S phase | 46 | 92 |
| Prophase | 46 | 92 |
| Metaphase | 46 | 92 |
| Anaphase | 92 | 92 |
| After mitosis (each cell) | 46 | 46 |
📌 Chromosome number is determined by centromeres
Chromatin vs Chromatid: Do NOT Confuse These
Core Concept
- Chromatin = form of DNA packaging
- Chromatid = one physical copy of a chromosome
They describe different ideas.
Chromatin
- DNA + histone proteins
- Loose, thread-like
- Found during:
- Interphase
- Telophase
- Allows gene expression
- Describes packing style, not number
- Not visible under light microscopy
Chromatid
- One replicated copy of a chromosome
- Formed after S phase
- Two identical chromatids joined at a centromere = one chromosome
- Describes copy number
- Visible during mitosis
Analogy
- Chromatin: loose pages spread on a table
- Chromosome: closed, bound book
- Chromatid: one of two identical books tied together
Comparison Table
| Feature | Chromatin | Chromatid |
|---|---|---|
| What it describes | DNA packaging | DNA copy |
| Loose or tight | Loose | Tight |
| Seen when | Non-dividing cell | After DNA replication |
| Number concept | No | Yes |
| Centromere related | No | Yes |
| Visible in microscope | No | Yes (mitosis) |
🚨 Common exam error
❌ “Chromatin becomes chromatids”
✅ Chromatin condenses to form chromosomes; chromosomes may contain one or two chromatids
Final MRCP-Grade One-Liners 🧠
- Cell cycle order: G1 → S → G2 → M
- G0 is a quiescent state entered from G1
- S phase = DNA synthesis (46 chromosomes, 92 chromatids)
- Prophase = chromatin condensation
- Anaphase = chromatid separation
- Chromosome number is counted by centromeres
Cheat sheet (exam-oriented)
- Cell cycle phases: G1 → S → G2 → M
- DNA replication occurs in S phase
- Mitosis = division of nucleus; cytokinesis = division of cytoplasm
- Prophase: chromatin condenses into chromosomes
- Prometaphase: nuclear envelope breaks down
- Metaphase: chromosomes align at equator
- Anaphase: sister chromatids separate
- Telophase: nuclear membranes reform, chromosomes decondense
- Somatic cells are diploid (46 chromosomes)
- Gametes are haploid (23 chromosomes)
Flash cards
Q1: What happens to chromatin during prophase?
A: It condenses into visible chromosomes.
Explanation: Condensation allows accurate segregation during mitosis.
Q2: Which phase of the cell cycle is not part of mitosis?
A: Interphase.
Explanation: Interphase includes G1, S, and G2 phases.
Q3: During which phase is DNA replicated?
A: S phase.
Explanation: Each chromosome forms two sister chromatids.
Q4: What structure connects sister chromatids?
A: Centromere.
Explanation: It holds chromatids together until anaphase.
Q5: When does the nuclear envelope begin to break down?
A: Prophase/prometaphase.
Explanation: This allows spindle attachment.
Q6: What is the metaphase plate?
A: The equatorial plane of the cell.
Explanation: Chromosomes align here in metaphase.
Q7: Which phase shows maximal chromosome condensation?
A: Metaphase.
Explanation: Chromosomes are shortest and thickest.
Q8: What triggers chromatid separation?
A: Cleavage of cohesin proteins.
Explanation: Allows movement in anaphase.
Q9: What happens to chromosomes in telophase?
A: They decondense into chromatin.
Explanation: Nuclear structure is restored.
Q10: What motor proteins drive cytokinesis?
A: Actin and myosin.
Explanation: They form the contractile ring.
Q11: What cells undergo mitosis?
A: Somatic cells.
Explanation: For growth and repair.
Q12: How many daughter cells are produced in mitosis?
A: Two.
Explanation: Both are genetically identical.
Q13: What is a kinetochore?
A: Protein complex on centromere.
Explanation: Spindle fibres attach here.
Q14: Which phase ensures equal genetic distribution?
A: Anaphase.
Explanation: Chromatids move to opposite poles.
Q15: What happens to nucleolus during prophase?
A: It disappears.
Explanation: Reflects reorganisation of nucleus.
Q16: Which phase immediately follows prophase?
A: Metaphase (or prometaphase).
Explanation: Depends on classification.
Q17: What prevents chromosome tangling?
A: Condensation in prophase.
Explanation: Shorter structures segregate efficiently.
Q18: What defines diploid cells?
A: Two copies of each chromosome.
Explanation: Typical of somatic cells.
Q19: What is cytokinesis?
A: Cytoplasmic division.
Explanation: Occurs after nuclear division.
Q20: Which mitotic phase restores nuclear membranes?
A: Telophase.
Explanation: Marks end of mitosis.
MCQs to test yourself
1. Chromatin first becomes microscopically visible as chromosomes during:
A. Telophase
B. Anaphase
C. Prophase
D. Interphase
E. Cytokinesis
Answer: C – Chromatin condensation begins in prophase.
2. Which of the following is false regarding interphase?
A. DNA replication occurs
B. Cell grows metabolically
C. Chromosomes are condensed
D. It includes G1, S, and G2
E. It precedes mitosis
Answer: C – Chromosomes are not condensed in interphase.
3. Nuclear envelope breakdown mainly occurs in:
A. Telophase
B. Prometaphase
C. Anaphase
D. Interphase
E. Cytokinesis
Answer: B – Allows spindle attachment.
4. Maximum chromosome condensation is seen in:
A. Prophase
B. Metaphase
C. Telophase
D. G2 phase
E. Cytokinesis
Answer: B – Important for karyotyping.
5. Which event defines anaphase?
A. Chromatin condensation
B. Nuclear membrane formation
C. Sister chromatid separation
D. Spindle formation
E. DNA replication
Answer: C
6. Which of the following is false about mitosis?
A. Produces identical cells
B. Occurs in somatic cells
C. Results in haploid cells
D. Includes prophase
E. Allows tissue repair
Answer: C
7. Centromeres divide during:
A. Prophase
B. Metaphase
C. Anaphase
D. Telophase
E. G1 phase
Answer: C
8. Which structure attaches chromosomes to spindle fibres?
A. Centrosome
B. Centriole
C. Kinetochore
D. Nucleolus
E. Golgi apparatus
Answer: C
9. Which phase immediately follows metaphase?
A. Prophase
B. Anaphase
C. Telophase
D. Interphase
E. Cytokinesis
Answer: B
10. Which of the following is false about telophase?
A. Chromosomes decondense
B. Nuclear membrane reforms
C. Spindle disassembles
D. Sister chromatids separate
E. Two nuclei appear
Answer: D
11. Actin–myosin contraction is seen in:
A. Prophase
B. Metaphase
C. Anaphase
D. Telophase
E. Cytokinesis
Answer: E
12. DNA content doubles in:
A. G1 phase
B. S phase
C. G2 phase
D. Prophase
E. Metaphase
Answer: B
13. Which phase is not part of mitosis?
A. Prophase
B. Metaphase
C. Anaphase
D. Telophase
E. Interphase
Answer: E
14. Which of the following is false regarding chromosomes in prophase?
A. They are visible
B. They are condensed
C. They consist of two chromatids
D. They are aligned at equator
E. They shorten and thicken
Answer: D
15. Somatic cells are best described as:
A. Haploid
B. Polyploid
C. Diploid
D. Aneuploid
E. Triploid
Answer: C
16. Which structure disappears early in mitosis?
A. Centromere
B. Kinetochore
C. Nucleolus
D. Centrosome
E. Spindle
Answer: C
17. Which phase ensures equal distribution of DNA?
A. Prophase
B. Metaphase
C. Anaphase
D. Telophase
E. Interphase
Answer: C
18. Which is false about metaphase?
A. Chromosomes are maximally condensed
B. Chromosomes align at equator
C. Nuclear membrane is intact
D. Spindle fibres are attached
E. Kinetochore function is critical
Answer: C
19. Mitotic spindle is primarily composed of:
A. Actin
B. Myosin
C. Microtubules
D. Intermediate filaments
E. Collagen
Answer: C
20. Which phase marks the end of nuclear division?
A. Prophase
B. Metaphase
C. Anaphase
D. Telophase
E. Cytokinesis
Answer: D
Summary for quick exam revision
Mitosis is the process by which somatic cells divide to form two genetically identical diploid daughter cells and occurs during the M phase of the cell cycle. DNA replication happens earlier in the S phase of interphase. Prophase is the first stage of mitosis and is defined by condensation of chromatin into visible chromosomes, disappearance of the nucleolus, and initiation of spindle formation. Prometaphase involves breakdown of the nuclear envelope and attachment of spindle fibres to kinetochores. In metaphase, chromosomes are maximally condensed and aligned at the equatorial plate. Anaphase is characterised by separation of sister chromatids and their movement to opposite poles, ensuring equal genetic distribution. Telophase restores nuclear membranes and chromosomes decondense back into chromatin. Cytokinesis then divides the cytoplasm via an actin–myosin contractile ring. The key exam pearl is that chromatin condensation into chromosomes occurs in prophase, not interphase or metaphase.