Prophase of Mitosis: Chromosome Condensation and Asters
Biology · Cell Cycle and Cell Division · NEET
Prophase is the first stage of mitosis. Two things mark it: (1) the loose chromatin condenses into thick, compact chromosomes, each made of two sister chromatids joined at the centromere, and (2) the centrosome (already doubled in S phase) moves to opposite poles, and each centrosome shoots out microtubules called asters. By the end of prophase, the Golgi complex, endoplasmic reticulum, nucleolus and nuclear envelope are no longer seen. Memory hook: "PRO = PRoduce chromosomes + PRepare the spindle" — condense the DNA, and build the asters.
Prophase: chromatin has condensed into compact chromosomes (each with two chromatids joined at the centromere), the two centrosomes have moved to opposite poles and radiate asters, and the nuclear envelope (dashed) is fading from view.
Your doubts, answered
Does the nuclear envelope break down in prophase or metaphase?
This is the most common trap. In prophase the nuclear envelope is not yet completely gone — NCERT says that at the END of prophase, cells viewed under the microscope do not clearly show the Golgi, ER, nucleolus and nuclear envelope. The COMPLETE disintegration of the nuclear envelope is what marks the START of metaphase. So think: prophase = envelope disappears from view; metaphase = envelope is fully gone and chromosomes spread into the cytoplasm.
When exactly do asters appear?
Asters appear in prophase. Each centrosome radiates out short microtubules called asters. The two asters together with the spindle fibres form the mitotic apparatus. Remember: the centrosome duplicated earlier in S phase, but it only MOVES to opposite poles and forms asters during prophase.
Is the centromere split during prophase?
No. During prophase the chromosome is still made of two sister chromatids joined at a single centromere. The centromere splits only in anaphase, when sister chromatids separate. If a question says centromere splits in prophase, it is wrong.
Are spindle fibres attached to the chromosomes in prophase?
No. Spindle fibres form during prophase, but they attach to the kinetochores of chromosomes only at metaphase. In prophase the apparatus is being built; in metaphase it grabs the chromosomes and lines them up at the equator.
What is the difference between an aster and a spindle fibre?
Asters are the star-shaped bundles of short microtubules that radiate directly out of each centrosome. Spindle fibres are the microtubules that stretch between the two poles and later connect to chromosomes. Asters + spindle fibres together = the mitotic apparatus. Asters are a feature of animal cells (which have centrioles); typical plant cells lack centrioles and asters.
⚠️ The NEET trap ✗ The nuclear envelope and nucleolus disappear during prophase, so the nuclear envelope disintegrates in prophase. ✓ They stop being visible by the end of prophase, but the COMPLETE disintegration of the nuclear envelope marks the START of metaphase (this is where chromosomes spread into the cytoplasm). 🧠 'Not seen' in prophase is not the same as 'fully gone.' Fully gone = metaphase begins.
Real NEET questions
NEET 2017
Which of the following options gives the correct sequence of events during mitosis?
A · Condensation → nuclear membrane disassembly → crossing over → segregation → telophase
B · Condensation → nuclear membrane disassembly → arrangement at equator → centromere division → segregation → telophase ✓
C · Condensation → crossing over → nuclear membrane disassembly → segregation → telophase
D · Condensation → arrangement at equator → centromere division → segregation → telophase
Solution: Mitosis begins with chromosome condensation (prophase), then nuclear-membrane disassembly, then alignment at the equator (metaphase), then centromere splitting (anaphase), then telophase. Options with 'crossing over' are wrong because crossing over is meiotic, not mitotic. Note that condensation and the start of nuclear-membrane breakdown are prophase events.
NEET 2016 Phase 1
Which of the following is not a characteristic feature during mitosis in somatic cells?
A · Spindle fibres
B · Disappearance of nucleolus
C · Chromosome movement
D · Synapsis ✓
Solution: Spindle fibres form and the nucleolus disappears — both are normal prophase features of mitosis. Chromosome movement also occurs. Synapsis (pairing of homologous chromosomes) is a meiosis-only event and never occurs in mitosis, so it is the correct answer to 'not a feature.'
NEET 2022
Which one of the following never occurs during mitotic cell division?
A · Spindle fibres attach to kinetochores of chromosomes
B · Movement of centrioles towards opposite poles
C · Pairing of homologous chromosomes ✓
D · Coiling and condensation of the chromatids
Solution: Coiling/condensation of chromatids and movement of centrioles to opposite poles are prophase events of mitosis, and spindle fibres do attach to kinetochores. Pairing (synapsis) of homologous chromosomes is meiotic and never happens in mitosis, so it is the answer.
Solved Cell Cycle and Cell Division NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Chromatin condenses into compact chromosomes (each with two sister chromatids joined at the centromere), the centrosome moves to opposite poles and forms asters, spindle fibres begin to assemble, and by the end the Golgi, ER, nucleolus and nuclear envelope are no longer visible.
What are asters?
Asters are star-shaped radial arrays of short microtubules given out by each centrosome during prophase. The two asters plus the spindle fibres form the mitotic apparatus.
How many chromatids does each chromosome have in prophase?
Two sister chromatids, joined together at the centromere. They separate only in anaphase.
Do plant cells form asters?
Typical higher plant cells lack centrioles, so they usually do not form asters. They still build a spindle (anastral spindle). Asters are characteristic of animal cell division.
Is the nuclear envelope present at the start of prophase?
Yes. It is present early in prophase and only stops being clearly visible by the end of prophase. Its complete disintegration marks the beginning of metaphase.
