Before a nucleus can divide, every chromosome has to be copied, packed down to a fraction of its length, and then physically dragged across the cell. None of that happens by accident — and getting the vocabulary right here makes mitosis and meiosis far easier later.
📘 What you need to know
A eukaryotic nucleus can divide in two ways: mitosis and meiosis.
Mitosis gives genetically identical nuclei. It is used for growth, repair, replacement and asexual reproduction, and keeps the chromosome number the same (usually diploid, 2n).
Meiosis gives genetically different nuclei with half the chromosome number (haploid, n). It produces gametes and generates genetic diversity.
The nucleus must divide before the cell does, or one daughter cell would be anucleate.
DNA replicates during interphase, giving each chromosome two identical chromatids joined at the centromere — the sister chromatids.
At anaphase the centromere splits and each chromatid becomes a chromosome again.
DNA is packed by coiling around histones into nucleosomes (eight histones each), forming chromatin, which supercoils in prophase into visible chromosomes.
Microtubules of tubulin lengthen and shorten, and motor proteins move chromosomes along them.
Two ways to divide a nucleus
Feature
Mitosis
Meiosis
Daughter nuclei
Genetically identical
Genetically different
Chromosome number
Maintained, usually diploid (2n)
Halved, haploid (n)
Number of nuclei produced
Two
Four
Used for
Growth, repair, replacement, asexual reproduction
Producing gametes
Significance
Keeps the genome the same in every cell
Generates genetic diversity
Why divide the nucleus first? If the cytoplasm split before the nucleus, one daughter cell would end up with no nucleus at all — an anucleate cell. It would have no instructions, could not make proteins, and could not divide again.
Chromatid, sister chromatids, chromosome
These three words describe the same DNA at different moments, and examiners test the difference deliberately.
During interphase, before nuclear division, the DNA replicates. Each chromosome now consists of two identical strands of DNA called chromatids, joined together at a narrow region called the centromere. Because they are identical copies, they are the sister chromatids.
At anaphase, the centromere splits. One chromatid from each chromosome is pulled into each daughter cell, and once separated, those chromatids are called chromosomes again.
Notice that replication does not change the chromosome number — it doubles the amount of DNA in each chromosome.
Packing the DNA down to size
The scale of the problem is worth pausing on. Human DNA can be more than 50 000 µm long, and it has to fit inside a nucleus averaging less than 5 µm across. That is a ten-thousandfold reduction.
The solution is repeated coiling:
The DNA is loosely coiled around histone proteins, forming a complex called chromatin.
Histones package the DNA into nucleosomes. Each nucleosome is a strand of DNA coiled around eight histone proteins.
During prophase, the chromatin is condensed by supercoiling — repeated coiling of the whole molecule — to form the compact chromosomes you can see under a microscope.
Histone proteins and enzymes make this supercoiling possible in eukaryotic cells.
Chromosomes are not permanent structures. They exist as condensed, visible objects only around nuclear division.
Moving the chromosomes
Condensing the DNA is only half the job. The chromosomes then have to be moved to the equator, and later dragged apart. That is done by microtubules and microtubule motors.
Microtubules are tubulin fibres, part of the cell’s cytoskeleton.
They can lengthen and shorten, which is what lets them move things. Two types of tubulin, alpha-tubulin and beta-tubulin, form dimers that are added to or removed from the ends of a microtubule to change its length.
A dimer is simply a compound made of two subunits.
Motor proteins carry the chromosomes along the microtubules towards the equator of the cell, ready for division.
Think of a microtubule as a scaffolding pole that can be built longer or taken apart at the ends, with a porter walking along it carrying the load. Adding and removing dimers is the building; the motor protein is the porter.
Worked examples
WORKED EXAMPLE
Counting chromatids
A cell has 12 chromosomes. After DNA replication in interphase, state the number of chromosomes, the number of chromatids, and the number of DNA molecules in the nucleus.
Step 1: Chromosomes
Replication does not change the number. Each still has one centromere.
12 chromosomesStep 2: Chromatids
Each chromosome now has two.
12 × 2 = 24 chromatidsStep 3: DNA molecules
Each chromatid contains one DNA molecule.
24 DNA molecules12 chromosomes, 24 chromatids, 24 DNA moleculesCount centromeres to count chromosomes. It never fails.
WORKED EXAMPLE
Why replicate first?
Explain why DNA replication must occur before nuclear division.
Step 1: What division does
Nuclear division shares the existing chromosomes between two nuclei.
Step 2: What would happen without replication
Each nucleus would receive only half of the genetic material, so neither would have a complete genome.
Step 3: What replication achieves
Copying first means each chromosome has two identical chromatids, so both daughter nuclei get a full set.
It ensures both daughter nuclei receive a complete copy of the genomeName the stage: replication happens in S phase of interphase.
WORKED EXAMPLE
Interpreting a drug effect
A drug prevents tubulin dimers from being added to microtubules. Suggest the effect on nuclear division.
Step 1: What the dimers are for
Adding and removing dimers is how microtubules lengthen and shorten.
Step 2: What that stops
Spindle microtubules could not form or change length, so chromosomes could not be moved to the equator or pulled apart.
Step 3: Consequence
Nuclear division would stall, and the cell could not divide correctly.
No spindle movement, so the chromosomes cannot be separatedSeveral anti-cancer drugs work in roughly this way.
💡 Exam tip
Count centromeres, not arms, when a question asks for the number of chromosomes in a diagram.
Use sister chromatids for the two identical copies, and remember they become chromosomes again once separated.
Give the number eight for histones per nucleosome — it is a favourite one-mark detail.
Say supercoiling, not just “coiling”, when describing condensation in prophase.
Name tubulin and motor proteins if asked how chromosomes move.
Link mitosis to growth, repair, replacement and asexual reproduction — all four.
⚠ Common mix-up
Saying replication doubles the chromosome number. It doubles the DNA, not the number of chromosomes.
Calling a chromatid a chromosome before anaphase. It only earns that name once the centromere has split.
Confusing chromatin with chromatid. Chromatin is the DNA-histone complex; a chromatid is one copy of a chromosome.
Thinking chromosomes are always visible. They are only condensed enough to see around nuclear division.
Saying meiosis produces two cells. It produces four haploid nuclei.
Forgetting histones exist only in eukaryotes. Prokaryotic DNA is packaged differently.
Up next: Mitosis — the DNA is copied and packed, the spindle equipment is ready. Now we follow the four phases in order and watch the chromosomes actually move.
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