Difficulty: Introductory | Prerequisites: None, though familiarity with basic cell structure (organelles, nucleus, membrane) will help.
Cell division is one of the most fundamental processes in biology, and it underpins everything from growth and tissue repair to sexual reproduction and genetic diversity. This set of notes covers the vocabulary you need (chromosomes, chromatids, homologous pairs, ploidy), the cell cycle that prepares a cell to divide, and the step-by-step mechanics of mitosis. If you can sketch what happens to chromosomes at each stage of mitosis and explain why the daughter cells are genetically identical, you have the core of this topic. You should already be comfortable with basic cell structure (nucleus, membrane, organelles), but no prior knowledge of division is assumed.
Cells copy their DNA during S phase, then divide it equally through mitosis (prophase, metaphase, anaphase, telophase) and split via cytokinesis, producing two genetically identical diploid daughter cells. The whole point is growth and repair: same DNA in, same DNA out. Understanding the vocabulary (chromosome vs. chromatid, diploid vs. haploid) is half the battle.
DNA molecule
A double-stranded helical molecule that carries genetic information. Think of it as the instruction manual for building and running a cell.
Chromosome
A single DNA molecule packaged with histone proteins into a compact structure. In eukaryotes, histones wind the DNA tightly so it fits inside the nucleus. A gene is one functional segment of a chromosome. Before replication, each chromosome is one double-stranded DNA molecule; after replication, it contains two joined copies (sister chromatids).
In simple terms, a chromosome is one complete "package" of DNA, and genes are individual instructions within that package.
Chromatid
One copy of a replicated chromosome, joined to its identical partner at the centromere. Before they separate, the two copies are called sister chromatids. After separation, each chromatid is considered its own chromosome.
Think of it as one half of an "H" shape: the two vertical bars are sister chromatids, and the crossbar is the centromere holding them together.
Centromere
The region where sister chromatids are joined together. It is also where kinetochore proteins attach, which is how spindle fibres grab and pull chromatids apart during division.
Kinetochore
A protein structure on the centromere that serves as the attachment point for spindle microtubules. In simple terms, it is the handle that the cell's machinery grips to move chromosomes around.
Homologous pair (homologues)
Two chromosomes that carry the same genes at the same loci but may carry different versions (alleles) of those genes. One comes from each parent. They are not identical; they are matched.
Think of it as two editions of the same textbook: same chapters in the same order, but the wording may differ.
Allele
A specific version of a gene at a given locus. Homologous chromosomes carry the same gene but potentially different alleles (e.g., one allele for brown eyes, another for blue).
Diploid (2n)
A cell that contains two complete sets of chromosomes, one from each parent. Most human somatic (body) cells are diploid, with 2n = 46.
Haploid (n)
A cell that contains only one set of chromosomes. Gametes (sperm and egg cells) are haploid. Meiosis converts diploid cells to haploid; fertilisation restores the diploid number by combining two haploid cells.
Spindle fibres (microtubules)
Protein structures that attach to kinetochores and pull chromosomes to opposite poles of the cell during division.
The cell cycle is the full sequence a cell goes through from one division to the next. Most of a cell's life is spent in interphase, not actively dividing.
Interphase has three sub-phases:
G1 (Gap 1): The cell grows, carries out its normal functions, and prepares the materials needed for DNA replication. Chromosomes exist as single, unreplicated structures. For an organism with a diploid number of 4 (2n = 4), the cell has 4 chromosomes.
S phase (Synthesis): DNA replication occurs at the replication fork. Each chromosome is copied, producing two identical sister chromatids joined at the centromere. The cell now has 4 chromosomes, each made of 2 chromatids (so 8 chromatids total, but still 4 chromosomes).
G2 (Gap 2): The cell continues to grow and produces proteins needed for division. All chromosomes are in their replicated (two-chromatid) form, ready for mitosis.
Key point: DNA synthesis happens well before division begins. Whether a cell is heading into mitosis or meiosis, the DNA is replicated during S phase in exactly the same way.
Mitosis divides one diploid cell into two genetically identical diploid daughter cells. The mnemonic PMAT-C helps: Prophase, Metaphase, Anaphase, Telophase, Cytokinesis. Some sources treat cytokinesis as separate from mitosis proper, but it is always tested alongside it.
A useful memory aid: P = "pre" (preparing), M = "middle" (chromosomes line up in the middle), A = "away" (chromatids pull away), T = "torn" (the cell envelope reforms, the cell looks torn in two), C = "cut" (the cytoplasm is cut in half).
Prophase
Chromatin condenses into visible chromosomes.
Each chromosome is already replicated (two sister chromatids joined at the centromere).
The nuclear envelope begins to break down.
Spindle fibres start forming from the centrioles and extending toward the chromosomes.
There is no interaction between homologous chromosomes in mitosis. This is a critical difference from meiosis.
Metaphase
Chromosomes align along the metaphase plate (the middle of the cell).
Spindle fibres attach to kinetochores at each chromatid's centromere.
Each chromosome is oriented so that one sister chromatid faces each pole.
Anaphase
Non-kinetochore and kinetochore microtubules pull sister chromatids apart.
Each chromatid is now considered an individual chromosome.
The chromatids are still identical at this point; they were copies of the same original.
For a 2n = 4 organism, anaphase produces 8 chromatids moving to opposite poles (4 toward each pole), which equals 4 chromosomes per future daughter cell.
Telophase and Cytokinesis
The chromosomes arrive at opposite poles and begin to decondense.
A new nuclear envelope forms around each set of chromosomes.
Cytokinesis divides the cytoplasm, producing two separate daughter cells.
Each daughter cell is diploid (2n = 4) and genetically identical to the parent cell.
Result of mitosis: Two daughter cells, each with the same chromosome number and the same genetic information as the original cell.
Real-world application: Mitosis is how your body grows from a single fertilised egg into trillions of cells, and how it repairs damaged tissue. When you cut your skin, the cells around the wound undergo mitosis to close the gap. Cancer occurs when mitosis becomes uncontrolled.
Students often think the chromosome number doubles during S phase. It does not. The number of chromosomes stays the same; each chromosome simply goes from one chromatid to two joined sister chromatids. A cell with 4 chromosomes before S phase still has 4 chromosomes after, just with twice the DNA content.
Students frequently confuse chromatids with chromosomes. After replication, a chromosome consists of two sister chromatids. After they separate in anaphase, each chromatid becomes its own chromosome. The distinction matters for counting.
Students sometimes believe homologous chromosomes pair up in mitosis. They do not. Pairing (synapsis) is exclusive to meiosis I. In mitosis, each chromosome lines up and separates independently.
Students mix up "identical" and "homologous." Sister chromatids are identical copies. Homologous chromosomes are matched (same genes, same loci) but carry potentially different alleles.
⚠️ Be prepared to draw chromosomes at each stage of mitosis for a given diploid number (e.g., 2n = 4). This is a classic exam question and appears directly in these tutoring notes.
⚠️ Know the difference between chromosome number and chromatid number at every stage: G1, S, G2, and each phase of mitosis.
⚠️ Understand that mitosis produces genetically identical cells. If an exam asks what distinguishes mitosis from meiosis, "identical daughter cells" and "no crossing over" are central answers.
⚠️ The PMAT-C mnemonic and what happens at each step is high-yield. Exams commonly ask you to name the phase from a description or diagram.
True or False: After S phase, a cell with 2n = 4 has 8 chromosomes.
False. It still has 4 chromosomes, each made of 2 sister chromatids (8 chromatids total).
Fill in the blank: During ______, chromosomes line up along the middle of the cell.
Metaphase.
True or False: Homologous chromosomes pair up during mitosis.
False. Pairing occurs only in meiosis I.
Fill in the blank: The protein structure on the centromere where spindle fibres attach is called the ______.
Kinetochore.
True or False: The two daughter cells produced by mitosis are genetically identical to the parent cell.
True.
Q: A cell has a diploid number of 6 (2n = 6). How many chromatids are present at the end of G2, just before mitosis begins?
A: 12 chromatids (6 chromosomes, each consisting of 2 sister chromatids).
Q: During which phase of mitosis do sister chromatids separate and begin moving to opposite poles?
A: Anaphase. The centromeres split, and spindle fibres pull the sister chromatids apart.
Q: A student observes a cell under the microscope and sees chromosomes lined up in a single row at the centre of the cell. What phase is this?
A: Metaphase. Chromosomes align along the metaphase plate.
Q: Explain why the daughter cells of mitosis are genetically identical to the parent cell.
A: During S phase, each chromosome is replicated to produce identical sister chromatids. Mitosis separates these identical copies so that each daughter cell receives one complete set of chromosomes with exactly the same genetic information as the original.
Q: How does the role of the kinetochore differ from the role of the centromere?
A: The centromere is the region of DNA where sister chromatids are physically joined. The kinetochore is a protein complex built on the centromere that provides the attachment point for spindle microtubules. The centromere is the location; the kinetochore is the structure that spindle fibres grab.
This material connects directly to meiosis, which uses the same PMAT framework but adds homologous pairing, crossing over, and a second division. Understanding mitosis first makes meiosis far easier to learn.
The cell cycle also connects to cell signalling and cancer biology: checkpoints at G1, G2, and the metaphase-to-anaphase transition regulate whether division proceeds, and mutations in checkpoint genes can lead to uncontrolled proliferation.
The concept of ploidy (diploid vs. haploid) bridges into genetics and inheritance, where understanding how chromosome number changes through meiosis and fertilisation is essential for Punnett squares and predicting offspring genotypes.
mitosis, cell division, cell cycle, interphase, G1 phase, S phase, G2 phase, prophase, metaphase, anaphase, telophase, cytokinesis, PMAT, sister chromatids, centromere, kinetochore, spindle fibres, microtubules, diploid, 2n, chromosome number, chromatid number, DNA replication, replication fork, homologous chromosomes, alleles, identical daughter cells, somatic cell division, BIO 311C, intro to biology, eukaryotic cell division