Source: Professor Partridge, University of Texas at Austin
Difficulty: Introductory to Intermediate | Prerequisites: Cell signaling basics (receptors, cascades), DNA structure and replication. Review chromosome structure if you are unsure what a chromatid is.
Mitosis is the process by which a cell duplicates its contents and splits into two genetically identical daughter cells. It is how your body grows, repairs wounds, and replaces cells that die. The cell cycle includes a long preparatory phase (interphase) and the division itself (mitosis plus cytokinesis). Understanding this topic is essential because the cell cycle's checkpoints and controls are the basis for understanding cancer, and the mechanics of chromosome separation come back in meiosis. You should already be comfortable with DNA replication and basic chromosome terminology before tackling this material.
The cell cycle alternates between interphase (G1, S, G2), where the cell grows and copies its DNA, and mitosis, where duplicated chromosomes are separated into two identical daughter cells. Checkpoints at G1, G2, and M ensure the cell only proceeds when conditions are right. Cyclin-CDK complexes drive transitions between phases, and failures in these controls can lead to cancer.
Interphase
The portion of the cell cycle between divisions, consisting of G1, S, and G2. The cell spends most of its life here, growing and preparing to divide.
G1 Phase (Gap 1)
The first growth phase. The cell increases in size, produces proteins, and gathers the nutrients and nucleotides it needs for the next phase. Think of it as stocking the pantry before cooking.
S Phase (Synthesis)
The phase in which DNA replication occurs. Each chromosome is duplicated, producing two sister chromatids joined at the centromere. After S phase, the cell has twice the normal amount of DNA.
G2 Phase (Gap 2)
A second growth phase. The cell continues to grow and checks that DNA replication was completed correctly, with no errors. Think of it as the quality-control step before division.
Mitosis
The division of the cell's nucleus into two genetically identical nuclei. Comprises prophase, prometaphase, metaphase, anaphase, and telophase.
Prophase
Chromosomes condense and become visible. Two centrioles begin moving to opposite poles of the cell, and spindle fibres start forming from them.
Prometaphase
The nuclear envelope breaks down. Spindle fibres attach to kinetochores on the chromosomes, and chromosomes begin moving toward the centre of the cell.
Metaphase
Chromosomes align along the metaphase plate (the cell's equator). Spindle fibres are attached to the centromere of each chromosome via the kinetochore.
Anaphase
Cohesion proteins are cleaved, and sister chromatids are pulled to opposite poles by the shortening of kinetochore microtubules. Think of it as the moment the zipper splits.
Telophase
The cell begins to pinch in (cleavage furrow forms), and a new nuclear envelope starts forming around each set of chromosomes.
Cytokinesis
The physical division of the cytoplasm. Two complete, identical daughter cells are formed.
Centromere
The region of a chromosome where two sister chromatids are joined, and where kinetochore proteins assemble.
Kinetochore
A protein complex on the centromere that serves as the attachment point for spindle fibres. In simple terms, the handle that the spindle grabs to pull chromatids apart.
Sister Chromatids
Two identical copies of a chromosome, produced during S phase and held together at the centromere by cohesion proteins until anaphase.
Cohesion Proteins (Cohesins)
Proteins that hold sister chromatids together from S phase until they are cleaved at the start of anaphase.
Spindle Fibres (Microtubules)
Protein filaments that form the mitotic spindle. They attach to chromosomes and pull them apart during anaphase.
Karyotype
A display of all the chromosomes in a cell, arranged by size and shape. Used to detect chromosomal abnormalities.
Diploid (2n)
Having two complete sets of chromosomes, one from each parent. Human somatic cells are diploid (2n = 46).
Cyclin-CDK Complex
A regulatory protein pair that drives the cell cycle forward. Cyclin levels rise and fall with each phase; when a cyclin binds its CDK partner, the CDK becomes active and phosphorylates target proteins to push the cell into the next phase.
Telomere
A repetitive DNA sequence at the end of a chromosome that protects it from degradation. Telomeres shorten with each round of replication.
Telomerase
An enzyme that extends telomeres by adding DNA to chromosome ends using an RNA template. Hyperactive in cancer cells, allowing unlimited division. In normal cells, telomerase activity declines, limiting a cell to roughly 50 divisions.
Interphase is where the cell spends most of its time. It has three sub-phases:
G1 (Gap 1): the cell grows, produces proteins, and accumulates the nutrients and nucleotides needed for DNA synthesis.
S (Synthesis): DNA replication occurs. Each chromosome is copied, yielding two sister chromatids connected at the centromere. The cell now has two copies of each of its two chromosome sets.
G2 (Gap 2): continued growth, protein synthesis, and a final check for replication errors before the cell commits to division.
Prophase: chromatin condenses into visible chromosomes. Two centrioles begin migrating to opposite poles; spindle fibres start growing from them.
Prometaphase: the nuclear envelope dissolves. Spindle fibres from each pole attach to the kinetochores on the sister chromatids. Chromosomes begin moving toward the centre.
Metaphase: all chromosomes line up on the metaphase plate (the equator). Each chromosome's centromere is attached to spindle fibres from both poles via its kinetochore.
Anaphase: cohesion proteins holding the sister chromatids together are cleaved. Kinetochore microtubules shorten, pulling the separated chromatids toward opposite poles.
Telophase: the cell begins to pinch inward (cleavage furrow). A new nuclear envelope forms around each cluster of chromosomes.
Cytokinesis: the cytoplasm divides completely, producing two genetically identical diploid daughter cells.
The cell remains diploid at every stage of mitosis.
Three types of spindle microtubules:
Kinetochore microtubules: attach directly to the kinetochore at the centromere. These are the primary fibres responsible for pulling sister chromatids apart during anaphase. They shorten by depolymerisation.
Astral microtubules: radiate from each centrosome toward the cell membrane, anchoring the spindle apparatus at each pole.
Polar microtubules: extend from opposite poles and overlap in the middle. They push against each other to elongate the cell during anaphase and telophase.
Three major checkpoints prevent the cell from progressing if something is wrong:
G1 checkpoint: checks cell size, nutrient availability, and DNA integrity. If conditions are not met, the cell enters G0 (a resting state) rather than proceeding to S phase.
G2 checkpoint: confirms that DNA replication is complete and checks for replication errors or DNA damage.
M checkpoint (spindle assembly checkpoint): verifies that all chromosomes are properly attached to spindle fibres and correctly aligned at the metaphase plate before anaphase begins.
Cyclin-dependent kinases (CDKs) are enzymes that phosphorylate target proteins to drive the cell through each transition. CDKs are always present but inactive until a cyclin binds to them.
Each phase transition has a specific cyclin. For example, G1/S cyclin must reach a threshold concentration and bind its CDK partner for the cell to move from G1 into S phase. This ensures transitions happen only when the cell is ready.
Kinases phosphorylate their target proteins, triggering a phosphorylation cascade analogous to signal transduction cascades.
Growth factors: extracellular signaling molecules that stimulate other cells to divide.
Density-dependent inhibition: at high cell density, cells slow or stop dividing. This prevents overcrowding in healthy tissue.
Anchorage dependence: most normal cells must be attached to an extracellular surface (such as a basement membrane) in order to divide. Cancer cells often lose this requirement.
Telomeres are repetitive DNA sequences at chromosome ends. Each round of DNA replication shortens them slightly because the replication machinery cannot copy the very end of a linear chromosome (there is no primer site for the final fragment).
Normal cells can divide roughly 50 times before telomere shortening triggers senescence (permanent growth arrest).
Telomerase is the enzyme that extends telomeres by adding repeats using an RNA template. In normal somatic cells, telomerase activity is low or absent. In cancer cells, telomerase is often hyperactive, allowing the cells to divide indefinitely, effectively becoming "immortal."
This is one of the hallmarks of cancer: loss of checkpoint control, loss of anchorage dependence, loss of density-dependent inhibition, and reactivation of telomerase.
Students often think mitosis is the whole cell cycle. Mitosis is only the division phase. Interphase (G1 + S + G2) takes up the vast majority of the cycle.
Students sometimes say the cell "splits its DNA in half" during mitosis. Each daughter cell receives a full, identical copy of the genome. The DNA was duplicated during S phase.
Students frequently confuse centromeres and centrioles. The centromere is the region on the chromosome where sister chromatids are joined. Centrioles are the structures at each pole from which spindle fibres grow.
Students sometimes believe cancer is caused by cells dividing "too fast." The core problem is that checkpoint controls fail, so cells divide when they should not, and telomerase keeps them from ever stopping.
⚠️ Be able to list the stages of mitosis in order and state what happens at each stage. This is a near-guaranteed exam question.
⚠️ Know the three types of spindle microtubules and what each one does (kinetochore, astral, polar).
⚠️ Know what each checkpoint checks: G1 (size, nutrients, DNA), G2 (replication complete, no damage), M (spindle attached, chromosomes aligned).
⚠️ Understand cyclin-CDK regulation. Be able to explain how a specific cyclin reaching threshold concentration triggers a phase transition.
⚠️ Be prepared to explain the connection between telomerase, telomeres, and cancer. This is a favourite exam topic.
True or False: After mitosis and cytokinesis, each daughter cell is haploid. (False: both daughter cells are diploid.)
Fill in the blank: The phase of interphase in which DNA replication occurs is called ______ phase. (S / Synthesis)
True or False: The kinetochore is part of the centriole. (False: the kinetochore is a protein complex on the centromere of the chromosome.)
Fill in the blank: The enzyme that extends telomeres and is hyperactive in cancer cells is called ______. (Telomerase)
True or False: At the M checkpoint, the cell checks whether DNA replication is complete. (False: the M checkpoint checks that spindle fibres are attached and chromosomes are aligned. DNA replication is checked at the G2 checkpoint.)
Q: Walk through the stages of mitosis in order, stating the key event at each stage.
A: Prophase (chromosomes condense, centrioles move apart, spindle fibres form), prometaphase (nuclear envelope dissolves, spindle fibres attach to kinetochores), metaphase (chromosomes align on the metaphase plate), anaphase (cohesins cleaved, sister chromatids pulled to opposite poles), telophase (cell pinches in, nuclear envelopes reform), cytokinesis (cytoplasm divides, two identical diploid daughter cells formed).
Q: What is the difference between kinetochore microtubules, astral microtubules, and polar microtubules?
A: Kinetochore microtubules attach to the kinetochore at the centromere and pull sister chromatids apart by depolymerising. Astral microtubules extend from the centrosome to the cell membrane and anchor the spindle at each pole. Polar microtubules extend from opposite poles and overlap in the middle, pushing against each other to elongate the cell.
Q: Explain how cyclin-CDK complexes regulate the cell cycle.
A: CDKs are always present in the cell but remain inactive until a cyclin partner binds them. Each cell cycle transition has a specific cyclin. When that cyclin reaches a threshold concentration, it binds and activates its CDK, which then phosphorylates target proteins to drive the cell into the next phase. After the transition, the cyclin is degraded, deactivating the CDK until the next cyclin accumulates.
Q: Why do cancer cells divide without limit while normal cells do not?
A: Normal cells are limited by checkpoint controls (G1, G2, M), density-dependent inhibition, anchorage dependence, and telomere shortening. Cancer cells typically have mutations that disable one or more checkpoints, lose density-dependent inhibition and anchorage dependence, and reactivate telomerase to maintain telomere length indefinitely.
Q: What is the role of telomeres, and why do they shorten with each cell division?
A: Telomeres are repetitive, non-coding DNA sequences at chromosome ends that protect the coding DNA from degradation. They shorten because DNA polymerase cannot replicate the very end of a linear chromosome (there is no upstream primer site for the final Okazaki fragment on the lagging strand). After roughly 50 divisions, telomeres become critically short and the cell stops dividing.
Mitosis connects directly to cell signaling: the cyclin-CDK system is a phosphorylation cascade, and growth factors are ligands that bind receptors to stimulate division. Understanding signal transduction cascades makes the checkpoint logic much clearer.
It also connects forward to meiosis: meiosis reuses the same mechanical steps (prophase, metaphase, anaphase, etc.) but applies them differently to produce haploid gametes instead of diploid daughter cells. If you understand mitosis well, meiosis is largely a matter of learning what is different.
The telomere and cancer material connects to biotechnology: understanding DNA replication limits and enzyme activity (telomerase) is relevant to gene therapy and cancer research.
Mitosis, cell cycle, interphase, G1 phase, S phase, G2 phase, prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis, sister chromatids, centromere, kinetochore, spindle fibres, spindle microtubules, kinetochore microtubules, astral microtubules, polar microtubules, centriole, centrosome, cohesion proteins, cohesins, metaphase plate, cleavage furrow, diploid, 2n, karyotype, cell cycle checkpoints, G1 checkpoint, G2 checkpoint, M checkpoint, spindle assembly checkpoint, cyclin, CDK, cyclin-dependent kinase, growth factors, density-dependent inhibition, anchorage dependence, telomere, telomerase, cancer, cell division, cell proliferation