Mitosis and Cell Division — CXC CSEC Biology Revision Notes

What you'll learn

Mitosis is one of the most frequently examined topics in CXC CSEC Biology, appearing in both multiple-choice and structured questions. This guide covers the complete process of mitotic cell division, the behaviour of chromosomes at each stage, and the biological significance of mitosis in growth, repair, and asexual reproduction. You'll master the precise terminology and sequencing examiners expect, supported by worked examples from past paper styles.

Key terms and definitions

Mitosis — a type of nuclear division that produces two daughter nuclei with the same number of chromosomes as the parent nucleus, maintaining the diploid number.

Chromosome — a thread-like structure made of DNA and protein, carrying genetic information in the form of genes.

Chromatid — one of two identical copies of a replicated chromosome, joined at the centromere.

Centromere — the region where two sister chromatids are joined together and where spindle fibres attach during cell division.

Spindle fibres — protein structures that form during cell division and attach to centromeres to pull chromatids apart.

Diploid — a cell or nucleus containing two complete sets of chromosomes (2n), one from each parent.

Cytokinesis — the division of the cytoplasm following nuclear division, completing cell division.

Cell cycle — the sequence of events in a cell from one division to the next, including interphase and mitotic division.

Core concepts

The cell cycle and interphase

Before mitosis begins, the cell undergoes interphase, which occupies approximately 90% of the cell cycle. CXC CSEC Biology examiners frequently test understanding that interphase is not part of mitosis itself but the preparation phase.

During interphase, three key processes occur:

DNA replication — each chromosome is copied to produce two identical chromatids joined at the centromere
Organelle synthesis — mitochondria, ribosomes, and other organelles are replicated
Cell growth — the cell increases in size and accumulates nutrients and energy stores (ATP)

At the end of interphase, each chromosome consists of two sister chromatids. For example, a human cell entering mitosis has 46 chromosomes, but each chromosome is composed of two chromatids, giving 92 chromatids in total. This distinction appears regularly in CSEC exam questions.

The stages of mitosis

Mitosis divides the nucleus and is traditionally described in four continuous stages. CXC examiners expect you to name these stages in sequence and describe the chromosome behaviour at each stage.

Prophase

Prophase marks the beginning of mitosis and involves several simultaneous changes:

Chromosomes condense and become visible under a light microscope as pairs of chromatids joined at the centromere
The nuclear membrane begins to break down and disappear
Centrioles (in animal cells) move to opposite poles of the cell
Spindle fibres begin to form between the poles

In plant cells, spindle fibres form without centrioles, since plant cells lack these structures. This comparison between plant and animal cell division is a common exam point.

Metaphase

This stage is characterized by chromosome alignment:

Chromosomes line up along the equator (middle) of the cell
Spindle fibres from both poles attach to the centromere of each chromosome
The cell is now ready for chromosome separation

Metaphase can be identified in microscope diagrams by the characteristic line of chromosomes across the cell centre. CSEC practical questions may show cells at different stages and ask you to identify metaphase using this feature.

Anaphase

During anaphase, the actual separation of genetic material occurs:

Centromeres split, separating sister chromatids
Spindle fibres shorten, pulling individual chromatids to opposite poles of the cell
Each chromatid is now an independent chromosome
The cell begins to elongate

This is the critical stage ensuring each daughter cell receives identical genetic information. Examiners often ask you to explain what happens to the chromatids during anaphase.

Telophase

Telophase reverses the changes of prophase:

Chromosomes reach the poles and begin to uncoil, becoming less visible
A nuclear membrane reforms around each set of chromosomes
Spindle fibres break down
Two nuclei are now present in the cell

Telophase overlaps with cytokinesis, though these are technically separate processes.

Cytokinesis: completing cell division

Cytokinesis divides the cytoplasm to produce two separate daughter cells. The mechanism differs between plant and animal cells:

In animal cells:

The cell membrane pinches inward at the equator
A cleavage furrow forms and deepens
The cell divides into two separate cells

In plant cells:

A cell plate forms across the middle of the cell
The cell plate develops from vesicles containing cell wall material
The cell plate fuses with the existing cell wall
Two separate cells with complete cell walls are formed

This difference relates to the rigid cell wall in plant cells preventing simple pinching. Caribbean crops like sugarcane (Saccharum officinarum) and banana (Musa species) rely on mitotic cell division during growth, with the cell plate mechanism enabling tissue expansion in their stems and roots.

Significance of mitosis in living organisms

CXC CSEC Biology papers regularly include questions asking you to state or explain the significance of mitosis. You must be able to give specific, detailed examples:

Growth

Mitosis enables multicellular organisms to increase in size by producing more cells:

A young coconut palm (Cocos nucifera) grows from a seedling to a 25-metre tree through millions of mitotic divisions
Humans grow from a single fertilized egg to approximately 37 trillion cells through repeated mitosis
The daughter cells are genetically identical to parent cells, maintaining consistent tissue characteristics

Repair and regeneration

Mitosis replaces damaged or dead cells:

Skin cells are continuously replaced through mitotic division in the basal layer of the epidermis
When you cut your hand, mitosis produces new skin cells to close the wound
In lizards common to the Caribbean like Anolis species, mitosis enables tail regeneration after autotomy (self-amputation when escaping predators)
Red blood cells in humans are replaced every 120 days through mitosis in bone marrow

Asexual reproduction

Many organisms reproduce asexually through mitosis, producing offspring genetically identical to the parent:

Vegetative propagation in plants: dasheen (Colocasia esculenta) produces corms through mitotic division, a method used by Caribbean farmers for crop propagation
Binary fission in bacteria: a single E. coli bacterium divides by mitosis to produce two identical daughter cells
Budding in yeast: a small bud grows through mitosis and eventually separates
Runners in strawberry plants grown in Trinidad and Tobago produce new plantlets at nodes

The advantage of asexual reproduction through mitosis is rapid population increase without requiring a mate. The disadvantage is lack of genetic variation, meaning all offspring are susceptible to the same diseases.

Chromosome number during mitosis

Understanding chromosome numbers before and after mitosis is essential for CXC CSEC Biology:

If the parent cell is diploid (2n), both daughter cells are also diploid (2n) with the same chromosome number as the parent.

Example: In humans:

Parent cell: 46 chromosomes (2n = 46)
After DNA replication in interphase: 46 chromosomes, each consisting of 2 chromatids
After mitosis: Each daughter cell has 46 chromosomes (2n = 46)

Example: In maize/corn (Zea mays), commonly grown in Caribbean agriculture:

Parent cell: 20 chromosomes (2n = 20)
After mitosis: Each daughter cell has 20 chromosomes (2n = 20)

This maintenance of chromosome number distinguishes mitosis from meiosis, which halves the chromosome number. Exam questions often present scenarios requiring you to calculate chromosome numbers after division.

Worked examples

Example 1: Identifying stages from diagrams

Question: The diagram shows four cells from the root tip of a banana plant undergoing mitosis.

Cell A: Chromosomes aligned at the cell equator Cell B: Chromatids moving to opposite poles Cell C: Chromosomes visible but scattered in the nucleus Cell D: Nuclear membrane reforming around two chromosome groups

(a) Identify the stage of mitosis shown in Cell B. (1 mark) (b) Name the stage shown in Cell A. (1 mark) (c) Arrange the cells in the correct sequence of mitosis. (2 marks)

Model answer: (a) Anaphase ✓

(b) Metaphase ✓

(c) Cell C (prophase) → Cell A (metaphase) → Cell B (anaphase) → Cell D (telophase) ✓✓ [Award 2 marks for completely correct sequence; 1 mark if one error]

Examiner note: Questions presenting diagrams of mitotic stages are extremely common in CXC CSEC Biology Paper 2. Learn to identify each stage by its key features: metaphase = line of chromosomes; anaphase = chromatids moving apart; telophase = two nuclei forming.

Example 2: Calculating chromosome numbers

Question: A cell from the leaf tissue of a cassava plant (Manihot esculenta) contains 36 chromosomes.

(a) State the number of chromosomes in each daughter cell after mitosis. (1 mark) (b) Explain why the chromosome number remains constant. (2 marks) (c) State ONE significance of mitosis in the cassava plant. (1 mark)

Model answer: (a) 36 chromosomes ✓

(b) During interphase, DNA replication occurs / each chromosome is copied ✓ During anaphase, sister chromatids separate and one copy moves to each pole / each daughter cell receives one chromatid from each chromosome ✓

(c) Growth / allows the plant to increase in size OR Repair / replaces damaged cells in leaves OR Asexual reproduction / produces stem cuttings for propagation ✓ [Accept any one valid significance with appropriate example]

Examiner note: When explaining chromosome constancy, you must reference both DNA replication and the separation mechanism. Simply stating "chromosomes copy themselves" is insufficient for full marks.

Example 3: Comparing plant and animal mitosis

Question:

(a) State TWO differences between mitosis in plant cells and animal cells. (2 marks) (b) Explain why plant cells cannot undergo cytokinesis in the same way as animal cells. (2 marks)

Model answer: (a)

Plant cells lack centrioles / centrioles present in animal cells ✓
Plant cells form a cell plate during cytokinesis / animal cells form a cleavage furrow ✓ [Accept: spindle forms differently; timing of cell wall formation]

(b) Plant cells have a rigid cell wall ✓ The cell wall prevents the membrane from pinching inward / a cell plate must form to divide the cell ✓

Examiner note: Comparison questions require you to state differences explicitly, not just describe one type. Use comparative language: "whereas," "while," "unlike."

Common mistakes and how to avoid them

Mistake: Describing interphase as a stage of mitosis. Correction: Interphase precedes mitosis and is part of the cell cycle but not part of mitosis itself. Mitosis consists only of prophase, metaphase, anaphase, and telophase. When asked to "describe the stages of mitosis," do not include interphase unless the question specifically asks about the complete cell cycle.
Mistake: Confusing chromatids and chromosomes, stating that the chromosome number doubles during mitosis. Correction: The number of chromosomes remains constant. During DNA replication, each chromosome becomes composed of two chromatids, but these are counted as one chromosome until the chromatids separate in anaphase. Only when centromeres split do we count each chromatid as an individual chromosome.
Mistake: Writing that chromosomes "copy themselves" or "replicate" during mitosis. Correction: DNA replication occurs during interphase, before mitosis begins. During mitosis, chromosomes do not replicate; they are separated and distributed to daughter cells. Be precise about when processes occur in the cell cycle.
Mistake: Stating vague significance such as "mitosis is important for survival." Correction: Examiners want specific significance. State precisely: "mitosis enables growth by increasing cell number," or "mitosis allows repair by replacing damaged tissue," or "mitosis produces genetically identical offspring in asexual reproduction." Include examples where possible.
Mistake: Mixing up the order of stages, particularly placing anaphase before metaphase. Correction: Learn the sequence using a mnemonic: "I Prefer Making Apple Tea" (Interphase, Prophase, Metaphase, Anaphase, Telophase). Remember that chromosomes must align (metaphase) before they can separate (anaphase) — logical sequencing helps.
Mistake: Forgetting that cytokinesis is separate from nuclear division. Correction: Mitosis refers specifically to nuclear division. Cytokinesis divides the cytoplasm and follows telophase. When describing cell division, mention both processes: "After nuclear division by mitosis, cytokinesis divides the cytoplasm to produce two separate cells."

Exam technique for "Cell Division: Mitosis and Its Significance"

"State" questions (1-2 marks): Provide brief, factual answers without explanation. Example: "State the stage of mitosis when chromosomes align at the equator" requires only "Metaphase." No additional description is needed. Write one clear sentence per mark.
"Describe" questions (3-4 marks): Give a sequential account of what happens. For describing mitosis, name each stage and give one key feature: "During prophase, chromosomes condense and become visible. During metaphase, chromosomes align at the equator..." Award yourself one mark per valid statement. Use accurate terminology — "chromosomes become visible" scores marks; "you can see the DNA" does not.
"Explain" questions (2-3 marks): Provide reasons and mechanisms. Example: "Explain why daughter cells have the same chromosome number as the parent cell" requires you to mention DNA replication AND chromatid separation. Link cause and effect explicitly: "Because DNA replicates in interphase, each chromatid contains identical genetic information, so when chromatids separate, each daughter cell receives the same genes."
Diagram labeling and identification: When identifying stages from microscope images or diagrams, look for definitive features: nuclear membrane present/absent, chromosome position (scattered/aligned/separated), number of nuclei visible. In Paper 3 (Alternative to SBA), you may need to identify stages in prepared slides of root tips from onion or bean plants. Practice recognizing stages in different cell types.

Quick revision summary

Mitosis produces two genetically identical diploid daughter cells through four stages: prophase (chromosomes condense, nuclear membrane breaks down), metaphase (chromosomes align at equator), anaphase (chromatids separate and move to poles), and telophase (nuclear membranes reform). DNA replication during interphase ensures each daughter cell receives identical genetic information. Cytokinesis then divides the cytoplasm. Mitosis is significant for growth, repair, and asexual reproduction. Plant and animal cells differ in centriole presence and cytokinesis mechanism (cell plate versus cleavage furrow). Chromosome number remains constant: diploid parent produces diploid daughters.