Cell Division, Cell Diversity And Cellular Organisation¶

Part of Module 2: Foundations in biology.

This topic explains how one cell becomes many cells, and how genetically similar cells can still perform very different functions. It links division, variation, specialisation, and whole-organism organisation. Understanding how cells divide accurately — and how errors in that process are caught — is essential groundwork for later topics on genetics and disease.

Learning Objectives¶

Chromosomes and Chromosome Structure¶

Chromosomes are thread-like structures located inside the nucleus of eukaryotic cells. They consist of DNA tightly coiled around proteins called histones. The combination of DNA and histone protein forms a compact structure that allows the very long DNA molecule to fit inside the nucleus.

Humans have 46 chromosomes in each body cell, arranged in 23 pairs. The first 22 pairs are autosomes, which are identical in form in both males and females. The 23rd pair are the sex chromosomes: females carry two X chromosomes (XX), males carry one X and one Y chromosome (XY). This is visible in a karyotype — a photograph of stained chromosomes arranged in homologous pairs.

Homologous Chromosomes¶

Each pair of autosomes consists of two homologous chromosomes — one inherited from each parent (paternal and maternal). Homologous chromosomes carry the same genes at the same loci (positions), but may carry different versions of those genes (alleles). They are the same length and have the same centromere position, but are not genetically identical because they may carry different alleles for the same traits.

Chromatids and the Centromere¶

The number of chromatids per chromosome depends on where the cell is in the cell cycle: - Before DNA replication: each chromosome consists of a single chromatid. - After DNA replication (S phase): each chromosome consists of two identical sister chromatids joined at the centromere. Sister chromatids are genetically identical copies produced during replication.

Diploid and Haploid¶

• Diploid cells (2n) contain two copies of each chromosome — one from each parent. In humans, 2n = 46.
• Haploid cells (n) contain one copy of each chromosome. Human gametes (egg and sperm) are haploid with n = 23.

The Cell Cycle¶

Organisms need new cells to grow and to repair damaged tissues. Cells that are capable of dividing follow a regulated sequence of steps called the cell cycle. The cycle has three main phases:

1. Interphase — the cell grows and prepares for division; this occupies the majority of the cycle.
2. Mitosis — the nucleus divides.
3. Cytokinesis — the cytoplasm divides to produce two daughter cells.

Interphase in Detail¶

Interphase is not a resting phase. During interphase, the cell carries out its normal functions while simultaneously preparing for division. It consists of three sub-phases:

• G1 phase: the cell grows in size, synthesises new proteins, and replicates organelles (such as mitochondria and chloroplasts) in preparation for cell division.
• S phase (Synthesis phase): DNA is replicated. Each chromosome goes from having one chromatid to having two sister chromatids joined at the centromere.
• G2 phase: the cell continues to grow, and the newly replicated DNA is checked for errors before the cell proceeds to mitosis.

Cell Cycle Checkpoints¶

Before the cell can progress to the next phase, it must pass checkpoints — quality control mechanisms that assess whether the processes of each phase have been completed correctly.

• G1 checkpoint: checks that the cell has the chemical signals required for DNA replication and that the existing DNA is undamaged. Cells with DNA damage may be directed to repair pathways or to apoptosis (programmed cell death).
• G2 checkpoint: checks that DNA has been replicated completely and without significant errors. If errors are detected, repair mechanisms are activated before the cell is permitted to enter mitosis.
• Metaphase checkpoint (spindle assembly checkpoint): checks that every chromosome is properly attached to a spindle fibre. Division will not proceed if any chromosome is unattached, preventing unequal distribution of genetic material to daughter cells.

Failure at checkpoints — particularly loss of checkpoint function — is a hallmark of cancer. Tumour cells often bypass these controls, allowing damaged or incompletely replicated DNA to be passed to daughter cells.

Mitosis¶

Mitosis is the form of nuclear division that produces two genetically identical daughter cells, each containing the same DNA as the parent cell. It is a continuous process, but is described in four stages.

Mitosis is used for: - Growth: a single-celled zygote divides by mitosis to form a multicellular organism. - Replacement of damaged or dead tissues: cells are constantly dying (e.g. skin, gut lining, red blood cells) and are replaced by mitosis. - Asexual reproduction: organisms such as bacteria, hydra, and some plants reproduce using mitosis to form genetically identical offspring. - Development: mitosis is used to form the different parts of an organism's body plan. - Production of stem cells: stem cells divide by mitosis to maintain their population.

Stages of Mitosis¶

Before mitosis begins, DNA has already been replicated during interphase. Each chromosome therefore consists of two identical sister chromatids joined at the centromere.

Prophase: Chromosomes condense and become visible under a microscope (they become shorter and thicker). Centrioles (bundles of protein present in animal cells) migrate to opposite poles of the cell and begin forming spindle fibres. The nucleolus disappears and the nuclear envelope breaks down, freeing the chromosomes into the cytoplasm.

Metaphase: Chromosomes align along the equator (metaphase plate) of the cell. Each chromosome attaches to a spindle fibre by its centromere. At this point the metaphase checkpoint ensures all chromosomes are correctly attached.

Anaphase: The centromeres divide, separating each pair of sister chromatids. Spindle fibres contract and shorten, pulling each chromatid toward an opposite pole of the cell. The chromatids take on a characteristic 'V' shape as they are pulled by the centromere.

Telophase: The chromatids (now referred to as chromosomes again) reach the poles of the cell and uncoil, becoming long and thin once more. A nuclear envelope reforms around each set of chromosomes, and the nucleolus begins to reform, producing two nuclei.

Cytokinesis¶

After mitosis, the cytoplasm divides in cytokinesis, producing two physically separate daughter cells. In animal cells, the membrane pinches inward at the equator. In plant cells, a new cell wall (cell plate) forms across the equator. Both daughter cells are genetically identical to each other and to the original parent cell.

Meiosis¶

Meiosis is a form of cell division that produces four haploid cells, each genetically distinct from one another. It consists of two successive divisions: meiosis I and meiosis II.

Before meiosis begins, DNA is replicated during interphase, so each chromosome consists of two sister chromatids.

Meiosis I — Reduction Division¶

Meiosis I is described as a reduction division because the chromosome number is halved: diploid cells (2n) produce haploid cells (n).

Prophase I: Chromosomes condense and homologous chromosomes pair up to form bivalents. Centrioles migrate to opposite poles and spindle fibres begin to form. The nuclear envelope breaks down. Crucially, during prophase I, the non-sister chromatids of the homologous chromosomes twist around one another and crossing over occurs at points called chiasmata (singular: chiasma).

Metaphase I: Bivalents (homologous pairs) align along the equator of the cell. The orientation of each bivalent — which homologue faces which pole — is completely random. This is independent segregation (or random assortment).

Anaphase I: Homologous chromosomes separate and are pulled to opposite poles by the spindle fibres. Sister chromatids remain joined at the centromere. The chromosome number is halved at each pole.

Telophase I: The chromosomes reach the poles and uncoil. Nuclear envelopes reform and cytokinesis occurs, producing two haploid cells. Each cell contains one chromosome from each homologous pair (still as two sister chromatids).

Meiosis II — Separation of Sister Chromatids¶

Meiosis II resembles mitosis and produces the final four haploid cells.

Prophase II: Chromosomes condense again. Centrioles migrate to opposite poles and spindle fibres reform. Nuclear envelopes break down.

Metaphase II: Chromosomes align at the equator; each chromosome attaches to spindle fibres at its centromere.

Anaphase II: Centromeres divide and sister chromatids are pulled to opposite poles.

Telophase II and Cytokinesis: Nuclear envelopes reform and cytokinesis produces four haploid cells. These four cells are the gametes (egg and sperm cells in animals).

Meiosis and Genetic Variation¶

Meiosis serves two biological purposes:

1. Production of haploid gametes: enables sexual reproduction; when two gametes fuse at fertilisation, the diploid chromosome number is restored.
2. Generation of genetic variation: increases diversity, which is the raw material for natural selection.

Crossing Over¶

Crossing over (recombination) occurs during prophase I. Homologous chromosomes condense and pair up; chromatids twist around one another and break at chiasmata, then rejoin to the chromatid of the homologous chromosome. This swaps alleles between homologous chromosomes, producing chromatids with new combinations of alleles that did not exist in either parent chromosome. Every chiasma represents one exchange event; multiple chiasmata can occur per bivalent.

The outcome is that each of the four daughter cells carries a unique combination of alleles, increasing genetic variation in the offspring.

Independent Segregation¶

During metaphase I, each bivalent aligns at the equator independently of all other bivalents. The orientation of the maternal versus paternal chromosome is random for every pair. This random arrangement means that, when homologues separate in anaphase I, each daughter cell receives a random mix of paternal and maternal chromosomes.

The number of genetically distinct gametes possible from independent segregation alone is 2ⁿ, where n is the number of chromosome pairs. For humans (n = 23):

2²³ = 8,388,608 possible chromosome combinations per parent

The number of possible genetically different zygotes from two parents is (2ⁿ)²:

(2²³)² ≈ 7 × 10¹³ possible zygote combinations

This enormous genetic diversity — before even considering crossing over or random fertilisation — explains why sexually reproducing populations contain so much phenotypic variation.

Random Fertilisation¶

Beyond meiosis itself, the random fusion of a sperm and egg cell at fertilisation adds another layer of variation. Any of the millions of possible gametes from each parent may combine, generating further diversity in the offspring.

Specialisation and Differentiation¶

All cells in a multicellular organism originate from the same zygote, but they become structurally and functionally diverse through a process called differentiation. During differentiation, cells switch on or off specific genes, producing only the proteins needed for their particular function. The resulting specialised cells have structural features directly adapted to what they do.

Specialised Animal Cells¶

Erythrocytes (red blood cells) transport oxygen around the body. Their specialisations: - Flattened biconcave disc shape increases the surface area to volume ratio, improving diffusion of oxygen. - No nucleus or other organelles — this maximises internal space for haemoglobin, the oxygen-binding protein. - Flexible membrane allows them to deform and squeeze through narrow capillaries.

Neutrophils are white blood cells that defend the body against pathogens. Their specialisations: - Flexible cell membrane enables them to engulf pathogens by phagocytosis. - Contain many lysosomes, which carry digestive enzymes to break down engulfed material. - Multi-lobed nucleus allows the cell to deform and squeeze through gaps in capillary walls to reach sites of infection.

Sperm cells (male gametes) carry genetic information to the female gamete. Their specialisations: - Flagellum (tail) propels the cell toward the egg. - Many mitochondria in the mid-piece supply the ATP needed for flagellar movement. - Acrosome at the head contains digestive enzymes that break down the protective layers of the egg cell, allowing penetration.

Squamous epithelial cells line the surfaces of organs such as the lungs and blood vessels. They are very thin and permeable, enabling efficient diffusion of gases such as oxygen and carbon dioxide across their surfaces.

Ciliated epithelial cells line organs such as the trachea and bronchioles. They have numerous cilia — hair-like extensions — that beat rhythmically to sweep mucus and trapped pathogens away from the lungs. In the fallopian tubes, similar cells move egg cells toward the uterus.

Specialised Plant Cells¶

Palisade cells are the main site of photosynthesis in leaves. Their specialisations: - Contain many chloroplasts to absorb light for photosynthesis. - Thin cell walls allow rapid diffusion of carbon dioxide into the cell. - Tall, thin shape allows many palisade cells to pack tightly together, forming a continuous layer near the upper leaf surface where light intensity is highest.

Root hair cells absorb water and mineral ions from the soil. Their specialisations: - Root hair extensions greatly increase the surface area available for absorption. - Thin, permeable cell wall allows easy entry of water by osmosis and ions by active transport. - Contain many mitochondria to supply ATP for active transport of mineral ions.

Guard cells control the opening and closing of stomata. Their specialisations: - Come in pairs flanking each stoma; the gap between them opens or closes. - When light is present, guard cells absorb water by osmosis and become turgid, causing them to bow outward and open the stoma. - When light is absent or the plant is water-stressed, guard cells lose water, become flaccid, and the stoma closes, reducing water loss by transpiration. - Thin outer walls and thicker inner walls cause the cell to bend when turgid, producing the opening.

Tissues, Organs, and Organ Systems¶

Cells do not work in isolation. The levels of biological organisation are:

Cell → Tissue → Organ → Organ System → Organism

A tissue is a group of similar cells working together to carry out a particular function. An organ is a group of different tissues working together. An organ system is a group of organs working together to carry out a broad physiological role.

Animal Tissues¶

Squamous epithelium provides a thin lining for surfaces and cavities (e.g. alveoli in the lungs, capillary walls). Being only one cell thick, it allows rapid diffusion of gases and other small molecules.

Ciliated epithelium lines the trachea and bronchi. It is composed of ciliated epithelial cells and goblet cells. Goblet cells secrete mucus to trap pathogens and particles; ciliated cells use their cilia to sweep the mucus toward the throat, keeping airways clear.

Cartilage is a connective tissue that cushions between bones and provides structural support to organs such as the ears, nose, and trachea. It is composed of chondrocyte cells embedded in an extracellular matrix.

Muscle tissue consists of elongated muscle fibres (bundles of cells) that contract and relax to generate movement. There are three types: - Smooth muscle: found in the walls of organs such as the gut and blood vessels; involuntary control. - Cardiac muscle: found only in the heart; contracts rhythmically and involuntarily. - Skeletal (striated) muscle: attached to bones; voluntary control.

Plant Tissues¶

Xylem tissue transports water and dissolved mineral ions from roots to the rest of the plant. It consists of dead xylem vessel cells that have lost their end walls and organelles, forming hollow tubes through which water can flow continuously. The walls of these cells are strengthened with lignin, a waterproof polymer that also provides structural support. Lignification causes the cell to die, but the empty, lignin-reinforced tubes are highly effective water-conducting vessels.

Phloem tissue transports sugars (primarily sucrose) and amino acids from source regions (where they are produced or stored) to sink regions (where they are used). It is composed of: - Sieve tube elements: living cells with perforated end walls (sieve plates) that allow sap to flow between cells. These cells have very few organelles, which reduces resistance to flow. - Companion cells: closely associated with sieve tube elements and connected via plasmodesmata. They contain many mitochondria to supply ATP, actively loading sugars into the sieve tubes and providing metabolic support to the sieve tube elements.

Stem Cells¶

What Stem Cells Are¶

All cells in a multicellular organism originate from undifferentiated precursor cells called stem cells. Stem cells have two defining features that distinguish them from other unspecialised cells: 1. They can divide by mitosis to produce more undifferentiated (stem) cells — self-renewal. 2. They can differentiate into specialised cell types.

Stem cells are used by organisms for growth, development, and tissue repair throughout life.

Types of Stem Cells — Potency¶

Stem cells are classified by their potency — the range of cell types they can differentiate into:

• Totipotent stem cells: can differentiate into any cell type and can form a complete organism. Found only in the very earliest stages of embryo development (the first few cleavage divisions).
• Pluripotent stem cells: can differentiate into most cell types (all three embryonic germ layers) but cannot form a complete organism on their own. Found in the inner cell mass of the blastocyst (approximately 7 days after fertilisation). This is the source of embryonic stem cells used in research.
• Multipotent stem cells: can differentiate into a limited range of related cell types. Adult stem cells are typically multipotent — e.g. haematopoietic stem cells in bone marrow can produce erythrocytes and neutrophils but not muscle cells.
• Unipotent stem cells: can differentiate into only one cell type; used for specific tissue maintenance.

Locations of Stem Cells¶

Embryonic stem cells are found in the inner cell mass of the early embryo (blastocyst stage). The earliest stem cells are totipotent; by around day 7, they become pluripotent. These cells have attracted significant research interest because of their broad differentiation potential.

Adult stem cells are found in specific tissues of the mature organism. They are typically multipotent or unipotent. A key example is the bone marrow, which contains haematopoietic stem cells that continuously produce red blood cells (erythrocytes) and white blood cells (including neutrophils) to replace those that have worn out. Red blood cells have a lifespan of approximately 120 days and must be continuously replaced.

Plant stem cells are found in meristematic tissue (meristems) at the tips of shoots and roots (apical meristems). They are pluripotent and divide continuously to produce new cells for growth. Meristematic tissue is also found between xylem and phloem tissues in an area called the vascular cambium, which produces new xylem and phloem cells as the plant grows in girth.

Stem Cells in Research and Medicine¶

Because stem cells can differentiate into specialised cells, they have significant potential in medicine and research:

• Treating neurological conditions: Parkinson's disease and Alzheimer's disease both involve loss of specific nerve cells in the brain. Transplanted stem cells could potentially regenerate these neurons and reduce symptoms.
• Drug testing: stem cell cultures can be used to test new drugs for toxicity and side effects before human trials, reducing the need for animal testing.
• Studying development: stem cells allow scientists to study how a single cell gives rise to a complete, complex organism — helping to identify when and why developmental errors occur.
• Identifying causes of genetic disorders: by studying stem cells derived from individuals with genetic conditions, scientists can identify at which stage of development problems arise.

Common Confusions¶

Mitosis and meiosis are not distinguished simply by number of divisions. Their biological outcomes are fundamentally different: mitosis produces two genetically identical diploid cells; meiosis produces four genetically varied haploid cells.

In meiosis I, homologous chromosomes separate. In meiosis II, sister chromatids separate. Keeping these two separations distinct prevents many errors in understanding chromosome numbers and genetic outcomes.

Differentiation and specialisation are not synonyms: differentiation is the cellular process (switching genes on and off) that leads to the outcome of specialisation (having a particular structure suited to a particular function).

Stem cells are defined by their capacity to both self-renew and differentiate — simply being unspecialised does not make a cell a stem cell.

Crossing over and independent segregation are separate mechanisms that both contribute to genetic variation, and both occur within meiosis. Random fertilisation adds a third source of variation not arising from the division itself.

Key Terms¶

• Cell cycle: the sequence of growth, DNA replication, and division that produces new cells; consists of interphase, mitosis, and cytokinesis.
• Interphase: the phase occupying most of the cell cycle; the cell grows, carries out normal functions, replicates DNA (S phase), and prepares organelles (G1, G2).
• G1 phase: growth phase before DNA replication; new proteins and organelles are produced.
• S phase: DNA synthesis phase; each chromosome is replicated to form two sister chromatids.
• G2 phase: growth phase after DNA replication; replicated DNA is checked for errors.
• Checkpoint: a regulatory mechanism in the cell cycle that ensures each phase is completed accurately before progression to the next.
• Chromatid: one of the two identical copies of a replicated chromosome, joined to its partner at the centromere.
• Sister chromatids: the two identical chromatids of a replicated chromosome, joined at the centromere.
• Centromere: the constriction point on a chromosome where sister chromatids are joined and where spindle fibres attach.
• Mitosis: nuclear division producing two genetically identical diploid daughter cells; used in growth, repair, and asexual reproduction.
• Cytokinesis: division of the cytoplasm following nuclear division, producing two separate daughter cells.
• Spindle fibres: protein fibres that attach to centromeres and pull chromatids or chromosomes to the poles of the cell during division.
• Diploid (2n): a cell or organism containing two complete sets of chromosomes, one from each parent.
• Haploid (n): a cell containing one complete set of chromosomes; typical of gametes.
• Homologous chromosomes: a pair of chromosomes carrying the same genes at the same loci, one from each parent; may carry different alleles.
• Autosomes: the 22 pairs of chromosomes identical in males and females (not the sex chromosomes).
• Sex chromosomes: the 23rd chromosome pair; XX in females, XY in males.
• Meiosis: cell division producing four haploid, genetically distinct cells; consists of meiosis I (homologues separate) and meiosis II (sister chromatids separate).
• Bivalent: the paired structure formed when homologous chromosomes associate during prophase I of meiosis.
• Chiasma (plural: chiasmata): the point at which non-sister chromatids of homologous chromosomes exchange segments during crossing over.
• Crossing over (recombination): the exchange of DNA segments between non-sister chromatids of homologous chromosomes during prophase I; produces new allele combinations.
• Independent segregation (random assortment): the random orientation of bivalents at the metaphase I plate, resulting in random distribution of maternal and paternal chromosomes to daughter cells.
• Differentiation: the process by which a cell becomes specialised by switching on and off specific genes, producing only the proteins needed for its function.
• Specialised cell: a cell with structural features adapted to carry out a particular function.
• Tissue: a group of similar cells working together to carry out a function.
• Organ: a structure composed of different tissues working together to carry out a function.
• Organ system: a group of organs working together to perform a broad physiological role.
• Squamous epithelium: a single layer of flattened cells lining surfaces; allows rapid diffusion.
• Ciliated epithelium: epithelial tissue bearing cilia; moves mucus across surfaces.
• Xylem: plant tissue that transports water and minerals; composed of dead, lignified, hollow vessel cells.
• Lignin: a waterproof polymer deposited in xylem cell walls; provides structural support and waterproofing.
• Phloem: plant tissue that transports sugars and amino acids; composed of living sieve tube elements and companion cells.
• Sieve tube elements: living phloem cells with perforated sieve plates; transport sucrose and amino acids.
• Companion cells: phloem cells supporting sieve tube elements; rich in mitochondria.
• Stem cell: an undifferentiated cell with the capacity to self-renew by mitosis and to differentiate into specialised cell types.
• Totipotent: able to differentiate into any cell type and form a complete organism (e.g. very early embryo cells).
• Pluripotent: able to differentiate into most cell types but not form a complete organism (e.g. embryonic stem cells).
• Multipotent: able to differentiate into a limited range of related cell types (e.g. adult bone marrow stem cells).
• Unipotent: able to differentiate into only one cell type.
• Meristem: plant tissue containing actively dividing stem cells, found at shoot and root tips (apical meristems) and between xylem and phloem (vascular cambium).
• Vascular cambium: meristematic tissue between xylem and phloem in plants; produces new vascular tissue as the plant grows in girth.

Connected Pages¶

• 2.1.1 Cell structure
• 2.1.5 Biological membranes
• 3.1.1 Exchange surfaces
• 4.2.2 Classification and evolution