Cell division, cell diversity and cellular organisation — OCR A-Level Biology

The cell cycle describes the sequence of events by which a cell grows and divides to produce two genetically identical daughter cells. It has two main stages: interphase and mitotic (M) phase. Interphase is subdivided into G1 (first growth phase, where the cell grows and organelles are synthesised), S (synthesis phase, where DNA is replicated so each chromosome becomes two sister chromatids) and G2 (second growth phase, where the cell continues to grow and prepares for division). The mitotic phase includes mitosis, in which the replicated chromosomes are separated into two nuclei, and cytokinesis, in which the cytoplasm divides to form two separate cells. Checkpoints regulate progression through the cycle, ensuring DNA is replicated and undamaged before division.

The cell cycle is an ordered sequence of interphase (G1, S, G2) and mitotic phases, and its progress is controlled at checkpoints so that DNA is replicated and segregated only when conditions are suitable. Regulation involves cyclins, whose concentrations rise and fall through the cycle, binding to and activating cyclin-dependent kinases (CDKs); the active complexes phosphorylate target proteins that move the cycle forward. Checkpoints, such as the G1 checkpoint assessing cell size, DNA damage and nutrient availability, the G2 checkpoint confirming replication is complete and undamaged, and the spindle assembly checkpoint during mitosis, can halt progress. Loss of control, for example through mutated proto-oncogenes or tumour suppressor genes, can lead to uncontrolled division and tumour formation.

Mitosis is nuclear division producing two genetically identical daughter nuclei, and it is conventionally described in stages. In prophase, chromosomes condense and become visible, the nuclear envelope breaks down and the spindle forms. In metaphase, chromosomes line up along the equator (metaphase plate) of the cell and spindle fibres attach to centromeres. In anaphase, spindle fibres shorten and pull sister chromatids apart to opposite poles. In telophase, chromosomes decondense, nuclear envelopes reform around each set of chromosomes and the spindle disassembles. Cytokinesis then divides the cytoplasm, completing the formation of two daughter cells.

This specification point asks you to recognise the cell cycle and the stages of mitosis in prepared sections of plant tissue, typically a root tip. In a root tip squash or longitudinal section, cells are not synchronised, so a single field of view shows cells at different points in the cycle. You identify each stage from chromosome behaviour and nuclear appearance: in interphase the nucleus is intact with visible nucleolus and no distinct chromosomes; in prophase chromosomes condense and become visible; in metaphase they align at the equator (metaphase plate); in anaphase chromatids separate and move to opposite poles; in telophase two nuclei reform and the cell plate begins to form. You also relate the proportion of cells in each stage to the relative duration of that stage, since more cells are seen where the stage lasts longer.

This point asks you to explain why mitosis matters across the life cycle of organisms. Mitosis produces genetically identical daughter cells, so it maintains a constant chromosome number and preserves the genotype. Its significance includes growth of multicellular organisms by increasing cell number, repair and replacement of damaged or worn-out cells and tissues, asexual reproduction producing clones, and, in plants, vegetative propagation. Because the genetic material is copied and divided equally, offspring cells are genetically identical to the parent cell (barring mutation), which is why mitosis supports continuity and stability of the genome. You should be able to link each of these roles to the underlying feature of identical chromosome copies being separated into two nuclei.

Meiosis matters in life cycles because it halves the chromosome number and reshuffles alleles, so gametes carry one set of chromosomes and fusion at fertilisation restores the diploid number. In animals, meiosis produces gametes directly and keeps the chromosome number constant across generations. In plants and many algae, meiosis produces spores that grow into a haploid gametophyte, and gametes form later by mitosis, so the haploid stage is multicellular. In fungi, meiosis follows fusion in the zygote. Genetic variation arises from crossing over in prophase I and independent assortment in metaphase I, giving offspring a new allele combination.

Meiosis has two divisions after one round of DNA replication. In prophase I homologous chromosomes pair as bivalents and crossing over occurs at chiasmata. In metaphase I bivalents line up on the equator and independent assortment occurs. In anaphase I homologous chromosomes separate to opposite poles. Telophase I and cytokinesis give two haploid cells. In prophase II the nuclear envelope breaks down; in metaphase II chromosomes line up singly on the equator; in anaphase II sister chromatids separate; telophase II and cytokinesis give four haploid cells. Each product is genetically different.

Multicellular organisms build many cell types from one fertilised egg. Specialisation means a cell becomes adapted, in shape, organelle content and membrane features, to carry out one function efficiently. Differentiation is the process producing these adaptations, usually through selective gene expression rather than loss of genes. Learn to link structure to function: a red blood cell loses its nucleus and packs haemoglobin for oxygen transport; a sperm cell keeps many mitochondria and a flagellum for motility; a root hair cell extends a long projection to increase surface area for water and mineral uptake; a palisade mesophyll cell holds many chloroplasts near the leaf surface for photosynthesis; a neurone is elongated with many ion channels for conduction.

Cells are organised in a hierarchy of increasing complexity. Similar cells specialised for the same function group together as a tissue, such as squamous epithelium, ciliated epithelium, cartilage or xylem. Different tissues combine to form an organ with a particular overall function, for example the stomach contains epithelium, muscle and connective tissue. Organs then work together as an organ system, such as the digestive system, to carry out a major body function. Learn the order: cell, then tissue, then organ, then organ system, then organism. In an MCQ, identify the level described in the stem and check the option matches that level exactly. A single named cell type is not a tissue; a tissue is not an organ; and an organ is not a system. Watch for options that place a structure at the wrong level of the hierarchy.

Stem cells are unspecialised cells that can divide repeatedly and can differentiate into other cell types. Their key features are the ability to self-renew by mitosis, so the stem cell population is maintained, and potency, the range of cell types they can become. Totipotent cells can form any cell type including extra-embryonic tissue; pluripotent cells can form most body cells but not extra-embryonic tissue; multipotent cells are restricted to a related family of cells, such as blood stem cells forming red and white blood cells; unipotent cells form one cell type. Differentiation occurs when a cell expresses particular genes, switching on some and off others, so it develops the structure and proteins suited to a specialised function. In multiple-choice questions, match the potency term to the described capability and link differentiation to selective gene expression.

In bone marrow, multipotent stem cells divide by mitosis and differentiate along different pathways to produce distinct blood cells. Erythrocyte production (erythropoiesis) is stimulated by erythropoietin: the cell loses its nucleus and organelles, becomes packed with haemoglobin and adopts a biconcave shape, giving a large surface area for oxygen transport. Neutrophil production involves retaining a lobed nucleus and forming many lysosomes and granules, enabling phagocytosis of pathogens. Both cell types therefore arise from the same common stem cell but express different genes, producing different structures suited to different functions — a clear illustration of differentiation.

Meristems hold undifferentiated stem cells that divide by mitosis and then differentiate into specialised tissues. In the root and shoot apical meristems, a single stem cell lineage can give rise to either xylem vessels or phloem sieve tubes, so the same genetic information produces two distinct outcomes. Xylem vessel elements lose their contents and lignify their walls to form hollow dead tubes carrying water and mineral ions upward. Phloem sieve tube elements keep a living companion cell, lose the nucleus and most organelles, and form sieve plates for translocation of assimilates. Differentiation therefore means selective gene expression and structural change, not a change in the genes present.

Stem cells are undifferentiated cells able to divide and differentiate into specialised cell types. Their potential uses fall into research and medicine. In research, pluripotent and multipotent stem cells provide models for studying development, gene expression and disease, and can be used to test drugs and toxicity. In medicine, stem cells may be used to replace damaged or lost tissues, for example bone marrow transplants to restore blood cells, or potential therapies for repairing tissue such as nerve or heart tissue. A useful method is to separate each use into research or medicine, then state the property of stem cells that makes it possible, such as self-renewal or potency. Recognise that many medical applications remain experimental and raise ethical issues, so the word potential matters.