Difficulty: Intermediate | Prerequisites: Basic cell biology, membrane structure
This set of notes covers the physical machinery inside the cell (the endomembrane system and cytoskeleton) and the two modes of cell division (mitosis and meiosis). You should already be comfortable with the basic structure of a eukaryotic cell and the role of membranes. Understanding these structures and processes is essential for later topics in genetics, development and cancer biology.
The endomembrane system (ER, Golgi, lysosomes, vesicles) manufactures, modifies and ships proteins and lipids. The cytoskeleton (microtubules, microfilaments, intermediate filaments) provides structure, enables movement and drives cell division. Mitosis produces two genetically identical daughter cells; meiosis produces four genetically unique haploid cells for sexual reproduction.
Nuclear envelope
A double membrane (inner and outer) that defines the boundary of the nucleus and controls molecular traffic through nuclear pores.
Signal sequence
A short amino acid sequence on a newly made protein that directs it to the correct cellular compartment.
Nuclear localisation signal
A specific signal sequence that targets a protein to the nucleus by binding to importin proteins, allowing passage through nuclear pores.
Endoplasmic reticulum (ER)
A network of membrane-enclosed tubes and sacs continuous with the outer nuclear envelope. It comes in two forms: rough ER (studded with ribosomes, makes and processes proteins) and smooth ER (lacks ribosomes, synthesises lipids, steroids and carbohydrates, stores Ca²⁺).
Lumen
The continuous interior space of the endoplasmic reticulum.
Golgi apparatus
A stack of flattened membrane-bound discs that receives proteins and lipids from the ER, modifies them (e.g. glycosylation) and sorts them to their final destinations. Material flows from the cis (receiving) face to the trans (shipping) face.
Glycosylation
The attachment of carbohydrate chains to proteins, typically occurring in the ER and Golgi.
Vesicle
A small membrane-enclosed sac that transports substances between organelles or to the cell surface.
Lysosome
A vesicle derived from the Golgi that contains digestive enzymes for breaking down macromolecules (proteins, nucleic acids, lipids, carbohydrates).
Second messenger
An intermediate signalling molecule in the cytoplasm that relays signals from a receptor to a target inside the cell. Examples include Ca²⁺, cyclic AMP (cAMP) and cGMP.
Cytoskeleton
The network of intracellular rods and fibres that supports the cell, enables movement and organises organelles. Made up of microtubules, microfilaments, intermediate filaments and coat proteins.
Microtubules
Hollow tubes made of alpha- and beta-tubulin dimers. The thickest cytoskeletal element. Functions include vesicle transport (via dynein and kinesin motor proteins), beating of cilia and flagella, and moving chromosomes during cell division.
Microfilaments (actin filaments)
Double helices of actin protein. Used for muscle contraction (with myosin), cell shape changes, cytoplasmic streaming and the contractile ring during cytokinesis.
Intermediate filaments
Rope-like protein fibres that provide structural support. They form a meshwork inside the cell, shaping cells and anchoring organelles.
Motor proteins
Proteins that use ATP to move along cytoskeletal tracks. Key examples: kinesin and dynein (move along microtubules), myosin (moves along actin filaments).
Coat proteins
Proteins (e.g. clathrin) that form crosslinked fibres on membrane surfaces, shaping and budding vesicles for intracellular transport.
Centrosome
The microtubule organising centre of an animal cell. Contains a pair of centrioles.
Centriole
A small cylindrical organelle near the nucleus in animal cells, involved in organising the spindle fibres during cell division.
Kinetochore
A protein complex on each chromatid at the centromere where spindle microtubules attach.
Centromere
The constricted region of a chromosome that holds sister chromatids together and serves as the attachment point for the kinetochore.
Sister chromatids
The two identical copies of a chromosome that are produced by DNA replication and joined at the centromere.
Mitosis
Division of the nucleus in eukaryotic cells, producing two daughter nuclei each with the same chromosome number as the parent. Phases: prophase, prometaphase, metaphase, anaphase, telophase.
Cytokinesis
Division of the cytoplasm following mitosis, producing two separate daughter cells. In animal cells, a cleavage furrow (contractile ring of actin and myosin) pinches the cell in two.
Diploid (2n)
A cell containing two complete sets of homologous chromosomes.
Haploid (1n)
A cell containing one complete set of chromosomes.
Meiosis
A two-stage cell division that produces four haploid daughter cells from one diploid parent. Meiosis I is the reductional division (chromosome count halved); meiosis II is the equational division (chromosome count unchanged).
Homologous chromosomes
Pairs of chromosomes that match in size, shape and gene content, one inherited from each parent.
Chiasma
The physical site where homologous chromosomes cross over and exchange segments during meiosis I.
Binary fission
The process by which prokaryotes (bacteria and archaea) divide, producing two daughter cells. Simpler than mitosis, with no spindle apparatus.
The route a secreted protein takes through the cell:
Ribosome on rough ER translates the mRNA. The growing polypeptide enters the ER lumen via its signal sequence.
The protein is folded and may be glycosylated inside the ER.
Transport vesicles bud off the ER and carry the protein to the Golgi apparatus.
The Golgi further modifies, sorts and packages the protein. Material moves from the cis face (nearest the ER) to the trans face (nearest the plasma membrane).
Secretory vesicles bud from the trans Golgi and fuse with the plasma membrane, releasing the protein outside the cell (exocytosis).
Proteins destined for the lysosome are tagged and diverted from the Golgi into lysosomal vesicles, where acidic enzymes break down macromolecules.
The smooth ER handles lipid and steroid synthesis, carbohydrate metabolism and calcium storage (important for muscle contraction).
Three main fibre types, from thickest to thinnest:
Microtubules (tubulin): move vesicles (via kinesin towards the + end, dynein towards the − end), form the mitotic spindle, and drive cilia/flagella.
Microfilaments (actin): power muscle contraction (with myosin), cell crawling (pseudopodia, filopodia, lamellipodia), cytoplasmic streaming and the cleavage furrow during cytokinesis.
Intermediate filaments (various proteins): provide mechanical strength, anchor organelles, maintain cell shape. They do not have motor proteins.
Coat proteins such as clathrin shape membranes and bud vesicles for intracellular trafficking.
Pseudopodia: broad, blobby extensions (e.g. amoeba movement)
Filopodia: thin, finger-like projections (sensing the environment)
Lamellipodia: flat, sheet-like extensions (cell crawling)
Mitosis divides the nucleus so each daughter cell gets a complete, identical set of chromosomes.
G2 Interphase: The cell has already replicated its DNA. It grows and synthesises proteins needed for division.
Prophase: Chromatin condenses into visible chromosomes. Each consists of two sister chromatids joined at the centromere.
Prometaphase: The nuclear envelope breaks down. Spindle microtubules (from the centrosomes) attach to kinetochores on the chromatids.
Metaphase: Chromosomes align at the metaphase plate (the cell's equator).
Anaphase: Sister chromatids separate and are pulled to opposite poles by shortening spindle microtubules.
Telophase: Nuclear envelopes reform around each set of chromosomes, which begin to decondense.
Cytokinesis: The cytoplasm divides. In animal cells, a cleavage furrow (ring of actin and myosin) pinches the cell in two.
Result: two diploid (2n) daughter cells, genetically identical to the parent.
Meiosis consists of two consecutive divisions that produce four haploid cells.
Meiosis I (reductional division): Homologous chromosomes pair up, cross over at chiasmata, then separate. The chromosome number is halved.
This is where genetic diversity is generated: independent assortment of homologues and crossing over shuffle allele combinations.
Meiosis II (equational division): Sister chromatids separate, much like mitosis. The chromosome number stays the same.
Result: four haploid (1n) daughter cells, each genetically unique.
Mitosis: one division, two diploid daughters, genetically identical.
Meiosis: two divisions, four haploid daughters, genetically unique.
Somatic cells undergo mitosis; germ cells (gametes) undergo meiosis.
Cyclins are regulatory proteins whose concentrations rise and fall during the cell cycle.
Cyclin-dependent kinases (CDKs) are always present but only active when bound to the appropriate cyclin. They phosphorylate target proteins to advance the cell through checkpoints.
Kinases in general add phosphate groups to target proteins, switching them on or off.
Students confuse centromere and centrosome. The centromere is the region on the chromosome where sister chromatids are joined; the centrosome is the microtubule organising centre of the cell.
Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids. Students often get these reversed.
Cytokinesis is not part of mitosis. Mitosis is nuclear division; cytokinesis is cytoplasmic division. They typically happen together but are distinct processes.
The smooth ER is not simply "ER without ribosomes doing nothing." It has specific and important functions: lipid synthesis, detoxification and calcium storage.
⚠️ Know the order of mitotic phases and what happens at each one. Exam questions often present a description and ask you to name the phase.
⚠️ Be able to compare mitosis and meiosis in terms of number of divisions, daughter cell ploidy and genetic variation.
⚠️ Understand the endomembrane pathway from rough ER → transport vesicles → Golgi → secretory vesicles → cell exterior.
⚠️ Know the three cytoskeletal fibre types, their protein composition and their functions.
⚠️ Cyclin/CDK regulation of the cell cycle is a favourite exam topic, especially in the context of cancer (uncontrolled cell division).
True or False: Microtubules are the thinnest element of the cytoskeleton. (False, they are the thickest; microfilaments are the thinnest.)
Fill in the blank: The phase of mitosis in which chromosomes line up at the cell's equator is __________. (Metaphase.)
True or False: Meiosis I separates sister chromatids. (False, meiosis I separates homologous chromosomes; meiosis II separates sister chromatids.)
Fill in the blank: The contractile ring that pinches the cell in two during cytokinesis is made of __________ and __________. (Actin (microfilaments) and myosin.)
True or False: Lysosomes are derived from the rough ER. (False, they are derived from the Golgi apparatus.)
Q: Trace the path of a secreted protein from synthesis to export, naming every organelle it passes through.
A: The protein is synthesised by a ribosome on the rough ER and enters the ER lumen. It is folded and may be glycosylated in the ER. Transport vesicles carry it to the cis face of the Golgi apparatus, where it is further modified as it moves to the trans face. Secretory vesicles bud from the trans Golgi and fuse with the plasma membrane, releasing the protein outside the cell.
Q: Compare the outcomes of mitosis and meiosis in terms of daughter cell number, ploidy and genetic identity.
A: Mitosis produces two diploid (2n) daughter cells that are genetically identical to the parent. Meiosis produces four haploid (1n) daughter cells that are genetically unique, owing to crossing over and independent assortment.
Q: A cell is observed with its chromosomes aligned on the metaphase plate and homologous pairs visible. Is it in mitosis or meiosis? Which stage?
A: This is metaphase I of meiosis. In mitosis, individual chromosomes (not homologous pairs) align at the metaphase plate. Homologous pairs lined up together is a hallmark of meiosis I.
Q: Explain how cyclins and CDKs work together to regulate the cell cycle.
A: Cyclin proteins accumulate during specific phases of the cell cycle and bind to CDKs, activating them. The activated CDK then phosphorylates target proteins that drive the cell past a checkpoint. After the checkpoint is passed, the cyclin is degraded and the CDK becomes inactive until the next cyclin accumulates.
Cell division connects to genetics: mitosis maintains chromosome number in somatic cells, while meiosis generates the genetic diversity that underlies inheritance patterns (covered in the Genetics notes). The cytoskeleton and endomembrane system connect to signal transduction, because vesicle trafficking and receptor-mediated endocytosis depend on both systems working together.
Endomembrane system, rough ER, smooth ER, Golgi apparatus, lysosome, vesicle, exocytosis, endocytosis, signal sequence, nuclear envelope, nuclear pore, cytoskeleton, microtubule, microfilament, actin, tubulin, intermediate filament, motor protein, kinesin, dynein, myosin, clathrin, centrosome, centriole, kinetochore, centromere, sister chromatids, mitosis, prophase, metaphase, anaphase, telophase, cytokinesis, cleavage furrow, meiosis, meiosis I, meiosis II, diploid, haploid, homologous chromosomes, crossing over, chiasma, cyclin, CDK, binary fission, BIO 151 Exam 2