Mitosis (M phase of the cell cycle) is divided into karyokinesis (nuclear division — including prophase, metaphase, anaphase, telophase) and cytokinesis (cytoplasm division). Both are required to produce two complete daughter cells.

Turner's syndrome (45,X or 45,XO) occurs when one sex chromosome is missing. These individuals have only 45 chromosomes with a single X chromosome and no Y chromosome, developing as phenotypically female but with gonadal dysgenesis.

The mitotic spindle apparatus is fully formed and functional during metaphase. However, spindle formation begins in late prophase/prometaphase. At metaphase, all chromosomes are attached to spindle fibres and aligned at the equatorial plate.

Klinefelter's syndrome (47,XXY) individuals have male phenotype because they have a Y chromosome directing male development. However they have reduced testosterone, small testes, infertility, and often gynaecomastia (breast development).

Klinefelter's syndrome (47,XXY) involves additional sex chromosomes — males have one extra X chromosome. Down's syndrome (autosomal trisomy 21), Turner's syndrome (45,X — missing a sex chromosome), and Cri-du-chat (chromosome 5 deletion) differ.

Crossing over is the physical exchange of corresponding segments between non-sister chromatids of homologous chromosomes during pachytene of meiosis I. It produces recombinant chromosomes with new combinations of alleles.

Benign tumours are localised, encapsulated, non-invasive, and do not metastasise. They grow slowly and do not invade surrounding tissue. Malignant tumours (cancers) invade and metastasise, making them life-threatening.

Genetic recombination (recombination) is the reshuffling/exchange of genetic material between homologous chromosomes during crossing over in meiosis I. It produces new combinations of alleles (recombinant chromosomes), increasing genetic diversity.

Metastasis is the process by which cancer cells detach from the primary tumour, enter blood or lymph vessels, travel to distant sites, and establish new (secondary/metastatic) tumours. Metastasis makes cancer much harder to treat.

Meiosis II resembles mitosis — sister chromatids separate (just like mitosis) rather than homologous chromosomes separating (as in meiosis I). The key difference is that meiosis II starts with haploid cells while mitosis starts with diploid cells.

Meiosis occurs during: sexual cell (gamete) formation in animals (oogenesis and spermatogenesis) and spore formation in plants (the sporophyte produces spores by meiosis). Both result in haploid cells.

Down's syndrome (trisomy 21) is caused by non-disjunction of chromosome 21 during meiosis. The affected individual has three copies of chromosome 21 (total 47 chromosomes) instead of the normal two.

Cancer is caused by mutations in somatic (body) cells — accumulated mutations in proto-oncogenes, tumour suppressor genes, and DNA repair genes lead to uncontrolled cell proliferation. Germline (sex cell) mutations cause inherited cancer predispositions.

At metaphase, each chromosome consists of two identical sister chromatids joined at the centromere — the result of DNA replication in S phase. Metaphase chromosomes are maximally condensed, making them ideal for karyotyping.

Karyokinesis (nuclear division) includes all four stages: Prophase (chromosome condensation, spindle formation), Metaphase (chromosome alignment), Anaphase (chromosome separation), and Telophase (nuclear envelope reformation).

Chromatin is the complex of DNA and proteins (histones and non-histones) that forms chromosomes. During interphase it appears as a network of fine threads — diffuse and active in gene expression. It condenses into visible chromosomes during division.

Cytokinesis is the division of the cytoplasm following karyokinesis. In animal cells, an actin-myosin contractile ring forms a cleavage furrow. In plant cells, a cell plate (new cell wall) forms between daughter nuclei.

Chromosomes first become visible during prophase as they condense. In interphase (before prophase), chromosomes are decondensed and not visible. During prophase they gradually become visible as distinct condensed structures.

The mitotic apparatus consists of: asters (star-shaped arrays of microtubules around each centriole), spindle fibres (microtubules connecting poles to chromosomes and to each other), and the two poles (centrosomes).

Karyokinesis (from Greek: karyon = nucleus, kinesis = movement) is the division of the nucleus. It encompasses prophase, metaphase, anaphase, and telophase of mitosis or meiosis.

Apoptosis (Greek: 'falling off', like leaves from a tree) is programmed cell death — an orderly, genetically controlled self-destruction process. It is essential for development (digit formation), immune regulation, and tumour suppression.

Necrosis is cell death due to tissue damage — injury, infection, or ischaemia. It is uncontrolled and causes inflammation. Unlike apoptosis (which is clean and programmed), necrosis releases cellular contents causing inflammatory response.

Turner's syndrome (45,X) survivors have: female external appearance (due to absence of Y chromosome), short stature (~150 cm), webbed neck, low posterior hairline, shield chest, streak gonads (no functional ovaries), and infertility.

Apoptosis (programmed cell death) involves cell shrinkage, chromatin condensation, membrane blebbing, and fragmentation into apoptotic bodies which are engulfed by phagocytes. It is a clean, regulated process unlike necrosis.

Turner's syndrome (45,X or 45,XO) results from absence of one sex chromosome. Affected individuals are phenotypically female with short stature, webbed neck, shield chest, absent ovaries (gonadal dysgenesis), and infertility.

Klinefelter's syndrome variants exist with 48,XXXY (48 chromosomes) and 49,XXXXY (49 chromosomes). More X chromosomes correlate with more severe intellectual disability and physical abnormalities.

Klinefelter's syndrome (47,XXY) results from an extra X chromosome in males. Affected individuals are phenotypically male but have small testes, infertility, reduced testosterone, gynaecomastia, and often tall stature.

Down's syndrome (trisomy 21) is characterised by: flat broad face, epicanthal folds (skin fold at inner corner of eye), protruding tongue, intellectual disability, hypotonia, and often congenital heart defects. It occurs in ~1/700 births.

Down's syndrome (trisomy 21) results from non-disjunction of chromosome 21 during meiosis, producing a gamete with 24 chromosomes. When fertilised by a normal gamete (23 chromosomes), the zygote has 47 chromosomes (trisomy 21).

Non-disjunction is the failure of homologous chromosomes (meiosis I) or sister chromatids (meiosis II or mitosis) to separate properly during cell division. It results in gametes with abnormal chromosome numbers (aneuploidy).

Metaphase I begins with nuclear membrane breakdown. Bivalents align at the equatorial plate. Kinetochore fibres from opposite poles attach to the kinetochores of homologous (not sister) chromosomes, preparing for their separation.

Diakinesis is the final substage of prophase I — chromosomes are maximally condensed, homologous chromosomes have largely separated but remain connected at chiasmata, and the nuclear envelope begins to break down.

Crossing over (recombination) occurs during the pachytene stage of prophase I. The synaptonemal complex is fully formed and non-sister chromatids of homologous chromosomes exchange segments at chiasmata.

A bivalent (also called a tetrad) is the paired structure formed during prophase I of meiosis when homologous chromosomes synapse. It consists of two homologous chromosomes, each made of two sister chromatids (4 chromatids total).

Prophase I of meiosis is divided into five substages: Leptotene (chromosomes condense), Zygotene (synapsis begins), Pachytene (crossing over occurs), Diplotene (bivalents separate), and Diakinesis (maximum condensation).

Meiosis occurs specifically during gametogenesis (gamete formation in animals) and sporogenesis (spore formation in plants). It reduces the chromosome number and introduces genetic variation through crossing over.

Meiosis is reductive division — it produces four haploid (n) daughter cells from one diploid (2n) parent cell, halving the chromosome number. This is essential for sexual reproduction to maintain species chromosome number.

Cancer cells are characterised by: high nucleus-to-cytoplasm ratio, enlarged and irregular nuclei, prominent nucleoli, frequent mitotic divisions (often abnormal), loss of contact inhibition, and ability to invade surrounding tissue.

A tumour (neoplasm) is an abnormal mass of cells produced by unregulated cell growth and division. Tumours can be benign (localised, non-invasive) or malignant (invasive, metastatic — these are cancers).

Uncontrolled mitosis results in tumour formation, cancer, and malfunction of normal tissue. Cancer occurs when cell cycle checkpoints fail, allowing cells to divide without normal controls, forming malignant tumours.

Spindle microtubules are composed of tubulin protein — alpha and beta tubulin dimers that polymerise to form hollow cylindrical tubes. GTP hydrolysis drives microtubule dynamics (assembly and disassembly).

During telophase, the chromosomes decondense and uncoil back into dispersed chromatin as the nuclear envelope reforms around each set of chromosomes, forming two daughter nuclei.

The mitotic apparatus (spindle fibres + asters) is organised by the end of prophase (late prophase/prometaphase). The spindle fibres attach to chromosomes at their kinetochores, ready to align them at metaphase plate.

During interphase, chromosomes are decondensed and exist as dispersed chromatin — too thin and diffuse to be visible even by electron microscopy. Chromosomes only become visible as they condense during prophase.

Cytokinesis is the physical division of the cytoplasm into two daughter cells following karyokinesis. In animal cells it occurs by cleavage furrow; in plant cells by cell plate formation in the middle of the cell.

Karyokinesis is the division of the nucleus — it includes prophase, metaphase, anaphase, and telophase of mitosis. Cytokinesis is the subsequent division of the cytoplasm to produce two separate daughter cells.

Mitosis is equational division — it produces two daughter cells each with the same chromosome number (2n) as the parent cell. Meiosis reduces the chromosome number to half (n). Mitosis is used for growth and repair.

The average human cell cycle takes about 24 hours: G1 (~10 hours), S phase (~8 hours), G2 (~4 hours), and M phase (~1 hour). However this varies greatly between different cell types and organisms.

The S (synthesis) phase is the phase during which DNA replication occurs. It follows G1 and precedes G2. After S phase, the cell enters G2 (pre-mitotic/post-synthetic gap) before entering mitosis.

The mitotic phase (M phase) is the period of actual cell division — it includes karyokinesis (nuclear division) and cytokinesis (cytoplasm division). Interphase is the preparatory period between divisions.