Cell Cycle and Cell Division

The One Thing

The cell cycle is an ordered sequence of growth, DNA duplication, and division that ensures genetic material is accurately distributed to new cells. Mitosis maintains chromosome number and genetic similarity, whereas meiosis halves chromosome number and generates genetic variation for sexual reproduction.

Reactions, Processes and Experiments

Key Terms

• Cell cycle: The series of events from the formation of a cell until it divides into daughter cells.
• Interphase: The active phase between divisions, consisting of G1, S, and G2 phases.
• G1 phase: The first growth phase in which the cell increases in size and produces proteins and organelles.
• S phase: The synthesis phase in which DNA is replicated, so each chromosome forms two sister chromatids.
• G2 phase: The second growth phase in which the cell prepares proteins and structures needed for division.
• G0 phase: A resting or non-dividing state in which some cells temporarily or permanently leave the active cell cycle.
• M phase: The division phase, including nuclear division and usually cytoplasmic division.
• Karyokinesis: Division of the nucleus during mitosis or meiosis.
• Cytokinesis: Division of the cytoplasm to form separate daughter cells.
• Chromosome: A condensed DNA-protein structure that carries genetic information.
• Sister chromatids: Identical copies of a chromosome joined at the centromere after DNA replication.
• Centromere: The chromosome region where sister chromatids are joined and spindle fibres attach.
• Spindle fibres: Microtubule structures that attach to chromosomes and help move them during division.
• Mitosis: A type of equational division in which one diploid cell forms two genetically identical daughter cells with the same chromosome number.
• Prophase: The stage in which chromosomes condense, the nucleolus and nuclear envelope disappear, and the spindle begins to form.
• Metaphase: The stage in which chromosomes arrange at the equatorial plane of the cell.
• Anaphase: The stage in which centromeres divide and sister chromatids move to opposite poles.
• Telophase: The stage in which chromosomes reach the poles, become less condensed, and new nuclear envelopes form.
• Meiosis: A reduction division in which one diploid cell produces four genetically different haploid cells through two divisions.
• Meiosis I: The reductional division in which homologous chromosomes separate, reducing the chromosome number by half.
• Meiosis II: The equational division in which sister chromatids separate, resembling mitosis.
• Synapsis: Pairing of homologous chromosomes during prophase I of meiosis.
• Bivalent or tetrad: A paired unit of homologous chromosomes containing four chromatids during prophase I.
• Crossing over: Exchange of genetic material between non-sister chromatids of homologous chromosomes.
• Chiasmata: Visible points where crossing over has occurred between homologous chromosomes.
• Homologous chromosomes: A pair of chromosomes, one from each parent, carrying genes for the same traits at corresponding positions.
• Haploid: Having one set of chromosomes, represented as n.
• Diploid: Having two sets of chromosomes, represented as 2n.
• Genetic variation: Differences in genetic composition produced by crossing over, independent assortment, and random fertilisation.

Easily Confused

• Interphase and a resting stage: Interphase is metabolically active and includes G1, S, and G2; it is not a resting stage.
• S phase and chromosome-number doubling: DNA replication occurs in S phase and doubles DNA amount, but it does not double chromosome number.
• Mitosis and meiosis: Mitosis produces two genetically similar diploid cells, whereas meiosis produces four genetically different haploid cells.
• Mitosis and cytokinesis: Mitosis refers to nuclear division; cytokinesis refers to division of the cytoplasm.
• Karyokinesis and cytokinesis: Karyokinesis separates the nucleus, whereas cytokinesis separates the cytoplasm.
• Anaphase of mitosis and anaphase I of meiosis: Sister chromatids separate in mitotic anaphase, whereas homologous chromosomes separate in anaphase I and sister chromatids remain together.
• Meiosis I and meiosis II: Homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II.
• Synapsis and crossing over: Synapsis is the pairing of homologous chromosomes during zygotene; crossing over is the exchange of genetic material during pachytene.
• Bivalent or tetrad and chiasmata: A bivalent or tetrad is the paired unit of homologous chromosomes; chiasmata are the visible points where crossing over has occurred.
• Homologous chromosomes and sister chromatids: Homologous chromosomes are a maternal and paternal pair, whereas sister chromatids are identical copies of one chromosome.
• Diploid and haploid: Diploid cells have two chromosome sets, represented as 2n; haploid cells have one set, represented as n.
• Animal and plant cytokinesis: Animal cells form a cleavage furrow, whereas plant cells form a cell plate.
• Chromosome number and DNA amount: DNA amount doubles during S phase, but chromosome number remains unchanged.

What Gets Asked

• Identify the phase of the cell cycle or division from its events. Marks are lost by placing DNA replication outside S phase or treating interphase as a resting stage.
• Compare mitosis and meiosis. The required distinctions include two versus four daughter cells, genetic similarity versus genetic difference, and diploid versus haploid products.
• Explain chromosome behaviour during anaphase. In mitosis and meiosis II, sister chromatids separate; in meiosis I, homologous chromosomes separate while sister chromatids remain together.
• Describe the substages of prophase I. The specific associations are synapsis in zygotene, crossing over in pachytene, and clearly visible chiasmata in diplotene.
• Apply chromosome notation to humans. Somatic cells are diploid, with 2n = 46, whereas gametes are haploid, with n = 23.
• Explain the importance of cell-cycle regulation and meiosis. Relevant points include checkpoints and genetic stability, prevention of chromosome doubling after fertilisation, and variation from crossing over, independent assortment, and random fertilisation.