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Next Toppers - 11th ScienceWhole chapter · Hinglish
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Must-know facts
25 facts
- 01Human cells in culture divide about once every 24 hours; yeast can complete a cycle in about 90 minutes.
- 02In the 24-hour human cycle, cell division proper takes only about an hour; interphase is more than 95% of the cycle.
- 03Interphase order: G1 → S → G2. Then the M phase: karyokinesis, then cytokinesis.
- 04S phase: DNA 2C → 4C, but the chromosome number stays 2n.
- 05The centriole duplicates in the cytoplasm during S phase in animal cells.
- 06G0 (quiescent stage) is entered from G1; cells stay metabolically active but stop multiplying. Example: heart cells.
- 07In animals mitosis occurs only in diploid somatic cells; exception: male honey bees (haploid).
- 08Plants show mitosis in both haploid and diploid cells.
- 09Mitosis is equational division: parent and daughters have the same chromosome number.
- 10End of prophase: no Golgi complex, ER, nucleolus or nuclear envelope visible.
- 11Metaphase: the easiest stage for studying chromosome morphology; kinetochores are disc-shaped structures on the centromere.
- 12Anaphase of mitosis: centromeres split, chromatids separate and move to opposite poles, centromere leading.
- 13Telophase: chromosomes decondense; nuclear envelope, nucleolus, Golgi complex and ER reform.
- 14Plant cytokinesis: cell plate forms in the centre and grows outward; it represents the middle lamella.
- 15Karyokinesis without cytokinesis gives a syncytium, e.g. liquid endosperm of coconut.
- 16Onion root tip cells have 16 chromosomes.
- 17Meiosis: two divisions, one DNA replication, four haploid cells.
- 18Prophase I order: leptotene, zygotene, pachytene, diplotene, diakinesis.
- 19Synapsis and the synaptonemal complex appear in zygotene; a synapsed pair is a bivalent or tetrad.
- 20Crossing over happens in pachytene, between non-sister chromatids, at recombination nodules, by the enzyme recombinase.
- 21Chiasmata (X-shaped) become visible in diplotene after the synaptonemal complex dissolves; diplotene can last months or years in some vertebrate oocytes.
- 22Diakinesis: terminalisation of chiasmata.
- 23Anaphase I separates homologues; anaphase II separates sister chromatids.
- 24Interkinesis is short and has no DNA replication.
- 25Meiosis I ends in a dyad of cells; meiosis II ends in a tetrad of four haploid cells.
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Key terms
21 terms
- Cell cycle
- The ordered series of events in which a cell copies its genome, makes its other parts and divides into two.
- Interphase
- The long stretch between two divisions, made up of G1, S and G2, when the cell grows and copies its DNA.
- Karyokinesis
- Division of the nucleus, in which the separated chromosomes are shared between two daughter nuclei.
- Cytokinesis
- Division of the cytoplasm that completes cell division.
- G0 (quiescent stage)
- A stage entered from G1 by cells that stop multiplying while staying metabolically active.
- Chromatid
- One of the two identical copies of a replicated chromosome, joined to its sister at the centromere.
- Centromere
- The point where the two sister chromatids of a chromosome are held together.
- Kinetochore
- Small disc-shaped structure on the centromere where spindle fibres attach.
- Metaphase plate
- The plane at the cell's equator in which chromosomes line up at metaphase.
- Mitotic apparatus
- The two asters together with the spindle fibres.
- Cell plate
- The precursor of the new wall in a dividing plant cell; it becomes the middle lamella.
- Syncytium
- A multinucleate cell formed when nuclear division is not followed by cytokinesis.
- Synapsis
- The pairing of homologous chromosomes during zygotene.
- Synaptonemal complex
- The structure formed along a pair of homologues during synapsis; it dissolves at the start of diplotene.
- Bivalent (tetrad)
- A pair of synapsed homologous chromosomes, four chromatids in all.
- Recombination nodule
- A site in pachytene where crossing over occurs between non-sister chromatids.
- Crossing over
- Enzyme-driven exchange of genetic material between homologous chromosomes; the enzyme is recombinase.
- Chiasmata
- X-shaped points where recombined homologues stay linked after the synaptonemal complex dissolves.
- Terminalisation
- The movement of chiasmata towards the chromosome ends during diakinesis.
- Interkinesis
- The short gap between meiosis I and meiosis II, with no DNA replication.
- Dyad and tetrad of cells
- Two cells after meiosis I; four haploid cells after meiosis II.