Cell Cycle and Cell Division

Biology · Class 11

Lesson 13 of 13 · 15 min

Chapter review

Watch a class

The whole chapter on YouTube

Whole chapter line by line

Competition Wallah · Hinglish · Lecture · Open on YouTube

Full chapter revision with diagrams

Next Toppers - 11th Science · Hinglish · Whole chapter · Open on YouTube

Loading the full lesson

Must-know facts

25 facts

  1. 1Human cells in culture divide about once every 24 hours; yeast can complete a cycle in about 90 minutes.
  2. 2In the 24-hour human cycle, cell division proper takes only about an hour; interphase is more than 95% of the cycle.
  3. 3Interphase order: G1 → S → G2. Then the M phase: karyokinesis, then cytokinesis.
  4. 4S phase: DNA 2C → 4C, but the chromosome number stays 2n.
  5. 5The centriole duplicates in the cytoplasm during S phase in animal cells.
  6. 6G0 (quiescent stage) is entered from G1; cells stay metabolically active but stop multiplying. Example: heart cells.
  7. 7In animals mitosis occurs only in diploid somatic cells; exception: male honey bees (haploid).
  8. 8Plants show mitosis in both haploid and diploid cells.
  9. 9Mitosis is equational division: parent and daughters have the same chromosome number.
  10. 10End of prophase: no Golgi complex, ER, nucleolus or nuclear envelope visible.
  11. 11Metaphase: the easiest stage for studying chromosome morphology; kinetochores are disc-shaped structures on the centromere.
  12. 12Anaphase of mitosis: centromeres split, chromatids separate and move to opposite poles, centromere leading.
  13. 13Telophase: chromosomes decondense; nuclear envelope, nucleolus, Golgi complex and ER reform.
  14. 14Plant cytokinesis: cell plate forms in the centre and grows outward; it represents the middle lamella.
  15. 15Karyokinesis without cytokinesis gives a syncytium, e.g. liquid endosperm of coconut.
  16. 16Onion root tip cells have 16 chromosomes.
  17. 17Meiosis: two divisions, one DNA replication, four haploid cells.
  18. 18Prophase I order: leptotene, zygotene, pachytene, diplotene, diakinesis.
  19. 19Synapsis and the synaptonemal complex appear in zygotene; a synapsed pair is a bivalent or tetrad.
  20. 20Crossing over happens in pachytene, between non-sister chromatids, at recombination nodules, by the enzyme recombinase.
  21. 21Chiasmata (X-shaped) become visible in diplotene after the synaptonemal complex dissolves; diplotene can last months or years in some vertebrate oocytes.
  22. 22Diakinesis: terminalisation of chiasmata.
  23. 23Anaphase I separates homologues; anaphase II separates sister chromatids.
  24. 24Interkinesis is short and has no DNA replication.
  25. 25Meiosis I ends in a dyad of cells; meiosis II ends in a tetrad of four haploid cells.

Common traps

Where marks are lost

Saying the chromosome number doubles in S phase.

Only the DNA doubles (2C to 4C). A 2n cell stays 2n; each chromosome now has two chromatids.

Counting chromatids as chromosomes.

Count centromeres: two chromatids joined at one centromere are one chromosome. They become two chromosomes only when the centromere splits in anaphase or anaphase II.

Putting crossing over in zygotene because that is where pairing starts.

Zygotene pairs (synapsis); pachytene crosses over; diplotene shows the chiasmata. Pair, cross, show.

Thinking chiasmata are where crossing over happens at the moment you see them.

Crossing over is finished by the end of pachytene. Chiasmata are the X-shaped links left behind, visible in diplotene and terminalised in diakinesis.

Mixing up anaphase of mitosis with anaphase I.

Mitotic anaphase splits centromeres and separates chromatids. Anaphase I keeps centromeres whole and separates homologues. Anaphase II is the one like mitosis.

Believing DNA is copied again during interkinesis.

Interkinesis has no DNA replication. Meiosis has two divisions after a single S phase, which is why the end cells are haploid.

Calling G0 a dead or metabolically inactive stage.

G0 cells are metabolically active; they only stop multiplying, and can be called back to divide.

Writing that plant cytokinesis starts at the edge like a furrow.

A rigid wall stops a furrow. The cell plate starts in the centre and grows outward to the side walls.

Thinking mitosis never occurs in haploid cells.

Male honey bees, some social insects, some lower plants and the haploid stages of plants divide by mitosis.

Calling interphase a resting phase in which nothing happens.

Interphase is when the cell grows and copies its DNA; it is more than 95% of the cycle.

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.
Test yourself: 10 questionsExam-style questions on Cell Cycle and Cell Division, with full solutions.Start
Chapter review | Cell Cycle and Cell Division | Lumi Learn