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5.1.35 · The Mitotic Cell Cycle

The Cell Cycle — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 10 parts Lesson 5 of 12 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

5.1.3 The Cell Cycle — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the phases of the cell cycle and the checkpoints that regulate it, and calculate the proportion of time or cells in each phase.

  • Three parts
  • Movement between phases
  • Interphase = G1 + S + G2
  • G1 (first gap)
  • S (synthesis)
  • G2 (second gap)

Part 1 of 8The cell cycle

1Cell cycle
Cell cycle the regulated sequence of events between one cell division and the next, in which a cell grows, replicates its DNA and divides into two daughter cells.
2Three parts
  • Three parts interphase, nuclear division and cytokinesis.
    • (mitosis, the M phase)
3Length
  • Length it varies with environmental conditions, cell type and organism, for example about 20 hours in onion root tip cells and about 10 hours in human intestinal epithelium.
4Movement between phases
  • Movement between phases it is triggered by chemical signals called cyclins.

Part 2 of 8Interphase

1Interphase
G1 + S + G2
  • the cell is not dividing but is very active, and it takes most of the cycle.
    • (often 90% or more)
2G1 (first gap)
  • G1 (first gap) the cell grows, makes RNA, enzymes, other proteins and organelles, and carries out its normal functions.
  • A signal to divide again is received at some point in G1.
  • Cells that do not divide again, such as neurones, stay in a resting state called G0.
3S (synthesis)
  • DNA is replicated semi-conservatively, so each chromosome becomes two identical sister chromatids, and histones are made.
  • S phase is relatively short.
4G2 (second gap)
  • G2 (second gap) the cell continues to grow and makes proteins needed for mitosis such as tubulin for the spindle.
  • New DNA is checked and errors are usually repaired, and ATP is stored.

Part 3 of 8M phase and cytokinesis

1Mitosis (M phase)
  • Mitosis (M phase) it follows interphase, cell growth stops, and the nucleus divides into two genetically identical nuclei.
2Cytokinesis
  • Cytokinesis it follows mitosis, and the whole cell divides with one nucleus moving into each daughter.
  • In animal cells the cytoplasm constricts between the nuclei, and in plant cells a new cell wall forms.

Part 4 of 8Checkpoints and control

1
Checkpoint
  • Checkpoint a control point at which the cell decides whether to proceed, so cells divide only when appropriate and safe.
2
G1 checkpoint
  • G1 checkpoint it checks cell size, nutrients, growth factors and DNA damage, and is the most important restriction point.
  • If conditions are poor the cell stays in G1 or enters G0.
3
G2 checkpoint
  • G2 checkpoint it checks that DNA replication is complete and that no damage has occurred.
4
Spindle (metaphase) checkpoint
  • Spindle (metaphase) checkpoint it checks that all chromosomes are attached to the spindle at the equator before anaphase,
    • so each daughter cell gets the right number.
5
Regulators
  • Regulators cyclins rise and fall in concentration, and cyclin-dependent kinases (CDKs) are active only when bound to cyclin.
  • Active cyclin–CDK complexes phosphorylate other proteins to trigger the next stage.
6
p53
  • p53 this protein stops the cycle if DNA is damaged and can trigger repair or apoptosis.
7
Mutations
  • Mutations mutations in genes for control proteins can lead to cancer.

Part 5 of 8Time spent in each phase

1Mitotic index
Mitotic index number of cells in mitosis ÷ total number of cells observed, which is a proportion and not a time.
2Time in a stage
(cells in the stage ÷ total cells) × length of the complete cell cycle.
PhaseTypical duration in a fast-dividing human cell (about 24 h cycle)
G1About 11 hours
SAbout 8 hours
G2About 4 hours
MAbout 1 hour

Part 6 of 8Mitotic index and its uses

High mitotic index
  • High mitotic index it is found in actively growing tissue such as root tips, embryos, bone marrow and the intestinal lining.
Low mitotic index
  • Low mitotic index it is found in tissue that rarely divides.
Biopsy
  • Biopsy a high mitotic index may indicate a tumour.
Cancer drugs
  • Cancer drugs they target dividing cells by blocking S phase or the spindle, which is why they also affect hair follicles and the gut lining.
    • (DNA replication)
Growth factors
  • Growth factors they bind to receptors on the cell surface membrane and start the cycle, and plant meristems respond to hormones such as cytokinins.

Part 7 of 8Reading graphs of the cycle

1DNA content per cell
  • DNA content per cell it is low in G1, rises steadily during S, plateaus at double the level in G2,
    • and falls suddenly to the original value as the cell divides at the end of M.
2Cyclin amount
  • Cyclin amount it rises to a peak just before mitosis and then falls rapidly as it is degraded, so the cell can exit mitosis.
3S phase
  • DNA doubles in S phase and does not halve.

Part 8 of 8More from the specification

1Telomeres
  • Non-coding DNA sequences located at the ends of linear chromosomes that prevent the degradation of essential genes during successive cycles of DNA replication.
2Tissue Repair and Replacement
  • Mitosis occurs in multicellular organisms to replace damaged or dead cells,
  • allowing tissue renewal such as replenishing specialised red blood cells and skin cells from adult stem cells.
3Mitotic Progeny and Parent Cell
  • The mitotic cell cycle results in two genetically identical daughter cells that maintain the exact chromosome number and genome of the original parent cell.
4Adult Stem Cells
  • Undifferentiated multipotent cells found in adult tissues that divide repeatedly to produce differentiated, specialised cells for tissue maintenance.
5Telomeres
  • Non-coding DNA sequences located at the ends of linear chromosomes that prevent the degradation of essential genes during successive cycles of DNA replication.
6Tissue Repair and Replacement
  • Mitosis occurs in multicellular organisms to replace damaged or dead cells,
  • allowing tissue renewal such as replenishing specialised red blood cells and skin cells from adult stem cells.
7Mitotic Progeny and Parent Cell
  • The mitotic cell cycle results in two genetically identical daughter cells that maintain the exact chromosome number and genome of the original parent cell.
8Adult Stem Cells
  • Undifferentiated multipotent cells found in adult tissues that divide repeatedly to produce differentiated, specialised cells for tissue maintenance.
9Malignant tumour
  • A rapidly growing tumour that invades tissues and may metastasise.
10Telomere
  • Repetitive non-coding DNA at the ends of a chromosome that prevents loss of genes during replication.
11Multicellular organism
  • An organism made of more than one cell.
12Asexual reproduction
  • Reproduction from one parent giving genetically identical offspring.

Key terms

TermMeaning
Cell cycleOrdered series of events from one cell division to the next.
InterphasePhase between divisions (G1, S, G2).
CheckpointControl point at which the cell decides whether to proceed.
CyclinProtein whose level varies in the cycle and activates a CDK.
CDKCyclin-dependent kinase, an enzyme that triggers the stages of the cycle.
Mitotic indexThe proportion of cells in a sample that are undergoing mitosis.
G0A resting state in which a cell has left the cycle.

Avoid these mistakes

1
Stating that interphase is a resting phase — the cell grows and replicates DNA.
2
Mixing up the order G1, S, G2, M.
3
Treating mitotic index as a time — it is a proportion of cells.
4
Saying DNA replication occurs in G2 — it occurs in S phase.

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