Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

2.1.3Scaling Up

Mitosis and the Cell Cycle

5 min read 9 parts Lesson 4 of 16 1 quiz

0%read

Section 1 of 9The cell cycle

Mitosis and the cell cycle
DNA Replication — 3D-Rendered Step-by-Step Simulation
DNA Replication — 3D Step-by-Step Simulation
r083-cyclin-cdk-cell-cycle
water cycle

The cell cycle is the series of events by which a body cell grows and divides to produce two identical daughter cells. It has three main stages.

1Growth and DNA replication
Growth and DNA replication the cell grows larger, increases the number of sub-cellular structures such as ribosomes and mitochondria, and makes extra protein. The DNA in each chromosome is copied (replicated), so that there are two identical copies of each chromosome joined together.
2Mitosis
Mitosis the copies of chromosomes are separated and pulled to opposite ends of the cell, and two nuclei form.
3Cytokinesis
Cytokinesis the cytoplasm and cell membrane divide to form two new daughter cells.

The cell cycle is controlled by genes. If control fails, cells may divide uncontrollably, which results in a tumour; this is how cancer begins.

Section 2 of 9Stages of mitosis

Mitosis itself is a continuous process, but it is helpful to describe it in stages.

1
The chromosomes (each made of two identical copies) condense and become visible, and the nuclear membrane breaks down.
2
The chromosomes line up along the centre of the cell.
3
Fibres pull the copies apart, one to each end of the cell.
4
Membranes form around each set of chromosomes, forming two nuclei.

The result is two nuclei with identical sets of chromosomes. After cytokinesis, there are two cells genetically identical to each other and to the parent cell.

Section 3 of 9Why mitosis is important

The Cell Cycle
asexual reproduction
Growth
Growth a multicellular organism grows by making more cells.
Repair and replacement
Repair and replacement damaged or worn-out cells, such as skin cells or red blood cells, are replaced.
Asexual reproduction
Asexual reproduction some organisms (e.g. many plants, yeast, bacteria use a related process) produce genetically identical offspring, called clones, by mitosis.

Since the daughter cells are genetically identical, there is no variation from mitosis. That is an advantage when the parent is well adapted, but a disadvantage if the environment changes.

Section 4 of 9Calculations and data

Cell cycle: interphase (G1, S, G2) and mitosis
cell cycle landscape

You may need to analyse the proportion of cells in different stages.

Calculations and data
In a root tip, a high proportion of cells in mitosis shows a region of rapid growth.
If a student counts 200 cells in a root tip and 30 are in mitosis, the percentage is 30 / 200 x 100 = 15%.
A related use is working out how long a cell spends in mitosis: if a cell cycle takes 24 hours and 15% of cells are dividing, mitosis takes about 3.6 hours.

Doubling is also tested

  • starting from one cell, after n divisions there are 2n cells.
  • After 5 divisions there are 32 cells.
  • In standard form this becomes useful for rapid growth, such as tumours where cells divide repeatedly.

StageWhat happens to DNA
Interphase (growth)DNA is replicated
MitosisCopies are separated into two nuclei
CytokinesisCell divides into two

Section 5 of 9Cancer and cell division

If mutations damage the genes that control the cell cycle, a cell can divide without stopping and form a mass of cells called a tumour.

Benign
A benign tumour grows in one place and does not invade other tissues; a malignant tumour invades neighbouring tissues and spreads through the blood to form secondary tumours elsewhere, which is cancer.
Risk factors include smoking, UV radiation and some inherited genes.
Treatments aim to remove the tumour or to kill the dividing cells.

This lesson connects to stem cells, which divide by mitosis, and to inheritance, where a different division (meiosis) is used to make gametes.

Section 6 of 9Observing mitosis in a root tip

Root tip
Stained root tip
root tip landscape
Stained root tip
root tip growth zones
Root tip
Micrograph of stained cells in mitosis
Dividing plant cells illustrating stages of mitosis
Dividing plant cells illustrating stages of mitosis

The tip of a growing onion or garlic root is a good place to see mitosis, because the cells there divide rapidly.

1
In a typical preparation, the tip is warmed in acid to separate the cells, stained to make the chromosomes visible, and squashed flat under a coverslip so that the layer is one cell thick.
2
Under the microscope you can see cells in different stages: those with a clear nucleus are not dividing, while those with visible threads of condensed chromosomes are in mitosis.
3
Counting several fields and calculating the percentage in mitosis gives an estimate of how fast the tissue is growing.
4
Cells further up the root, in the region of elongation, show very few dividing cells.

Remember that because mitosis is brief compared with the whole cycle, most cells in any sample appear to be in the growth stage, not dividing.

Section 7 of 9Exam-style questions

Nucleus, Chromosome and DNA
Nucleus, Chromosome and DNA
r094-mitosis-vs-meiosis

  1. Describe what happens to the DNA before mitosis begins. [2]2 marks

  2. A root tip contains 250 cells; 40 are in mitosis. Calculate the percentage of cells in mitosis. [2]2 marks

  3. Explain why the cells produced by mitosis are genetically identical and why this is important in a healing wound. [3]3 marks

Section 8 of 9Key terms

THE PROCESS OF CELL DIVISION BY MITOSIS — INTERACTIVE SIMULATION
Cell division by mitosis in five labelled steps
TermMeaning
Cell cycleSeries of stages in which a cell grows and divides.
MitosisDivision of the nucleus producing two identical nuclei.
CytokinesisDivision of the cytoplasm and membrane.
ReplicationCopying of DNA.
CloneAn organism genetically identical to its parent.
TumourAn abnormal mass of cells formed by uncontrolled division.
BenignA tumour that does not spread.
MalignantA tumour that invades and spreads (cancer).

Section 9 of 9Exam technique

1
Saying mitosis produces gametes; that is meiosis.
2
Missing out the DNA replication stage before division.
3
Forgetting that cytokinesis, the division of the cytoplasm, is separate from nuclear division.
4
Saying daughter cells have half the chromosomes. They have the same number as the parent.
Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.