Mitosis and the cell cycleEdexcel A-Level Biology A: Revision notes
Section 1
The cell cycle
The cell cycle is the sequence of events from one cell division to the next. It has two main parts.
- Interphase: the longest part, when the cell is not dividing. In G1 the cell grows and makes proteins and organelles. In the S phase the DNA is replicated. In G2 the cell grows further and prepares for division.
- Mitosis followed by cytokinesis: the nucleus divides and then the cytoplasm divides.
Most cells in a tissue are in interphase at any one time, because mitosis is a short part of the cycle.
DNA replication happens in interphase (S phase), not during mitosis.
Section 2
Preparing for mitosis: DNA replication
In the S phase each chromosome is copied. DNA replication is semi-conservative: each original strand acts as a template, and complementary bases pair up, so each new DNA molecule has one old and one new strand.
After replication each chromosome consists of two identical sister chromatids joined at a centromere. This is why every daughter cell can receive an exact copy of all the genetic information.
Section 3
The stages of mitosis
- Prophase: chromosomes condense and become visible, the nuclear envelope breaks down and the spindle forms from microtubules.
- Metaphase: chromosomes line up on the equator, attached to spindle fibres by their centromeres.
- Anaphase: centromeres divide and the chromatids, now separate chromosomes, are pulled to opposite poles.
- Telophase: chromatids reach the poles and uncoil, nuclear envelopes re-form and the cytoplasm divides (cytokinesis).
The result is two daughter cells with the same number and type of chromosomes as the parent.
In anaphase it is chromatids that are pulled apart, not pairs of homologous chromosomes; that happens in meiosis.
Section 4
Why mitosis matters
Because DNA is copied exactly and shared equally, mitosis produces genetically identical daughter cells. It is used for:
- growth: increasing the number of cells in an organism
- replacement and repair of damaged cells
- asexual reproduction: for example runners in strawberry plants, producing clones with the same genes as the parent.
An advantage of asexual reproduction is that good characteristics are passed on and no mate is needed. The disadvantage is a lack of variation, so a whole population may be vulnerable to the same disease.
Section 5
Core practical 5: root tip squash
Method. Cut about 5 mm from a growing root tip, where cells divide in the meristem. Warm it in hydrochloric acid, which breaks down the middle lamella so the cells separate. Rinse, place on a slide, add a stain that binds to DNA (such as toluidine blue or acetic orcein), then lower a coverslip and squash the tissue gently to make a single thin layer. View with a light microscope, starting at low power.
Mitotic index = number of cells in mitosis ÷ total number of cells counted. For example 32 dividing cells out of 200 gives 0.16. Multiplying by the length of the cell cycle estimates the time spent in mitosis: 0.16 × 18 h = 2.9 h.
Count cells in several fields of view to improve reliability. Take care with hydrochloric acid and stains, wear eye protection.
Say what the stain binds to (DNA) and why you squash the tissue (a thin layer lets light through).
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Mitosis and the cell cycle
- A student prepares a root tip squash from garlic roots to observe the stages of mitosis. She cuts the last few millimetres from the tip of a root, warms it in hydrochloric acid, adds a stain that binds to DNA, and then squashes it under a coverslip before viewing it with a light microscope.State the purpose of (i) warming the root tip in hydrochloric acid and (ii) adding the stain.2 marks
- Using a light microscope, the student examines a root tip squash and counts 200 cells. She finds 168 cells in interphase, 14 in prophase, 6 in metaphase, 4 in anaphase and 8 in telophase. The whole cell cycle in this species takes 18 hours.Calculate the mean time that a cell in this root tip spends in mitosis. Give your answer in hours to two significant figures.2 marks
- A chemotherapy drug prevents spindle fibres from forming. Tumour cells exposed to the drug are found with condensed chromosomes, each made of two chromatids, scattered through the cell. The cells do not divide and many later die.Explain why chromosomes cannot be separated into two daughter nuclei when spindle formation is prevented.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).