Section 1 • Specification 3.14

Introduction to the Cell Cycle

Understanding how cells divide to produce genetically identical daughter cells for growth and asexual reproduction

One of the most awe-inspiring processes of life is the way organisms reproduce. Amoeba produce more Amoeba, liver cells generate more liver cells, and bougainvillea create new bougainvillea — all through the remarkable process of cell division.

Most new biological material results from the process of nuclear division known as mitosis, followed by division of the rest of the cell. This produces genetically identical offspring from a single parent cell.

Asexual reproduction is the production of genetically identical offspring from a single parent cell or organism. Growth (an increase in cell numbers) is also the result of mitotic cell division.

The production of offspring by sexual reproduction is also mostly dependent on mitosis to produce new cells after the gametes (sex cells) have fused. In mitosis, the chromosomes of a cell are duplicated and the genetic information is then equally shared out between the two daughter cells.

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Mnemonic: MIRGAS

Mitosis = Repair, Growth, Asexual reproduction

🎯 Learning Objectives

Understand the role of the cell cycle in producing genetically identical daughter cells Describe the phases of the cell cycle Explain the stages of mitosis Understand how cyclins and CDKs control the cell cycle

🧠 Did You Know? — Permanent Cells

Some cells do not enter the cell cycle once they have formed — they must last a lifetime. Examples include nerve cells, the light-sensitive cells of your retina, the transparent cells of the lens of your eye, and cardiac muscle.

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Checkpoint Questions

Test your understanding

Question 1

What type of cell division produces genetically identical daughter cells?

Question 2

Which of the following is NOT a function of mitosis?

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Exam Practice

Answer in your notes

Question 1

4 marks

Describe the role of the cell cycle in the growth of a multicellular organism.

Section 2

Phases of the Cell Cycle

The cell cycle consists of interphase (G₁, S, G₂), mitosis, and cytokinesis

Cells divide on a regular basis through a sequence of events known as the cell cycle. The length of the cell cycle is variable — it can be very rapid, taking 24 hours or less, or it can take a few years.

Interphase is a period of non-division when cells increase in mass and size, carry out normal cellular activities and replicate their DNA ready for division. It consists of three stages:

G₁ (Gap 1) — The cell grows, takes in material, and produces organelles. This phase is highly variable: in actively dividing cells it's very short (hours or days), but in other cells it can be months or even years.

S Phase (Synthesis) — DNA replication occurs. Chromosomes replicate and become double-stranded chromatids ready for the next cell division.

G₂ (Gap 2) — Organelles and other materials needed for cell division are synthesised. Before a cell can divide, it needs two of everything.

Mitosis — Nuclear division occurs (covered in detail in the next section).

Cytokinesis — The final stage when the cytoplasm divides and new cells separate.

Cell Cycle Diagram

Hover over phases for details

Interphase ~90% of cycle

G₁

S

G₂

M

G₁ (~45%)

S (~35%)

G₂ (~10%)

M (~10%)

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Checkpoint Questions

Test your understanding

Question 1

During which phase of interphase does DNA replication occur?

Question 2

A cell culture divides every 48 hours. If S phase takes 35% of the cycle, how long does S phase last?

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Exam Practice

Answer in your notes

Question 1

3 marks

A cell culture is dividing with a cell cycle of 24 hours. Calculate the time spent in each phase given: G₁ = 45%, S = 33%, G₂ = 14%, M = 8%. Show your working.

Section 3

Mitosis — Nuclear Division

The process by which a cell divides to produce two genetically identical daughter cells

Mitosis is the division of the nucleus that results in each daughter cell receiving an identical set of chromosomes. It is a continuous process, but for convenience we divide it into stages: prophase, prometaphase, metaphase, anaphase, and telophase, followed by cytokinesis.

Interactive Mitosis Animation

Click each stage to see the cellular changes

Interphase

The cell is not dividing. This is the longest phase of the cell cycle.

DNA exists as diffuse chromatin (not visible as individual chromosomes)
Nuclear envelope and nucleolus are intact
DNA replication occurs during S phase
Centrioles replicate ready for cell division

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Mnemonic: IPMAT-C

Interphase, Prophase, Metaphase, Anaphase, Telophase, Cytokinesis — or try "I Passed My Anatomy Test Completely!"

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Checkpoint Questions

Test your understanding

Question 1

Why do chromosomes only become visible as a cell goes into mitosis?

Question 2

During which stage of mitosis do chromosomes line up along the metaphase plate (cell equator)?

Question 3

What happens to sister chromatids during anaphase?

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Exam Practice

Answer in your notes

Question 1

6 marks

Describe the events that occur during mitosis, from prophase to telophase.

Section 4

Chromosomes & DNA Packaging

How DNA is organised into chromosomes using histones and nucleosomes

A chromosome is made up of a mass of coiled threads of DNA and proteins. If a chromosome were the same length as five consecutive letters on this page, the DNA molecule it contained would stretch the length of a football pitch!

When the DNA molecules condense, they need to be packaged very efficiently. This is achieved with the help of positively charged basic proteins called histones.

The DNA winds around the histones to form dense clusters known as nucleosomes (like beads on a string). These interact to produce more coiling and then supercoiling to form the dense chromosome structures visible through the microscope during cell division.

In the supercoiled areas, the genes are not available to be transcribed to make proteins. This is why chromosomes only become visible during cell division — when transcription is paused.

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Chromatids vs Chromosomes

After DNA replication, each chromosome consists of two identical sister chromatids joined at the centromere. During anaphase, these separate to become individual chromosomes.

🧬 Levels of DNA Packaging

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DNA Double Helix

Base genetic structure

2nm

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Nucleosome

DNA wound around histones

10nm

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Chromatin Fibre

Coiled nucleosomes

30nm

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Chromosome

Supercoiled, visible at division

700nm+

🔬 Human Karyotype — 46 Chromosomes

22 pairs of autosomes + 1 pair of sex chromosomes (XX or XY)

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X/Y

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Checkpoint Questions

Test your understanding

Question 1

What is the role of histones in DNA packaging?

Question 2

How many chromosomes are found in a normal human somatic (body) cell?

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Exam Practice

Answer in your notes

Question 1

6 marks

Explain how DNA is packaged into chromosomes, including the role of histones and nucleosomes.

Section 5

Control of the Cell Cycle

How cyclins and cyclin-dependent kinases (CDKs) regulate cell division

The cell cycle is controlled by chemical signals made in response to different genes. This control occurs at checkpoints when the cell cycle moves from one phase to the next.

The key regulators are small proteins called cyclins which build up during interphase. These bind to enzymes called cyclin-dependent kinases (CDKs) to form active complexes.

The cyclin-CDK complex phosphorylates target proteins, changing their shape and activity, which triggers the next stage of the cell cycle.

Examples of cyclin-CDK actions include:

Phosphorylation of chromatin proteins — causes chromosomes to condense and become denser
Phosphorylation of nuclear envelope proteins — leads to breakdown of the nuclear membrane during prometaphase

🔄 Cyclin-CDK Mechanism

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Cyclins accumulate gradually during interphase

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Cyclins bind to CDKs, activating them

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Active cyclin-CDK phosphorylates target proteins

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Cell progresses past checkpoint to next phase

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Cyclins are degraded; CDKs become inactive again

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Checkpoint Questions

Test your understanding

Question 1

What type of enzyme is a CDK (cyclin-dependent kinase)?

Question 2

Which of the following is NOT controlled by the cyclin-CDK complex?

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Exam Practice

Answer in your notes

Question 1

5 marks

Describe the role of cyclins and cyclin-dependent kinases (CDKs) in controlling the cell cycle.

Reference

Subject Vocabulary

Key terms and definitions for this topic

Mitosis

The process by which a cell divides to produce two genetically identical daughter cells

Asexual Reproduction

The production of genetically identical offspring from a single parent or organism

Sexual Reproduction

The production of offspring that are genetically different from the parent organisms by the fusing of two sex cells (gametes)

Meiosis

A form of cell division in which the chromosome number of the original cell is halved, leading to the formation of gametes

Cell Cycle

A regulated process of three stages (interphase, mitosis and cytokinesis) in which cells divide into two genetically identical daughter cells

Interphase

The period between active cell divisions when cells increase their size and mass, replicate their DNA and carry out normal metabolic activities

Cytokinesis

The final stage of the cell cycle — division of the cytoplasm at the end of mitosis to form two independent, genetically identical cells

Histones

Positively charged proteins involved in the coiling of DNA to form dense chromosomes in cell division

Nucleosomes

Dense clusters of DNA wound around histones (described as "beads on a string")

Chromatid

One strand of the replicated chromosome pair that is joined to the other chromatid at the centromere

Centromere

The region of a chromosome where sister chromatids are joined and where spindle fibres attach during cell division

Karyotype

A way of displaying an image of the chromosomes of a cell to show the pairs of autosomes and sex chromosomes

Cyclins

Small proteins that build up during interphase and are involved in the control of the cell cycle by their attachment to cyclin-dependent kinases

CDKs

Cyclin-dependent kinases: enzymes involved in the control of the cell cycle by phosphorylating other proteins, activated by attachment to cyclins