Cell: The Building Block of Life · Lesson 7 of 9
How do Normal Cells Grow and Divide?
“Cells divide with impressive discipline—until the rules are ignored.”
• Explain why growth requires an increase in cell number. • Describe how an onion root tip can be used to observe dividing cells. • State the roles of cell division in growth, repair and reproduction. • Compare the outcomes and purposes of mitosis and meiosis. • Explain how fertilisation restores chromosome number. • Describe the requirements and uses of cell culture. • Relate errors in cell division to possible biological problems.
A small cut gradually closes, new roots grow from an onion bulb and a single fertilised egg develops into a body containing trillions of cells. Cells cannot simply expand forever. They grow only to a certain size and then divide, increasing the number of cells available for growth, repair and reproduction.
Old, dead or damaged cells must be replaced. Growth of a multicellular organism also depends mainly on producing more cells rather than making each existing cell enormous. Prokaryotic and eukaryotic cells both divide, but eukaryotic cell division is organised through a controlled sequence called the cell cycle. The detailed stages are studied later; here the focus is on why cells divide and the outcomes of the two major types.
Activity: Let us enhance our skills
Fresh onion root tips contain many cells that divide continuously. A prepared root-tip squash spreads these cells into a thin layer so that different appearances can be observed under a microscope.
| Stage of preparation | Action | Purpose |
|---|---|---|
| Grow roots | Place an onion bulb over water for 5–6 days. | Produces actively growing root tips. |
| Preserve | Transfer short root pieces to aceto-alcohol and later to 70% ethanol. | Preserves the tissue. |
| Soften | Treat washed root tips with dilute hydrochloric acid and rinse. | Makes the tissue easier to separate. |
| Stain | Add aceto-carmine and gently warm with caution. | Improves chromosome contrast. |
| Squash | Cut the tip, cover it and press gently. | Spreads cells into a thin observable layer. |
| Observe | Use a microscope. | Reveals cells with different division-related appearances. |
Acids, stains, aceto-alcohol, ethanol and a spirit lamp require supervision. Wear suitable protection, use small quantities, keep flammable liquids away from flame and warm the slide only as directed.
Problem
A student cuts a piece from the middle of an old onion root instead of using its freshly growing tip. Why might fewer dividing cells be observed?
- 1.The root grows mainly near its tip.
- 2.Cells in this growing region divide frequently to produce new cells.
- 3.Older regions contain more cells that have already taken on established roles.
- 4.Therefore, the older middle region is likely to show fewer actively dividing cells than the tip.
Cell Division
The process by which new cells are formed from pre-existing cells.
| Need | How cell division helps |
|---|---|
| Growth | Increases the number of cells. |
| Repair | Replaces cells in damaged tissue. |
| Maintenance | Replaces cells that are regularly lost or worn out. |
| Asexual reproduction | Produces new cells with inherited information like the parent cell. |
| Sexual reproduction | Produces gametes with half the chromosome number. |
Mitosis
Mitosis is the most common type of division in body growth and repair. One parent cell produces two daughter cells. Each receives the same number of chromosomes and largely the same genetic information as the parent cell. This consistency allows a tissue to replace lost cells while continuing its function.
Meiosis
Meiosis produces gametes and occurs in cells of reproductive organs. In animals, it occurs in the testes to produce sperm and in ovaries to produce eggs. In plants, it occurs in anthers and ovaries as part of gamete formation. The parent cell divides twice and produces four daughter cells, each with half the chromosome number.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Daughter cells | Two | Four |
| Chromosome number | Same as parent cell | Half of parent cell |
| Genetic relationship | Daughter cells are genetically identical to the parent cell | Gametes contribute to variation among offspring |
| Main roles | Growth, repair, maintenance and asexual reproduction | Gamete production for sexual reproduction |
Problem
Identify the suitable type of division for replacing skin cells after a cut and for producing sperm cells.
- 1.Replacement skin cells must retain the same chromosome number and information needed for skin function.
- 2.Mitosis produces two similar daughter cells with the parent chromosome number, so it supports healing.
- 3.Sperm cells must contain half the chromosome number so that fertilisation does not double it in every generation.
- 4.Meiosis produces four cells with half the chromosome number, so it produces gametes.
Problem
Why must gametes contain half the chromosome number of the parent cell?
- 1.Meiosis reduces the chromosome number by half in each gamete.
- 2.During fertilisation, a gamete from one parent combines with a gamete from the other parent.
- 3.The two half sets together restore the normal full chromosome number.
- 4.If gametes retained the full number, fertilisation would produce a cell with twice the expected chromosome number.
Bridging Science and Society
The growth of plant or animal cells outside the body under carefully maintained conditions.
Cells used in culture are placed in a nutrient-rich medium. Suitable temperature, moisture and acidic or alkaline conditions are maintained, and sterile handling prevents unwanted microorganisms from taking over. Cell culture supports the study of cell behaviour and the production of biochemicals, food products, medicines and vaccines.
| Requirement | Why it matters |
|---|---|
| Nutrient-rich medium | Supplies materials needed for growth. |
| Suitable temperature | Supports normal cellular activity. |
| Suitable acidic or alkaline condition | Maintains a workable chemical environment. |
| Moisture | Maintains the aqueous conditions needed by cells. |
| Sterile conditions | Prevents contamination by other organisms. |
Mitosis and meiosis must remain controlled. Errors in mitosis can contribute to uncontrolled division, tumour formation or abnormal chromosome numbers in body cells. Errors in meiosis may produce gametes with abnormal chromosome numbers and may be associated with developmental problems, early pregnancy loss or reduced fertility.
Pause and Ponder
Problem
Predict what would happen if cells repairing a skin cut divided through meiosis instead of mitosis.
- 1.Meiosis would reduce the chromosome number by half.
- 2.It would produce four cells rather than two matching replacement cells.
- 3.The new cells would not have the normal chromosome number required for ordinary skin-cell function.
- 4.Repair would therefore not proceed normally.
Mitosis is not simply 'division in animals' and meiosis is not simply 'division in plants'. Both processes occur in plants and animals, but in different cells and for different purposes.
Quiz
Which description best matches Cell division?
Which description best matches Cell culture?
Which term matches this description: The process by which new cells are formed from pre-existing cells.
Which term matches this description: The growth of plant or animal cells outside the body under carefully maintained conditions.
Which statement is a key takeaway from this lesson?
Practice Problems
- Why do onion root-tip cells show several different appearances under a microscope?
- List three roles of mitosis in a multicellular organism.
- Compare the number and chromosome content of cells produced by mitosis and meiosis.
- Explain how meiosis and fertilisation work together to maintain chromosome number.
- Why are sterile conditions important in cell culture?
- Describe one possible consequence of an error in mitosis and one of an error in meiosis.
Key Takeaways
• New cells arise through cell division. • Mitosis produces two genetically identical cells with the parent chromosome number. • Meiosis involves two divisions and produces four gametes with half the chromosome number. • Fertilisation restores the full chromosome number. • Cell culture requires nutrients and carefully controlled, sterile conditions. • Errors in division can disturb normal growth or reproduction.