Mitosis

1. Overview

Mitosis is a fundamental biological process that allows organisms to grow, heal, and reproduce asexually. It ensures that every new cell created contains an exact copy of the genetic information required for the organism to function, maintaining continuity across generations of cells.

Key Definitions

• Mitosis: Nuclear division giving rise to genetically identical cells.
• Chromosome: A thread-like structure of DNA, carrying genetic information in the form of genes.
• Gene: A length of DNA that codes for a specific protein.
• Genetically Identical: Cells that have the exact same DNA sequence and chromosome number.
• Daughter Cells: The two new cells produced following the division of a parent cell.
• Stem Cells: Unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions.
• Differentiation: The process by which a stem cell becomes a specialised cell (e.g., a muscle cell or a neuron).

Core Content

There are no specific Core-only objectives for this topic; all requirements are part of the Supplement/Extended curriculum.

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Extended Content (Extended Curriculum Only)

The Nature of Mitosis

Mitosis is defined as nuclear division. Its primary purpose is to ensure that when a cell divides, each resulting daughter cell receives a full, identical set of chromosomes.

• Genetic Consistency: Because the DNA is copied exactly, the daughter cells are genetically identical to each other and to the original parent cell.
• Chromosome Number: Mitosis maintains the chromosome number. For example, a human body cell with 46 chromosomes will divide to produce two daughter cells, each also containing 46 chromosomes.

The Process: Step-by-Step

1. Replication: Before mitosis begins, the exact replication of chromosomes occurs. Each chromosome is copied to create two identical "sister" strands (chromatids) joined together.
   • 📊A single chromosome (straight line) showing an arrow leading to a replicated chromosome (X-shape), with a label indicating the two identical chromatids.
2. Alignment and Separation: During mitosis, these copies of chromosomes separate.
3. Division: The nucleus divides into two, and the cell membrane moves to separate the cytoplasm, forming two distinct cells.
   • 📊A mother cell with 4 chromosomes in the center, followed by a stage showing the copies being pulled to opposite sides of the cell, and finally two separate daughter cells each containing 4 chromosomes.

The Role of Mitosis

Mitosis is essential for four main biological functions:

1. Growth: Increasing the number of cells in an organism (e.g., a seedling growing into a tree).
2. Repair of damaged tissues: Creating new cells to close a wound or fix an injury.
3. Replacement of cells: Substituting cells that die naturally or wear out (e.g., skin cells or red blood cells).
4. Asexual Reproduction: Producing offspring that are clones of the parent (common in bacteria, fungi, and some plants).

Stem Cells

Stem cells are unique because they are unspecialised.

• They divide by mitosis to produce more daughter cells.
• These daughter cells have the potential to undergo differentiation, where they develop specific structures to perform specific functions (such as becoming a red blood cell, a white blood cell, or a nerve cell).

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Key Equations

There are no mathematical equations for this topic. However, remember the ratio: 1 Parent Cell $\rightarrow$ 2 Genetically Identical Daughter Cells.

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Common Mistakes to Avoid

• ❌ Wrong: Thinking that growth is just cells getting larger.
  • ✓ Right: Growth involves an increase in the number of cells through mitosis.
• ❌ Wrong: Stating that chromosomes replicate during mitosis.
  • ✓ Right: Chromosomes replicate before mitosis begins (during a phase called interphase).
• ❌ Wrong: Confusing mitosis with meiosis (the division used for making gametes).
  • ✓ Right: Mitosis results in identical cells; meiosis results in non-identical cells with half the chromosome number.
• ❌ Wrong: Using the term "cell division" when "nuclear division" is specifically asked for in the definition of mitosis.

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Exam Tips

• Keywords: Whenever you describe mitosis, always use the phrase "genetically identical." It is almost always a marking point in exam schemes.
• Command Words: If a question asks you to "State the role of mitosis," ensure you list at least three: growth, repair, and replacement.
• Stem Cells: When describing stem cells, you must mention two things: their ability to divide (staying unspecialised) and their ability to differentiate (becoming specialised).
• Contextual Questions: You may be asked about mitosis in the context of cancer. Cancer is the result of uncontrolled mitosis (uncontrolled cell division).
• Diagrams: You are not required to name the stages (Prophase, Metaphase, etc.), but you should be able to recognise a diagram showing chromosomes being pulled apart to opposite poles.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) Define the term mitosis. [2]

(b) State two roles of mitosis in multicellular organisms. [2]

(c) Explain why it is important that chromosomes are replicated before mitosis occurs. [1]

Worked Solution:

(a)

1. Mitosis is a type of nuclear division. This is the basic definition.

2. That results in two genetically identical daughter cells. Identical to each other and the parent cell.

How to earn full marks:

• Mention that it is nuclear division $\boxed{}$
• Mention that the cells produced are genetically identical $\boxed{}$

(b)

1. Growth This is a key role.

2. Repair of damaged tissues Another important function.

How to earn full marks:

• State growth $\boxed{}$
• State repair of damaged tissues $\boxed{}$

(c)

1. To ensure each daughter cell receives a full set of chromosomes. Maintaining the correct chromosome number.

How to earn full marks:

• State that each daughter cell gets a full set of chromosomes $\boxed{}$

Common Pitfall: Many students forget to mention that mitosis is specifically nuclear division. Also, be sure to emphasize that the daughter cells are genetically identical to the parent cell, not just similar.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

(a) State two differences between stem cells and specialised cells. [2]

(b) Describe how stem cells are involved in the repair of a damaged organ, such as the liver. [4]

Worked Solution:

(a)

1. Stem cells are unspecialised, whereas specialised cells are specialised. This is the fundamental difference.
2. Stem cells can divide by mitosis to produce more stem cells and cells that can differentiate; specialised cells cannot. Ability to differentiate is key.

How to earn full marks:

• State that stem cells are unspecialised, specialised cells are specialised $\boxed{}$
• State that stem cells can differentiate, but specialised cells cannot $\boxed{}$

(b)

1. Stem cells divide by mitosis. Mitosis is the key process.
2. Producing more stem cells. To maintain the stem cell population.
3. And cells that can differentiate/specialise into liver cells. To replace the damaged cells.
4. These new liver cells replace the damaged cells, repairing the organ. Restoring function.

How to earn full marks:

• State that stem cells divide by mitosis $\boxed{}$
• State that this produces more stem cells and cells that can differentiate $\boxed{}$
• State that these cells differentiate into liver cells $\boxed{}$
• State that the new liver cells replace the damaged cells $\boxed{}$

Common Pitfall: Students often confuse the roles of stem cells and specialized cells. Remember that stem cells are like blank slates that can become specialized, while specialized cells have a specific job and can't change into other types of cells.

Exam-Style Question 3 — Extended Response [9 marks]

Question:

A scientist is studying the growth of a plant root tip. They observe cells undergoing mitosis.

(a) State what happens to the copies of chromosomes during mitosis. [2]

(b) Explain how mitosis contributes to the growth of the root tip. [4]

(c) Suggest one reason why it is important that the chromosome number remains constant during mitosis. [1]

(d) Some chemicals can disrupt mitosis, preventing cell division. Suggest how such chemicals could be used in medicine or agriculture, giving a specific example in each case. [2]

Worked Solution:

(a)

1. The copies of chromosomes separate. This is the primary event.
2. Each copy goes to a separate daughter cell. Ensuring each cell gets a complete set.

How to earn full marks:

• State that the copies of chromosomes separate $\boxed{}$
• State that each copy goes to a separate daughter cell $\boxed{}$

(b)

1. Mitosis produces new cells. Basic principle of cell division.
2. Increasing the number of cells in the root tip. Leading to growth.
3. These new cells can elongate and differentiate. Contributing to overall root length.
4. This elongation leads to increased root length and therefore growth. Linking cell division to macroscopic growth.

How to earn full marks:

• State that mitosis produces new cells $\boxed{}$
• State that this increases the number of cells in the root tip $\boxed{}$
• State that these new cells can elongate and differentiate $\boxed{}$
• State that this leads to increased root length $\boxed{}$

(c)

1. To ensure that the daughter cells function correctly. Maintaining proper cellular processes.

How to earn full marks:

• State that it ensures the daughter cells function correctly $\boxed{}$

(d)

1. Medicine: To treat cancer by stopping the uncontrolled division of cancer cells. Targeting rapidly dividing cells.
2. Agriculture: As herbicides to prevent the growth of weeds. Controlling unwanted plant growth.

How to earn full marks:

• State a medical application (e.g., cancer treatment) and explain how the chemical works $\boxed{}$
• State an agricultural application (e.g., herbicide) and explain how the chemical works $\boxed{}$

Common Pitfall: When explaining the role of mitosis in growth, don't just say "it makes more cells." Explain how more cells lead to increased size or length. Also, remember that chemicals that disrupt mitosis can be used to control cell growth in both medicine and agriculture.

Exam-Style Question 4 — Extended Response [8 marks]

Question:

(a) Define asexual reproduction. [2]

(b) State why mitosis is important in asexual reproduction. [2]

(c) Paramecium caudatum is a single-celled organism that reproduces asexually by binary fission, a process involving mitosis.

(i) Describe what happens to the chromosomes of Paramecium caudatum before binary fission. [2]

(ii) Suggest one advantage and one disadvantage of Paramecium caudatum reproducing asexually by binary fission. [2]

Worked Solution:

(a)

1. Asexual reproduction is reproduction involving only one parent. Parental involvement.
2. Resulting in offspring that are genetically identical to the parent. Key characteristic of asexual reproduction.

How to earn full marks:

• State that it involves only one parent $\boxed{}$
• State that the offspring are genetically identical to the parent $\boxed{}$

(b)

1. Mitosis produces genetically identical cells. Mitosis is the key process.
2. Therefore, asexual reproduction results in offspring genetically identical to the parent. Maintaining genetic consistency.

How to earn full marks:

• State that mitosis produces genetically identical cells $\boxed{}$
• State that this results in offspring genetically identical to the parent $\boxed{}$

(c) (i)

1. The chromosomes are replicated. Duplication is essential.
2. Resulting in two identical copies of each chromosome. Ensuring each daughter cell gets a full set.

How to earn full marks:

• State that the chromosomes are replicated $\boxed{}$
• State that this results in two identical copies of each chromosome $\boxed{}$

(c) (ii)

1. Advantage: Rapid reproduction. Quick population growth.
2. Disadvantage: Lack of genetic variation. Vulnerable to environmental change.

How to earn full marks:

• State a valid advantage (e.g., rapid reproduction) $\boxed{}$
• State a valid disadvantage (e.g., lack of genetic variation) $\boxed{}$

Common Pitfall: Many students forget to mention the genetic similarity between parent and offspring in asexual reproduction. Also, when discussing advantages and disadvantages, be sure to link them directly to the organism's survival and adaptation.