Cambridge O Level5090

Nuclear division

Biology 5090 Chapter Notes

What this chapter covers

Nuclear divisionAsexual and sexual reproductionSexual reproduction in plantsSexual reproduction in humans
ShareWhatsAppPost
Nuclear division notes

Unable to load PDF

The notes viewer could not load. Please refresh the page.

Read online free. Download a watermarked copy with a free account.

Read the notes

The full Nuclear division notes as text: skim, search, and jump between subtopics.

~10 min read

In this chapter

  1. 1Chromosomes and the Cell Cycle
  2. 2Mitosis: Division for Growth and Repair
  3. 3Meiosis: Division for Gamete Formation
  4. 4Comparing Mitosis and Meiosis

1. Chromosomes and the Cell Cycle

The nucleus of a cell contains the genetic instructions for life, packaged into structures called chromosomes. Chromosomes are long, coiled threads of DNA. In the body cells (somatic cells) of most organisms, chromosomes exist in matching pairs, one inherited from each parent. This is called the diploid state (symbolised as 2n). For example, humans have 46 chromosomes in their diploid cells, arranged as 23 pairs. Gametes (sex cells like sperm and eggs) contain only one chromosome from each pair, so they are haploid (n). For humans, the haploid number is 23. Before a cell can divide, it must first make a perfect copy of all its DNA. During this process, each chromosome is replicated. The original chromosome and its copy are called sister chromatids, and they are joined together at a point called the centromere. This replicated chromosome still counts as a single chromosome until the sister chromatids are pulled apart during division. This entire process of growth, DNA replication, and division is known as the cell cycle.

Key term

Diploid (2n): A cell or nucleus containing two complete sets of chromosomes, one from each parent.

Examiner insight

Examiners expect you to know the specific diploid and haploid numbers for humans (46 and 23) as a standard example.

Common pitfall

Confusing the number of chromosomes with the number of chromatids. A replicated chromosome (made of two chromatids) is still counted as one chromosome until the chromatids separate.

Fun fact

If you could unravel all the DNA in all the chromosomes from just one of your cells, it would stretch about 2 metres long!

Worked example 13 marks

A diploid body cell of a cat has 38 chromosomes. Answer the following questions:(a) How many pairs of chromosomes are there in a cat's skin cell?(b) How many chromosomes will be in a cat's egg cell?(c) How many chromatids will be present in a skin cell just before it divides by mitosis?

  1. 1

    (a) The cell is diploid with 38 chromosomes. Chromosomes exist in pairs in diploid cells. Therefore, the number of pairs is 38 / 2 = 19 pairs.

  2. 2

    (b) An egg cell is a gamete, which is haploid (n). It contains half the number of chromosomes of a diploid cell. Therefore, the number of chromosomes is 38 / 2 = 19 chromosomes.

  3. 3

    (c) Before mitosis, each chromosome replicates to form two sister chromatids. So, if there are 38 chromosomes, there will be 38 x 2 = 76 chromatids.

Recap

  • Chromosomes are thread-like structures in the nucleus that carry genetic information.
  • Diploid (2n) cells have two sets of chromosomes, one from each parent.
  • Haploid (n) cells have one set of chromosomes.
  • Before cell division, each chromosome replicates to form two identical sister chromatids.
  • Human somatic cells are diploid with 46 chromosomes, while human gametes are haploid with 23 chromosomes.

Quick check

  1. What is the difference between a diploid and a haploid cell?2 marks
  2. What are sister chromatids?1 mark

2. Mitosis: Division for Growth and Repair

Mitosis is a type of nuclear division that produces two daughter nuclei that are genetically identical to each other and to the parent nucleus. It is fundamental for life. The main purposes of mitosis are: growth of multicellular organisms (from a single zygote to a full adult), repair of damaged tissues (e.g., healing a cut), replacement of worn-out cells (e.g., skin cells or red blood cells), and asexual reproduction in some organisms. The process ensures that every new body cell receives a complete and identical set of chromosomes. Although it's a continuous process, it can be described in stages. First, the replicated chromosomes condense and become visible. Then, they line up along the centre (equator) of the cell. After that, the sister chromatids of each chromosome are pulled apart to opposite ends (poles) of the cell. Finally, a new nucleus forms around each set of chromosomes, and the cytoplasm divides, resulting in two separate, identical diploid cells.

Key term

Mitosis: A type of nuclear division resulting in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, essential for growth and repair.

Examiner insight

Marks are often awarded for linking mitosis to specific, concrete examples like repairing a wound, replacing red blood cells, or the growth of a plant root tip.

Common pitfall

Stating that mitosis 'creates' cells for growth, instead of specifying it 'increases the number of cells' for growth. The cells then enlarge.

Worked example 13 marks

Explain the importance of mitosis in a multicellular organism like a human.

  1. 1
    1. Growth: Mitosis allows an organism to grow from a single cell (zygote) into a complex, multicellular adult by increasing the number of cells.
  2. 2
    1. Repair and Replacement: Mitosis produces new cells to replace those that are old, damaged, or have been lost. For example, it replaces skin cells that are constantly shed or cells to heal a wound.
  3. 3
    1. Genetic Stability: By producing genetically identical cells, mitosis ensures that all body cells have the same genetic information, allowing them to function correctly.

Worked example 22 marks

A cell with 16 chromosomes in its nucleus divides by mitosis. State the number of chromosomes present in each of the daughter cells and explain your answer.

  1. 1

    Number of chromosomes: 16.

  2. 2

    Explanation: During mitosis, the chromosomes are replicated exactly and then separated into two new nuclei. This process ensures that each daughter cell receives a full, identical set of chromosomes, so the chromosome number is maintained from the parent cell to the daughter cells.

Recap

  • Mitosis is nuclear division that produces two genetically identical daughter cells.
  • The daughter cells are diploid (2n), just like the parent cell.
  • Mitosis is essential for growth, repair of tissues, and replacement of cells.
  • It is also the basis for asexual reproduction in some organisms.
  • The process ensures the chromosome number is maintained in the new cells.

Quick check

  1. State two reasons why mitosis is important.2 marks
  2. Are the cells produced by mitosis genetically identical or different?1 mark

3. Meiosis: Division for Gamete Formation

Meiosis is a special type of nuclear division that reduces the chromosome number by half, creating four genetically different cells. In animals, this process occurs in the reproductive organs (testes and ovaries) to produce gametes (sperm and eggs). Meiosis involves two separate divisions, known as Meiosis I and Meiosis II. In Meiosis I, pairs of homologous chromosomes are separated, which halves the chromosome number. In Meiosis II, the sister chromatids of each chromosome are separated (similar to mitosis). The final result is four haploid(n) cells. Crucially, these cells are genetically different from the parent cell and from each other. This variation is introduced in two main ways: 1) 'Crossing over', where homologous chromosomes exchange segments of DNA, and 2) 'Independent assortment', where the chromosome pairs line up randomly before being separated. This genetic variation is the raw material for evolution and is why siblings from the same parents look different.

Key term

Meiosis: A type of reduction division in which the chromosome number is halved from diploid to haploid, resulting in four genetically different gametes.

Examiner insight

Examiners look for the understanding that meiosis has two key outcomes: halving the chromosome number (producing haploid cells) AND creating genetic variation.

Common pitfall

Confusing meiosis with mitosis, particularly by stating that meiosis produces two cells or that the cells are genetically identical.

Fun fact

The number of possible chromosome combinations in a human gamete from meiosis is over 8 million (2 to the power of 23), and that's before accounting for the genetic shuffling from crossing over!

Worked example 13 marks

A diploid cell in a human contains 46 chromosomes. If this cell undergoes meiosis, how many chromosomes will be in each of the resulting cells? Explain why this number is important.

  1. 1

    Number of chromosomes: Each resulting cell will have 23 chromosomes.

  2. 2

    Explanation: Meiosis is a reduction division, meaning it halves the chromosome number. This is important because the resulting cells are gametes (sex cells).

  3. 3

    During fertilisation, two gametes (e.g., a sperm with 23 chromosomes and an egg with 23 chromosomes) fuse. This restores the full diploid number (46 chromosomes) in the zygote, ensuring the offspring has the correct number of chromosomes for its species.

Worked example 22 marks

Explain how meiosis produces genetic variation.

  1. 1
    1. Meiosis produces genetically different cells, unlike mitosis which produces identical cells.
  2. 2
    1. This happens because during meiosis, the paternal and maternal chromosomes are shuffled and dealt out randomly into the gametes (independent assortment).
  3. 3
    1. This creates new combinations of chromosomes in the gametes, ensuring that offspring from sexual reproduction are genetically different from their parents and each other.

Recap

  • Meiosis is a reduction division that produces gametes.
  • It involves two cell divisions, resulting in four daughter cells.
  • The daughter cells are haploid (n), containing half the chromosomes of the parent cell.
  • The daughter cells are genetically different from the parent cell and from each other.
  • Meiosis is the source of genetic variation in sexually reproducing organisms.

Quick check

  1. Where in the human body does meiosis occur?2 marks
  2. How many daughter cells are produced by meiosis, and what is their ploidy level?2 marks

4. Comparing Mitosis and Meiosis

Understanding the differences between mitosis and meiosis is a common exam topic. While both are types of nuclear division, they have very different purposes and outcomes. Here is a point-by-point comparison:

Purpose:

  • Mitosis: For growth, repair, cell replacement, and asexual reproduction.
  • Meiosis: For the production of gametes for sexual reproduction.

Location:

  • Mitosis: Occurs in all somatic (body) cells throughout an organism's life.
  • Meiosis: Occurs only in germline cells within the reproductive organs (gonads).

Number of Divisions:

  • Mitosis: One round of division.
  • Meiosis: Two consecutive rounds of division (Meiosis I and Meiosis II).

Number of Daughter Cells:

  • Mitosis: Produces two daughter cells.
  • Meiosis: Produces four daughter cells.

Chromosome Number (Ploidy):

  • Mitosis: Daughter cells are diploid (2n) and have the same chromosome number as the parent cell.
  • Meiosis: Daughter cells are haploid (n) and have half the chromosome number of the parent cell.

Genetic Variation:

  • Mitosis: Daughter cells are genetically identical to the parent cell and to each other (clones).
  • Meiosis: Daughter cells are genetically different from the parent cell and from each other, creating variation.

Key term

Somatic Cell: Any cell of a living organism other than the reproductive cells (gametes).

Examiner insight

Comparison questions are very common. Use a table or comparative words like 'whereas' or 'while' and structure your answer point-by-point to show the contrast clearly and gain maximum marks.

Common pitfall

Making vague comparisons like 'they are different'. You must state precisely *how* they are different for each feature (e.g., 'Mitosis produces two cells, whereas meiosis produces four').

Worked example 13 marks

Complete the table to compare mitosis and meiosis.

  1. 1

    Feature | Mitosis | Meiosis

  2. 2
  3. 3

    Number of daughter cells | Two | Four

  4. 4

    Chromosome number in daughter cells | Diploid (same as parent) | Haploid (half of parent)

  5. 5

    Genetic variation | None (cells are identical) | Yes (cells are different)

Worked example 23 marks

A scientist observes a cell dividing. They note that the resulting cells have half the number of chromosomes as the starting cell. Which type of division are they observing, and what is the function of this type of division in mammals?

  1. 1

    Type of division: Meiosis.

  2. 2

    Reason: Meiosis is a reduction division that halves the chromosome number.

  3. 3

    Function: In mammals, the function of meiosis is to produce gametes (sperm and eggs) for sexual reproduction.

Recap

  • Mitosis produces two diploid cells, while meiosis produces four haploid cells.
  • Mitosis results in genetically identical cells, whereas meiosis results in genetically unique cells.
  • Mitosis involves one division, while meiosis involves two divisions.
  • Mitosis occurs in somatic cells for growth and repair.
  • Meiosis occurs in reproductive organs to produce gametes.

Quick check

  1. Which division process is essential for creating genetic variation?1 mark
  2. A cell divides to produce two new cells, each with 46 chromosomes. Was this mitosis or meiosis?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the term 'haploid nucleus'.1 mark
  2. State one location in a flowering plant where meiosis occurs.1 mark
  3. Explain the role of mitosis in the repair of damaged tissues.3 marks
  4. A cell in the ovary of a dog contains 78 chromosomes. This cell divides by meiosis. State the number of cells produced and the number of chromosomes in each.2 marks
  5. Describe how a chromosome appears just before mitosis, compared to how it appears long after division is complete.2 marks
  6. Explain why it is essential that gametes are haploid for sexual reproduction to be successful.3 marks
  7. Compare the outcomes of mitosis and meiosis. Your answer should refer to the number of daughter cells, their chromosome number, and their genetic relationship to the parent cell.4 marks
  8. A rice plant has 24 chromosomes in its root cells. A pollen grain nucleus from this plant fertilises an ovule nucleus. How many chromosomes will be in the zygote that is formed? Explain your answer.4 marks
  9. Stem cells are unspecialised cells that divide by mitosis to produce daughter cells which can then become specialised. Explain why division by mitosis is important for the function of stem cells.3 marks
  10. Describe the sequence of events involving chromosomes during mitosis that ensures the two daughter cells are genetically identical.5 marks

Go deeper

Practise and revise with member-only material for this chapter.

Workbook

Exam-style practice questions with full worked solutions.

Unlock with a plan

Chapter at a Glance

A single-page summary of every key idea for fast revision.

Unlock with a plan

Free notes are just the start.

Unlock every Workbook and Chapter at a Glance, and generate your own worksheets and predicted papers.

Explore plans

Related chapters

Cell structure and functionConcept and use of a classification systemDiffusion and osmosisBiological moleculesEnzyme actionPhotosynthesis