5.1 Cell Division
Cell division is a fundamental biological process controlled by genetic material inside the cell's nucleus. Explore the structural hierarchy connecting genes, DNA and chromosomes, and investigate the mechanisms and importance of mitosis and meiosis.
Learning Objectives
- Explain the relationship between genes, deoxyribonucleic acid (DNA), and chromosomes.
- Describe the structure of a chromosome and interpret a human karyotype.
- Differentiate between autosomes and sex chromosomes.
- Describe the stages and events of mitosis and meiosis.
- Compare and contrast the similarities and differences between mitosis and meiosis.
- Justify the importance of mitosis and meiosis in living organisms.
Hierarchy of Genetic Material
Genes form DNA, DNA coils into chromosomes, and chromosomes are located inside the nucleus.
Short Explanation
Definition
Genetics is the study of genes, heredity, and variations in organisms.
Inside every cell's nucleus lies the genetic blueprint of life. Chromosomes are thread-like structures composed of DNA molecules tightly coiled around proteins.
DNA and Genes
Each DNA molecule is a double helix built from nucleotide building blocks: deoxyribose sugar, a nitrogenous base and a phosphate group. Specific segments along the DNA strand are called genes, which code for individual characteristics.
Human Chromosomes
Humans possess 46 chromosomes (23 homologous pairs): 44 autosomes and 2 sex chromosomes (XX in females, XY in males).
Two Types of Cell Division
To grow and repair tissues, somatic cells divide via mitosis, producing two genetically identical diploid daughter cells. To reproduce sexually, reproductive cells divide via meiosis, undergoing crossing over to produce four genetically unique haploid gametes.
Chromosome Structure and Karyotype
A chromosome consists of two sister chromatids joined at a centromere. A karyotype arranges homologous chromosomes by shape and size.
Every species has a fixed number of chromosomes. A human body cell (somatic cell) contains 46 chromosomes (23 pairs). Homologous chromosomes are pairs of chromosomes with similar shapes and sizes.
Chromosome Structure
A chromosome consists of two sister chromatids joined together at a central point called the centromere. Each chromosome carries several types of genes, approximately 200 to 300 genes per chromosome.
| Chromosome type | Number in humans | Function |
|---|---|---|
| Autosomes | 22 pairs (44 chromosomes) | Carry genes that control somatic characteristics such as eye colour, hair type and tongue-rolling ability. |
| Sex chromosomes | 1 pair (2 chromosomes) | Carry genes that determine gender: male \(44 + \text{XY}\), female \(44 + \text{XX}\). |
Somatic cells are diploid (\(2n = 46\)), whereas gametes (sperm and ovum) are haploid (\(n = 23\)).
Mitosis and Meiosis
Both processes begin after interphase, when DNA replication occurs.
Interphase
Interphase is the stage before mitosis or meiosis begins. During this stage, DNA replication occurs, and cells undergo growth to prepare for cell division.
Mitosis
Mitosis is a cell division process that forms two identical daughter cells. Daughter cells have the same number of chromosomes and genetic content as the parent cell (diploid). It occurs in somatic cells of humans and animals, and in meristematic tissues of plants such as root tips and shoot tips.
| Stage | Key events |
|---|---|
| Prophase | Chromosomes shorten, thicken and become visible. Each chromosome consists of two sister chromatids joined at the centromere. Spindle fibres begin to form. Nuclear membrane and nucleolus disintegrate. |
| Metaphase | Chromosomes line up at the equatorial plane. Spindle fibres attach to the centromere. |
| Anaphase | Centromere splits into two. Sister chromatids separate and move toward opposite poles of the cell. |
| Telophase | Chromatids reach the opposite poles. Nuclear membrane and nucleolus reform. Cytoplasm divides to produce two diploid daughter cells. |
Meiosis
Meiosis is a cell division process where the parent cell divides into four non-identical daughter cells (gametes), each containing half the number of chromosomes of the parent cell (haploid). It occurs in reproductive organs: testes and ovaries in humans, and anther and ovary in plants.
| Stage | Key events |
|---|---|
| Prophase I | Chromosomes shorten, thicken and become visible. Homologous chromosomes pair together. Crossing over occurs. |
| Metaphase I | Homologous chromosomes arrange themselves at the equatorial plane. Spindle fibres attach to centromeres. |
| Anaphase I | Homologous chromosomes separate and move toward opposite poles. |
| Telophase I | Cytoplasm divides to end Meiosis I. |
| Prophase II | Spindle fibres begin to form. |
| Metaphase II | Chromosomes arrange at the equatorial plane. Spindle fibres attach to centromeres. |
| Anaphase II | Centromeres split into two. Sister chromatids separate and move toward opposite poles. |
| Telophase II | Four non-identical haploid daughter cells are formed. |
Comparison Between Mitosis and Meiosis
Both are types of cell division, and DNA replication occurs prior to division during interphase.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Site of occurrence | Somatic cells | Reproductive cells/organs |
| Number of daughter cells | Two | Four |
| Crossing over | Does not occur | Occurs during Prophase I |
| Genetic identity | Daughter cells are genetically identical to the parent cell | Daughter cells are genetically distinct / non-identical |
| Variation | Produces no variation | Produces genetic variation |
| Chromosome number | Same as parent cell (\(2n\), diploid) | Half of parent cell (\(n\), haploid) |
Importance of Mitosis and Meiosis
Each type of cell division plays a vital role in living organisms.
Importance of Mitosis
Mitosis promotes growth in living organisms by increasing the number of cells. It replaces damaged or dead cells through wound healing and tissue repair. It also enables asexual reproduction in single-celled organisms such as Amoeba sp.
Importance of Meiosis
Meiosis produces gametes for sexual reproduction: sperm and ovum in animals, and pollen grains and embryo sac in plants. It ensures the diploid number of chromosomes is restored and maintained from generation to generation upon fertilisation. It also introduces genetic variation in offspring through crossing over.
Abnormal Cell Division
Uncontrolled and abnormal cell division leads to cancer, where cells divide much faster than normal.
Try to Answer First
Answer in your mind, then press “Check Answer”.
Name the three chemical components that make up a single DNA nucleotide.
In which phase of cell division does crossing over take place, and what is its significance?
State two main differences between autosomes and sex chromosomes in humans.
Common Mistakes
- Confusing the terms chromatid and chromosome. A chromosome may consist of two sister chromatids joined at the centromere after replication.
- Stating that mitosis occurs in all human cells. Mitosis occurs in somatic cells, while gametes are produced via meiosis in reproductive organs.
- Stating that crossing over occurs in mitosis. Crossing over occurs exclusively in Prophase I of meiosis, not in mitosis.
- Mixing up the chromosome counts. Humans have 22 pairs of autosomes (44 autosomes) and 1 pair of sex chromosomes (2 sex chromosomes), not 23 pairs of autosomes.
Concept Misunderstandings
DNA replication occurs during prophase of mitosis or meiosis.
DNA replication occurs beforehand during interphase, prior to prophase.
Mitosis produces different cells because wounds heal with fresh skin.
Mitosis produces genetically identical daughter cells so that the replacement cells have the exact same structure and function as the original parent cells.
Centromeres split during Anaphase I of meiosis.
In Anaphase I, homologous chromosome pairs separate; centromeres only split during Anaphase II, and during anaphase of mitosis.
Summary
- The nucleus contains chromosomes, which consist of DNA and proteins.
- Genes are specific functional segments along a DNA double helix.
- Humans have 46 chromosomes (44 autosomes + 2 sex chromosomes: XY for male, XX for female).
- Mitosis produces 2 identical diploid daughter cells for growth, tissue repair, and asexual reproduction.
- Meiosis produces 4 non-identical haploid gametes for sexual reproduction and introduces genetic variation via crossing over.
Short Activity
A. Objective Quiz
1 Which phase of mitosis involves chromosomes lining up along the equatorial plane of the cell?
2 How many autosomes and sex chromosomes are present in a normal human somatic cell?
B. Fill in the Blanks
3 The exchange of genetic material between homologous chromosomes during Prophase I of meiosis is known as .
4 Cell division by takes place in plant meristematic tissue at root and shoot tips to allow growth.
C. Drag and Drop: Match the Term to Its Description
Drag each term to the description it matches. On a phone, tap an answer first, then tap a matching box. Tap the Answer Choices area to return a selected item.
C. Match the Term to Its Description
Write the correct term next to each description.
| Description | Term |
|---|---|
| 1. Basic repeating unit of DNA made of deoxyribose sugar, nitrogenous base, and phosphate group | |
| 2. Central region joining two sister chromatids where spindle fibres attach | |
| 3. An arranged display of homologous chromosomes sorted by shape and size | |
| 4. A haploid reproductive cell produced by meiosis containing 23 chromosomes |
Answer choices: Nucleotide • Centromere • Karyotype • Gamete
