2. Meiosis

1. Core Content

Introduction

Revision of the structure of a cell with an emphasis on the parts of the nucleus, the centrosome and the cytoplasm

Structure of chromosomes:

• Chromosomes consist of DNA (which makes up genes) and protein

• The number of chromosomes in a cell is a characteristic of an organism (e.g. humans have 46 chromosomes)

• Chromosomes which are single threads become double (two chromatids joined by a centromere) as a result of DNA replication

Differentiate between:

• Haploid (n) and diploid (2n) cells in terms of chromosome number

• Sex cells (gametes) and somatic cells (body cells)

• Sex chromosomes (gonosomes) and autosomes

Revision of the process of mitosis

Meiosis – The process

Definition of meiosis

Site of meiosis in plants and in animals

Meiosis is a continuous process, but the events are divided into different phases for convenience

Events of interphase:

• DNA replication takes place

• Chromosomes which are single threads, become double

• Each chromosome will now consist of two chromatids joined by a centromere

• DNA replication helps to double the genetic material so that it can be shared by the new cells arising from cell division

The events of the following phases of Meiosis I, using diagrams:

• Prophase I

- Including a description of crossing over

• Metaphase I

- Including the random arrangement of chromosomes

• Anaphase I

• Telophase I

The events of each phase of Meiosis II, using diagrams:

• Prophase II

• Metaphase II

- Including the random arrangement of chromosomes

• Anaphase II

• Telophase II

Importance of meiosis

The importance of meiosis:

• Production of haploid gametes

• The halving effect of meiosis overcomes the doubling effect of fertilisation, thus maintaining a constant chromosome number from one generation to the next

• Mechanism to introduce genetic variation through:

- Crossing over

- The random arrangement of chromosomes at the equator

Abnormal meiosis

Non-disjunction and its consequences

Non-disjunction of chromosome pair 21 during Anaphase I in humans to form abnormal gametes with an extra copy of chromosome 21

The fusion between an abnormal gamete (24 chromosomes) and a normal gamete (23 chromosomes) may lead to Down syndrome

Comparison of mitosis and meiosis

Similarities of mitosis and meiosis

Differences between mitosis and meiosis

5. Informal Assessments