Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Describe the stages of meiosis I and meiosis II.
• Explain how crossing over and independent assortment create genetic variation.
• Compare and contrast meiosis with mitosis.
• Relate errors in meiosis to genetic disorders.
• Explain the role of meiosis in sexual reproduction.

Lesson summary

Meiosis is a fundamental process in sexual reproduction, ensuring genetic diversity in offspring. Without meiosis, the chromosome number would double with each generation, leading to non-viable offspring. This week, we will delve into the intricacies of meiosis and its critical role in the continuity of life. The significance of this topic is especially relevant to South African learners because it underpins our understanding of inheritance patterns of traits, including susceptibility to certain diseases prevalent in our population, such as diabetes and heart disease, which have a genetic component.

Lesson notes

2.1 What is Meiosis? Meiosis is a type of cell division that reduces the number of chromosomes in a cell by half, producing four genetically different haploid cells. These haploid cells are called gametes (sperm in males, ova in females). This reduction is crucial because during fertilization, two gametes fuse, restoring the diploid chromosome number in the offspring. Without meiosis, the chromosome number would double with each generation, resulting in genetic instability and ultimately, non-viable organisms. 2.2 Stages of Meiosis: Meiosis consists of two main divisions: Meiosis I and Meiosis I

I. Each division has phases similar to mitosis: prophase, metaphase, anaphase, and telophase, but with distinct characteristics. 2.2.1 Meiosis I: Prophase I: This is the longest and most complex phase of meiosis

I. Chromosomes condense:* DNA coils and becomes visible as chromosomes. Each chromosome consists of two identical sister chromatids held together at the centromere.

Synapsis:* Homologous chromosomes (pairs of chromosomes carrying genes for the same traits) pair up side-by-side, forming a structure called a tetrad or bivalent.

Crossing Over:* During synapsis, non-sister chromatids within a tetrad can exchange genetic material. This process is called crossing over. The points where crossing over occurs are called chiasmata. Crossing over is a crucial source of genetic variation. Imagine you have two maize plants, one resistant to drought and the other to stalk borer. Crossing over can potentially create a new maize variety combining both traits.

Nuclear envelope breaks down:* The nuclear membrane disintegrates, and the spindle fibers begin to form.

Metaphase I: Tetrads align:* Homologous chromosome pairs (tetrads) line up along the metaphase plate (the center of the cell).

Independent Assortment:* The orientation of each tetrad is random. This means that the maternal and paternal chromosomes are randomly distributed to the daughter cells. This is another significant contributor to genetic variation. Consider that humans have 23 pairs of chromosomes. The number of possible combinations due to independent assortment is 2^23, which is over 8 million!

Anaphase I: Homologous chromosomes separate:* The homologous chromosomes are pulled apart by the spindle fibers to opposite poles of the cell. Sister chromatids remain attached at their centromeres. This is a crucial difference from mitosis, where sister chromatids separate.

Telophase I: Chromosomes arrive at poles:* The chromosomes arrive at opposite poles of the cell.

Cytokinesis:* The cell divides into two daughter cells. Each daughter cell is now haploid, meaning it contains half the number of chromosomes as the original diploid cell.

However, each chromosome still consists of two sister chromatids. 2.2.2 Meiosis II: Meiosis II is very similar to mitosis.

Prophase II: Chromosomes condense:* Chromosomes, each consisting of two sister chromatids, condense.

Spindle fibers form:* The spindle fibers begin to form.

Metaphase II: Chromosomes align:* Chromosomes line up along the metaphase plate.

Anaphase II: Sister chromatids separate:* The sister chromatids separate and are pulled apart by the spindle fibers to opposite poles of the cell. Now, each sister chromatid is considered an individual chromosome.

Telophase II: Chromosomes arrive at poles:* The chromosomes arrive at opposite poles of the cell.

Cytokinesis:* The cell divides, resulting in four haploid daughter cells. These daughter cells are gametes (sperm or ova). 2.3 Comparing Mitosis and Meiosis: | Feature | Mitosis | Meiosis | | ---------------- | --------------------------------------------- | ----------------------------------------------------------------------- | | Number of divisions | One | Two | | Daughter cells | Two | Four | | Chromosome number | Remains the same (diploid to diploid) | Halved (diploid to haploid) | | Genetic variation | No (daughter cells are genetically identical) | Yes (due to crossing over and independent assortment) | | Purpose | Growth, repair, asexual reproduction | Sexual reproduction (gamete formation) | | Where | Somatic cells (body cells) | Germ cells (cells that produce gametes - ovaries and testes) | 2.4 Errors in Meiosis: Non-Disjunction Sometimes, during meiosis, chromosomes fail to separate properly. This is called non-disjunction. It can occur during Anaphase I (homologous chromosomes fail to separate) or Anaphase II (sister chromatids fail to separate). Non-disjunction results in gametes with an abnormal number of chromosomes.

Down Syndrome (Trisomy 21): Occurs when an individual has three copies of chromosome 21 instead of the normal two. This usually results from non-disjunction during meiosis in the egg cell. People with Down Syndrome often have characteristic facial features, intellectual disability, and other health problems.

Turner Syndrome (XO): Occurs when a female is missing one X chromosome.