Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Define meiosis and contrast it with mitosis.
• Describe the stages of meiosis I and meiosis II.
• Explain how meiosis creates genetic variation.
• Understand the consequences of non-disjunction, like Down syndrome.
• Outline the process of gametogenesis (sperm and egg formation).

Lesson summary

This week, we delve into the crucial process of meiosis and its role in sexual reproduction. Understanding meiosis is fundamental to understanding how genetic variation arises, how traits are inherited, and ultimately, how species evolve. This is particularly relevant in South Africa, where we have immense biodiversity and a population with diverse genetic backgrounds. Understanding meiosis helps us understand inherited diseases common in some communities, the basis of genetic counselling and the future of livestock and crop improvements.

Lesson notes

2.1 Meiosis: The Foundation of Sexual Reproduction Meiosis is a type of cell division that reduces the chromosome number by half, creating four genetically different haploid daughter cells from a single diploid cell. This is essential for sexual reproduction because when two haploid gametes (sperm and egg) fuse during fertilization, they restore the diploid chromosome number in the resulting zygote. Without meiosis, chromosome numbers would double with each generation, leading to genetic chaos.

Diploid (2n): A cell containing two sets of chromosomes, one inherited from each parent. Human diploid cells have 46 chromosomes (23 pairs).

Haploid (n): A cell containing only one set of chromosomes. Human haploid gametes have 23 chromosomes. Meiosis is different to mitosis. Mitosis produces two identical diploid daughter cells from a single diploid cell. Mitosis is for growth, repair, and asexual reproduction. Meiosis is solely for sexual reproduction. 2.2 The Stages of Meiosis Meiosis consists of two successive nuclear divisions: Meiosis I and Meiosis I

I. Each division has phases similar to mitosis: prophase, metaphase, anaphase, and telophase.

Meiosis I: Separates homologous chromosome pairs.

Prophase I: This is the longest and most complex phase of meiosis.

Leptotene: Chromosomes start to condense.

Zygotene: Homologous chromosomes pair up precisely, gene by gene, in a process called synapsis. The resulting structure is a tetrad (or bivalent).

Pachytene: Crossing over occurs. This is the exchange of genetic material between non-sister chromatids of homologous chromosomes. The points where crossing over occurs are called chiasmata. Crossing over is crucial for creating genetic variation.

Diplotene: Homologous chromosomes begin to separate, but remain attached at the chiasmata.

Diakinesis: Chromosomes are fully condensed, and the nuclear envelope breaks down. The spindle apparatus forms.

Metaphase I: Homologous chromosome pairs (tetrads) align at the metaphase plate. The orientation of each pair is random, meaning that the maternal or paternal chromosome can face either pole (independent assortment). This is another major source of genetic variation.

Anaphase I: Homologous chromosomes separate and move to opposite poles of the cell. Sister chromatids remain attached.

Telophase I: Chromosomes arrive at the poles. The nuclear envelope may reform. Cytokinesis usually occurs, dividing the cell into two haploid daughter cells.

Meiosis II: Separates sister chromatids (very similar to mitosis).

Prophase II: Chromosomes condense again, and the nuclear envelope (if reformed) breaks down.

Metaphase II: Sister chromatids align at the metaphase plate.

Anaphase II: Sister chromatids separate and move to opposite poles of the cell.

Telophase II: Chromosomes arrive at the poles. The nuclear envelope reforms. Cytokinesis occurs, dividing each cell into two haploid daughter cells. The end result of meiosis is four haploid daughter cells, each genetically different from the original diploid cell and from each other. 2.3 Genetic Variation: The Power of Meiosis Meiosis generates genetic variation in three main ways: Crossing Over: Exchange of genetic material between homologous chromosomes during prophase

I. This creates new combinations of alleles on the same chromosome.

Independent Assortment: Random orientation of homologous chromosome pairs at the metaphase plate during metaphase I. The number of possible combinations is 2 n , where n is the haploid number of chromosomes. In humans (n=23), there are over 8 million possible combinations!

Random Fertilization: Any sperm can fertilize any egg. This further increases the genetic variation in offspring. Given the immense variation from meiosis and independent assortment, the chances of any two siblings (except identical twins) inheriting the exact same combination of genes are extremely low. 2.4 Non-Disjunction: When Things Go Wrong Non-disjunction occurs when chromosomes fail to separate properly during meiosis I or meiosis II. This can result in gametes with an abnormal number of chromosomes. If such a gamete participates in fertilization, the resulting zygote will have an abnormal chromosome number (aneuploidy).

Trisomy: The presence of an extra chromosome (2n+1). The most common example is Down syndrome (trisomy 21), where an individual has three copies of chromosome 21 instead of two. In South Africa, access to pre-natal screening for trisomy is growing, aiding informed decisions for expecting parents.

Monosomy: The absence of a chromosome (2n-1).

Example: A woman with trisomy 21 has a child. What is the probability that the child will also have trisomy 21?

Solution: The woman with trisomy 21 (47, XX+21) produces eggs. Due to non-disjunction, some of her eggs will have two copies of chromosome 21 and some will have 1 copy. If the egg containing 2 copies of chromosome 21 is fertilized by a normal sperm cell, then the child will have trisomy 21.