Lesson 3 — Meiosis


The Process of Meiosis

Read pages 563 - 565


In the previous lesson, you learned how cells can replicate by mitosis. At the end of mitosis, two identical daughter cells exist with the same number of chromosomes as the parent cell. This is great for growth and repair, but it generates very little variation in a species if it is used for reproduction. As you may recall from Biology 20, variation drives natural selection and allows a species to survive.

To have variety within the species, many organisms use meiosis and fertilization, also called sexual reproduction, to produce offspring. The main goals of meiosis are to produce cells with half the normal chromosome number and to vary the combinations of genes present on those chromosomes. 

Although meiosis takes longer, involves more than one parent, and costs more energy to complete, it results in variation. By encouraging genetic variation in a population by using meiosis and fertilization, a species will be better suited to overcome changes in the environment. Variation increases the chances for survival and attainment of reproductive age so that adaptive traits can be maintained in the population.



A pair of replicated homologous chromosome.
Alberta Education. ADLC.
Meiosis - Reduction Division

In Lesson 1, you learned that humans have 46 chromosomes organized into 22 homologous pairs and one sex pair. This picture demonstrates a replicated homologous pair. The homologous pairs have the same type of genes but they may not have the same alleles (forms of genes) as each other. 

For example, each homologous chromosome may carry the gene for blood type, but the allele on one chromosome codes for type A and the allele on the other chromosome codes for type B.

The cells that result from meiosis have only one complete set of chromosomes and are known as haploid, or n. Recall that the number of sets of chromosomes in a cell is referred to as its ploidy count. In humans, all our body cells, called the somatic cells, are diploid (2n). Only our gametes (sperm or eggs) are haploid (n).


To prepare for meiosis, the cell duplicates its chromosomes in S phase. Then, it goes through two division cycles: meiosis I and meiosis II.

The goal of meiosis I is to separate the homologous pairs of chromosomes. This reduces the number of chromosomes by half. Meiosis I is a reduction division. When meiosis I is complete, the chromosomes number is reduced in each daughter cell. However, they are still made up of two chromatids.

The goal of meiosis II is to pull apart the sister (replicated) chromatids, which is similar to the process of mitosis. Meiosis II follows a pattern exactly like mitosis and separates the two chromatids into two new daughter cells.

The final result of meiosis is four haploid No cells that have originated from one diploid cell. In humans, that means the starting cell has 46 chromosomes, and the resulting cells, known as gametes, have 23 chromosomes.

Consider the process of meiosis in detail.


Meiosis. A: Interphase. B: Meiosis I. C: Meiosis II. NCBI. Public Domain.

Interphase
The process of interphase is identical to mitosis. In the G1 phase, the cell grows.

The DNA of the chromosomes is replicated in the S phase. Each chromosome is replicated into two sister chromatids, which are held at the centromere.

Then, the cell prepares for meiosis in the G2 phase.

Meiosis I
In meiosis I, the homologous chromosomes separate, resulting in two haploid cells.

  1. Prophase I - just like in mitosis, chromosomes condense, spindle fibers form and the nuclear membrane disappears and in addition:
    • The homologous chromosomes come together and find their pair in a process called synapsis. Because each chromosome is made up of two sister chromatids, four chromatids are together in a pair. The temporary bundle they form is called a tetrad.
    • Genetic recombination occurs in a process called crossing-over.

  2. Metaphase I
    • The tetrads line up randomly along the middle of the cell. This is called independent assortment and contributes to the genetic variation of the daughter cells.

  1. Anaphase I
    • The homologous pairs are pulled apart by the shortening of the spindle fibres.
    • The sister chromatids are still held together at the centromere.

  2. Telophase I
    • The separated chromosomes arrive at the poles.

At the end of meiosis I, the two daughter cells are haploids ( n). The two cells contain one pair of homologous chromosomes. This means only one set of chromosome (n) is in each cell.  Each homologous chromosome still consists of two sister chromatids.

Meiosis II
In meiosis II, the two sister chromatids in each haploid cell from meiosis I separate, resulting in four haploid daughter cells. The process of meiosis II is similar to mitosis.

  1. Prophase II
    • The chromosomes condense and become visible. 

  2. Metaphase II
    • The sister chromatids align in the middle of the cell. 

  3. Anaphase II
    • The sister chromatids are pulled apart and move toward each pole.

  4. Telophase II
    • The separated chromatids arrive at opposite poles.

Four haploid daughter cells are produced at the end of meiosis II. Each daughter cells is genetically different from the others because of recombination of genetic materials during crossing-over in prophase I and independent assortment of homologous chromosomes in metaphase I.


Watch and Listen

To put your understanding of meiosis together, watch the following animation on stages of Meiosis.  While you are watching, notice the movement of chromosomes and compare the resulting haploid gametes to the diploid parent at the start.                                                                                                                         




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