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In this article, we will interpret photomicrographs and diagrams of cells in various stages of meiosis and identify the main stages of meiosis. We will also explain that crossing over and random orientation (independent assortment) of pairs of homologous chromosomes and sister chromatids during meiosis produces genetically distinct gametes. In the end, we will explain that the random fusion of gametes at fertilization produces individuals that are genetically different.
Meiosis resembles mitosis in many ways. The cell undergoes similar stages and employs the same techniques to organize and separate chromosomes. However, in meiosis, the cell has a more complicated task. It has to separate sister chromatids just like mitosis, but it also has to separate homologous chromosomes which refer to the similar but non-identical pairs of chromosomes that an organism gets from its two parents.
Identifying the Stages of Meiosis
We can use microscopes to observe and photograph the cells that are undergoing meiosis. Various stages of meiosis have different features which means that they can be identified from diagrams or photomicrographs.
Meiosis I and Meiosis II
In meiosis I, the homologous chromosomes pair side by side which means that the cells are undergoing meiosis I if you observe pairs of chromosomes in diagrams of photomicrographs. The number of cells formed can assist in differentiating between meiosis I and meiosis II. If two new cells are formed, then it is meiosis I, however, it is meiosis II if four new cells are formed.
Features of Various Stages of Meiosis I
In this section, we will discuss various features which distinguish different stages of meiosis I.
- Prophase I: If homologous pairs of chromosomes can be seen, then it is the prophase I stage of the meiosis I
- Metaphase I: In this stage of meiosis I, homologous pairs are lined up side by side along the equator of the spindle
- Anaphase I: Entire chromosomes are pulled to the opposite poles, however, the centromeres remain intact
- Telophase I: In this stage of the meiosis, we can observe two groups of condensed chromosomes around which nuclei membranes are forming
- Cytokinesis: In this stage, the cytoplasm divides and the cell membrane pinches inwards to create two cells
The figure below shows the different phases of meiosis I.
Features of Various Stages of Meiosis II
In this section, we will discuss various features which distinguish different stages of meiosis I.
- Prophase II: In this stage of meiosis II, single entire chromosomes are visible. In this stage, chromosomes condense and nuclei envelop break down if required. The centrosomes move apart and the spindle is created between them. The spindle microtubules start to capture chromosomes
- Metaphase II: In this stage, single, whole chromosomes are lined up along the equator of the spindle in a single file which means at a 90-degree angle to the old spindle
- Anaphase II: In this stage, the division of centromeres takes place and chromatids are pulled to opposite poles
- Telophase II: In this stage, nuclei are formed around four groups of condensed chromosomes
- Cytokinesis: In this stage, the division of cytoplasm takes place along with the formation of four haploid cells
The following figure shows different phases of meiosis II.
Meiosis as Sources of Genetic Variation
Having genetically distinct offspring can be beneficial for natural selection. There are many mechanisms of meiosis that enhance the genetic diversity of gametes that are produced. Both crossing over and independent assortment lead to various combinations of alleles in gametes.
In the next section of the article, we will discuss crossing over in detail.
Crossing Over
The process by which non-sister chromatids exchange alleles is referred to as crossing over.
Now, let us discuss the process of crossing over in detail.
Process of Crossing Over
- Homologous chromosomes pair up and are extremely close to each other during meiosis I
- Crossing over of non-sister chromatids takes place and they get entangled
- These crossing points are known as chiasmata
- The DNA molecules are stressed because of this entanglement
- Consequently, a section of chromatid from one chromosome can break and rejoin with the chromatid from the other chromosome
This swapping of alleles is critical as it leads to a combination of alleles on the two chromosomes. Usually, there are at least one chiasmata found in each bivalent during meiosis. There is a probability that the crossing over can take place further down the chromosomes away from the centromere.
In the next section of the article, we will discuss independent assortment in detail.
Independent Assortment
Independent assortment refers to the production of varying combinations of alleles in daughter cells because of the random alignment of homologous pairs along the equator of the spindle during metaphase I.
- The genetic variation between gametes increases because of the various combinations of chromosomes in daughter cells
- Homologous chromosomes pair up in prophase I and are pulled towards the equator of the spindle in metaphase I
- The arrangement of each pair can occur with either chromosome on top. This implies that it is entirely random
- The orientation of a single homologous pair is independent, i.e. it is not affected by the orientation of any other pair
- After that, the separation of homologous chromosomes takes place and they have pulled apart into different poles
- The alleles combination that ends up in each daughter cell is dependent on how the homologous chromosome pairs were lined up
- The number of different possible chromosome combinations can be worked out using the formula 2n, where n represents the chromosome number in a haploid cell
- For humans, this is 223 which is equal to 8,324,608 different combinations
Fusion of Gametes
- Due to meiosis, genetic variation is created between the gametes which an individual produces through crossing over and independent assortment
- It implies that each gamete carries significantly different alleles
- During the process of fertilization, any male gamete fuses with any female gamete to create a zygote
- The genetic variation between the zygotes occurs because of the random fusion of gametes at fertilization. Each will have a unique combination of alleles
- There is almost no probability of individual organisms that result from successive sexual reproduction are genetically identical
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