Meiosis, derived from Greek, is to lessen the number of chromosomes within eukaryotic cells. Meiosis is the process responsible for chromosomal reduction in plants, animals, and fungi, resulting in the production of germ cells needed for sexual reproduction. By converting a double set of chromosomes (diploid) into a single set (haploid), meiosis generates germ cells or spores. These haploid cells combine during sexual reproduction to form a diploid zygote, thereby conserving the chromosomal number of the species. In this blog post, we’ll review the phases, the mechanisms, and the purpose of meiosis.
What We Review
What is the Purpose of Meiosis?
It comes down to chromosomal numbers. If a human, with n = 46 chromosomes, or two pairs of n = 23 chromosomes, were to reproduce without chromosomal reduction, the egg cell and sperm cell would both have n = 46 chromosomes. When they fuse to become a gamete, the zygote (embryo) would have n = 92 chromosomes, or double the number necessary! This would result in genetic abnormalities in the child. Further, imagine if this child were to reproduce with another child with n = 92 chromosomes: their child would have 184 chromosomes! This number would be ever-increasing. Thus, a chromosomal reduction is necessary for each species’ continued existence. Here lies the purpose of meiosis.
Conserving Chromosomal Number
Before meiosis begins, the chromosomes in the nucleus of the cell undergo replication. Meiosis produces four daughter cells with half the chromosomes of the parent cell; or four haploid cells from a single diploid cell. Remember, haploid and diploid refer to the number of chromosomes in the cell: haploid cells contain one set of chromosomes (n) while diploid cells contain two full sets of chromosomes (2n). As you can see, the math doesn’t quite work out: the parent cell must first be converted to a 4n (tetraploid) cell before division begins. So a cell with n = 46 chromosomes will be converted to a cell with n = 92 chromosomes, which, after meiosis, will produce four cells with n = 23 chromosomes.
Meiosis begins much the same as mitosis does. After chromosomal replication, all chromosomes separate into sister chromatids (the identical two halves of a chromosome). However, here the similarities end. In meiosis, recombination or crossing over occurs. During recombination, the pairs of chromosomes line up and recombine, so that each chromosome has a piece of another in it. In this way, genetic diversity is ensured.
Thus, meiosis uses recombination to produce four haploid daughter cells which are not identical to their diploid parent cell or each other.
The Phases of Meiosis
Meiosis is split into two parts, or divisions, each of which consists of several phases. These are prophase I, metaphase I, anaphase I, and telophase I in meiosis I; and prophase II, metaphase II, anaphase II, and telophase II in meiosis II. You will need some terminology to understand these phases:
- Bivalent – a pair of homologous chromosomes held together by a chiasma.
- Chiasma – point of crossing over when chromosomes exchange genetic material.
- Centromere – the point of constriction of a chromosome.
- Dyad – half of a tetrad; one half of a synapsed pair of homologous chromosomes.
- Homologous chromosomes – the pair of chromosomes formed by an original chromosome and its duplication. These chromosomes are not identical.
- Metaphase plate – the midline of the cell.
- Monad – after separation, each chromosome of a tetrad forms a monad. A dyad without the synapsis to its homologous chromosome.
- Nuclear envelope – the double membrane which encloses the nucleus.
- Nucleolus – the center of rRNA production within the nucleus.
- Sister chromatids – the two identical chromatids which form a chromosome.
- Spindle fibers – a bundle of microtubules running from one pole of the cell to another, along which chromosomes move.
- Synapse/synapsis – the process whereby two homologous chromosomes come into physical contact with one another.
- Tetrad – a pair of homologous chromosomes held together by a chiasma.
Meiosis I
Here is a full picture of meiosis I (figure 1). We will go through each phase in detail. You should be able to understand the significance of each phase and the reason for each step. Once you understand this, you will be prepared to explain the role and purpose of meiosis.
Image Source: Wikimedia Commons
Figure 1: The four stages of meiosis I with the prophase stage further separated into four sub-phases
1. Prophase I
Prophase I is characterized by three main events: the condensation of chromatin into visible chromosomes, the synapsis of chromosomes in each homologous pair, and the crossing over of genetic material between these synapsed chromosomes. Scientists further divide Prophase I into five discrete phases: leptonema, zygonema, pachynema, diplonema, and diakinesis (figure 2).
Image Source: Wikimedia Commons
Figure 2: The different stages of the prophase of meiosis I
Leptonema
Also known as the leptotene stage, this phase is characterized by the condensation of chromatin to form visible chromosomes. The homology search begins.
Zygonema
This phase is also known as the zygotene stage. The homology search continues, with homologous chromosomes aligning into rough pairing, forming bivalents. The synaptonemal complex begins to form.
Pachynema
Pachynema is also known as the pachytene stage. This phase includes further development of the synaptonemal complex between homologous pairs of bivalents, leading to synapsis. At this phase, it is clear that each bivalent contains two pairs of sister chromatids. The sister chromatids of one pair are non-sister chromatids to the sister chromatids of the other pair. Together, the four chromatids are known as a tetrad. Crossing over or recombination of genetic material between pairs of non-sister chromatids occurs.
Diplonema
This phase is also known as the diplotene stage. The pairs of sister chromatids begin to separate. Nonsister chromatids remain in contact at points known as chiasmata where the genetic exchange has occurred during crossing over.
Diakinesis
Chromosomes separate further but are still attached through the chiasmata of the non-sister chromatids. Separation leads to the chiasmata moving toward the ends of the chromatids; this process is terminalization. The nuclear envelope and nucleolus deteriorate. Also, the centromeres of each chromosome attach to spindle fibers, before lining up on the metaphase plate. The chromosomes are still in pairs, which form tetrads.
2. Metaphase I
This phase is similar to the metaphase of mitosis. The spindle fibers attached to the centromere of each tetrad align the chromosomes so that one-half of each tetrad is oriented towards each pole.
Image Source: Wikimedia Commons
Figure 3: The alignment of tetrads during metaphase I of meiosis I
3. Anaphase I
At anaphase I, the chromosomes do not split into their sister chromatids. Instead, each tetrad is split into its chromosome pairs (dyads). These are pulled to opposite poles in a process known as disjunction. Anaphase ends with the same number of dyads at each pole as the haploid number of the parent cell.
Image Source: Wikimedia Commons
Figure 4: The splitting of chromosome dyads during anaphase I
4. Telophase I
In some organisms, telophase I is entered, and a nuclear membrane forms around the dyads at each pole, before a short interphase period is reached. In other organisms, telophase I is skipped, and meiosis II is entered.
Image Source: Wikimedia Commons
Figure 5: Formation of nuclear membrane isolating the two dyads
Meiosis II
Once again, we will explore the second meiosis phase much like the first one. Here is a full picture of meiosis II following telophase I:
Image Source: Wikimedia Commons
Figure 6: The four stages of meiosis II, with four haploid cells at the end of this phase
1. Prophase II
Sister chromatids form dyads connected by a centromere. These are situated at the center of the cell. No condensation of chromatic material or dissolving of nuclear membranes need occur.
Image Source: Wikimedia Commons
Figure 7: The sister chromatids from the dyads are attached by a centromere during prophase II
2. Metaphase II
Spindle fibers attached to the centromere of each sister chromatid align the dyads at the metaphase plate, with one-half of the dyad facing toward each pole.
Image Source: Wikimedia Commons
Figure 8: The metaphase plate forms separating each half of the dyad
3. Anaphase II
The spindle fibers attached to each sister chromatid shorten, and each is pulled to an opposing pole of the cell.
Image Source: Wikimedia Commons
Figure 9: Like in anaphase I, the sister chromatids are pulled towards opposite ends
4. Telophase II
The chromatids (monads) are situated at the poles of the cell. Cytokinesis occurs, wherein a nuclear membrane forms around each set of chromosomes. Then, the cell divides into two cells with a haploid number of chromosomes. Thus, four haploid gametes are formed, which can now recombine during sexual reproduction to form a zygote.
Image Source: Wikimedia Commons
Figure 10: Four haploid daughter cells form during telophase II
Key Points: Mechanisms and Purpose of Meiosis
The details of meiosis can be overwhelming; below we will highlight some key points from both phases of meiosis. During recombination chromosomes from each parent exchange the tips of their homologous chromosomes. This way each homologous chromosome pair has a little bit of the other one. The point at which chromosomes exchange material is called a chiasma.
Image Source: Wikimedia Commons
Figure 11: Genetic material exchange during meiosis and the resulting gametes
Why is Meiosis Important in Studying Biology?
Meiosis is important for three main reasons:
- It allows the sexual reproduction of diploid organisms.
- It enables genetic diversity.
- It aids in the repair of genetic defects.
1. Allows sexual reproduction of diploid organisms
As mentioned previously, meiosis allows the reduction of a diploid cell to a haploid gamete. These can then recombine with another haploid gamete to create a diploid zygote.
2. Enables genetic diversity
The crossing over or recombination of genes that occurs in meiosis rearranges the alleles present in each chromosome of a homologous pair, allowing the mixing of paternal and maternal genes, either of which can be expressed in the resultant offspring. This allows genetic diversity in a population, which is a buffer to genetic defects, susceptibility of the population to disease and changes in the environment. Without this recombination, the gene pool of populations would stagnate, and a single event could wipe out an entire population. Genetic diversity means that there will be certain individuals within any given population that will be better able to survive a loss of habitat, a change in food availability, a change in weather patterns, diseases or other catastrophic events, ensuring species continuity.
3. Aids the repair of genetic defects
The recombination which occurs in meiosis can further help in the repair of genetic defects in the next generation. If a genetic defect is present in a certain allele of one parent, recombination can replace this allele with the healthy allele of the other parent, allowing for healthy offspring.
How is Meiosis Different from Mitosis?
Mitosis is the production of two genetically identical diploid daughter cells from one diploid parent cell. Meiosis produces four genetically distinct haploid daughter cells from a single diploid parent cell. These germ cells can then combine in sexual reproduction to form a diploid zygote.
Meiosis only occurs in eukaryotic organisms which reproduce sexually, whereas mitosis occurs in all eukaryotic organisms, including those which reproduce asexually.
The table below summarizes the similarities and differences between meiosis and mitosis.
Meiosis |
Mitosis |
|
Similarities | |
| Can only occur in eukaryotes | |
| DNA replication occurs first | |
| Production of daughter cells based on parent cell’s genetic material | |
| Means of cell replication in plants, animals, and fungi | |
|
Differences | |
| Starts as diploid; ends as haploid | Starts as diploid; ends as diploid |
| Chromosome number is reduced | Chromosome number is conserved |
| Chromosome pairs undergo synapsis | No synapsis occurs |
| Used for sexual reproduction | Used for growth/healing/asexual reproduction |
| 2 nuclear divisions | 1 nuclear division |
| 8 phases | 5 phases |
| Daughter cell not identical to parent cell | Daughter cell identical to parent cell |
| Results in 4 daughter cells | Results in 2 daughter cells |
| Produces germ cells | Produces somatic cells |
| Occurs only in sexual organisms | Occurs in asexual and sexual organisms |
Wrapping Up Meiosis and Biology
We now know that meiosis is the process of chromosomal reduction which allows the production of haploid germ cells necessary for sexual reproduction. Meiosis furthermore fulfills the purpose of enabling genetic diversity and facilitating the repair of genetic defects through recombination.
The benefits that meiotic reproduction gives over mitotic reproduction are that mitotic reproduction produces identical cells, conserving the chromosomal set and the genes within, whereas meiosis allows for the expression of new traits because of the process of crossing over. Without meiosis maintaining genetic diversity within populations, organisms would not be able to adapt to suit their environment, evolve, or survive catastrophic events. A population’s genetic diversity is its most reliable tool in the fight for the species’ survival.
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