Meiosis is a type of cell division that occurs in sexually reproducing organisms and is essential for the production of genetically diverse offspring.
It involves the reduction of the chromosome number by half, which ensures that each daughter cell receives the correct number of chromosomes.
During meiosis, the genetic material of each parent is divided and shuffled in a process called crossing over, which generates genetic diversity.
Understanding the process of meiosis is crucial for understanding how genetics and evolution work and has practical applications in fields such as agriculture and medicine.
In this article, we will talk in more depth about the steps of meiosis and what this process means.
Meiosis, also called “reduction division,” is a process in which genetic material is swapped between homologous chromosomes (cross-over and recombination) and four daughter cells are made.
Oscar Hertwig, a German biologist, found and wrote about meiosis for the first time in 1876. He found it in the eggs of sea urchins. It was described again in 1883, at the level of chromosomes, by the Belgian zoologist Edouard Van Beneden, in Ascaris worms’ eggs.
The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologist August Weismann, who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained.
In 1911, the American geneticist Thomas Hunt Morgan observed crossover in Drosophila melanogaster meiosis and provided the first genetic evidence that genes are transmitted on chromosomes.
The term “meiosis” was coined by J.B. Farmer and J.B. Moore in 1905.
What is Meiosis?
Meiosis takes place in germ cells, which are designed to produce gametes in sexually reproducing organisms. Most of the stages of meiotic division are similar to those of mitosis.
- It has received each only one set of chromosomes (they are haploid, in contrast to the mother cell which contained homolog chromosomes and was diploid)
- It has each a distinct genetically composition, also different from that of the parental cell.
- Meiosis is divided into two phased: meiosis I and meiosis II.
- Haploid reproduction cells are the product of meiotic division and a post-meiotic differentiation phase.
- In animals, these cells are directly formed by differentiation (maturation) of the meiotic products.
- In plants, meiotic products progress through mitotic division to meiospores that can further develop to become reproductive cells.
Meiosis: Timing in the cell cycle and function
Meiosis (from the Greek word meion, “to reduce”) comes after the G2-phase when DNA replication (in the S-phase) is already concluded so that the cells bear 2n and 4c at the beginning of meiosis. The DNA is still uncoiling at that point. There are two different parts to meiosis: meiosis I, which is the true reductive division, and meiosis II.
The essential events that occur during meiosis are the same as they are during mitotic cell division:
- Two successive divisions without any DNA replication occurring between them.
- Pairing and formation of chiasmata and crossing over
- Segregation of homologous chromosomes, and
- Separation of sister chromatids.
Meiosis – I
Meiosis I is called a reduction division because it splits homologous chromosomes into two haploid cells (N chromosomes, which are 23 in humans). A normal human cell has 46 chromosomes and is called 2N because it has 23 pairs of identical chromosomes.
But after meiosis I, even though the cell has 46 chromatids, it is only type N and has only 23 chromosomes. This is because later, in Anaphase I, the sister chromatids will remain together as the spindle fibers pull the pair toward the pole of the new cell.
In meiosis II, an equational division similar to mitosis will occur whereby the sister chromatids are finally split, creating a total of 4 haploid cells (23 chromosomes, N)—two from each daughter cell from the first division.
It is the longest phase of meiosis. During prophase I, DNA is exchanged between homologous chromosomes in a process called homologous recombination. This often results in a chromosomal crossover. Crossover makes new combinations of DNA, which is a major source of genetic variation and may lead to new combinations of alleles that are good for the organism.
The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent.
The process of pairing the homologous chromosomes is called Synapsis. At this stage, non-sister chromatids may cross-over at points called chiasmata (plural; singular chiasma).
The first meiotic prophase is divided into the following five sub-stages. They are
- Leptotene (Leptonema),
- Zygotene (Zygonema)
- Panchytene (Pachynema)
- Diplotene (Diplonema)
The first stage of prophase-I is the leptotene stage, also known as leptonema, from Greek words meaning “thin threads.”
- In this stage of prophase I, individual chromosomes Each consists of two sister chromatids. change from the diffuse state they exist in during the cell’s period of growth and gene expression, and condense into visible strands within the nucleus.
- However, the two sister chromatids are still so tightly bound that they are indistinguishable from one another.
- During leptotene, lateral elements of the synaptonemal complex assemble.
- Leptotene is of very short duration, and progressive condensation and coiling of chromosome fibers take place.
- Each chromosome is attached at both ends to the nuclear envelope via a specialized structure called an attachment plate.
The zygotene stage begins as soon as an intimate pairing between the two members of each homologous chromosome pair is initiated by the process called synapsis or zygotene pairing.
This is called the “bouquet stage” because of the way the telomeres cluster at one end of the nucleus.
The pairing is completed in three different ways, as follows:
- Proterminal pairing: The two homologous chromosomes start pairing at the terminals, which gradually progresses towards the centromere.
- Procentric pairing: The pairing starts art the centromere and proceeds towards the end.
- Random or intermediate pairing: The pairing may be at many points towards the ends.
At this stage, the synapsis (pairing/coming together) of homologous chromosomes takes place, facilitated by an assembly of the central element of the synaptonemal complex.
- Pairing is brought about in a zipper-like fashion and may start at the centromere (pro-centric), at the chromosome ends (pro-terminal), or at any other portion (intermediate).
- Individuals in a pair are equal in length and in the position of the centromere.
- This pairing is highly specific and exact. The paired chromosomes are called bivalent or tetrad chromosomes.
- The pachytene stage, also known as pachynema, comes from Greek words meaning “thick threads.” This is the stage when chromosomal crossover (crossing over) occurs.
- Non-sister chromatids of homologous chromosomes may exchange segments over regions of homology.
- Sex chromosomes, however, are not wholly identical, and only exchange information over a small region of homology.
- At the sites where exchange happens, chiasmata form. When information is shared between non-sister chromatids, the information is recombined.
- This means that each chromosome has the same full set of information it had before, and there are no gaps.
Because you can’t tell the chromosomes apart in the synaptonemal complex, you can’t see the act of crossing over with a microscope, and you can’t see chiasmata until the next stage.
- During the diplotene stage, also known as diplonema, from Greek words meaning “two threads”, the synaptonemal complex degrades and homologous chromosomes separate from one another a little.
- The chromosomes themselves uncoil a bit, allowing some transcription of DNA.
- The homologous chromosomes of each bivalent, on the other hand, remain tightly bound at the chiasmata, the regions where crossing-over occurred.
- The chiasmata remain on the chromosomes until they are severed in anaphase I.
- In human fetal oogenesis, all developing oocytes reach this stage and stop before birth. This suspended state is referred to as the “dictyotene stage” and remains so until puberty.
- Lampbrush chromosomes are found in amphibians and some other animals. This happens when the chromosomes of some species grow very large.
- Chromosomes condense further during the diakinesis stage, from Greek words meaning “moving through.” This is the first point in meiosis where the four parts of the tetrad are actually visible.
- Sites of crossingover entangle together, effectively overlapping, making chiasmata clearly visible.
- Other than this observation, the rest of the stage closely resembles the prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.
During these stages, two centrosomes, which contain a pair of centrioles in animal cells, migrate to the two poles of the cell.
- These centrosomes, which were duplicated during S-phase, function as microtubule organizing centers, nucleating microtubules, which are essentially cellular ropes and poles.
- After the nuclear envelope breaks down, the microtubules move into the nucleus and attach to the chromosomes at the kinetochore. The kinetochore acts like a motor, pulling the chromosome along the attached microtubule toward the starting centriole, like a train on a track.
- There are four kinetochores on each tetrad, but the pair of kinetochores on each sister chromatid fuses and functions as a unit during meiosis I.
- Microtubules that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will interact with microtubules from the opposite centriole: these are called non-kinetochore microtubules or polar microtubules.
The third type of microtubules, the aster microtubules, radiates from the centrosome into the cytoplasm or contacts components of the membrane skeleton.
- Homologous pairs move together along the metaphase plate:
- Kinetochore microtubules from both centrioles attach to their respective kinetochores
- Homologous chromosomes align along an equatorial plane that bisects the spindle
- Continuous counterbalancing forces are exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes
- The independent assortment of chromosomes is due to the random orientation of each bivalent along the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line.
- Microtubules in the kinetochore (bipolar spindles) shorten.
- Recombination nodules are severed, and homologous chromosomes are pulled apart.
- Each chromosome has only one functional unit of a pair of kinetochores.
- Whole chromosomes are pulled toward opposing poles, forming two haploid sets.
- Each chromosome still contains a pair of sister chromatids.
- Disjunction occurs, leading to genetic diversity.
- Nonkinetochore microtubules lengthen, pushing the centrioles farther apart.
- The cell elongates in preparation for division down the center.
- The first meiotic division ends when the chromosomes arrive at the poles
- Each daughter cell has half the number of chromosomes
- Each chromosome consists of a pair of chromatids
- The microtubules that make up the spindle network disappear
- A new nuclear membrane surrounds each haploid set
- The chromosomes uncoil back into chromatin
- Cytokinesis occurs, completing the creation of two daughter cells
- Sister chromatids remain attached during telophase I.
Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage.
Meiosis – II
Meiosis II is the second part of the meiotic process. Mechanically, the process is similar to mitosis, though its genetic results are fundamentally different. The end result is the production of four haploid cells (23 chromosomes, N in humans) from the two haploid cells (23 chromosomes, N, each chromosome consisting of two sister chromatids) produced in meiosis I.
The four main steps of Meiosis II are:
- Prophase II,
- Metaphase II,
- Anaphase II, and
- Telophase II.
- In prophase II we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids.
- Centrioles move to the Polar Regions and arrange spindle fibers for the second meiotic division.
- In metaphase II, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes (centrioles) at each pole.
- The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis-I, perpendicular to the previous plate.
- This is followed by anaphase II, where the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart.
- The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.
- The process ends with telophase II
- Telophase II is similar to telophase I
- Telophase II is marked by uncoiling and lengthening of the chromosomes
- The spindle disappears during telophase II
- Nuclear envelopes reform during telophase II
- Cleavage or cell wall formation occurs during telophase II
- Four daughter cells are produced, each with a haploid set of chromosomes.
Meiosis is now complete and ends up with four new daughter cells.
What is the significance of Meiosis?
Meiosis is a type of cell division that occurs in sexually reproducing organisms. It is significant because it helps to generate genetic diversity among offspring by shuffling the combinations of alleles present on the chromosomes that are inherited from the parents.
During meiosis, each parent’s genetic material is divided and shuffled in a process called crossing over, which results in the production of genetically unique gametes (sex cells such as sperm or eggs).
When these gametes fuse during fertilization, they produce offspring with a unique combination of genetic material.
Meiosis is also important because it reduces the chromosome number by half, ensuring that each daughter cell receives the correct number of chromosomes.
This is necessary because, during normal cell division (mitosis), the chromosome number is doubled.
If meiosis didn’t happen, the daughter cells that were made would have twice as many chromosomes as usual. This could cause genetic disorders and other problems.
Final words on Meiosis
In conclusion, meiosis is a type of cell division that happens in organisms that reproduce sexually and is important for making offspring with different genes.
During meiosis, the chromosome number is halved, which ensures that each daughter cell receives the correct number of chromosomes.
Meiosis is also important because it generates genetic diversity through the process of crossing over, which shuffles the combinations of alleles present on the chromosomes inherited from the parents.
Understanding the process of meiosis is crucial for understanding how genetics and evolution work, and it has practical applications in fields such as agriculture and medicine.