Meiosis, or reduction division, is a process during which exchange of genetic material between the homologous chromosomes (crossing-over and recombination) takes place and such a division of the genetically material occurs the four daughter cells.
Meiosis was discovered and described for the first time in sea urchin eggs in 1876 by the German biologist Oscar Hertwig. 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?
The meiosis occurs in the germ cells, which are designed to form gametes in sexually reproducing organisms. Most of the stages of the meiotic division are similar to 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 = 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. Then DNA is still uncoiled. Two different divisions are distinguished within meiosis, i.e. meiosis I (the true reductive division) and meiosis II.
The meiotic division takes place at the end of the G2 phase of the interphase, as in the case of mitotic cell division, the essential events that take place during meiosis are:
- 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 separates homologous chromosomes, producing two haploid cells (N chromosomes, 23 in humans), so meiosis I is referred to as a reduction division. A regular diploid human cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes. However, after meiosis I, although the cell contains 46 chromatids, it is only considered as being N, with 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. The new combinations of DNA created during crossover are a significant source of genetic variation and may result in beneficial new combinations of alleles.
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 consisting 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 of its ends to the nuclear envelope via a specialized structure called an attachment plate.
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 of 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, 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. The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process.
Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope, and chiasmata are not visible 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. However, the homologous chromosomes of each bivalent remain tightly bound at 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 develop to this stage and stop before birth. This suspended state is referred to as the dictyotene stage and remains so until puberty.
In some species, the chromosomes expand enormously, producing lampbrush chromosomes, found in amphibians and some other organisms.
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 tetrads are actually visible.
Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible.
Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.
During these stages, two centrosomes, containing 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.
The microtubules invade the nuclear region after the nuclear envelope disintegrates, attaching to the chromosomes at the kinetochore. The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating 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: As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line.
Kinetochore (bipolar spindles) microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since 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. During this time disjunction occurs, which is one of the processes leading to genetic diversity as each chromosome can end up in either of the daughter cells. Nonkinetochore microtubules lengthen, pushing the centrioles farther apart. The cell elongates in preparation for division down the center.
The first meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear and a new nuclear membrane surround each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, 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 of the chromosomes 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, which is similar to telophase I, and is marked by uncoiling and lengthening of the chromosomes and the disappearance of the spindle. Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, 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?
- The meiosis maintains a definite and constant number of chromosomes in the sexually reproducing organisms.
- By crossing over, the meiosis provides an opportunity for the exchange of genes and thus, gametes causes genetic variation within the species. The variation serves as the raw material for the evolutionary process.
- Meiosis has an impact on the genetic consequences due to events, such as
- The pairing of the homologous chromosomes
- The process of crossing over and recombination
- Segregation of homologous chromosomes.