Meiosis

Unlike mitosis, meiosis does not occur in the somatic cells. Instead, it occurs in the gametes. The doubling of the gametic chromosomes occurs due to meiosis. It occurs in two stages. The first meiotic stage leads to the reduction of the chromosome number from the diploid to haploid. The second meiotic division is the same as the mitotic division. A single chromosomal duplication precedes the two divisions. Meiosis also consists of stages such as prophase-I, metaphase-I, anaphase-I, and telophase-I in its meiosis-I stage. The meiosis-II stage also consists of prophase-II, metaphase-II, anaphase-II, and telophase-II. Let us discuss each of the phases in details.

Meiosis-I		Meiosis-II
Prophase-I	Leptonema or leptotene	Prophase-II
	Zygonema or zygotene
	Pachynema or pachytene
	Diplonema or diplotene
	Diakinesis
Metaphase-I		Metaphase-II
Anaphase-I		Anaphase-II
Telophase-I		Telophase-II

Table: Different stages involved in Meiosis-I and Meiosis-II

Meiosis-I:

Following are the subphases of meiosis-I:

Image: Different stages of Meiosis-I and Meiosis-II

Prophase-I:

It shows a similarity with the mitotic prophase. However, a slight difference in the substages makes the prophase-I different from the mitotic prophase. The meiotic prophase-I consists of five substages such as leptotene, zygotene, pachytene, diplotene, and diakinesis. The leptotene stage involves the coiling of the chromosomes. It helps in committing the cell to enter the meiosis. The chromosomes look like threads in leptotene. They get an orientation of the bouquet. It is known as the bouquet configuration. Each chromosome in the leptotene looks like a single chromosome. The pachytene stage reveals the two chromosomes. The DNA replication occurs well before the leptotene stage.

The zygonema stage or the zygotene is the early mid-phase of the prophase-I. The chromosomes get shortened in this stage. The chromosomes in the stage are known as homologous chromosomes. The pairing of these chromosomes occurs only in the meiosis. It does not occur during mitosis. Mitotic recombination occurs very rarely.  The homologous chromosomes are a pair of essentially identical chromosomes. They, later on, involve synapsis. The genes in the homologous chromosomes belong to the common ancestor. Hence, they get retained. Synapsis involves a point-by-point pairing of the homologous chromosomes. It occurs mainly during the zygonema stage. Mainly the dipteran tissues such as the Drosophila salivary glands undergo synapsis. Parallel, dense elements showing lateral position surround the medial complex. These elements form a ribbon-like tripartite structure. This structure is known as a synaptonemal complex. It occurs in the central axes of the paired homologous chromosomes present in a pachytene bivalent. Synaptonemal complex helps in maintaining the parallel configuration of the lateral elements.

Synapses also play a crucial role in forming the zipper-like structures along the length of the chromatids. It also reduces the chromosomal threads into the half. Their appearance becomes like bivalents instead of a single chromosomal look. The bouquet-like arrangement also involves the role of telomeres (chromosomal ends). The pachytene stage, each paired chromosome separates into two sister chromatids. Exceptions include the centromeric regions. The division occurs longitudinally. It forms chromatin tetrads. Meaning, four chromatids occur due to the longitudinal division. Next step involves a localized breakage. It exposes the non-sister chromatids. These non-sister chromatids exchange the genetic material with each other.

The process of exchange is known as the crossing over. It plays a crucial role in giving a different set of alleles to the progeny, slightly differing from the parental alleles. Hence, it gives rise to the new genetic material (recombinants). The disassembly of the synaptonemal complex commits the cell to the diplotene stage. Each pair of sister-chromatids in a tetrad start separating in this phase (except the places of exchange). The cross-shaped structure arising due to the overlapping chromatids is known as chiasmata. The process of terminalisation brings the chiasma towards the end of the tetrads. Diakinesis involves tight coiling of the chromosomes. The nucleolus and the nuclear envelope disappear. The first meiotic division produces two secondary gametocytes containing the dyads.

 Metaphase-I:

It involves a complete breakdown of the nuclear envelope. It shows an alignment of the bivalents on the equatorial plane. It gives rise to spindle formation followed by the microtubule attachment.

Anaphase-I, Telophase-I, and cytokinesis:

In the anaphase, the chromosomal disjoining occurs. The chromosomes migrate to the opposite poles, thereby moving the centromeres. However, the centromeres do not get separated from their sister chromatids. They remain attached. It leads to the formation of a new nuclear envelope. Next, the cell divides into two through a process known as cytokinesis.

Meiosis-II:

The second meiotic division mimics the mitotic division. The chromosomes start getting condensed in the prophase-II. The chromosomes get aligned on the equator in the metaphase-II. The centromeres split, moving the chromatids apart on the opposite poles. The anaphase-II plays a crucial role in proper centromere splitting. After the telophase-II, the microscopic observations reveal well-defined chromosomes.

Gene segregation:

Half the numbers of chromosomes occur in a haploid cell. Thus, it is a result of meiosis. A diploid cell enters the meiotic phase and results in haploid cells. Two meiotic divisions occur after the DNA replication which occurs only once (in the S phase). However, the haploid nuclei fuse together if the diploid nuclei are required. Thus, meiosis helps in maintaining the chromosome number. The chromosomes aligning at the equatorial plane may either be paternally derived ones or maternally derived ones. There involves no restriction in the alignment of the chromosomes. Thus, nuclei derived out of meiosis contain a combination of both the paternal and the maternal chromosomes. The process of variation leads to variation. It gives rise to recombinants.

Meiotic drive is a condition in which a meiotic division gives rise to an unequal recovery of the gametes produced by a heterozygote. It involves an intragenomic conflict.

References:

[1] The Cell, Bruce Alberts

[2] All About Mitosis and Meiosis, Elizabeth Cregan

[3] Mitosis and Meiosis, Part 1

Copyrights, 2019 All Rights Reserved

Mitosis

Prokaryotes and eukaryotes differ in their cellular structures, the pathways, the machinery, and different processes. The cellular reproductive capacity of the eukaryotes is much higher than the prokaryotes. The cells follow a typical cycle for undergoing division and proliferation. The cell cycle studies help in knowing the pattern of growth and reproduction. The cellular compartmentalization bifurcates the functioning of every important cellular component. The nucleus and the cytoplasm have different roles to play. They involve great interactions and function in an efficient way. The study of cell cycle not only helps us in knowing about the cell division but also its way of differentiation.

Different types of cells exist in eukaryotic organisms. The cells either show haploidy or diploidy or any other condition. Each of them shows the presence of well-defined chromosomes. The G1, S, G2, and the M phases constitute the phases of a cell cycle. The M phase or the mitotic phase belongs to the somatic cell cycle. It is the final phase of the cell cycle and lasts for one hour. The interphase, lasting for 23 hours involves the G1, S, and the G2 phases. The division phase or mitosis plays a crucial role in cellular division. It consists of four subphases such as prophase, metaphase, anaphase, and telophase. Studying mitosis helps in finding out the mitotic index. It helps in determining the fraction of cells undergoing mitosis in a given sample. The mitotic index study helps in determining the division of cancer cells undergoing rapid division.

Image: Different stages of Mitosis

Mitotic apparatus:

Before knowing about the mitotic phase in detail, it is important to know the components involved in the mitosis phase. The mitotic apparatus consists of three main components such as the asters, the spindle, and the traction fibers. Each centrosome involves the formation of the asters. The spindle apparatus consists of a gelatinous structure. The connection of the centromeres to the centrosomes occurs with the help of the traction fibers.

Prophase:

The initial phase of the mitosis involving chromosome visibility is known as prophase. The word prophase indicates the initial stage involved in the division. The chromosomes occur in the highly condensed state. They show clear visibility under the microscope. The formation of the spindle apparatus starts during this phase. The spindle apparatus looks like a slender structure tapering towards the ends. The slenderness of the spindle arises due to the tubulin fibers. These tubulin fibers help in the chromosomal movements during the other phases of the mitotic phase. The chromosomes get attached to these fibers with the help of kinetochores. Each chromosome becomes longitudinally double. Exceptions include the regions near or at the centromere. Three types of microtubules include the polar microtubules, the kinetochore microtubules, and the astral microtubules. Hence, these structures help in organizing the spindle formation. The assembly occurs outside the nucleus during the prophase. The prophase further involves three stages. They include the early prophase, the middle prophase, and the late prophase.

The centrioles are the other organelles involved in the spindle formation. These structures show a composition of tubulin. They help in organizing the mitotic spindle. During the early prophase, these structures move apart, thereby indicating the beginning of the spindle formation. The early prophase chromosomes show a reduced structure. After some time, their visibility gets clarified. The nucleolus gets disappeared in this phase.

The middle prophase leads to the movement of centrioles further apart. It proceeds with the mitotic spindle formation. The late prophase depicts the movement of the centrioles towards the opposite sides. The spindle formation starts. It is the time for the chromosomes to coil and produce a series of compact structures known as gyres.

Metaphase:

The nuclear envelope starts getting disappeared in this phase. The kinetochores are an example of well-defined proteins playing a crucial role in chromosome-spindle attachment. They attach themselves very well to the centromere (structures of the chromosomes joining the sister chromatids). Hence, the chromosomes get aligned on the equatorial plane of the spindle. It occurs between the two spindle poles. It is also known as a metaphase plate. The chromosome gets aligned perfectly on the spindle. The cellular processing technique helps in arresting the cells. Hence, they get visualized under an electron microscope. The microscopic observations reveal the scaffolding patterns of the proteins surrounding the uncoiled DNA. Thus, it helps in studying the double-stranded DNA.

Anaphase:

The centromeres of the sister chromatids undergo a separation during the anaphase. They form two daughter chromosomes. The traction fibers help in the separation of the chromatids. They separate in such a way that their movement follows towards the spindle pole. The kinetochores also separate during this process. The process of disjunction leads to the conversion of the sister chromatids into the independent chromosomes. The microtubules start getting shorter and shorter. Thus, the two independent chromosomes move toward the poles. Due to the disjunction, the chromosomes appear in different shapes. They include V, J, T, X or rod-shaped structures. The shape, however, depends on the position of the centromere. The J-shaped chromosomes are known as the sub-metacentric chromosomes. The V-shaped chromosomes are known as the metacentric. Improper centromere split leads to chromosomal abnormalities. The factor defining the movement of the chromosomes towards the poles is known as the mitotic center. Example of the mitotic center includes the centriole.

Telophase:

Now the chromosomes align in two groups. They place themselves at the opposite ends of the cell. The uncoiling of the chromosomes starts in telophase. The spindle apparatus disappears. This phase helps in the completion of the nuclear division. The dissolution of the kinetochore microtubules occurs during this phase. The polar microtubules get elongated in this phase.

Cytokinesis:

One cell undergoes a division to give rise to two cells. Cytokinesis occurs after the telophase of the mitosis. The two cells have their own set of nuclei and organelles. First, the cell cytoplasm separates and cleaves at the middle of the telophase cell. Thus the cell divides into two. The mitotic spindle helps in determining the cleavage site. The thin ring of the actin filaments gets cleaved first. Hence, it plays a crucial role in the process of cleavage.

References:

[1] The Cell, Bruce Alberts

[2] All About Mitosis and Meiosis, Elizabeth Cregan

[3] Mitosis and Meiosis, Part 1

Copyrights, 2019 All Rights Reserved