DIAKINESIS: CHARACTERISTICS AND SUBPHASES - BIOLOGY - 2025

The diakinesis is the fifth and final subphase prophase I of meiosis, during which the chromosomes, filamentous before meiosis contract at the maximum. The contraction of the chromosomes makes them more maneuverable during subsequent division movements that lead to the formation of haploid cells, or gametes.

At the end of diakinesis, the nuclear spindle is formed whose attachment to the kinetochores of the chromosomes through microtubules pulls them towards the poles of the cell. This phenomenon inspired the term diakinesis, derived from the Greek words that mean movements in opposite directions.

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Place in meiosis

The function of meiosis is to produce four haploid cells from one diploid cell. To do this, in meiosis, the chromosomes must be classified and distributed so that their number is reduced by half.

Meiosis consists of two stages, called meiosis I and II, each subdivided into five phases, called prophase, prometaphase, metaphase, anaphase, and telophase. The homonymous stages of meiosis I and II are distinguished by adding "I" or "II".

In meiosis I, the original cell divides in two. In meiosis II a new division produces four gametes.

Viewed at the level of a pair of alleles, the original cell would have A, a. Before meiosis, DNA replication makes this cell have A, A; a, a. Meiosis I produces a cell with A, A and another with a, a. Meiosis II divides both cells into gametes with A, A, a, a.

Meiosis prophase I is the longest and most complex phase of meiosis. It consists of five subphases: leptotene, zygotene, pachytene, diplotene, and diakinesis.

During this process, chromosomes condense (contract), homologous chromosomes recognize each other (synapses), and randomly exchange segments (crossover). The nuclear membrane disintegrates. The nuclear spindle appears.

Previous subphases (leptotene to diplotene)

During leptotene, the chromosomes that during the preceding period of cell growth and gene expression had replicated and were in a diffuse state, begin to condense, becoming visible under a light microscope.

During zygotene the homologous chromosomes begin to line up. Synapse takes place, accompanied by the formation of a protein structure, called the synaptonemal complex, between paired chromosomes

During pachytene, homologous chromosomes line up completely, forming bivalents, or tetrads, each of which contains two pairs of sister chromatids, or monads. In this sub-phase the crossover between each of these pairs takes place. The contact points of the crossed chromatids are called chiasms.

During diplotene, chromosomes continue to shorten and thicken. The synaptonemal complex almost completely disappears. Homologous chromosomes start to repel each other until they are joined only by chiasms.

Diplotene can last a long time, up to 40 years in women. Meiosis in human ovules stops in diplotene by the seventh month of fetal development, progressing to diakinesis and meiosis II, culminating in fertilization of the ovum.

characteristics

In diakinesis, the chromosomes reach their maximum contraction. The nuclear, or meiotic, spindle begins to form. The bivalents begin their migration towards the cell equator, guided by nuclear use (this migration is completed during metaphase I).

For the first time in the course of meiosis, the four chromatids of each bivalent can be observed. The crossover sites overlap, making the chiasms clearly visible. The synaptonemal complex disappears completely. The nucleoli also disappear. The nuclear membrane disintegrates and turns into vesicles.

The condensation of chromosomes during the transition from diplotene to diakinesis is regulated by a particular complex of proteins called condensin II. In diakinesis, transcription ends and the transition to metaphase I begins.

Importance

The number of chiasms observed in diakinesis allows a cytological estimate of the total length of the genome of an organism to be made.

Diakinesis is an ideal stage to perform chromosome counts. The extreme condensation and repulsion between bivalents allow a good definition and separation of the same.

During diakinesis, the nuclear spindle has not fully attached to the chromosomes. This allows them to be well separated, allowing their observation.

Recombination events (crossovers) can be observed in diakinesis cells by conventional cytogenetic techniques.

In men with Down syndrome, the presence of the extra chromosome 21 is not detected in most cells in pachytene due to its concealment in the sex vesicle.

This structural complexity makes individual chromosome identification difficult. In contrast, this chromosome can be easily visualized in the vast majority of cells in diakinesis.

The relationship thus evidenced between chromosome 21 and the XY complex during pachytene could be the cause of spermatogenic failure in Down syndrome, as has been generally observed in cases of hybrid animals, in which the association of an additional chromosome with this complex it produces male sterility.

Observation of recombination

Observation of chiasms during diakinesis allows direct examination of the number and location of recombinations on individual chromosomes.

As a result, it is known, for example, that one crossover can inhibit a second crossover in the same region (chiasmatic interference), or that females have more chiasms than males.

However, this technique has some limitations:

1) Diakinesis is very short-lived, so finding suitable cells can be difficult. For this reason, if the type of study allows it, it is preferable to use cells obtained during pachytene, which is a sub-phase of much longer duration.

2) Obtaining cells in diakinesis requires the extraction of oocytes (females), or the performance of testicular biopsies (males). This represents a serious drawback in human studies.

3) Due to their high condensation, chromosomes from cells in diakinesis are not optimal for staining procedures, such as G, C, or Q banding. This problem also makes it difficult to observe other morphological details that are more evident in non-chromosomes. contracted.

References

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