Botany - B.Ye. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter I. Cytology
1.4. Cell Formation

The following modes of Cell formation exist: copulation, Water/54.html">Free cell formation, budding, with the most common being nuclear and cellular division.

Copulation is the fusion of two Cells to form a new one, occurring in lower plants.

Free cell formation is rare and involves multiple nuclear divisions initially, with cell walls forming around the nuclei only afterward. This process is observed during endosperm development in the seeds of certain plants.

Budding is The formation of new cells from a parent cell through its outward protrusion (evagination). Often, the new cells do not detach, forming a chain instead. Budding is characteristic of some Fungi.

The universal mode of cell reproduction is division. The forms of division include direct (amitosis) and indirect nuclear division (Mitosis and Meiosis). Amitosis is the direct division of The Nucleus and The Cell, occurring in temporary Tissues such as the nucellus, endosperm, perisperm, Ovary wall cells, tuber parenchyma, and others. Mitosis is the primary type of nuclear and cellular division; plant growth occurs predominantly through mitotic division. First, the nucleus divides, followed by the cell. Between two consecutive cell divisions, the nucleus is in interphase—a period characterized by high metabolic activity, ATP accumulation, and chromosome self-duplication. During mitosis, two cells are formed from one. The Selection/36.html">Biological essence of mitosis lies in the even distribution of the hereditary material contained within the Chromosomes between the two daughter cells. The complex of processes resulting in the formation of two cells from one is called the mitotic cycle. The mitotic cycle comprises karyokinesis (nuclear division) and cytokinesis (cellular division). Karyokinesis includes four phases: prophase, metaphase, anaphase, and telophase.

Prophase begins with an increase in nuclear volume and The Development of chromosomes. In early prophase, chromosomes appear as thin threads that gradually spiral, thicken, and shorten. In late prophase, it becomes distinguishable that each chromosome is a Structure formed by two parallel and closely apposed longitudinal halves—chromatids—connected by a centromere. At this time, the nucleolus and nuclear envelope dissolve. The degree of chromonema coiling changes throughout mitosis, reaching its maximum at the transition from prophase to metaphase. Prophase is the longest phase of mitosis, occupying about three-quarters of the entire division process.

Metaphase sets in when the chromosomes finally settle at the equator of the cell. By this time, the mitotic spindle has already formed: achromatic fibers stretch from the poles of the cell to the chromosomes. The chromosomes arrange themselves perpendicularly to the spindle fibers at an equal distance from both poles. During this period of mitosis, each chromosome consists of two maximally shortened chromatids, with a distinct longitudinal cleft visible between them, making the chromosomes countable.

Anaphase begins with the division of the centromere and ends with the migration of daughter chromosomes to opposite poles of the cell. Centromere division occurs synchronously in all chromosomes. At the end of anaphase, the spindle at the equator thickens and takes on a barrel shape, known as the phragmoplast.

Telophase begins with the decoiling of chromonemata and the return of chromosomes to the state typical of interphase; nucleoli and new nuclei form by the end of telophase. Simultaneously, the mitotic spindle breaks down at the poles, and its fibers condense at the equator, where a Cell wall forms from the phragmoplast, dividing the Cytoplasm into two equal parts. This is The final stage of the mitotic cycle—cytokinesis.

Meiosis is a complex process of nuclear division accompanied by fundamental alterations in nuclear material, wherein four haploid daughter cells are produced from a single diploid parent cell. Meiosis is observed during the formation of Gametes (gametogenesis) and spores in higher spore-bearing plants (sporogenesis). Meiosis consists of two consecutive divisions: the First Division (reductional), during which the chromosome number is reduced and conjugation—the pairing of homologous chromosomes—takes place. The Second Division is called equational and proceeds via a mitotic type (Fig. 10).

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The longest and most complex phase is the prophase of the first division, which is conventionally divided into the following stages: leptonema, zygonema, pachynema, diplonema, and diakinesis. Prior to meiosis, DNA molecules self-duplicate (replicate) during interphase. Leptonema is characterized by the appearance of thin, randomly arranged chromosome threads in the nucleus, which show a tendency toward parallel alignment by the end of the stage.

During zygonema, homologous chromosomes approach one another and begin to conjugate; the homologs unite to form bivalents. Each bivalent consists of four chromatids (tetrads).

In pachynema, the chromatids thicken, twist, and exchange homologous segments—a process geneticists call Crossing Over. Crossing over leads to profound transformations of chromosomes, making maternal and paternal homologs distinct from the original ones, which creates conditions for generating diverse genetic material and ensures genetic variation in offspring during sexual reproduction. At the diplonema stage, chromosomes thicken due to the coiling of chromonemata.

During diakinesis, the bivalents shorten to their maximum extent, nucleoli dissolve, and the bivalents locate at the periphery of the nucleus. At the end of diakinesis, homologous chromosomes remain connected at certain points, which determines the shape of the bivalent. The length of chromosomes decreases several-fold from pachynema to diakinesis.

Prophase of the first division transitions into metaphase: the nuclear envelope dissolves, the mitotic spindle forms, and bivalents align in a single plane at the spindle equator. In anaphase, bivalents separate into chromosomes and move toward the poles. In telophase, chromosome decoiling generally does not occur, making this phase transitional to the second division.

Interphase between the First and Second divisions is shortened or entirely absent. The processes observed during the second division resemble mitosis. During prophase II, the mitotic spindle forms, and chromosomes migrate toward the cell equator. In metaphase II, chromosomes arrange in a single row along the equatorial plane, and spindle fibers attach to them. In anaphase II, chromosome centromeres divide, and chromatids migrate to the cell poles due to the shortening of achromatin fibers. During telophase, chromosomes decoil, the mitotic spindle disappears, and nucleoli and the nuclear envelope form. Telophase concludes with the division of the cytoplasm.

Thus, As a result of two consecutive divisions of a diploid parent cell, four haploid daughter cells are formed. The Biological Significance of meiosis lies in the fact that mature sex cells receive a haploid set of chromosomes, whereas the species-specific chromosome Complement is restored upon Fertilization. This preserves the karyotype and nuclear DNA content characteristic of each species. The exchange of segments between homologous chromosomes (crossing over), as well as the independent assortment of homologous chromosomes into different daughter cells, enhances hereditary Variability through The Emergence of novel Gene combinations.