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In eukaryotic cells, cellular reproduction has two main events:

1. The copying of cellular components, and

2. The cleaving of the cell.

These two events are a representation of the two critical phases between one cell division process and the next during a cell's lifetime. The phases are:

• Interphase, and

• The Mitotic or M Phase

These two phases of the cell cycle are then divided into four (4) stages—G1, S, G2, and M—each with substages, which include their own sub-divisions.

1. Interphase:

It is important to note that although interphase is often included when discussing mitosis, it is not technically a part of mitosis. However, the three stages of interphase, G1, S, and G2, are a crucial part of the cell cycle wherein cellular components are copied and prepared for division (Fowler et al., 2021).

A. Gap 1 or G1 occupies the time between the end of mitotic cell division and the start of DNA duplication in the S phase.

a) During the G1 stage, the cell is actively growing and metabolizing. Organelles are needed for protein synthesis, resulting in an increase in the number and size of organelles.

b) Before leaving G1, the cell accesses whether or not the cellular condition is appropriate for DNA replication. If the cell is not ready, it will not move forward in the process but instead, stay back. When this happens, the cell enters a phase called G0.

c) This phase can last for hours, months, or even years, depending on the mitotic rate or the measurement of the time it takes for a cell to grow and divide. Various conditions and tissue types are factors that can affect the mitotic rate.

B. The Synthesis or S stage is the part of the cell cycle in which the cell's DNA pack chromosomes are duplicated.

d) During the S phase, cellular growth continues, as does the rate of protein and enzyme synthesis.

e) By the time DNA replication is complete, the number of chromosomes has doubled, and the cell can move into the next stage.

f) The cell replication that takes place in the S phase is critical as it allows for the genetic make-up of each divided cell to be the same.

C. Gap 2 or G2 Phase: In this stage, the cells continue to grow and function normally in preparation for cell division. Before moving on to M Phase, the cell accesses itself for any damage that may have occurred during the duplication and distribution of chromosomes. This is done through G2 -M DNA, an internal control mechanism called a checkpoint. If defective cellular processes and/or alterations of genome integrity are revealed, these surveillance mechanisms respond and either arrest or delay cell cycle progression.

g) G2 -M DNA is the first of three checkpoints located between each cell cycle stage (Fowler et al., 2021).

2. Mitosis and Cytokineses: the cleavage or separation and division of the cell's nucleus.

A. Mitosis

h) Mitosis involves the dividing of the nucleus and DNA (1 nucleus with 2 DNA and 2 nuclei each with 1 DNA).

i) Cell Division is the dividing of a single cell into two cells over two phases.

B. Cytokinesis is the dividing of the cytoplasm and plasma membrane of a parental cell equally, resulting in the production of two genetically identical daughter cells (Fowler et al., 2021).

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Prophase:

• The chromosomes condense, allowing them to become visible

• The spindle apparatus or spindle pole is assembled, then attached to the centromeres of duplicated chromosomes, thus producing spindle fibers

• The nuclear membrane and nucleoli disappear
(Fowler et al., 2021).

Metaphase:

• Spindles line up the duplicated chromosomes along the metaphase plate, one spindle to each half or chromatid sister

Anaphase:

• The centromere of each duplicated chromosome is split in two

• The paired chromatids are pulled apart

Telophase:

• The chromatids arrive at opposite poles, and de-condensation begins

• The Mitotic spindle breaks down, and spindle fibers continue to push poles apart

• The nuclear envelope reforms and cytokinesis occurs

• Two new genetically identical daughter cells are produced
(Hirose et al., 2011).

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According to the textbook (Fowler et al., 2021), meiosis is a nuclear division process wherein a single germ cell goes through two consecutive cell divisions—meiosis I and meiosis II—resulting in the production of four (4) haploid gamete sperm or egg cells, each generically unique and with half the chromosomes as the diploid parent cell.

Meiosis helps exclusively in producing gametes, thus ensuring genetic diversity. Cells that cannot undergo meiosis are considered somatic (of the body) cells, and use the process of mitosis to reproduce.

Stages of Meiosis I - Key Points

Prophase I:

• Chromosomes condense —"supercoiling"

• Homologous chromosomes—one maternal and one paternal—are made physically paired

• Segments of Chromatids can cross over at break points called Chiasmata, which are found within each chromosome pair

Metaphase I:

• Homologous pairs move to the middle of the cell, where each pair lines up randomly every time meiosis takes place

• Mendel called this "independent assortment”

• Every time meiosis happens, the pairs line up differently

Anaphase I:

• Spindle fibers separate the homologous pairs into two groups, each with a mix of maternal and paternal chromosomes

Telophase I:

• New haploid nuclei are formed for two new daughter cells
(Fowler, et al., 2021)

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Prophase II:

The chromosomes condense

Metaphase II:

The sister chromatids line up at the meta-phase plate

Anaphase II:

The chromatids of each chromosome are separated

Telophase II:

Each daughter cell from meiosis I form two more cells, creating a total of 4 haploid cells

(Fowler et al., 2021).

Mendel’s Law of Independent Assortment

Mendel's law of independent assortment states that genes are assorted independently of each other during meiosis, meaning if one gene ends up in one gamete, the other gene is just as likely to end up in that gamete as the other gamete. This is because of the way that homologous chromosomes line up on the metaphase plate in meiosis I (Fowler et al., 2021).

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Hirose, Y., Suzuki, R., Ohba, T., Hinohara, Y., Matsuhara, H., Yoshida, M., Itabashi, Y., Murakami, H., & Yamamoto, A. (2011). Chiasmata promote monopolar attachment of sister chromatids and their co-segregation toward the proper pole during meiosis I. PLoS Genetics, 7(3). https://doi.org/10.1371/journal.pgen.1001329

Fowler, S., Roush, R., & Wise, J. (2021). Concepts of biology. OpenStax College.

Meiosis phases: Phases of meiosis. (22, February 2). Gentaur. https://gentaur.co.uk/learning/meiosis-phases-phases-of-meiosis/

O'Connor, C. (2008). Mitosis and cell division. Nature Education 1(1). http://www.nature.com/scitable/topicpage/mitosis-and-cell-division-205

Wong, E. (2021, January 3). Anatomy of a cell: An overview. Biology LibreTexts. https://bio.libretexts.org/Bookshelves/Cell_and_Molecular_Biology/Book%3A_Cells_-_Molecules_and_Mechanisms_(Wong)/01%3A_Anatomy_of_a_Cell_-_A_Very_Brief_Overview