Cell Division: Mitosis, Meiosis, and Cellular Structures

Types of Cells in Multicellular Organisms

Diploid Cells

They have two copies of each chromosome type. It is symbolized as 2n. In humans, somatic cells, which constitute the body, have 2x23 chromosomes = 46.

Haploid Cells

They have a single copy of each type of chromosome. It is symbolized as n. In humans, these are the reproductive cells (eggs and sperm) and have 23 chromosomes. n is the number of different types of chromosomes present in each cell.

Mitosis

Inside the life cycle of a cell, we can identify:

• The interphase or growth period, which is the interval between two successive divisions.
• The cell division, the stage where two daughter cells are formed from an initial cell.

Cell Division

Cell division comprises:

1. The division of the nucleus or mitosis, also called karyokinesis.
2. The division of the cytoplasm or cytokinesis.

A) Division of the Nucleus: Mitosis

Mitosis is a continuous process with four stages to facilitate study:

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

1. Prophase

• The chromatin condenses into chromosomes.
• Each chromosome is composed of two identical strands, the chromatids, which are connected by a constriction: the centromere.
• In the centromeres, kinetochores form, protein structures that organize the formation of microtubules during mitosis.

2. Metaphase

• Because of the kinetochoric microtubules, the chromosomes are equidistant.
• They are called the equatorial plate or metaphase plate.

3. Anaphase

• Kinetochoric microtubules shorten.
• The sister chromatids are stretched in opposite directions.
• Simultaneously, the two poles of the mitotic spindle move away.
• At the end of anaphase, the anaphase chromosomes are at the poles and decondense.

4. Telophase

• Chromosomes decondense into chromatin.
• The nucleolus reforms.
• The endoplasmic reticulum and nuclear envelope vesicles form two new nuclear envelopes.

B) Division of the Cytoplasm: Cytokinesis

• In animal cells, protozoa, and many algae, the division of the cytoplasm occurs by strangulation or segmentation.
• In plant cells, the division of the cytoplasm occurs by cytokinesis with a septum. It is the fusion of vesicles of the Golgi complex, forming a structure called the phragmoplast or medial plate.
• There are pores called plasmodesmata.

Meiosis

The genetic material is replicated in the S phase of the cell cycle. The segregation of chromosomes in the first meiotic division reduces the DNA content in half.

The Nucleus

Consists of:

• A double membrane called the nuclear envelope.
• Nucleoplasm.
• Chromatin.
• Nucleolus.

During the cell cycle, the nucleus varies in appearance:

• Interphase nucleus: Intact nuclear envelope. Fibers of lax chromatin.
• Dividing nucleus: Fibers of condensed chromatin, giving rise to chromosomes. The nuclear envelope disappears.

Generally, there is only one nucleus in each cell.

• Syncytium: Multinucleated cell.
• Plasmodium: Multinucleated cell formed by successive cell divisions without cytokinesis.

Nuclear Envelope

Consisting of a double membrane separated by a perinuclear space of 200 Å. Underneath it is the inner membrane of the nuclear lamina. It originates from the endoplasmic reticulum. The nuclear envelope is perforated by nuclear pores. Each pore is surrounded by a pore complex.

Functions of the Nuclear Envelope

• Separates the nucleoplasm from the cytoplasm.
• Regulates the exchange of substances.
• The nuclear lamina is involved in the condensation of chromatin to form chromosomes.

Nucleoplasm

Colloidal dispersion in a gel state. It contains proteins related to the synthesis and packaging of nucleic acids, nucleotides, water, and ions. It is the environment where DNA duplication and gene transcription to form RNA occur.

Nucleolus

It is composed of RNA and proteins, which form the pre-ribosomal region. A pyknotic nucleus with very compacted chromatin is found in sperm cells and during aging.

Chromatin

Formed by DNA associated with histone proteins. It has various degrees of condensation depending on the different stages of the cell cycle and mitosis. There are as many strands of chromatin as the cell has chromosomes. The chromatin fiber is composed of a 100 Å "string of pearls" called nucleosomes. This fiber is formed by a succession of nucleosomes.