Cell Division: Mitosis & Meiosis – Chromosomal Behavior and the Cell Cycle

Cell Division: Mitosis & Meiosis

Introduction to Cell Division

Cell division is a fundamental biological process by which cells reproduce, enabling growth, development, and tissue repair in multicellular organisms. There are two main types of cell division: mitosis and meiosis. Mitosis results in genetically identical daughter cells, while meiosis produces gametes with half the chromosome number of the parent cell.

Chromosomal Behavior and Mendelian Genetics

Historical Observations of Chromosome Behavior

• Walther Flemming (1882): First documented chromosome behavior during cell division.

• Observed consistency in chromosome number in different cells of an organism and among organisms within the same species.

• Chromosome number is maintained through their replication and equal division during mitosis.

• Walter Sutton & Theodor Boveri: Observed chromosome behavior during the production of reproductive cells (gametes).

• Chromosome number is reduced during gamete (sperm & egg) production (meiosis).

• Chromosome number is restored during fertilization.

• Chromosomes behave in the same way as Mendel’s hereditary factors, supporting the chromosomal theory of inheritance.

Key Terms:

• Chromosome: A structure composed of DNA and proteins that carries genetic information.

• Homologous chromosomes: Chromosome pairs, one from each parent, that are similar in shape, size, and genetic content.

• Gametes: Reproductive cells (sperm and egg) with half the chromosome number of somatic cells.

The Cell Cycle

Phases of the Cell Cycle

The cell cycle is the ordered sequence of events that leads to cell division and duplication. It consists of interphase (G1, S, G2) and the mitotic (M) phase.

• G1 phase: Cell growth and preparation for DNA replication.

• S phase (DNA synthesis): DNA replication occurs, resulting in duplicated chromosomes.

• G2 phase: Further growth and preparation for mitosis.

• M phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Diagram: The cell cycle is often depicted as a circular process, with interphase occupying the majority of the cycle and the M phase representing cell division.

DNA Replication and Sister Chromatids

Formation of Sister Chromatids

During the S phase of the cell cycle, DNA replication duplicates the genetic material, resulting in the formation of sister chromatids.

• Before DNA replication: Each chromosome consists of a single DNA molecule; homologous chromosomes are present (one from each parent).

• After DNA replication: Each chromosome consists of two identical sister chromatids joined at the centromere.

Key Terms:

• Sister chromatids: Two identical copies of a chromosome connected by a centromere, formed after DNA replication.

• Centromere: The region where sister chromatids are joined.

Stages of Mitosis

Phases of Mitosis

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets, resulting in two daughter cells. The stages are:

• Prophase: Chromosomes condense and become visible; spindle fibers begin to form.

• Prometaphase: Nuclear envelope breaks down; spindle fibers attach to chromosomes at the centromere.

• Metaphase: Chromosomes align at the cell's equatorial plate (metaphase plate).

• Anaphase: Sister chromatids are pulled apart toward opposite poles of the cell.

• Telophase: Nuclear envelopes reform around the separated chromosomes; chromosomes de-condense.

• Cytokinesis: Division of the cytoplasm, resulting in two separate daughter cells.

Example: Skin cells undergo mitosis to replace damaged or dead cells.

Meiosis: Reduction Division

Overview of Meiosis

Meiosis is a specialized form of cell division that produces gametes (sperm and egg cells) with half the chromosome number of the parent cell. It consists of two consecutive divisions: meiosis I and meiosis II.

• Meiosis I: Homologous chromosomes separate, reducing the chromosome number by half (reduction division).

• Meiosis II: Sister chromatids separate, similar to mitosis, resulting in four haploid daughter cells.

Key Terms:

• Chiasma: The site where homologous chromosomes exchange genetic material (crossing over).

• Haploid (n): A cell with one set of chromosomes.

• Diploid (2n): A cell with two sets of chromosomes.

Comparison of Mitosis and Meiosis

Major Differences Between Mitosis and Meiosis

Mitosis and meiosis are both forms of cell division, but they serve different purposes and have distinct outcomes.

Feature	Mitosis	Meiosis
Number of divisions	One	Two
Number of daughter cells	Two	Four
Chromosome number in daughter cells	Diploid (2n)	Haploid (n)
Genetic identity	Identical to parent	Genetically unique
Function	Growth, repair, asexual reproduction	Sexual reproduction (gamete formation)

Key Equations

Chromosome Number During Cell Division

• Diploid to Haploid Transition (Meiosis):

$n = \frac{2n}{2}$

• DNA Replication: Each chromosome duplicates to form two sister chromatids.

$\text{Chromosome} \xrightarrow{\text{Replication}} \text{Sister Chromatids}$

Summary

• Cell division is essential for life, enabling growth, development, and reproduction.

• Mitosis produces identical cells for growth and repair; meiosis generates genetically diverse gametes for sexual reproduction.

• Chromosomal behavior during cell division underlies Mendelian inheritance and genetic continuity.