Cell Division and Chromosomes: Study Guide for Cell Biology

Cell Division

Overview of Cell Division

Cell division is a fundamental process by which cells reproduce, allowing for growth, repair, and maintenance in multicellular organisms. The two main types of cell division are mitosis and meiosis.

• Mitosis results in two genetically identical daughter cells and occurs in somatic (body) cells.

• Meiosis produces gametes (sperm and egg cells) with half the chromosome number of the parent cell.

• Cell division is essential for growth, healing, and replacing worn-out cells.

Example: Skin cells divide by mitosis to replace cells lost due to injury.

Phases of the Cell Cycle

The cell cycle consists of a series of phases that prepare a cell for division and ensure accurate replication and distribution of genetic material.

• Interphase: The cell grows, replicates its DNA, and prepares for division. It includes G1 (Gap 1), S (Synthesis), and G2 (Gap 2) phases.

• Mitosis: The nucleus divides, followed by cytokinesis (division of the cytoplasm).

• Cytokinesis: The cytoplasm divides, resulting in two separate daughter cells.

Key Point: The cell spends most of its life cycle in interphase.

Mitosis

Stages of Mitosis

Mitosis is divided into distinct phases, each characterized by specific events involving chromosomes and cellular structures.

• Prophase: Chromosomes condense, the nuclear envelope breaks down, and the mitotic spindle begins to form.

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

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

• Telophase: Chromatids reach the poles, nuclear envelopes reform, and chromosomes decondense.

Example: During metaphase, chromosomes are lined up in the middle of the cell, ready to be separated.

Key Structures in Mitosis

• Chromosome: A structure composed of DNA and proteins, visible during cell division.

• Sister Chromatids: Two identical copies of a chromosome connected by a centromere.

• Centrosome: Organelle that organizes the spindle fibers during mitosis.

• Spindle Fibers: Microtubules that attach to chromosomes and help separate them.

• Cleavage Furrow: In animal cells, the indentation that begins cytokinesis.

• Cell Plate: In plant cells, the structure that forms to separate two daughter cells during cytokinesis.

Events Before Cell Division

• The cell's DNA is replicated during the S-phase of interphase.

• The cell increases in size and prepares necessary organelles for division.

• Genetic material is duplicated to ensure each daughter cell receives a complete set.

Equation:

$\text{DNA Replication:} \quad \text{1 chromosome} \rightarrow \text{2 sister chromatids}$

Chromosomes and Chromosome Number

Somatic Cells vs. Gametes

Chromosome number varies between somatic cells and gametes due to the processes of mitosis and meiosis.

• Somatic cells are diploid (2n), containing two sets of chromosomes.

• Gametes are haploid (n), containing one set of chromosomes.

• In humans, somatic cells have 46 chromosomes, while gametes have 23 chromosomes.

• In mosquitoes (example from questions), if somatic cells have 6 chromosomes, gametes have 3 chromosomes.

Equation:

$\text{Diploid number} = 2n$ $\text{Haploid number} = n$

Table: Chromosome Number in Humans

Cell Type	Number of Chromosomes
Body Cells (Somatic)	46
Sperm Cells (Gametes)	23

Key Terms and Definitions

• Chromosome: A thread-like structure of nucleic acids and protein found in the nucleus, carrying genetic information.

• Centrosome: The main microtubule organizing center in animal cells, crucial for spindle formation.

• Spindle Apparatus: Structure that separates chromosomes during mitosis and meiosis.

• Cytokinesis: The division of the cytoplasm, resulting in two daughter cells.

• Interphase: The phase of the cell cycle when the cell is not dividing but is growing and replicating DNA.

• Cleavage Furrow: The indentation that begins the process of cytokinesis in animal cells.

• Cell Plate: The structure that forms in plant cells during cytokinesis to separate the two daughter cells.

Summary Table: Stages of Mitosis

Stage	Main Events
Prophase	Chromosomes condense, spindle forms, nuclear envelope breaks down
Metaphase	Chromosomes align at the metaphase plate
Anaphase	Sister chromatids separate and move to opposite poles
Telophase	Nuclear envelopes reform, chromosomes decondense
Cytokinesis	Cytoplasm divides, forming two daughter cells

Applications and Importance

• Mitosis is essential for growth, tissue repair, and asexual reproduction in multicellular organisms.

• Errors in mitosis can lead to cancer or genetic disorders.

• Understanding the cell cycle is crucial for fields such as medicine, genetics, and biotechnology.

Additional info: The study guide covers topics from chapters on Cell Division, Chromosomes, and the Cell Cycle, which are central to college-level Cell Biology.