Cell Division: DNA Replication & Mitosis

Why Divide?

Organisms grow by increasing in size. Instead of simply expanding, cells divide to facilitate organism growth.

Cell Division

Cell division is required for:

• Growth and repair of multicellular organisms.
• Reproduction of unicellular organisms.
• Optional form of reproduction of multicellular organisms.

Three Basic Steps of Cell Division:

1. Copy the DNA
2. Separate the copies
3. Divide the cytoplasm to create two new cells

Mitotic cell division (mitosis) is the most common method, producing 2 daughter cells that are genetically identical to the parent cell and to each other (clones).

DNA Replication

Division requires DNA replication. DNA is replicated by copying existing material, following Watson-Crick base pairing: A=T and G=C.

Cellular Reproduction of Prokaryotes

Binary Fission (Prokaryotic Fission)

For example, the reproduction of bacteria.

Steps:

1. Copy the DNA
2. Separate the copies
3. Divide the cytoplasm to create two new cells

Cellular Reproduction in Eukaryotes

Asexual reproduction by cell division!.

• Unicellular eukaryotes (e.g., Paramecium via binary fission).
• Multicellular eukaryotes:
  • E.g., Hydra (Budding)
  • E.g., Plants and animals (Cloning)

Overview of Mitosis

Produces 2 daughter cells genetically identical to the parent cell and to each other ("clones").

5 Stages:

1. Interphase (G1, S, G2)
2. Prophase
3. Metaphase
4. Anaphase
5. Telophase & Cytokinesis

Interphase

Three parts:

1. G1: Cell growth & differentiation
2. S: Synthesis of DNA & duplication of chromosomes
3. G2: Cell growth & preparation for cell division

G1: Differentiation

Three types of cells based on the ability to divide and differentiate:

1. Stem cells: Totipotency or pluripotency.
   • Ability to self-renew and differentiate into any (some) cell type (e.g., bone marrow, skin, intestines, ovaries & testes, meristem cells in plants).
2. Differentiated cells with limited ability to divide
   • E.g., liver cells
3. Permanently differentiated cells, unable to divide
   • E.g., brain cells

S: DNA Replication

DNA is copied in an uncondensed form, then condensed for sorting.

Mitotic Cell Division Overview

G1: cell growth and differentiation, S: synthesis of DNA; chromosomes are duplicated, G2: cell growth and preparation for cell division, prophase, metaphase, anaphase, telophase and cytokinesis.

Stages of Mitosis

Prophase ('Pro' = Before)

1. Chromosomes condense.
2. Microtubules form a spindle.
3. Nuclear envelope breaks down.
4. Chromosomes join with spindle microtubules by centromeres.

Metaphase

Chromosomes line up along the equator of the cell, forming the "metaphase plate."

Anaphase

Sister chromatids separate. Shortening of the spindle fibers pulls them to the opposite poles of the cell.

Telophase

• Nuclear envelope forms around each group of chromosomes.
• Chromosomes relax.
• New cell plate begins to form.

Cytokinesis

Cytoplasm is divided between the two daughter cells.

Cytokinesis in Plant Cells

New cell wall is deposited via vesicles. Carbohydrate-filled vesicles bud off the Golgi apparatus and move to the equator of the cell. The vesicles fuse to form a new cell wall and plasma membrane between the daughter cells, resulting in complete separation of the daughter cells.

Summary

What is cell division, and why do we do it?

Three Stages of Cell Division:

• Copy the DNA
• Separate the DNA
• Divide the cell contents equally
• Binary fission (prokaryotic division)
• Eukaryotic DNA replication and cell division
• 5 stages of mitosis