Cell Division

Mitosis and Meiosis

Section Objectives

• Relate the function of a cell to its organization in tissues, organs, and organ systems.

• Sequence the events of the cell cycle.

Cell Size LimitationsCell Size Limitations

• The cells that make up a multicellular organism come in a wide variety of sizes and shapes.

• Considering this wide range of cells sizes, why then can’t most organisms be just one giant cell? What limits cell size?

1. Diffusion limits cell size1. Diffusion limits cell size

• Although diffusion is a fast and efficient process over short distances, it becomes slow and inefficient as the distances become larger.

• Because of the slow rate of diffusion, organisms can’t be just one giant-sized cell.

2. DNA limits cell size2. DNA limits cell size• The cell cannot survive unless there is

enough DNA to support the protein needs of the cell.

• In many large cells, more than one nucleus is present.

• Large amounts of DNA in many nuclei ensure that cell activities are carried out quickly and efficiently.

3. Surface area-to-volume ratio3. Surface area-to-volume ratio

• As a cell’s size increases, its volume increases much faster than its surface area.

• Volume mm3 (inside) and Area mm2 (outside).

Surface area = 6 mm2 Volume = 1 mm3

Surface area = 24 mm2 Volume = 8 mm3

1 mm 1 mm

1 mm

2 mm 2 mm

2 mm

4 mm 4 mm

4 mm

Faced with all of this. . .

• A cell must divide or die.• Most chose to divide or reproduce

in a process called mitosis.

Cell Reproduction - MitosisCell Reproduction - Mitosis• Cell division is the process by which new

cells are produced from one cell.

• Cell division results in two cells that are identical to the original, parent cell.

The discovery of chromosomesThe discovery of chromosomes• Structures, which contain DNA and

become darkly colored when stained, are called chromosomes.

• Chromosomes are the carriers of the genetic material that is copied and passed from generation to generation of cells.

The structure of eukaryotic chromosomesThe structure of eukaryotic chromosomes

Centromere

Chromosome

Sister chromatids

Supercoil within chromosome

Continued coiling within supercoil

Histone H1

Nucleosome

DNA

The Cell CycleThe Cell Cycle

• The cell cycle is the sequence of growth and division of a cell.

• The majority of a cell’s life is spent in the growth period known as interphase.

Interphase

The Cell CycleThe Cell Cycle

• Following interphase, a cell enters its period of nuclear division called mitosis.

• Following mitosis, the cytoplasm divides, separating the two daughter cells.

Mitosis

Interphase: A Busy TimeInterphase: A Busy Time

• During the first part, the cell grows and protein production is high.

Rapid growth

and metabolic activity

Interphase

Interphase: A Busy TimeInterphase: A Busy Time

• The first part of Interphase is called the G1 phase: the cell grows and makes protein.

Interphase: A Busy TimeInterphase: A Busy Time

• The second stage of interphase is the S phase: it stands for synthesis of DNA. This is when the cell makes an extra set of chromosomes.

Interphase: A Busy TimeInterphase: A Busy Time

• The third part of interphase is a second growth phase called G2. During this time, mitochondria and other organelles are made and parts needed for cell division are put together.

Now the cell is ready!

• Mitosis consists of 4 stages. These four stages allow for the nucleus to divide.• Prophase – for “preparing”• Metaphase – for “middle”• Anaphase – for “away”• Telophase – for “two”

Prophase: Preparing the cell for dividing.Prophase: Preparing the cell for dividing.

• During prophase, the chromatin coils to form visible chromosomes.

Spindle fibers

Disappearing nuclear envelope

Doubled chromosome

Prophase:What form the DNA is in.Prophase:What form the DNA is in.

• The two halves of the doubled structure are called sister chromatids. These make a chromosome.

Sister chromatids

Centromere

Metaphase: Chromosomes in the MiddleMetaphase: Chromosomes in the Middle

• During metaphase, the chromosomes move to the equator of the spindle.

Centromere

Sister chromatids

Anaphase: Chromatids AwayAnaphase: Chromatids Away

• During anaphase, the centromeres split and the sister chromatids are pulled apart to opposite poles of the cell.

Telophase: Two new cells are formed.Telophase: Two new cells are formed.

• During telophase, two new daughter cells are formed. The cells separate as the cell cycle goes into the next interphase.

Nuclear envelope reappears

Two daughter cells are formed

CytokinesisCytokinesis

• Following telophase, the cell’s cytoplasm divides in a process called cytokinesis.

• Cytokinesis differs between plants and animals because of the need to make a cell wall.

• Toward the end of telophase in animal cells, the plasma membrane pinches in along the equator.

Results of MitosisResults of Mitosis

• When mitosis is complete, unicellular organisms remain as single cells.

• In multicellular organisms, cell growth and reproduction result in groups of cells that work together as tissue to perform a specific function.