cell division

Learning Objectives

In this lesson, you will learn

1. What is cell division? 
2. What are the different types of cell division?
3. Cell division in prokaryotes versus eukaryotes.
4. What is mitosis? 
5. What is meiosis?
6. Difference between diploids and haploids.

WHAT IS CELL DIVISION?

Cell division is the process in which one cell called the parent cell divides to form two new cells referred to as daughter cells. Depending on the type of organism, there are several types of cell division.

WHAT ARE THE TYPES OF CELL DIVISION?  

There are three main types of cell division: binary fission, mitosis, and meiosis. Binary fission is used by simple organisms like bacteria. More complex organisms gain new cells by either mitosis or meiosis.

CELL DIVISION IN PROKARYOTES

Binary fission

1. Cytoplasm
2. Chromosome
3. DNA replication
4. Cytokinesis
5. Two identical daughter cells

Most prokaryotic cells divide by the process of binary fission. Binary fission can be described as a series of steps, although it is actually a continuous process. The steps include DNA replication, chromosome segregation, and finally the separation into two daughter cells.

1. DNA replication – Just before the cell divides, its DNA is copied in a process called DNA replication. This results in two identical chromosomes instead of just one. This step is necessary so that when the cell divides, each daughter cell will have its own chromosome.
2. Chromosome segregation – The two chromosomes separate and move to opposite ends (known as poles) of the cell. This occurs as each copy of DNA attaches to different parts of the cell membrane.
3. Separation – A new plasma membrane starts growing into the center of the cell, and the cytoplasm splits apart, forming two daughter cells. As the cell begins to pull apart, the new and the original chromosomes are separated. The two daughter cells that result are genetically identical to each other, and to the parent cell. A new cell wall must also form around the two cells.

CELL DIVISION IN EUKARYOTES

Cell division is more complex in eukaryotes than prokaryotes. Prior to dividing, all the DNA in a eukaryotic cell’s multiple chromosomes is replicated. Its organelles are also duplicated. Then, when the cell divides, it occurs in two major steps:

1. Mitosis - The first step is mitosis, a multi-phase process in which the nucleus of the cell divides. During mitosis, the nuclear membrane breaks down and later reforms. The chromosomes are also sorted and separated to ensure that each daughter cell receives a diploid number (2 sets) of chromosomes. In humans, that number of chromosomes is 46 (23 pairs). Mitosis is described in greater detail later.
2. Cytokinesis - The second major step is cytokinesis. As in prokaryotic cells, the cytoplasm must divide. Cytokinesis is the division of the cytoplasm in eukaryotic cells, resulting in two genetically identical daughter cells.

Mitosis

Mitosis is used when a cell needs to be replicated into exact copies of itself. Everything in the cell is duplicated. The two new cells have the same DNA, functions and genetic code. The original cell is called the mother cell and the two new cells are called daughter cells.

Examples of cells that are produced through mitosis include cells in the human body for the skin, blood, and muscles.

Cell cycle for mitosis

Cells go through different phases called the cell cycle. The “normal” state of a cell is called the interphase. The genetic material is duplicated during the interphase stage of the cell. When a cell gets the signal that it is to duplicate, it will enter the first stage of mitosis called the prophase.

• Prophase – During prophase, the chromatin condenses into chromosomes and the nuclear membrane and nucleolus break down.
• Metaphase – During metaphase, the chromosomes line up along the middle of the cell.
• Anaphase – During anaphase the chromosomes separate and move to opposite sides of the cell.
• Telophase – During telophase the cell forms two membranes around each set of chromosomes and the chromosomes uncoil. The cell walls then pinch off and split down the middle. The two new cells or daughter cells are formed. The splitting of the cells is called cytokinesis or cell cleavage.

1. Interphase
2. Prophase
3. Metaphase
4. Anaphase
5. Telophase
6. Cytokinesis (daughter cells)

Mitosis starts with prophase in which the chromosome is condensed. The cell proceeds to metaphase where the chromosomes are aligned on the metaphase plate. Then the chromosomes are separated in anaphase and the cell’s cytoplasm is pinched apart during telophase. Cytokinesis is the final process that breaks the cell membrane and divides the cell into two.

Interphase is the normal state of a cell. It may also be known as the resting state. This is when the cell is making sure it has all the nutrients and energy to survive. It is getting ready for another division – duplicating its nucleic acids so when it’s time for prophase it has everything.

Meiosis

When it is time for the entire organism to reproduce, meiosis is used. There are two main differences between mitosis and meiosis. First, the meiosis process has two divisions. When meiosis is complete, a single cell produces four new cells instead of just two. The second difference is that the new cells only have half the DNA of the original cell. This is important for life on Earth as it allows for new genetic combinations to occur which produces variety in life.

Examples of cells that undergo meiosis include cells used in sexual reproduction called gametes.

DIPLOIDS AND HAPLOIDS

The cells produced from mitosis are called diploids because they have two complete sets of chromosomes.

The cells produced from meiosis are called haploids because they only have half the number of chromosomes as the original cell.