Biology: Cell Division

INTERPHASE

This is the period before cell division occurs.

• Cell grows to its mature size - cell doubles in size and produces all the structures needed to carry out its functions.
• The cell makes a copy of its DNA, and prepares to divide into two cells in a process called replication. At the end of DNA replication, the cell contains two identical sets of DNA.
• The cell produces structures that it will use to divide during the rest of the cell cycle.

MITOSIS

During mitosis, one copy of the DNA is distributed into each of the two daughter cells.

Chromosome is a doubled rod of condensed chromatin.

Chromatid is an identical rod, or strand, of chromosome.

• PROPHASE  - the chromatin in the nucleus condenses to form chromosomes. Structures called spindle fibers form a bridge between the ends of the cell. The nuclear membrane breaks down.
  
• METAPHASE - the chromosomes line up across the center of the cell. Each chromosome attaches to spindle fiber at its centromere, which holds the chromatids together.
  
• ANAPHASE - The centromeres split. The two chromatids separate. One chromatid moves along the spindle fiber to one end of the cell. The other chromatid moves to the opposite end. The cell becomes stretched out as the opposite ends pull apart.
  
• TELOPHASE - The chromosomes begin to stretch out and lose their rod-like appearance. This occurs in the two regions at the ends of the cell. A new nuclear membrane forms around each region of chromosomes.

During mitosis chromatids separate from each other and move to opposite ends of the cell. Then two nuclei form around the chromatids at the two ends of the cell.

CYTOKINESIS

During cytokinesis, the cytoplasm divides, distributing the organelles into each of the two new cells.

Cytokinesis marks the end of the cell cycle. Two new cells have formed. Each daughter cell has the same number of chromosomes as the original parent cell. At the end of cytokinesis, each cell enters interphase, and the cycle begins again.

Source: Encyclopedia Britannica

STRUCTURE OF DNA

There are four kinds of nitrogen bases: adenine, thymine, guanine, and cytosine. The capital letters A,T, G, and C are used to represent the four bases.

Adenine (A) only pairs with thymine (T), while guanine (G) only pairs with cytosine (C). This pairing pattern is the key to understanding how DNA replication occurs.