Cell Division Why do cells divide?

Cells must divide in order for the surface area (cell membrane) to keep up with the volume of the cell.

Cell Cycle Stage 1 : Interphase Stage 2: Mitosis Stage 3: Cytokinesis

Stage 1: Interphase The period before cell division occurs. The cell grows to mature size and produces copies of organelles. DNA makes a copy of itself (DNA replication.) Cell stores energy and produces structures for division.

Stage 2: Mitosis The cell’s nucleus divides into two new nuclei. One copy of the DNA goes to each new daughter cell. There are four phases: prophase, metaphase, anaphase, and telophase.

Prophase The chromatin in the cell’s nucleus begins to get thicker and are now called chromosomes. Each identical rod, or strand, of the chromosome is called a chromatid. The two strands are held together by a structure called a centromere.

Metaphase The chromosomes attach to the spindle fibers and line up in the middle of the cell.

Anaphase The chromosomes get pulled by the spindle fibers to each side of the cell.

Telophase The two new nuclei of the daughter cells form, and a nuclear membrane appears around both.

Stage 3: Cytokinesis During cytokinesis, the cytoplasm divides, distributing the organelles into each of the two new cells. The animal cell’s membrane pinches in, and the plant cell forms a cell plate. Each daughter cell has the same number of chromosomes as the original parent cell.

DNA Replication- A cell makes a copy of its DNA before mitosis begins.

In 1953, two scientists, James Watson and Francis Crick, figured out the structure of the DNA molecule.

The DNA molecule looks like a twisted ladder called a “double helix”. The sides are made of sugar and phosphate. The rungs are made of nitrogen bases: adenine, thymine, guanine, and cytosine. These bases always pair together: A-T C-G

The length of the cell cycle varies from cell to cell. Most of the cell’s life is spent in interphase. Some cells, such as the human brain cells, never divide.

DNA Replication starts when the two sides of the DNA molecule unwind or “unzip” at the hydrogen bond holding the nitrogen bases together. Next free floating nucleotides (sugar, phosphate, and a nitrogen base) attach to their complimentary bases. A always pairs with T, while G always pairs with C.

DNA replication is needed so that the DNA and chromosomes in the daughter cells are exactly like the parent cell.