The Cell Cycle

All living are made of cells All living are made of cells. For an organism to grow, these cells must divide to make more cells. The process of cellular division is very important because it makes every cell in an organism genetically identical.

Interphase = 90% of the cell cycle The process of dividing into two cells is only a small part of the cell cycle. Most of the cell’s time is spent preparing to divide during interphase. Interphase = 90% of the cell cycle

Interphase Interphase has 3 important stages. The two gap stages (G1 and G2) are the gaps between activity when the cell grows and performs normal cellular processes.

Interphase Some cells enter a resting phase known as the G0 stage when they are not preparing to divide. Some cells, like neurons, stay in the G0 permanently.

Interphase For cellular division, the most important thing that happens during interphase is the synthesis or S stage. It is during this stage that DNA replication occurs.

Interphase: DNA Replication Every cell needs a complete set of DNA. This means that before a cell can divide, the DNA must be replicated (copied) to make 2 complete sets during the S stage.

S Stage: DNA Replication Before DNA can be replicated, it must unwind and the two strands must separate between the nitrogen bases.

Then, DNA polymerase enzymes add nucleotides to create two complete and identical sets of DNA. Image by Madprime

DNA Replication DNA replication is a semi-conservative process; there is not 1 new set and 1 old set. Each set of DNA contains 1 old strand and 1 new strand.

Watch the short video over DNA replication.

Mitosis Once the cell is ready to begin dividing, it enters mitosis. The purpose of mitosis is to separate the two sets of DNA into two different nuclei.

Mitosis Mitosis allows each new daughter cell to be genetically identical. This is because each cell has a nucleus with a complete set of chromosomes.

Each daughter cell would be identical and have 46 chromosomes. If the parent cell has 46 chromosomes before mitosis, how many chromosomes will be in each genetically identical daughter cell after mitosis? Each daughter cell would be identical and have 46 chromosomes.

Cytokinesis Once two nuclei have been formed, the cell can divide its cytoplasm, which includes all its organelles, into two new daughter cells. This is call cytokinesis.

Throughout the cell cycle, there are several checkpoints Throughout the cell cycle, there are several checkpoints. These are the points where the cell checks to make sure no mistakes have been made. If serious mistakes occurred, the cell may go through apoptosis. Apoptosis = Cell Death

Certain mutations can cause cells to ignore these checkpoints Certain mutations can cause cells to ignore these checkpoints. Cells can begin to divide very rapidly, and they do not stop dividing.

When someone’s cells begin uncontrollably dividing, they develop a tumor. Cancer is a form of uncontrolled cell division.

Binary Fission Mitosis only occurs in eukaryotes because only eukaryotic cells have a nucleus. Prokaryotes do NOT have a nucleus, but their cells divide in a very similar way through a process called binary fission. 19