Cell Cycle and Interphase Notes

Cell Cycle Reading Guide

Cell Cycle Overview

• The cell cycle, or cell division cycle, is the series of events that take place in a eukaryotic cell between its formation and the moment it replicates itself.
• The cell cycle occurs in eukaryotic cells.
• The two main phases of the cell cycle are interphase and the mitotic phase.
• Interphase occurs between divisions, while the mitotic phase is when the cell is replicating itself.
• During interphase, the cell forms and carries out its normal metabolic functions.
• A fertilized egg develops into a mature organism during the cell cycle, forming structures like hair, skin, and blood cells.

Interphase

• Cell division never occurs in interphase.
• During interphase, cells obtain nutrients and duplicate their chromatids or genetic material.
• Chromatids are found in the nucleus of a cell and are made of chromatin.
• Most cells spend the majority of their lifetime in interphase, such as human skin cells, which spend approximately 22 hours per day in interphase.
• Nerve cells may spend decades in interphase instead of dividing.
• The three stages of interphase are G1, S, and G2.

G1 Phase

• G1 stands for Growth 1.
• During G1, the cell creates cell organelles and begins metabolism.
• Cells sometimes exit the cell cycle from the G1 phase.
• After cytokinesis, the cell enters the G1 phase.
• The major activity during G1 is cell growth.
• New organelles are made in G1.
• Significant protein synthesis occurs during G1, including the production of proteins and enzymes.

S Phase

• S stands for Synthesis.
• During the S phase, the chromosomes of the cell are copied; this is DNA synthesis or replication.
• Each chromosome is copied during the S phase.
• A chromosome is originally made of one coiled DNA double helix molecule, called a chromatid.
• At the end of the S phase, each chromosome has two identical DNA double helix molecules.
• The centrosomes are also copied during the S phase and hold the duplicated chromosomes together.

G2 Phase

• G2 stands for Growth 2.
• During G2, the cell grows and prepares for cell division.
• G2 is the final and shortest phase of interphase.
• A typical cell spends about four to five hours in the G2 phase.
• The cell prepares for mitosis during the G2 phase by growing.
• Mitosis follows the G2 phase.

G0 Phase

• Cells sometimes exit the cell cycle, usually from G1, and enter the G0 phase.
• In the G0 phase, cells are alive and metabolically active but do not divide.
• Cells in the G0 phase do not copy their DNA and do not prepare for cell division.
• Heart muscle, eye, and brain cells enter the G0 phase when they are mature.
• If these cells are damaged during your lifetime, they cannot be replaced.

Cytokinesis

• G1 stage occurs after cytokinesis.
• During cytokinesis, the cytoplasm is divided.
• The new cells are called daughter cells, and they are identical to each other.

Chromosome Structure

• A chromosome consists of a centromere, a short arm (chromatid), and a long arm (chromatid).

Mitotic Phase

• During mitosis, the nucleus and nuclear material divide.
• Another name for mitosis is karyokinesis, because only the nucleus is dividing.

Numerical Information

• Human skin cells divide once a day and spend about 22 hours in interphase.
• A typical cell spends four to five hours in the G2 phase.
• Approximately 90 percent of cells are in interphase.