Interphase

Interphase is the longest portion of the cell cycle, where the cell performs its essential functions and prepares for division. Interphase is broken down into three sub-phases: first gap phase (G₁), synthesis phase (S phase), and second gap phase (G₂). These cell cycle phases are described briefly below:

First gap phase (G₁)

• The cell performs all of its necessary functions.

• Accumulates energy and proteins in preparation for the synthesis phase.

• The cell grows so that it is large enough to be divided in two.

• The first cell checkpoint ensures the cell is ready to move on to S phase.

Synthesis phase (S phase)

• The cell undergoes DNA Replication, copying exactly all of the DNA chromosomes.

• The chromosomes now form the recognizable X-shape, each half of the X being a full copy of the other side. Each half is called a sister chromatid.

• Organelles called centrosomes are duplicated.

Second gap phase (G₂)

• The cell continues to grow and accumulate energy for the mitotic phase.

• The organelles are duplicated for each daughter cell.

• The cytoskeleton is dismantled into microtubules to be used during mitosis.

• A second cell checkpoint ensures the cell is ready to enter the mitotic phase.

The mitotic phase of the cell cycle is when the cell separates its DNA chromosomes and cell components, leading to cell division and daughter cells. The mitotic phase consists of two sub-phases that occur simultaneously: mitosis and cytokinesis. Mitosis is the division of the DNA chromosomes and takes place over five sub-phases: prophase, prometaphase, metaphase, anaphase, and telophase. Cytokinesis is when cell division actually happens, producing daughter cells. In animal cells, an actin ring pinches the cell’s plasma membrane together until it has split into two cells, almost like pinching a piece of clay into two. Plant cells, however, build a cell plate inside itself until it fuses with its outer cell wall and separates the two new daughter cells. With cytokinesis complete, the cell cycle is also complete and may begin once again for each of the daughter cells.

Prophase

• In the first stage of mitosis, the chromosomes begin to condense and become visible.
• Centrosomes separate to opposite sides producing spindle microtubules. These microtubules move the chromosomes in the cell, like puppet strings.
• The nuclear envelope breaks down and moves to the edge of the cell, making way for mitosis.

Prometaphase

• The chromosomes fully condense but are still being directed to the middle of the cell.
• Spindle microtubules attach to each sister chromatid at structures called kinetochores.
• The centrosomes have moved to opposite poles of the cell.

Metaphase

• The chromosomes are aligned in the center of the cell on a straight line called the metaphase plate.
• A cell checkpoint ensures that the spindle microtubules have attached to each sister chromatid’s kinetochore correctly, in preparation to separate the chromosomes.

Anaphase

• The mitotic spindles shorten, pulling each of the sister chromatids apart. Now that the chromatids have separated, they are called daughter chromosomes. Two full sets of DNA chromosomes should now be on opposite sides of the cell.
• Unattached spindle microtubules elongate the cell into an oval shape, preparing it for cell division during cytokinesis.

Telophase

• Two new nuclear envelopes form around the DNA chromosomes.
• The chromosomes loosen into usable chromatin.
• the spindle microtubules break down and are recycled into the cytoskeleton for the new daughter cells after cytokinesis.

Gap 0

Immediately after the mitosis cell division, the new cell may enter a quiescent stage called gap zero (G₀) of the cell cycle, and will not continue to actively divide. This may be a temporary stage while the organism is under stress, before the cell moves into the G₁ after some outside stimulus. Or it can be permanent in the case of nerve or heart cells that do not actively divide once fully formed.