6.2 The Cell Cycle

Cells on the path to cell division go through a series of carefully regulated stages of growth and division that produce two genetically identical cells.

A cell moves in an orderly fashion.

During the interphase, there are three phases: G1, the S phase and the G2 phase.
Interphase follows the mitotic phase.
Nuclear division is when duplicated chromosomes are divided into daughter nuclei.
The cell divides in a process called cytokinesis in which the cytoplasm is divided and two daughter cells are formed.

The cell undergoes normal processes while also preparing for cell division during interphase.

Many internal and external conditions must be met for a cell to move from interphase to the mitotic phase.
Three stages of interphase are called G1, S, and G2.

The cell is active at the biochemical level during the G1 stage.

The cell has enough energy to complete the task of replicating each chromosomes in the nucleus, as well as accumulating the building blocks of chromosomal Chapter 6.

Nuclear DNA is in a semi-condensed configuration.

Each of the chromosomes is made of two sister chromatids.
During the S phase, the centrosome is duplicated.
Centrioles help organize cells.
Plants and most fungi do not have centrosomes with Centrioles.

Some cells are duplicated and the cytoskeleton is dismantled.

During G2, there may be additional cell growth.
Before the cell can enter the first stage of mitosis, final preparations need to be completed.

The nucleus and the cytoplasm must be divided to make two daughter cells.

The cell is divided into two identical daughter cells after the duplicated chromosomes are aligned, separated, and moved to opposite poles.
The separation of the cytoplasmic components into two daughter cells is called cytokinesis.

The division of the cell nucleus can be achieved through a series of phases, which are divided into prophase, prometaphase, metaphase, anaphase, and telophase.

There are five stages of animal cell mitosis--prophase, prometaphase, metaphase, anaphase, and telophase--visualized here.

This is a transmission electron microscope showing the cytokinesis.

The kinetochore is attached to the spindle.

The chromatids are sisters.

The kinetochore is attached to the spindle.

The chromatids are sisters.

The sister chromatids separate from the kinetochore.

The kinetochore and sister chromatids are not the same.

The Golgi apparatus and the reticulum fragment are dispersed to the center of the cell when the nuclear envelope breaks.

The centrosomes are moving to opposite poles.
As the microtubule fibers shorten, the centrosomes are pushed farther apart.
The sister chromatids become visible under a light microscope as they coil more tightly.

As more microtubules assemble and stretch across the length of the former nuclear area, reproduction at the Cellular Level spindle continues to develop.

The chymosomes are more visual.

The chromatids are attached to each other.

The chromosomes are very small.

The centrosome is where the microtubule was attached.

The non-kinetochore microtubules slide against each other at the metaphase plate.

The chromosomes reach the opposite poles.

Each daughter cell will have a different set of monomers that will be used to assemble the components.
Nuclear envelopes are around the chromosomes.

The page of movies shows different aspects of the disease.

You can watch the movie "DIC microscopy of cell division in a newt lung cell" to identify the phases of the disease.

The process of cytokinesis is different for plants that have cell walls than it is for other eukaryotes.

The start of cytokinesis occurs in animal cells that lack cell walls.

There is a contractile ring at the former metaphase plate.
The actin filaments pull the equator of the cell inward.
The actin ring contracts and the cell is cleaved in two, but not in plant cells because of the rigid cell walls.
The daughter cells need to form a wall.
During interphase, the Golgi apparatus breaks up into vesicles and is dispersed throughout the dividing cell.
Golgi vesicles move on microtubules to collect at the metaphase plate during telophase.
The cell wall at the center of the cell is merged with the cell plate.
A new cell wall is built by using the glucose that has accumulated between the layers.
On either side of the new cell wall are the Golgi membranes.

The actin fibers are drawn in by a ring of cells.

The cells are pinched in two.
Golgi vesicles coalesce at the former metaphase plate in a plant cell.
The cell plate is formed by the fusion of the vesicles.
The contents of the cell walls are new.

The classic cell-cycle pattern in which a newly formed daughter cell immediately enters interphase is not the case for all cells.

The cell has exited the cell cycle and is in a quiescent state.
Some cells enter G0 temporarily until an external signal kicks in.
Cells that aren't preparing to divide enter an alternate phase.
This is a temporary condition until G1 is triggered.
The cell will stay in G0 permanently.

The length of the cell cycle is very variable.

In humans, cell turnover varies from a few hours in early embryonic development to an average of two to five days for epithelial cells, or an entire human lifetime spent in G0 by specialized cells such as cortical neurons or cardiac muscle cells.
Each phase of the cell cycle has variations in the time that a cell spends.
The length of the cycle is approximately 24 hours when the cells are grown in culture outside the body.
The G1 phase lasts approximately 11 hours in a 24 hour cell cycle.
Internal and external mechanisms control the timing of events in the cell cycle.

The daughter cells need to be the same as the parent cells.

Every new cell produced from an abnormal cell may be passed on to other cells because of mistakes in the distribution of the chromosomes.
At the end of G1, at the G2-M transition, and during metaphase, there are checkpoints.

Three checkpoints control the cell cycle.

The G1 checkpoint is where integrity of the DNA is assessed.

There is a checkpoint called the G2 checkpoint.

The M checkpoint is where the attachment of each kinetochore is assessed.

The G1 checkpoint is used to determine if conditions are favorable for cell division to proceed.

The restriction point is the point at which a cell irreversibly commits to the cell-division process.
There is a check for damage to the genomic DNA at the G1 checkpoint, as well as adequate reserves and cell size.
A cell that doesn't meet all the requirements won't be released into the S phase.

If certain conditions are not met, the G2 checkpoint bars entry.

Cell size and reserves are assessed the same way as in the G1 checkpoint.
Ensuring that all of the chromosomes have been replicated and that the replicated DNA is not damaged is the most important role of the G2 checkpoint.

The end of the metaphase stage is near the M checkpoint.

The M checkpoint is used to determine if all the sister chromatids are attached to the microtubules.