The most basic function of the cell cycle is to accurately duplicate the vast amount of DNA in the chromosomes and then precisely separate the copies into two genetically identical daughter cells.
- After the S phase, chromosome segregation and cell division occur in the M phase (M for mitosis), which takes much less time (less than an hour in a mammalian cell).
- The M phase involves a series of dramatic events that begin with nuclear division, or mitosis.
- Mitosis begins with chromosome condensation: the duplicated DNA strands, packaged in elongated chromosomes, condense into the much more compact chromosomes required for their segregation.
- The nuclear envelope then breaks open and the replicated chromosomes, each consisting of a pair of sister chromatics, are attached to the microtubules of the mitotic spindle.
- As mitosis proceeds, the cell pauses briefly in a state called metaphase, when the chromosomes are aligned at the equator of the mitotic spindle and ready to separate.
- The sudden separation of the sister chromatics marks the beginning of anaphase, in which the chromosomes move to opposite poles of the spindle, where they decadence and form intact nuclei.
The Cell-Cycle Control System Is Similar in All Eukaryotes
Some features of the cell cycle, including the time required to complete certain events, vary greatly from one cell type to another, even in the same organism. The basic organization of the cycle and its control system, however, is essentially the same in all eukaryotic cells. The proteins of the control system first appeared over a billion years ago. Remarkably, they have been so well conserved over the course of evolution that many of them function perfectly when transferred from a human cell to a yeast cell. We can therefore study the cell cycle and its regulation in a variety of organisms and use the findings from all of them to assemble a unified picture of how eukaryotic cells divide. In the following section, we briefly review the three eukaryotic systems in which cell-cycle control is commonly studied—yeasts, frog embryos, and cultured mammalian cells.
The cell cycle control system can be genetically dissected in yeasts
Yeasts are tiny, single-celled fungi whose mechanisms of cell cycle control are remarkably similar to ours. Two species are commonly used for cell cycle studies. The fission yeast Schizosaccharomyces pommel is named after the African beer that is made from it. It is a rod-shaped cell that grows by stretching at its ends. Division occurs by the formation of a septum or cell plate in the centre of the rod. The budding yeast such armoires cerevisiae is used by both brewers and bakers. It is an oval cell that divides by forming a bud that first appears during G1 and grows steadily until it separates from the mother cell after mitosis.
Cell cycle steps
To split, the cell must perform several important tasks. It needs to grow, copy the genetic device (DNA), and physically split into two subsidiaries. Cells perform these tasks in a series of organized, predictable steps that make up the cell cycle. The cell cycle is a cycle, not a linear pathway, because at the end of each tower, two subsidiaries can start the same process from the start.
In eukaryotic cells or cells with a cell cycle stage heart, they are divided into two major phases: an interface and a mitotic phase (m).
- During the interface, the cells grow and make copies of the DNA.
- During the mitotic phase, cells separate their DNA into two sets, splitting the cytoplasm, forming two new cells.
Inter phase
Let’s enter the cell cycle just as a cell forms, by division of its mother cell. What must this newborn cell do next if it wants to go on and divide itself? Preparation for division happens in three steps:
G1 phase: During G1 phase, also called the first gap phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps.
S phase: In S phase, the cell synthesizes a complete copy of the DNA in its nucleus. It also duplicates a microtubule-organizing structure called the centrosome. The centrosomes help separate DNA during M phase.
G2 phase: During the second gap phase, or G2 phase, the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis. G2 phase ends when mitosis begins.
The G1 phase, S, and G2 phases together are known as inter phase. The prefix inters– means between, reflecting that interphase takes place between one mitotic (M) phase and the next.
Cell cycle exit and G0
What happens to the two daughter cells produced in one round of the cell cycle? This depends on what type of cells they are. Some types of cells divide rapidly, and in these cases, the daughter cells may immediately undergo another round of cell division. For instance, many cell types in an early embryo divide rapidly, and so do cells in a tumor.
Other types of cells divide slowly or not at all. These cells may exit the G1 phase and enter a resting state called G0 phase. In G0, a cell is not actively preparing to divide; it’s just doing its job. For instance, it might conduct signals as a neuron (like the one in the drawing below) or store carbohydrates as a liver cell. G0is a permanent state for some cells, while others may re-start division if they get the right signals.
