Cells undergo routine cycles of growth, replication and death
All cells go through a cycle of growth that includes normal functioning, preparing to reproduce or replicate, replication, and death.
This diagram of the cell cycle is one with which you should become very familiar. In this section, we'll go through it twice. Here we'll just give it a once-over, and then we'll look at it in more detail below, and in other sections that break out certain features even more finely.
There are four basic phases of cell growth, which we label (for historical reasons) G1, S, G2 and M.
The G1 phase of growth is the normal phase of growth, development and usefulness of the cell. In this phase a cell is growing and doing its job. If it's a fat cell it's storing fat molecules; if it's a muscle cell its contributing to the contraction and relaxation of a muscle fiber, and so on.
The remaining three phases are all part of the reproduction of the cell, culminating in the process of mitosis.
In S phase, all of the DNA of the cell, much of which was tightly coiled and packed for storage (it wasn't in use) is unwound and completely replicated by protein machinery specific to that purpose.
In the G2 phase, the cell puts on a little more mass in preparation to divide into two "daughter" cells.
M phase is mitosis, the process of cell division to two daughter cells identical to the parent cell.
A more detailed look
Now we'll take a more detailed look at the cell cycle using the figure below. If you click/tap on it you can download a .pdf version for your notes.
The section on mitosis (M-phase) below will be less detailed than the page dedicated to mitosis, so consider it to be an introduction to the process of cell division.
G1: Normal growth phase
This phase of the cell is the normal growth and functioning phase of the cell, usually the longest in the cycle. It's important to note that not all cells replicate that often. Nerve cells, particularly those in the brain and spinal cord of mammals, grow slowly and replicate infrequently.
The epithelial cells of mucous membranes, on the other hand, turn over very frequently. So do skin cells; they're constantly dying, sloughing off of your body and being replaced from layers beneath the surface.
So the G1 phase of a cell can be different depending on the type of cell and the job it does.
In the G1 phase, much of the DNA is coiled tightly for storage because is isn't used. All cells need to produce proteins and RNA molecules to fulfill their roles in tissues, but not all cells need access to the full genome of the organism.
One level of gene regulation – deciding which genes are in use and which are not – involves compact coiling of the DNA around protein molecules called histones.
So the G1 phase consists of growth to full size, and normal functioning using the part of the genome needed to produce the products – proteins and RNAs – that the cell needs to carry out its job.
S-phase
In S-phase a cell continues to perform its designated functions, but it also devotes some energy to preparing to divide into two new cells. The first step in division is replication of the genome – all of its DNA.
Each daughter cell must contain a complete and accurate copy of the genome, so at the end of S-phase, one cell must have two complete and identical copies. That's quite an accomplishment when you consider, for example, that the human genome, 23 chromosomes, contains about 3 billion DNA base pairs.
In S-phase, all DNA is unwound and made ready for the copying machinery, a set of enzymes that uncoil the double helix, then "walk" along it making a mirror image of each individual strand of each chromosome.
A few of the enzymes involved are
- Helicase – responsible for unwinding the double helix,
- DNA transcriptase – responsible for organizing the copying each strand, and
- DNA ligase – adds each nucleotide to the growing chain.
G2 phase
In G2 the cell nearly doubles in size, in preparation for splitting into two cells in the M-phase.
While the normal cell function can continue through G2 phase, it is not at its best. The DNA is not in its ideal state for cell function and
the cell is now devoting considerable energy toward growth and extension of its cell wall and membrane. At the end of G2 is where the fun begins.
M phase – Mitosis
The process of mitosis is divided into four parts that are characterized by some distinct visual changes in the cell – changes that can often be seen in a light microscope.
The cell diagrams here are little practice cells that only contain two chromosomes, a dark one and a light one, just for simplicity.
1. prophase
In prophase, the replicated strands of DNA, identical "sisters," coil up as tightly as DNA can to form sister chromatids, which are often visible under a light microscope. It's remarkable that the long strands of helical DNA can coil so tightly. The sister chromatids are held together at a special structure called the centromere.
In this phase the nuclear membrane, which normally sequesters the DNA from the rest of the cytoplasm, dissolves. It will be reformed – one nucleus in each daughter cell – after the cell divides.
2. metaphase
Metaphase is characterized by a profound visible change. Each pair of chromatids lines up more or less on the equator of the cell. Microscopically, what is happening is that microtubules, special protein fibers that can elongate and contract, are hooking up to each centromere and to two special structures, the polar bodies, located in opposite parts of the cell.
Sometimes we talk about a phase "1a", called prometaphase, in which the microtubules are attached, but the chromatids aren't yet lined up on the equator. I've skipped that here.
3. anaphase
In anaphase, the microtubules, now in bundles called spindle fibers, contract as the centromeres separate, and each daughter chromatid is pulled into its half of the dividing cell.
At the end of anaphase, there are two distinct clumps of DNA on two ends of a large cell, both of which are high-fidelity copies of the genome of the original cell. The single parent cell is ready to split into daughter cells.
4. telophase
In telophase, the daughter cells begin to get back to normal operation. Spindle fibers shrink, DNA uncoils and repacks according to the work that the cell needs to do, nuclear membranes re-form and a new segment of cell wall bridges the parent cell, which then pinches into two individual daughter cells.
Then we're back to G1 phase when these two cells finish pulling apart. They will grow and function until it's time to divide again.
Cell death
Once in a while something goes wrong inside a cell. It might be a mis-copied strand of DNA or some other problem, which sets off a series of responses leading to cell death, usually by a process called apoptosis.
Apoptosis (most people omit the second P sound and just say "ap'·a·toe'·sis") is more-or-less an explosion of the cell. Rapidly, nuclear and cell membranes weaken and dissolve, and the contents of the cell spill out to be cleaned up by other cells that have that job.
Sometimes cell death is said to be "programmed." When an organism is first developing, for example,
it may be advantageous for it to grow a certain kind of progenitor cell that won't persist into the grown organism; it's there to help with development of different tissues and will eventually die when it is no longer necessary for the survival of the organism.
When the cell death mechanism is compromised we can have problems. Cells have excellent monitoring systems that can signal for death when something like a cancer first develops. So when the cell-death mechanism is out of order, that cancerous cell might persist and grow, perhaps into a tumor.
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