Mitosis is the process by which cells reproduce – one cell splits into two "daughter cells."
While some of the details differ from cell type to cell type, this section will teach you most of the basic features of the process. Along the way some of the features that can differ by cell type will be discussed.
Some cells undergo mitosis on a regular schedule, such as bacteria when conditions are favorable. Other cells, such as mammalian nerve cells, hardly reproduce at all in a mature individual.
In favorable conditions (enough nutrients and a temperature of 37˚C, Escherichia coli (E. coli) cells divide by mitosis about every 20 minutes.
Different organisms deploy chromosomes in slightly different ways, both in cells and during replication of cells. Here we'll focus on diploid cells (humans have diploid cells), which contain two slightly different copies of each chromosome, forming what is called a homologous pair. Humans have 23 kinds of chromosomes (mostly categorized by their length, from longest to shortest), and there are 2 versions of each kind, making 23 homologous pairs or 46 total chromosomes.
In this section and the section on meiosis, we'll employ schematic drawings of chromosomes that look like this:
On the left is a homologous pair. One came from each parent, thus the ♂ and ♀ symbols. These drawings represent chromosomes that are maximally condensed. The DNA that forms a chromosome is capable of being loosely-coiled — the configuration optimal for use when the cell is performing its principal functions — or tightly coiled and compacted in preparation for cell replication. The coiled form ensures that extremely long DNA molecules can be efficiently segregated into "daughter" cells without tangling.
Here is an electron micrograph of an actual human chromosome during prophase of mitosis:
The pinched-off region is called the centromere. It is the point at which microtubules in the cell attach in order to align and segregate chromosomes into daughter cells.
Before cells can begin mitosis or the first stages of meiosis, protein machinery in the nucleus must make a copy of each chromosome, forming a tetraploid cell, a temporary state necessary for cell division to begin (right side of the panel above).
Let's first just review the basic features of the life cycle of a cell. If we begin after mitosis in G1-phase, That's where the cell does most of its business. In G1 the phase is doing the job it was "born" to do.
In S-phase, the cell prepares for division by reproducing its genome – making a copy of each strand of DNA present in the nucleus. Every cell must have an intact copy of each strand. After S-phase, a cell contains two copies of each chromosome of its genome.
G2-phase is an additional growth phase during which the cell adds extra bulk in preparation for mitosis. In this phase the cell prepares to eventually split into two daughter cells, making sure that each has enough membrane and contents to begin functioning right away in G1-phase.
Now let's take a look at each step of mitosis.
In the diagrams that follow, I'll use some highly-stylized cell diagrams like this one. It only has three chromosomes. That's fine to figure out how mitosis works.
The cell membrane is the outer circle and the nuclear membrane surrounds the uncoiled DNA inside. The small structure at the top is a centrosome, a collection of proteins that will eventually help segregate and separate identical pairs of copied chromosomes.
In S-phase, the DNA is only lightly packed and is usually not visible using a light microscope. The DNA replication machinery, a group of enzymes that unwind the double helix, copy each strand and check for errors, copies the DNA.
At some point in the cell cycle, the centrosome is also copied. Each dividing cell must have two centrosomes (this isn't a fixed rule, as plant cells and some other cells don't have centrosomes and mitosis still occurs just fine). This process doesn't necessarily happen in S-phase, but in cells in which a centrosome is necessary, it must occur before mitosis.
Once the DNA is copied, the nucleus is more crowded. When the cell is ready for mitosis, two centrosomes are present. At this state the cell contains two identical copies of each chromosome.
Remember that these diagrams are highly schematic. They don't show details of the membranes or any of the other organelles or other features of the cell. It's better that way when you're just learning about this process.
During prophase of mitosis, the cell begins preparation for division. Two key things occur:
The DNA would be a tangled mess, nearly impossible to separate, if it was not tightly coiled before division. In prophase each chromosome is wound as tightly as possible into short structures that are often visible in a light microscope. These are called chromatids.
Two copies of each chromosome now exist in each nucleus, and these "sister chromatids" pair up at a single point, often seen as a narrow point on the chromosome, called the centromere. The pairing, a weak binding of one sister chromatid to the other, is formed by a protein complex known as the centrosome.
The figure schematically shows compact sister chromatids paired and ready for mitosis.
During prophase, the centrosomes (top of the cell) begin to synthesize protein fibers called microtubules.
These will eventually attach to the centromeres of each sister chromatid and will aid in segregating one copy of the genome into each daughter cell.
Just to give you an idea of how chromosomes really look in a microscope, here's a human male karyotype (see caption below):
Just to give you a better idea of what a chromatid looks like through a microscope, the figure above is an arrangement of photographs of all of the pairs of chromosomes of a normal human male. The photos were taken in prophase or metaphase and were paired and arranged in order of length. Such a display of chromatids is called a karyotype. A normal human has 23 pairs of chromosomes in its karyotype. A human inherits one of each pair from each parent. The approximate location of the centromere, the point where sister chromatids attach, is shown on chromosome 3. (adapted from Wikipedia Commons image)
In prometaphase (sometimes just considered to be a part of prophase), the nuclear membrane disintegrates into small pieces and falls away.
As that occurs, spindle fibers formed in prophase attach to the centrosomes via a protein complex called the kinetochore.
Once those attachments are made, Prometaphase is complete.
The main distinguishing feature of metaphase is that pairs of sister chromatids line up along an imaginary disk, called the metaphase plate, an imaginary divider that roughly cuts the cell in two. It looks something like in the figure
The metaphase plate is sketched in magenta. Remember, the cell is three-dimensional; that's why it's a "plate" or "disc," not a line.
To accomplish this alignment, the microtubules contract and pull on the chromatid pairs at the centromeres. Once the chromatids are aligned, cells may have "checkpoint" mechanisms to check for proper alignment. This is a safeguard against segregating uneven amounts of sister chromatids into each daughter cell. That could be lethal to both daughter cells.
Anaphase is the chromatid-separation phase. In anaphase, the microtubules connecting one of each pair of sister chromatids to each of two ends of the cell are tightened and the link forming the centrosome is broken chemically, releasing the sister chromatids from their bond.
The microtubules retract and each sister chromatid is dragged, relatively rapidly, to its side of the cell.
Late in anaphase, certain microtubules (polar microtubules) not connected to the chromatids but connecting centrosomes, elongate to push the centrosomes apart and thereby elongate the cell.
None of this happens by magic. It is all driven by chemical reactions and interactions, but they're very highly specialized reactions and interactions beyond the scope of this page. Make sure you get the basic physical steps down before you tackle those.
Telophase is the daughter-cell separation phase.
In telophase, the polar microtubules continue to elongate and the microtubules connecting centrosome to chromatid break apart and detach.
Later in telophase, chromatid DNA begins to relax and unwind into the new nuclei in preparation to be accessed for normal function of the daughter cells.
Nuclear membranes begin to form around the chromatids and a cleavage furrow begins to mark the place where the cell membrane will pinch into two new cells.
At some point at the end of or just after (these are fuzzy lines, we have two new functional diploid (containing two versions of each chromosome, one from each parent) cells, each identical to the first (below).
Please note that the boundaries between prophase, metaphase, anaphase and telophase are not well defined. They are soft—the cell doesn't pause in between phases. Breaking up mitosis in this way really just helps us to remember what takes place when, and to have a common language to use when referring to events in cell division. There are small differences between cell types, too.
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