Mitosis is an important part of the cell cycle, which gives rise to the diverse tissue types that comprise an organism. Thus, it is a topic that that every student of the life sciences will encounter. Mitosis is a multi-step process. There are key phases and features that are often tested on exams. Knowing these key points, and which ones are often confusing on tests, will prepare you to do well.
Mitosis is one part of the cell cycle. The largest part of the cell cycle is interphase, which consists of three phases: G1, S and G2. G1 phase is when the cell grows, making more proteins and organelles that will be needed for two cells. S phase is when DNA is duplicated in preparation for mitosis. Since it is right before mitosis, G2 phase is when proteins that are necessary for mitosis are produced. The cell cycle has G1, S, and G2-mitosis checkpoints, which are pauses in the cell cycle during which the cell checks to see if everything is working properly before proceeding.
Mitosis has five general phases, which can be remembered by the acronym PMAT: prophase, metaphase, anaphase, and telophase. During prophase, the nuclear membrane breaks down and the early spindle forms. Some textbooks mention a pro-metaphase, which is when the duplicated chromosomes find their way to the middle of the cell. Metaphase is when the duplicated chromosomes align in the middle of a fully-formed spindle, ready to be pulled apart. Anaphase is when the duplicated chromosomes are pulled apart by the spindle fibers. Lastly, telophase is when the cell divides apart into two cells.
A common test question is about the difference between plant cells and animal cells during telophase. During telophase, the chromosomes have already been pulled apart and two new cells are forming. The formation of the two new cells is called cytokinesis. In animals, two cells form because the cell membrane between them is pinched together in a cleavage furrow, splitting one cell into two. Plant cells, however, have a firm cell wall that cannot be pinched, so cytokinesis works differently. A dividing plant cell splits into two cells by building bits of a new cell wall in its middle area. These bits merge to form the cell plate which divides one cell into two.
One common point of confusion is the difference between chromosomes and chromatids. Chromosomes are long DNA strands that are packed into dense finger-like structures by proteins during mitosis. During the S-phase of interphase, chromosomes duplicate, but remain stuck together like fingers in the shape of a X. These duplicated fingers are called sister chromatids. Two sister chromatids make up one chromosome. During anaphase, it’s the sister chromatids that are pulled apart. Once pulled apart, each finger is called a chromosome once again, and the term chromatid no longer applies. Each chromosome finger has a structure in the center called a centromere. The centromeres are where two sister chromatids are joined together.
