High school biology
The cell cycle and mitosis review
|Cell cycle||The series of growth and development steps a cell undergoes between its formation and reproduction|
|Interphase||Phase of the cell cycle where the cell grows and makes a copy of its DNA|
|Mitosis||Phase of the cell cycle where the cell separates its DNA into two sets and divides, forming two new cells|
|Cancer||A disease of uncontrolled cell growth|
The cell cycle
In eukaryotic cells, the cell cycle is divided into two major phases: interphase and mitosis (or the mitotic (M) phase).
Interphase is the longest part of the cell cycle. This is when the cell grows and copies its DNA before moving into mitosis. During mitosis, chromosomes will align, separate, and move into new daughter cells.
The prefix inter- means between, so interphase takes place between one mitotic (M) phase and the next.
Image of the cell cycle. Interphase is composed of G1 phase (cell growth), followed by S phase (DNA synthesis), followed by G2 phase (cell growth). At the end of interphase comes the mitotic phase, which is made up of mitosis and cytokinesis and leads to the formation of two daughter cells. Mitosis precedes cytokinesis, though the two processes typically overlap somewhat.
Interphase consists of three steps:
- G phase: first gap phase; the cell grows larger and organelles are copied
- S phase: synthesis phase; the cell synthesizes a complete copy of the DNA in its nucleus
- G phase: second gap phase; the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis
Cells that are meant to divide will complete G and enter mitosis. Other types of cells that divide slowly or not at all may exit the G phase and enter a non-dividing state called G. Some cells remain here indefinitely, while others may re-enter division under the right conditions.
Mitosis (the M phase)
The process of mitosis, or cell division, is also known as the M phase. This is where the cell divides its previously-copied DNA and cytoplasm to make two new, identical daughter cells.
Mitosis consists of four basic phases: prophase, metaphase, anaphase, and telophase.
Stages of mitosis
Early prophase. The mitotic spindle starts to form, the chromosomes start to condense, and the nucleolus disappears.
Late prophase (prometaphase). The nuclear envelope breaks down and the chromosomes are fully condensed.
Metaphase. Chromosomes line up at the metaphase plate, under tension from the mitotic spindle. The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles.
Anaphase. The sister chromatids separate from one another and are pulled towards opposite poles of the cell. The microtubules that are not attached to chromosomes push the two poles of the spindle apart, while the kinetochore microtubules pull the chromosomes towards the poles.
Telophase. The spindle disappears, a nuclear membrane re-forms around each set of chromosomes, and a nucleolus reappears in each new nucleus. The chromosomes also start to decondense.
Cytokinesis in animal and plant cells.
Cytokinesis in an animal cell: an actin ring around the middle of the cell pinches inward, creating an indentation called the cleavage furrow.
Cytokinesis in a plant cell: the cell plate forms down the middle of the cell, creating a new wall that partitions it in two.
Cancer and cell cycle regulation
The cell cycle is usually regulated by checkpoints. These are the factors that a cell considers when deciding whether or not to move forward through the cell cycle, and include both external cues (like molecular signals) and internal cues (like DNA damage).
Diagram of cell cycle with checkpoints marked. G1 checkpoint is near the end of G1 (close to the G1/S transition). G2 checkpoint is near the end of G2 (close to the G2/M transition). Spindle checkpoint is partway through M phase, and more specifically, at the metaphase/anaphase transition.
Cancer is a term that describes many different diseases caused by the same problem: uncontrolled cell growth.
Most cancers occur due to a series of mutations that make them divide more quickly, bypass checkpoints during cell division, and avoid apoptosis (programmed cell death).
Generally, mutations of two types of cell cycle regulators can promote the development of cancer:
- Positive regulators, which normally promote cell growth, may become hyperactivated (oncogenic).
- Negative regulators (tumor suppressors), which prevent the formation of tumors, may become inactivated.
Common mistakes and misconceptions
- Interphase is not part of mitosis. Although we often talk about interphase and mitosis together, interphase is technically not part of mitosis. However, both processes are part of the larger cell cycle, where interphase consists of the G, S, and G stages of the cell cycle.
- DNA replication occurs during interphase, not prophase. A common misconception is that DNA copies itself during prophase, but this is not true. In prophase, the DNA has already been copied while the cell was in interphase.
- The chromosome number is the same in the daughter cells as it was in the parent cell. Because DNA is duplicated during interphase before the cell undergoes mitosis, the amount of DNA in the original parent cell and the daughter cells are exactly the same.
- Both genetics, as well as external factors, can play a role in the development of cancer. Many types of cancer have a genetic component, so inheriting certain genes may make someone more likely to get these types of cancer. However, having these genes does not necessarily mean that cancer will develop, as factors such as lifestyle and environment also play a part.
Want to join the conversation?
- This might be a foolish question, but what if apoptosis happened to humans? Then "Survival of the Fittest" would be true, but the world population would decrease dramatically if that was a reality.(5 votes)
- I think that apoptiosis does happen to human. As the cell grow old, (ex) skin cell) they will just kill itself. Also, some mutated cell will detect their own problem and perform apoptosis.(13 votes)
- what is the role of immune system in recognizing bad cells(3 votes)
- So the immune cell will actively seek out dysfunctional cells and literally eat them.(6 votes)
- Is cytokinesis part of mitosis?(3 votes)
- I think it is not a part of mitosis itself, but is part of the mitotic phase which results in the two daughter cells. Mitosis deals only with the nucleus, while cytokinesis divides the cell after mitosis os finished.(5 votes)
- Is there a chemical or enzyme which causes the breakup of the nucleus?(5 votes)
- how long would apoptosis go on for(1 vote)
- Well, apoptosis is when a cell essentially says to itself, "Oh no, I have something wrong with me." and self-destructs itself. It basically commits suicide for the greater good of the colony to avoid spawning more defected cells. To make it simple, apoptosis cannot go on; it is a one-time thing. Hope this helps!(7 votes)
- What are the three parts of interphase? What is the characteristic of each?(1 vote)
- In chronological order :
- G1, or first growth phase, is when the cell synthetises its proteins by translating its RNA through ribosomes
- S, or synthesis phase, is when the cell replicates its DNA in order to divide
- G2, or second growth phase, is when the last proteins for division are produced, and also organelles like mitochondria or chloroplasts divide(5 votes)
- How long do these phases take? Can you give an example of a specific cell? Is it different for different types of cells?(2 votes)
- Yes it is different for different types of cells. A human cell ( I don’t know which) typically takes about 24 hrs for a cell cycle (most of the 23 hrs are interphase n’ the rest mitosis and cytokinesis), in labs. (Not in the human body). But most intestinal cell go through the phases more quickly.
Hope it helps..(3 votes)
- If a product was used to increase the growth rate of food plants, how would this affect mitosis in an onion plant? Would it increase the number of cells and therefore they would divide faster? How would a slide of an onion root look different during the different stages of mitosis?(3 votes)
- What does the mitotic index have to do with the speed of cell division?(2 votes)
- Where does karyokinesis fit in in mitosis if the nuclear membrane disintegrates?(2 votes)