Mitosis Overview
Sections
Somatic cells
• Most of the body's cells
• Diploid (2n) : 46 chromosomes
Reproductive cells: gametes
• Sperm and egg cells
• Haploid (1n): 23 chromosomes
Interphase
• Cell doubles in size and DNA during interphase
• 90-95% of cell cycle
• G1: cell grows without replicating DNA
• S: synthesis phase, DNA replicates
• G2: cell synthesizes proteins in preparation for mitosis
• Chromosomes condensed during mitosis: uncondensed (chromatin form) for rest of cell cycle
mitosis
- Somatic cell division
Prophase
• Centrosomes migrate to opposite sides of cell
• Mitotic spindles form from centrosomes
Prometaphase
• Nuclear envelope fragments
• Nucleolus disappears
• Sister chromatids attached to each other at their centromeres until anaphase
Metaphase
• Chromosomes line up at metaphase plate
Anaphase
• Sister chromatids move to opposite sides of cell
Telophase
• Nuclear envelope reforms
Cytokinesis
• Cytoplasm divides cell into two
• Produces 2 diploid daughter cells with identical genomes
Mitosis
• Division of nucleus into 2 daughter nuclei
Cytokinesis
• Division of cytoplasm
CLINICAL CORRELATION
Aneuploidy
• Sister chromatids do not separate properly during cell division (mostly meiosis but also occurs during mitosis)
• Daughter cells with extra or missing chromosome
• Common in cancer cells
Full-Length Text
- Here, we will learn mitosis and the eukaryotic cell cycle. We will specifically use human cells as our model.
- To begin, start a table to learn the two types of human cells.
- Denote that they include:
- Somatic cells, which describes most of the body's cells.
- Reproductive cells, which are sperm and egg cells aka gametes.
- These two types of cells contain different amounts of chromosomes.
- Denote that somatic cells contain 46 chromosomes total and that they are diploid because they contain two sets of 23 chromosomes, one set inherited from each parent.
- Denote that reproductive cells contain 23 chromosomes, and that they are therefore haploid.
In this tutorial we describe somatic cell division: mitosis.
To begin, let's learn the duration of phases in the cell cycle.
- First, draw a pie chart; it represents the cell cycle of a dividing human cell, which lasts about 24 hours.
- Denote that the cell cycle divides into two major periods:
- Interphase, in which the cell grows and prepares for cell division.
- Mitosis (M-phase), in which the cell divides.
- Show that 90-95% of the pie chart is interphase, and the remaining is mitosis.
- Mitosis takes up about 1 hour of the 24-hour human cell cycle.
- Illustrate that interphase itself comprises three separate phases:
- G1, in which the cell grows without replicating DNA.
- S, the "synthesis" phase in which DNA replicates.
- G2, during which the cell synthesizes proteins in preparation for mitosis.
- During interphase, the cell doubles in size and amount of DNA.
Now, before we learn the phases of mitosis, let's track a single chromosome through the cell cycle to learn how the amount of genetic material changes before and after cell division.
- Start with interphase.
- Draw an uncondensed chromatin fiber within a cell.
- Each chromatin fiber is a single chromosome.
- Indicate that the chromatin fiber replicates in the S phase.
- Now, show that in mitosis, the original chromosome and its duplicate condense.
- Label each condensed chromosome "sister chromatid."
- Indicate that the sister chromatids connect at the centromere.
- Show that by the end of the mitotic phase, sister chromatids separate, which evenly distributes genetic material between two daughter cells.
- As a clinical correlation, denote that aneuploidy results when sister chromatids do not separate properly during mitosis.
- This results in daughter cells with an extra or missing chromosome (or chromosomal region).
- Aneuploidy is common in somatic cancer cells, in which cell division is rapid and prone to errors.
Now, let's learn the discrete phases of the mitotic period. As a reminder, the original set of 46 chromosomes replicates in the S phase, doubling the genetic material in the cell. Thus, throughout mitosis there are 46 pairs of chromosomes in the cell.
- Illustrate that mitosis comprises 5 different phases:
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
- Indicate that cytokinesis, or the splitting of cytoplasm between two daughter cells, concludes the cycle.
- Cytokinesis is often considered part of telophase, and not a discreet phase in itself.
Let's draw each of these phases in sequence.
Start with prophase.
- Draw a cell that encloses an intact nucleus.
- Illustrate duplicated, condensed chromosomes within the nucleus.
- We will only include two of the 46 pairs of chromosomes for simplicity.
- Show a nucleolus within the nucleus.
- Outside of the nucleus, draw two centrosomes, each enclosing a pair of centrioles.
- Draw microtubules emerging from the centrosomes in all directions.
- These microtubules nucleate from the centrosome during interphase, but lengthen during prophase.
- Show that they form a mitotic spindle between the centrosomes.
Now, draw prometaphase.
- Draw a cell with a fragmented nuclear envelope.
- Prometaphase begins as soon as the envelope begins to fragment.
- Indicate that the nucleolus disappears.
- Show the centrosomes (and centrioles) at opposite poles of the cell.
- The centrosomes remain at these poles until cell division.
- Label them "mitotic spindle poles."
- Again, draw microtubules emerging from the centrosomes.
- Indicate that the spindle fibers attach to kinetochores on sister chromatids.
- Each sister chromatid has a kinetochore that faces the opposite pole.
- Importantly, show that the spindle fibers attach the sister chromatids in each chromosomal pair to opposite poles.
- Once attached, the chromosomes move about within the cell.
Next, let's draw metaphase.
- Draw a cell similar to prometaphase, but with chromosomes lined up along the metaphase plate, which is the region approximately midway between each spindle pole.
Now, draw anaphase.
- Draw a slightly elongated cell with a fragmented nucleus.
- Show that the spindle fibers separate the sister chromatids and pull them towards each spindle pole.
- Next, use arrows to show that the spindle poles themselves move farther away from each other and elongate the spindle.
Now, draw telophase, the last phase.
- Draw an elongated cell with a contractile ring in the middle.
- The contractile ring comprises actin and myosin and forms a "cleavage furrow" in the cell.
- Show that the daughter chromosomes arrive at the spindle poles, and that microtubule spindle fibers, those that were not attached to sister chromatids, overlap at the center of the cell.
- Indicate that a nuclear envelope forms around the chromatids as they decondense.
- The formation of these nuclear envelopes marks the end of mitosis.
- Show that the nucleolus reappears in this phase.
Finally, let's draw cytokinesis, in which the cytoplasm divides amongst the two daughter cells.
- Draw a cell with a now deepened cleavage furrow.
- Illustrate that the microtubule spindle disassembles in this phase.
- Show that the nuclear envelopes completely reform.
- Denote that mitosis describes the division of the nucleus into two daughter nuclei.
- Denote that cytokinesis describes the division of cytoplasm.
- Now, show that the contractile ring contracts and pinches the cell into two daughter cells.
- Label them diploid.
Mitosis results in the formation of two daughter cells, each with two sets of 23 chromosomes.
- Many factors regulate the progression of a dividing cell through this cycle, and several checkpoints ensure that both daughter cells receive a complete genome at the end of it.
References:
- Campbell, N. A. & Reece, J. B. Biology, 7th ed. (Pearson Benjamin Cummings, 2005).
- Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. Molecular Biology of the Cell, 5th ed. (Garland Science, 2008).
- Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. Essential Cell Biology, 3rd ed. (Garland Science, 2010).
- Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., Ploegh, H. & Matsudaira, P. Molecular Cell Biology, 6th ed. (W. H. Freeman and Company, 2008).
- Marieb, E. N. & Hoehn, K. Human Anatomy & Physiology, 10th ed. (Pearson, 2016).
- Polinsky, K. R. Tumor Suppressor Genes. (Nova Publishers, 2007).
- Coleman, W. B. & Tsongalis, G. J. The Molecular Basis of Human Cancer. (Springer Science & Business Media, 2001).