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.

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