1.9: Mitosis

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Introduction

Learning Objectives

Describe the phases of the cell cycle
Identify and explain each of the four phases of mitosis

Early in the semester we learned about cell theory. One of the components of cell theory is that cells must come from other cells. How does that happen though? There are actually several ways to replicate cells. Which process a cell uses depends on what type of organism the cell is and the reason why the cell is replicating.

One such method is binary fission -- a replication process used by simple organisms such as bacteria and archaea. In binary fission, the bacterium’s single circular chromosome is replicated with each copy moving to opposite sides of the cell. Then proteins begin pinching the cell in the middle, a septum forms, and the cell splits into two. This process is both simple and fast, meaning bacteria can replicate very quickly.

Another method is mitosis. This process is used by the more complex eukaryotic cells. In multicellular organisms like ourselves, mitosis is used for growth and repair. Consider how big you were when you were born. Are you the same size? Hopefully, no. But rather than your cells getting larger, your body has simply made more cells to create the growth you’ve experienced over the years. You’ve also probably experienced a few injuries before. But if you get a cut, it heals. This healing you see is because the cells that were killed and damaged by whatever cut your skin were replaced. Nearby healthy cells used mitosis to replace them.

Cell Cycle

In eukaryotic organisms like plants and animals, cell replication is a tightly regulated cycle. Cells must receive various signals and meet certain criteria before being able to divide. These checks and balances ensure that the cell has enough building material, that the DNA replicates correctly, and that the chromosomes are aligned properly to divide. This helps ensure that the organism is keeping up with normal rates of cell death and not wasting resources. When this system fails, things like cancer can result.

The cell cycle can be broken down into two main parts: interphase and the mitotic phase. Let’s look at interphase first. In interphase, there are three distinct phases: G₁, S, and G₂. In the G₁ phase, the cell is running its daily activities and is gathering needed materials for the next steps. Then the cell enters the S phase. S phase is unique in that this is when a cell replicates its entire genome (the complete set of DNA) of the cell. After all of the DNA has been replicated, the cell then enters the G₂ phase. In the G₂ phase, the cell makes final preparations to divide.

The diagram various stages of cell cycle. G1 is shown in purple, indicates cell growth. G1 checkpoint is located at the end of G1. S is shown in blue, indicates DNA synthesis. G2 is shown in grey, indicates cell growth. G2 checkpoint is located at the end of G2. M checkpoint is in Mitosis.

Figure 1: Stages of Cell Cycle. (Modified by Karen Marks, from OpenStax, CC BY 4.0, via Wikimedia Commons)

Looking at the figure above, do you think a cell spends more time in interphase or mitotic phase?

Mitosis in Plant and Animal Cells

Mitosis has five phases: prophase, prometaphase, metaphase, anaphase, and telophase. Once the cell has completed these phases, it will split in two via cytokinesis.

Table 1 Phases of Mitosis
PHASE	DESCRIPTION	APPEARANCE
Prophase	Chromosomes condense and become visible Nuclear envelope disappears Centrosomes move to opposite sides (poles)
Prometaphase	Kinetochores (attachment points) appear on chromosomes Spindle fibers form and attach to chromosomes
Metaphase	Chromosomes line up at equator Spindle fibers attach sister chromatids to opposite poles
Anaphase	Sister chromatids are pulled apart and towards opposite poles The cell begins to elongate
Telophase	Chromosomes are on opposite poles Nuclear envelope reforms around chromosomes Spindle fibers break down
Cytokinesis	The cell splits into two Forms a cleavage furrow (animals) or cell plate (plants)

Observe the phases of Mitosis in Plant Cells

Exercise 1: Observing the Phases of Mitosis in the Onion Root Tip

Materials:

Prepared slide of the onion root tip
Compound light microscope

Procedure:

Examine a slide of a longitudinal section of an onion root tip. Adjust the slide to view the region just above the root cap, where there are likely to be dividing cells.

Focus on the dividing cells using the 4x scanning objective lens, then switch to the 10x objective and then the 40x objective.

Survey the slide to find a cell in each phase of mitosis. Draw a cell for each phase in the boxes provided.

Magnification

Interphase

Prophase

Metaphase

Anaphase

Telophase/Cytokineses

Objective

10x

Objective

40x

Objective

Interphase

DNA is uncondensed and in the form of chromatin. Individual chromosomes are not visible. The nuclear membrane is intact. The nucleolus is visible.

Figure 2: Interphase in plant cells.

Prophase

Chromatin begins to condense into distinguishable chromosomes. These “puffy” structures are seen throughout the nucleus. Nucleoli begin to disappear. In late prophase (often called prometaphase) the nuclear membrane is no longer visible.

Figure 3: Prophase in plant cells.

Metaphase

The chromosomes line up in the middle of the cell. Spindle fibers attach to kinetochores at the centromere and extend to the poles of the cell.

Figure 4: Metaphase in plant cells.

Anaphase

Centromeres split, separating each former chromatid into two individual chromosomes. The chromosomes move toward opposite poles.

Figure 5: Anaphase in plant cells.

Telophase and Cytokinesis

Chromosomes reach the poles. The nuclear envelopes begin to reform. The formation of a cell plate forms between the two cells to carry out cytokinesis.

Figure 6: Telophase in plant cells.

Figure 7: Cytokinesis in plant cells.

Observe the phases of Mitosis in Animal Cells