Objectives

By the end of this lesson you should be able to:

  1. Describe the macrostructure of the chromosome
  2. Identify the phases in interphase
  3. Identify the phases in mitosis
  4. Describe the mechanism of cell division
  5. Identify the phase of mitosis illustrated on a microscope slide  
  6. Explain how cytokinesis in plants and animal cell is different
  7. Differentiate between mitosis and binary fission
  8. Explain how the cell cycle is regulated

Basic Concepts

All the genetic material in the cell of an organism (its genome), is packaged in the chromosome. The chromosome is a condensed form of chromatin.

Figure 1. The chromosome is the condensed form of chromatin. Creative commons image source.

Chromatin consists of long highly coiled hair-like structures. These predominate when the cell is not dividing. When the cell is dividing they condense and become thick (chromosome form). 

Human reproductive cells (sperm and eggs) are called gametes. They have23 chromosomes. During fertilization, gametes come together to form a zygote with 46 chromosomes. The zygotes can then undergo cell division and hence growth, in a process called mitosis and cytokinesis.

During cell division, the chromosome divides into two identical sister chromatids attached along their sides by proteins called cohesins. Each chromatid has a centromere region which consists of DNA sequences that enables the two chromatids to attach most closely, creating a “waist”.

Figure 2. Sister chromatids

Phases of the Cell Cycle

For cells to divide, a series of events has to take place. The cell first goes through a process called interphase where it spends most of its time. Interphase consists of the following 3 subphases:

  1. First gap phase (G1): Cell grows bigger, copies organelles, and make cellular building blocks that it will need in further steps
  2. Synthesis phase (S): Chromosomes are duplicated
  3. Second gap phase (G2): The cell grows some more (makes proteins and organelles). A single centrosome is duplicated into two.
Figure 3. The Cell Cycle. Creative Commons image source.

After interphase, the cell cycles to the mitotic (M) phase. This is where the cell divides into two identical daughter cells.

Stages of Mitosis

There are five stages of mitosis.

  1. Prophase
  2. Prometaphase
  3. Metaphase
  4. Anaphase
  5. Telophase

Mnemonic: PP-MAT

Figure 3. Phases of cell division. Creating Commons image source (modified)  

Before we get into what happens in each stage, let me introduce you to the mitotic spindle. This is a structure that is needed to make sure that identical daughter cells are produced. It consists of a centrosome, and rope-like structures called microtubules which are made of protein subunits. To lengthen, they simply add protein subunits (polymerization) and to get shorter, they shed protein subunits (depolymerization). The assembly of microtubules start at the centrosome.

Microtubules of the spindle attach to a region of proteins associated with the centromeres on each chromatid, called kinetochore. A chromosome’s two kinetochores face in opposite directions. More than one microtubule can be attached to the kinetochore. The number varies among species.

Figure 4. Spindle apparatus. Wiki Commons image source.

What Happens in Prophase

  • Chromatin condenses (more tightly coiled) becoming visible under a microscope as two sister chromatids joined at centromere
  • Mitotic spindle forms (centrosome and microtubules)
  • Centrosomes move away from each other as mitotic spindle polymerizes
  • Nucleolus disappears     

What Happens in Prometaphase

  • Nuclear envelope disintegrates
  • Microtubules extend and enter nuclear area
  • Chromosomes are more condensed
  • Some microtubules attach to kinetochores
  • Kinetochores that are not attached to a chromosome interact with each other from opposite sides

What Happens in Metaphase

  • The centrosomes are at opposite poles of the cell
  • Chromosomes assemble at an imaginary center called the metaphase plate

What Happens in Anaphase

  • Cohesion proteins are cleaved
  • Each chromatid becomes a chromosome
  • Chromosomes move towards poles as microtubules shorten
  • Nonkinetochore spindles lengthen causing the cell to elongate

What Happens in Telophase and Cytokinesis

  • Two daughter calls are formed
  • Nuclear envelop and nucleolus reappear
  • Chromosomes become less condensed
  • Microtubules depolymerize

Cytokinesis in Animal Versus Plant Cells

Cell division in animal cells is characterized by a cleavage furrow that forms near the old metaphase plate. The furrow is due to contractile proteins actin and myosin which eventually causes the cells to be pinched apart. In plant cells, Golgi vesicles move along the microtubules during telophase towards the metaphase plate where they fuse to form a cell plate. As more vesicles bring materials such as pectin and cellulose to the metaphase plate region, a cell wall is formed. The Golgi membranes become part of the plasma membrane.

Figure 5. Plant versus animal cell cytokinesis

Cell Division in Prokaryotes

Cell division in prokaryotes such as bacteria occurs via a simpler process called binary fission. In this process, bacterial chromosome exists as a single circular chromosome in the nuclear region of the cell. Copying of the DNA starts at a spot called the origin of replication. As replication progresses, the two copies of the chromosomes move to opposite poles of the cell. At the same time, the cell elongates, further separating the chromosome copies. After copying is complete, the membrane pinches inward at the center to produce two identical daughter cells.  

Figure 6. Binary fission mechanism. Creative Commons image source.

Regulation of the Cell Cycle

The cell cycle is regulated by checkpoints at different stages of interphase and mitosis. Three major checkpoints are located at G1, G2, and M phases of interphase. Each checkpoint acts as a “go-ahead” signal to proceed onward. Cyclin-dependent kinases (Cdks) play a key role in signaling. Remember that kinases are enzymes that causes phosphorylation. To be active, in the cell cycle, they must be attached to a protein called cyclin. It is the presence of Cdks that gives the “go ahead”.

There are other mechanisms, for example, protein complexes. At the M checkpoint, a protein complex prevents the chromosomes from separating until all the chromosomes are properly attached by microtubules at the metaphase plate. Once attached, the complex sets off a chain of molecular events that results in cleaving and separation of the chromatids.

Some chemical and physical factors may also affect cell division. For example, under laboratory conditions, it is observed that some cells fail to divide in the absence of certain growth factors in culture media.

An example of a physical factor affecting cell division is density-dependent inhibition, whereby cells stop dividing after reaching a certain quorum.

Figure 6. Cell cycle check points. Wiki Commons image source.

You might be wondering what happens in the case of cancer cells that continue to divide uncontrollably. What is happening to the cell cycle control mechanism? It is likely that after growth factors are depleted, cancer cells continue to replicate by discovering how to receive replication signals in novel ways. One way could be by making their own growth factors.

Textbook: Reece, J. B., & Campbell, N. A. (2011). Campbell biology. Boston: Benjamin Cummings / Pearson.

Courtney Simons
Administrator
Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University. He also holds Masters degrees in both Environmental Science and Instructional Design from Wright State University.
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