LESSON 1: CELL CYCLES AND CELL DIVISION

All cells follow a similar pattern of development for at least part of their lifespans: cells first grow, then divide to produce two new cells. This is true of unicellular prokaryotic and eukaryotic organisms, and it is true of the cells that make up multicellular eukaryotic organisms. Cells grow by adding materials to their cell membranes and cell walls, if present. Cells divide either by fission, as in prokaryotic organisms, mitosis, or meiosis. The process of cell division is inherently intricate – cells must partition their DNA and cell contents appropriately into the new cells that result from division. In addition, organisms need to control when and how often cells divide in order to develop properly and ultimately to survive.

Learning Objectives

• Understand the concept of a cell cycle, and how it plays an important role in different types of organisms.
• Understand several differences between the eukaryotic cell cycle and the prokaryotic growth cycle.
• Understand how the eukaryotic cell cycle is controlled, and why this is important.

Topics covered in this Lesson

• Prokaryotic Cell Cycle
• Eukaryotic Cell Cycle
• Control Eukaryotic Cycle

Prokaryotic Cell Cycle

The prokaryotic growth cycle is a relatively straightforward process. Essentially, prokaryotic organisms grow until reaching a critical size and then divide, only to repeat the process over and over again. The rate of growth and division in bacterial cells is greatly dependent on environmental factors, such as nutrient availability and temperature. Under optimal environmental conditions, bacterial populations can grow exponentially in size at very rapid rates, in some instances increasing from several individuals to several million or billion individuals in a matter of hours or days.

Learning Objectives

• Understand that cellular division in prokaryotes is greatly affected by environmental conditions.

• Be able to describe the structure of the prokaryotic chromosome and how the two copies of DNA split after replication into two different cells.

• Know how cytokinesis works to separate two daughter cells.

Eukaryotic Cell Cycle

The eukaryotic cell cycle is somewhat more complex than the prokaryotic growth cycle. Part of this complexity is due to the fact that eukaryotic cells contain multiple chromosomes within a membrane-bound nucleus, rather than a single circular chromosome within the cytoplasm. Because of this, the processes of DNA replication and partitioning are somewhat more complex. The process of cell division that leads to two, genetically identical daughter cells in eukaryotic organisms is called mitosis.

Learning Objectives

1. Know and understand the different stages of the eukaryotic cell cycle, and what happens during each one.

2. Understand that DNA must be replicated prior to mitosis, and why this is so.

3. Follow the path of a cell’s DNA/chromosomes through the different stages of mitosis.

4. Understand that mitosis is the division of one cell to produce two, genetically identical daughter cells.

Control Eukaryotic Cycle

As you now know, there are many different phases of the eukaryotic cell cycle. The entrance to each phase
is controlled by specific proteins, called cyclin-dependent kinases, that require the appropriate signal before allowing the cell to go on to the next phase. Signals to proceed into cellular division can come from inside the cell, outside the cell, and even outside the organism. If the signals are successfully passed on to the cyclin-dependent kinases, cellular division will occur.

Problems in the signaling pathway for cell division can lead to significant problems for a cell or organism. Sometimes a cell receives too much signal, or one of the “gate keepers” is malfunctioning. If a cell begins dividing out of control, it can ultimately lead to tumors and other cancerous growths.

Learning Objectives

• What types of enzymes control the transition between one phase of the eukaryotic cell cycle and the next? How do they work?

• Understand how signals external to the cell and even external to the organism can lead to cell division.

• What is cancer and how does it relate to the eukaryotic cell cycle?