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Lesson Objectives

  • Outline the process of DNA replication.
  • Compare and contrast cell division in prokaryotic and eukaryotic cells.
  • Describe the four phases of mitosis in eukaryotic cells.
  • Identify the stages of the cell cycle in prokaryotic and eukaryotic cells.

Lesson Vocabulary

  • anaphase
  • binary fission
  • cell cycle
  • cell division
  • chromosome
  • cytokinesis
  • DNA (deoxyribonucleic acid)
  • DNA replication
  • interphase
  • metaphase
  • mitosis
  • prophase
  • telophase

Introduction

Cell division is the process in which a cell divides to form two new cells. The original cell is called the parent cell. The two new cells are called daughter cells. All cells contain DNA. DNA is the nucleic acid that stores genetic information. Before a cell divides its DNA must be copied. That way, each daughter cell gets a complete copy of the parent cell’s genetic material.

Copying DNA

DNA stands for deoxyribonucleic acid. It is a very large molecule. It consists of two strands of smaller molecules called nucleotides. Before learning how DNA is copied, it’s a good idea to review its structure.

DNA Structure

As you can see in Figure below, each nucleotide includes a sugar, a phosphate, and a nitrogen base. The sugar in DNA is called deoxyribose. There are four different nitrogen bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). Chemical bonds between the bases hold the two strands of DNA together. Adenine always bonds with thymine, and cytosine always bonds with guanine. These pairs of bases are called complementary base pairs.

Nucleic Acid

Structure of DNA

Chromosomes

As a cell prepares to divide, its DNA first forms one or more structures called chromosomes. A chromosome consists of DNA and protein molecules coiled into a definite shape. Chromosomes are circular in prokaryotes and rodlike in eukaryotes. You can see an example of a human chromosome in Figure below. The rest of the time, DNA looks like a tangled mass of strings. In this form, it would be very difficult to copy and divide.

Chromosome

Human chromosome

DNA Replication

The process in which DNA is copied is called DNA replication. You can see how it happens in Figure below. An enzyme breaks the bonds between the two DNA strands. Another enzyme pairs new, complementary nucleotides with those in the original chains. Two daughter DNA molecules form. Each contains one new chain and one original chain.

DNA Replication

DNA replication

Cell Division in Prokaryotic and Eukaryotic Cells

How cell division proceeds depends on whether a cell has a nucleus. Prokaryotic cells lack a nucleus. Their DNA is in the cytoplasm. It forms just one circular chromosome. Eukaryotic cells have a nucleus holding their DNA. Their DNA forms multiple rodlike chromosomes, like the one in Figure 5.2. Eukaryotic cells also have other organelles. For these reasons, cell division is more complex in eukaryotic cells.

Prokaryotic Cell Division

You can see how a prokaryotic cell divides in Figure below. This type of cell division is called binary fission. The cell simply splits into two equal halves. Binary fission occurs in bacteria and other prokaryotes. It takes place in three continuous steps:

  1. The cell’s chromosome is copied to form two identical chromosomes. This is DNA replication.
  2. The copies of the chromosome separate from each other. They move to opposite poles, or ends, of the cell. This is called chromosome segregation.
  3. The cell wall grows toward the center of the cell. The cytoplasm splits apart, and the cell pinches in two. This is called cytokinesis.

Binary fission in a prokaryotic cell

Binary fission in a prokaryotic cell

Eukaryotic Cell Division

Before a eukaryotic cell divides, the nucleus and other organelles must be copied. Only then will each daughter cell have all the needed structures.

1. The first step in eukaryotic cell division, as it is in prokaryotic cell division, is DNA replication. As you can see in Figure below, each chromosome then consists of two identical copies. The two copies are called sister chromatids. They are attached to each other at a point called the centromere.

Chromosome and Centromere

2. The second step in eukaryotic cell division is division of the cell’s nucleus. This includes division of the chromosomes. This step is called mitosis. It is a complex process that occurs in four phases. The phases of mitosis are described below.

3. The third step is the division of the rest of the cell. This is called cytokinesis, as it is in a prokaryotic cell. During this step, the cytoplasm divides, and two daughter cells form.

These three steps are shown in Figure below.

Cell division in a eukaryotic cell

Cell division in a eukaryotic cell

Mitosis

Mitosis, or division of the nucleus, occurs only in eukaryotic cells. By the time mitosis occurs, the cell’s DNA has already replicated. Mitosis occurs in four phases, called prophase, metaphase, anaphase, and telophase. You can see what happens in each phase in Figure below. The phases are described below. You can also learn more about the phases of mitosis by watching this video:

http://www.youtube.com/watch?v=slZa6XE4WXE

Phases of mitosis

  1. Prophase: Chromosomes form, and the nuclear membrane breaks down. In animal cells, the centrioles near the nucleus move to opposite poles of the cell. Fibers called spindles form between the centrioles.
  2. Metaphase: Spindle fibers attach to the centromeres of the sister chromatids. The sister chromatids line up at the center of the cell.
  3. Anaphase: Spindle fibers shorten, pulling the sister chromatids toward the opposite poles of the cell. This gives each pole a complete set of chromosomes.
  4. Telophase: The chromosomes uncoil, and the spindle fibers break down. New nuclear membranes form.

The Cell Cycle

Cell division is just one of the stages that a cell goes through during its lifetime. All of the stages that a cell goes through make up the cell cycle.

Prokaryotic Cell Cycle

The cell cycle of a prokaryotic cell is simple. The cell grows in size, its DNA replicates, and the cell divides.

Eukaryotic Cell Cycle

In eukaryotes, the cell cycle is more complicated. The diagram in Figure below shows the stages that a eukaryotic cell goes through in its lifetime. There are two main stages: interphase and mitotic phase. They are described below. You can watch a eukaryotic cell going through the phases of the cell cycle at this link: http://www.cellsalive.com/cell_cycle.htm

Eukaryotic cell cycle

Eukaryotic cell cycle

Interphase is longer than mitotic phase. Interphase, in turn, is divided into three phases:

  1. Growth phase 1 (G1): The cell grows rapidly. It also carries out basic cell functions. It makes proteins needed for DNA replication and copies some of its organelles. A cell usually spends most of its lifetime in this phase.
  2. Synthesis phase (S): The cell copies its DNA. This is DNA replication.
  3. Growth phase 2 (G2): The cell gets ready to divide. It makes more proteins and copies the rest of its organelles.

Mitotic phase is when the cell divides. It includes mitosis (M) and cytokinesis (C).

Lesson Summary [B head]

  • DNA is the nucleic acid that stores genetic information. It must be copied before a cell divides. DNA replication is the process in which DNA is copied.
  • Cell division is the process in which a parent cell divides to form two daughter cells. It occurs by binary fusion in most prokaryotic cells. It is more complex in eukaryotic cells.
  • Mitosis is the process by which the nucleus of a eukaryotic cell divides. It happens in four phases: prophase, metaphase, anaphase, and telophase.
  • Cell division is just one stage of the cell cycle. The cell cycle includes all of the stages in the life of a cell. The cell cycle is more complex in eukaryotic than prokaryotic cells.

Lesson Review Questions

Recall

  1. What is DNA replication? When and why does it occur?
  2. What are chromosomes? When do chromosomes form?
  3. Identify the steps of cell division in a prokaryotic cell.
  4. List the phases of mitosis and what happens during each phase.

Apply Concepts

  1. A single-celled organism belongs to the Eukarya Domain. Apply lesson concepts to describe how the organism’s cells divide.

Think Critically

  1. Explain why cell division is more complicated in eukaryotic than prokaryotic cells.
  2. Compare and contrast the cell cycles of prokaryotic and eukaryotic cells.

Points to Consider

Cell division is how organisms grow and replace worn out or damaged cells. It’s also how they produce offspring. Producing offspring is known as reproduction.

  • How do you think prokaryotes reproduce?
  • How do you think multicellular eukaryotes reproduce?

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