Prokaryotes divide by fission

In prokaryotes, cell division results in the reproduction of the entire single-celled organism. The cell grows in size, replicates its DNA, and then essentially divides into two new cells, a process called fission.

reproductive signals. The reproductive rates of many prokaryotes respond to conditions in the environment. The bacterium Escherichia coli, a species that is commonly used in genetic studies, is a "cell division machine" that essentially divides continuously. Typically, cell division takes 40 minutes at 37°C. But if there are abundant sources of carbohydrates and salts available, the division cycle speeds up so that cells may divide in 20 minutes. Another bacterium, Bacillus subtilis, stops dividing when food supplies are low, then resumes dividing when conditions improve. These observations suggest that external factors, such as materials in the environment, control the initiation of cell division in prokaryotes.

replication of dna. A chromosome, as we saw in Chapter 4, is a DNA molecule containing genetic information. When a cell divides, all of its chromosomes must be replicated, and each of the two resulting copies must find its way into one of the two new cells.

Most prokaryotes have only one chromosome, a single long DNA molecule with proteins bound to it. In the bacterium E. coli, the DNA is a continuous molecule often referred to as a circular chromosome. If the bacterial DNA were actually arranged in a circle, it would be about 1.6 million nm (1.6 mm) in circumference. The bacterium itself is only about 1 |im (1,000 nm) in diameter and about 4 |im long. Thus the bacterial DNA, fully extended, would form a circle over 100 times larger than the cell! To fit it into the cell, the DNA must be packaged. The DNA molecule accomplishes some packaging by folding in on itself, and positively charged (basic) proteins bound to negatively charged (acidic) DNA contribute to this folding. Circular chromosomes appear to be characteristic of all prokaryotes, as well as some viruses, and

Step Cell Reproduction

9.1 Important Consequences of Cell Division

Cell division is the basis for (a) growth, (b) reproduction, and (c) regeneration.

9.1 Important Consequences of Cell Division

Cell division is the basis for (a) growth, (b) reproduction, and (c) regeneration.

Prokaryotic Plasma Membrane

are also found in the chloroplasts and mitochondria of eu-karyotic cells.

Functionally, the prokaryotic chromosome has two regions that are important for cell reproduction:

► The site where replication of the circle starts: the origin of replication, designated ori

► The site where replication ends: the terminus of replication, ter

The process of chromosome replication occurs as the DNA is threaded through a "replication complex" of proteins at the center of the cell. These proteins include the enzyme DNA polymerase, and their operation will be discussed further in Chapter 11. During the process of prokaryotic DNA replication, the cell grows and provides a mechanism for the ordered distribution of the DNA into the newly formed daughter cells.

segregation of dna. DNA replication actively drives the segregation of the replicated DNA molecules to the two new cells. The first region to be replicated is ori, which is attached to the plasma membrane. The two resulting ori regions separate as the new chromosome forms and new plasma membrane forms between them as the cell grows longer (Figure 9.2). By the end of replication, there are two chromosomes, one at either end of the lengthened bacterial cell.

cytokinesis. Cell separation, or cytokinesis, begins 20 minutes after chromosome replication is finished. The first event of cytokinesis is a pinching in of the plasma membrane to form a ring similar to a purse string. Fibers composed of a protein similar to eukaryotic tubulin (which makes up microtubules) are major components of this ring. As the membrane pinches in, new cell wall materials are synthesized, which finally separate the two cells.

and cell specialization. For example, an adult human has several trillion cells, all ultimately deriving from the fertilized egg, and many of them have specialized roles. We will discuss how cells specialize later in this book, in Part Three. For now, we will focus on cell reproduction.

Cell reproduction in eukaryotes, like that in prokaryotes, involves reproductive signals, DNA replication, segregation, and cytokinesis. But, as you might expect, events in eukary-

The bacterial chromosome is attached to the plasma membrane at the chromosome's ori region.

The bacterial chromosome is attached to the plasma membrane at the chromosome's ori region.

Extracellular matrix Plasma membrane Chromosome

The chromosomal DNA replicates. The attachment points separate as the cell grows.

The chromosomal DNA replicates. The attachment points separate as the cell grows.

Prokaryotes Dividing

Fission is complete; two new cells are formed.

Fission is complete; two new cells are formed.

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