Biology (Holt)

Prokaryotic Cell Reproduction
A prokaryote’s single DNA molecule is circular and is attached to the
inner cell membrane. Prokaryotes reproduce by a type of cell division
called binary fission. is a form of asexual reproduction
that produces identical offspring. In asexual reproduction, a single
parent passes exact copies of all of its DNA to its offspring.
Binary fission occurs in two stages: first, the DNA is copied (so
that each new cell will have a copy of the genetic information), and
then the cell divides. The prokaryote divides by adding a new cell
membrane to a point on the membrane between the two DNA
copies. As new material is added, the growing cell membrane
pushes inward and the cell is constricted in the middle, like a long
balloon being squeezed near the center. A new cell wall forms
around the new membrane. Eventually the dividing prokaryote is
pinched into two independent cells. Each cell contains one of the
circles of DNA and is a complete functioning prokaryote.

Eukaryotic Cell Reproduction
The vast amount of information encoded in DNA is organized into
units called genes. A is a segment of DNA that codes for a pro-
tein or RNA molecule. A single molecule of DNA has thousands of
genes lined up like train cars. Genes play an important role in deter-
mining how a person’s body develops and functions. When genes
are being used, the DNA is stretched out so that the information it
contains can be used to direct the synthesis of proteins.
As a eukaryotic cell prepares to divide, the DNA and the proteins
associated with the DNA coil into a structure called a ,
as shown in Figure 2. Before the DNA coils up, however, the DNA is
copied. The two exact copies of DNA that make up each chromosome
are called (KROH muh tihdz). The two chromatids of a
chromosome are attached at a point called a .The chro-
matids, which become separated during cell division and placed into
each new cell, ensure that each new cell will have the same genetic
information as the original cell.

centromere

chromatids

chromosome

gene

Binary fission

SECTION 1Chromosomes 119

Real Life

Escherichia colicells

can produce 1 million

new cells in less than

7 hours.

A variety of E. coliknown

as O157:H7 is sometimes

found in raw or under-

cooked meat. When such

meat is eaten, this bacteria

can cause life-threatening

intestinal bleeding and

kidney failure. Thorough

cooking is necessary

to destroy the bacteria.

Finding Information

Research out-

breaks of E. coli

O157:H7 in your

community or

state.

DNA double

helix

DNA and

proteins

Further coiling

within supercoil

Supercoil within

chromosome

Chromosome
(made of 2
chromatids)

Centromere

Figure 2 Chromosome

structure.A chromosome

consists of DNA tightly

coiled around proteins. The

chromosomes are formed as

a cell prepares to divide.