The Solar System

CHAPTER 26 | ASTROBIOLOGY: LIFE ON OTHER WORLDS 585

As a cell begins to

divide, its DNA

duplicates itself.

The duplicated

chromosomes move

to the middle.

The two sets of

chromosomes

separate, and...

Cell Reproduction by Division

the cell divides to

produce...

two cells, each

containing a full set

of the DNA code.

To divide, a cell must duplicate its

DNA. The DNA ladder splits, and

new bases match to the exposed bases

of the ladder to build two copies of the

original DNA code. Because the base

pairs almost always match correctly,

errors in copying are rare. One set of the

DNA code goes to each of the two new

cells.

Original DNA

Copy DNA

Copy DNA

3 3a

A cell is a tiny factory that uses the DNA code

to manufacture chemicals. Most of the DNA

remains safe in the nucleus of a cell, and the code

is copied to create a molecule of RNA (ribonucleic

acid). Like a messenger carrying blueprints, the

RNA carries the code out of the nucleus to the

work site where the proteins and enzymes are

made.

2a

A single cell from a human being contains about
1.5 meters of DNA containing about 4.5 billion
base pairs — enough to record the entire works of
Shakespeare 200 times. A typical adult human
contains a total of about 600 AU of DNA. Yet the
DNA in each cell, only 1.5 meters in
length, contains all of the information
to create a new human. A clone
is a new creature created from
the DNA code found in a single
cell.

DNA, coiled into a
tight spiral, makes up
thechromosomes that are
the genetic material in a cell. A
gene is a segment of a chromosome
that controls a certain function. When a
cell divides, each of the new cells receives a
copy of the chromosomes, as genetic information
is handed down to new generations.

2b

Cell membrane

(transport of raw

materials and

finished product)

Material

storage

Manufacture

of proteins

and enzymes

Energy

production

Nucleus

(information

storage)