Download Chapter 4 • Lesson 20

Chapter 4 • Lesson 20
The Structure and Function of DNA
Objectives: 3,1,1,4,1.2
Key Words
• genetics • trait • nucleic acid • nucleotide • DNA • RNA • double helix • complementary bases
• replication • chromosome • gene
Getting the Idea
DNA is a large macromolecule that is central to life. DNA in each cell directs all the cell's
activities. It is also responsible for the shape and features of the entire organism. Although
DNA controls many complicated structures and functions, in some ways this molecule is
very simple. The code it contains, which directs so many life processes, is written in a
language that uses only four letters.
Genetics and DNA
Genetics is the branch of biology concerned with heredity, the passing of characteristics
from parent to offspring. Each characteristic of an organism, such as cell structure or body
shape, is called a trait. Organisms inherit most of their traits from their parents. The genetic
information that determines these traits is contained in nucleic acids. Recall that nucleic
acids are large organic molecules made up of carbon, hydrogen, oxygen, nitrogen, and
phosphorus.
Each nucleic acid is made up of smaller units called nucleotides. A nucleotide consists of a
five-carbon sugar molecule bonded to a nitrogenous base and a phosphate group. Cells
contain two types of nucleic acids—DNA (deoxyribonucleic acid) and RNA (ribonucleic
acid). Each nucleic acid is named for the sugar it contains: deoxyribose in DNA and ribose
in RNA. You will learn more about RNA in the next lesson. DNA and RNA both contain fivecarbon sugar molecules. Although the specific sugars in DNA and RNA differ, the carbon
atoms of both sugars are arranged in a ring.
DNA carries the cell's genetic information. It also contains instructions for cellular activity
and for making the proteins organisms need for survival. In eukaryotic cells, most of the
DNA is located in the nucleus, where it is coiled into structures called chromosomes. Recall
that prokaryotic cells do not have nuclei. In prokaryotes, DNA floats freely in the cytoplasm.
Some of this DNA may be in the form of a circular ring called a plasmid.
The Structure of DNA
Each molecule of DNA is made up of linked nucleotides. The nucleotides are all the same,
except for the nitrogenous bases they contain. Each nucleotide includes one of four
nitrogenous bases. The four bases in DNA are adenine, thymine, guanine, and cytosine.
Scientists often refer to each base by the first letter in its name. Thus, A = adenine, T =
thymine, G = guanine, and C = cytosine.
A molecule of DNA is shaped like a twisted ladder. This shape is called a double helix. The
sides of the ladder are two strands that spiral around an imaginary axis. As shown in the
diagram, these strands are made up of alternating phosphate and sugar molecules that are
joined together by chemical bonds. The "rungs" of the twisted ladder shape are composed
of pairs of nitrogenous bases.
The bases in DNA always pair in the same way: adenine with thymine, A-T or T-A, and
cytosine with guanine, C-G or G-C. The nucleotides in each pair are known as
complementary bases. They are held together by weak hydrogen bonds. The sequence of
bases from rung to rung along the ladder stores the genetic information contained in the
DNA molecule.
An organism's traits are based on the proteins present during its development and
throughout its life. Cells use the sequence of nucleotides in DNA as a set of instructions for
making proteins. However, the proteins are not made directly by DNA. Instead, the
information in DNA is copied from the DNA molecule by RNA. The RNA then carries the
information to the cell's ribosomes, where proteins are made. You will learn more about this
process in the next lesson.
DNA Replication
Cells are living structures. They must be able to reproduce to make new cells like
themselves. Recall from Lesson 5 that the cell cycle is a continuous process in which cells
grow, make copies of their chromosomes, and divide to form daughter cells. Before a cell
divides, its DNA makes a copy of itself in a process called replication. The DNA molecule
replicates during the S phase of the cell cycle, so that each daughter cell receives an exact
copy of the parent cell's DNA.
In the first step of replication, the DNA molecule is unzipped down the middle by two
enzymes that break the weak hydrogen bonds between the complementary bases. The
nucleotides separate, breaking the DNA molecule into two complementary halves. The
sequence of bases on each half is used to construct a duplicate DNA molecule.
When the double helix separates into two strands, complementary sequences of bases are
exposed. For example, if one strand "reads" ACTTG, then its complementary strand "reads"
TGAAC, as shown below.
After the DNA molecule unzips, enzymes in the cytoplasm join nucleotides together to make
matching DNA strands. The DNA-synthesizing enzymes move nucleotides into the proper
positions so that A pairs with T and G pairs with C to form two new complementary strands.
The result is two new DNA molecules. Each is made up of half of the original DNA molecule
and a newly formed matching half. Finally, an enzyme called DNA polymerase checks the
arrangement of bases in each new DNA strand, decreasing the chance that the copy
contains an error. Each new molecule is identical to the original molecule of DNA. The
diagram below shows this process.
Chromosomes and Genes
A molecule of DNA can be quite long. The nucleus of a single human cell contains more
than 1 meter of DNA. To fit inside the nucleus, long sections of DNA are tightly coiled into
chromosomes. Chromosomes are structures that contain the genetic information that is
passed down from one generation to the next.
Every species has a characteristic number of chromosomes in its cells. An organism has
many more inherited traits than chromosomes. A trait is determined by one or more small
sections of a chromosome called genes. Each chromosome contains hundreds or
thousands of genes. Each gene codes for a specific protein. The diagram shows the
relationship among genes, chromosomes, and DNA.
The genetic information of each species
differs from that of every other species.
However, the DNA of all organisms has
some things in common. The more similar
the genetic codes of two organisms, the
more closely related the organisms are. The
similarity is measured by determining how
many genes the organisms share and how
alike the shared genes are. Similar genes
code for the same proteins, which may be
reflected in shared traits. For example, a
bird shares more genes with a reptile than with a fern plant or an insect. Even within a
species, the genetic code of each organism is likely to differ from that of any other
organism. These genetic differences explain why individual organisms have unique
combinations of traits.