DNA (deoxyribonucleic acid) is a molecule which is found in the nucleus of cells and contains
the chemical message for cell reproduction and the synthesis of proteins.
The DNA molecule consists of two strands, each of which is a chain of nucleotides. Each nucleotide
has three parts:
1. a five-carbon sugar called deoxyribose
2. a phosphate group
3. a nitrogen base NUCLEOTIDE
There are four different kinds of nitrogenous bases:
(A) Adenine (G) Guanine (T) Thymine (C) Cytosine
The Double Helix:
Each nucleotide bonds to other nucleotides to form a long strand. Two
of these strands bond together to form a molecule of DNA. The two strand twist
around to form a spiral structure called a double helix.
The double helix was first described in 1953 by James Watson, Francis
Crick and Rosalind Franklin. Their discovery is one of the most significant of
the twentieth century and has led to much understanding about DNA, its role, its
function, and how it is related to heredity and genetic disorders.
The DNA molecule looks something like a twisted ladder. The sides of
the ladder are formed by alternating sugar and phosphate units, and the rungs
consist of bonded pairs of nitrogen bases (also called base pairs.) The rungs are
uniform length. The DNA molecule has a right hand twist, with each full turn consisting of ten base
The two strands of DNA are held together by hydrogen bonds between the bases. In DNA,
adenine always bonds with thymine, and cytosine always bonds with guanine. This occurs because
adenine and thymine compliment each other in such a way that hydrogen bonds are established
between them. These bonds cannot form between adenine and cytosine or adenine and guanine
because of the difference in structure. The same kind of compliment exists between cytosine and
guanine. Adenine and thymine form two hydrogen bonds, while cytosine and guanine form three.
The fact that adenine always bonds to thymine and cytosine always bonds
to guanine has structural and functional consequences. The sequential
arrangement of nitrogen bases along one strand is the exact compliment of the
sequential arrangement of bases on the adjacent strand. Because of this
characteristic, the two strands are called complimentary strands.
DNA is a winning formula for packaging genetic material. Therefore
almost all organisms – bacteria, plants, yeast and animals – carry genetic
information encapsulated as DNA. One exception is some viruses that use RNA
Different species need different amounts of DNA. Therefore the copying
of the DNA that precedes cell division differs between organisms. For example, the DNA in E. coli
bacteria is made up of 4 million base pairs and the whole genome is thus one millimeter long. The
single-cell bacterium can copy its genome and divide into two cells once every 20 minutes.
The DNA of humans, on the other hand, is composed of approximately 3 billion base pairs, making up
a total of almost a meter-long stretch of DNA in every cell in our bodies.
In order to fit, the DNA must be packaged in a very compact form. In E. coli the single circular
DNA molecule is curled up in a condensed fashion, whereas the human DNA is packaged in 23 distinct
1. What three people discovered the double helix?
2. Describe, in your own words, the structure of the DNA molecule.
3. What is meant by base pairs? What bonds with what?
4. Do all organisms have the same genetic material? Do they all have the same amount of
Replication of DNA:
"It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible
copying mechanism for the genetic material" wrote Watson and Crick in the scientific paper that was published
in Nature, April 25, 1953.
The scientific world was stunned by Watson, Crick and Franklin's proposed structure for DNA
because it immediately suggested how DNA might be copied. If the two complementary strands of
DNA were to unwind, then each strand could serve as the template on which a new complimentary
strand could be built. This process would produce two new DNA molecules that are exact replicas of
the original DNA. Subsequent research has shown that this is exactly what happens.
The process of duplicating a DNA molecule is called replication. Replication begins when and
enzyme called DNA helicase attaches to a DNA molecule, moves along the molecule, and "unzips" the
two strands of DNA. DNA helicase acts by breaking the hydrogen bonds between the nitrogen bases.
After the DNA strands are separated, an enzyme called DNA polymerase builds the "new"
strands using the "old" strands as templates. The unpaired bases in each strand react with
complimentary bases that are floating freely in the nucleus of the cell.
These complimentary bases bond with the bases in the DNA strands by
forming new hydrogen bonds. This process results in two new DNA
molecules each of which consists of one "old" strand of DNA and one
"new" strand of DNA.
The process of replication maintains the complimentary nature of
the NDA molecule.. Since adenine (A) can only pair with thymine (T)
and cytosine (C) can only pair with guanine (G), the sequence of
nucleotides in each new strand exactly matches that in the original
Replication of DNA doesn't begin at one end of the molecule and
proceed to the other. Rather, DNA is copied simultaneously at many
points along the molecule. Enzymes join the segments of DNA to each
other as they are copied. Because of the size of the DNA molecule, if
replication doesn't occur in this way, it would take 16 days to copy just
one DNA molecule of a fruit fly. Actually, the replication of this fruit fly
DNA takes only about three minutes, because about 6,000 sites are being
1. How does the structure of DNA lend itself replication?
2. Explain in your own words the way in which DNA is replicated.
3. How can DNA replicate so quickly if the molecule is so long?
Accuracy & Repair
The entire process of DNA replication occurs with great accuracy. The cells has a built in
"proofreading" function, preformed by enzymes or proteins, depending on the organism. The degree of
accuracy is on the order of one error per billion. (Whenever a mistake or "inaccuracy" occurs, a change
in the DNA will result, which may result in a change to the proteins which are created, which may
result in a change in the organism, which may or may not harm or benefit the organism.)
DNA also has a mechanism for repair. In human cells, body heat, radiation, chemicals an other
factors can damage DNA. Body heat alone continuously breaks up bonds between deoxyribose and
purines. A group of 20 or more repairs enzymes recognizes an removes damaged nucleotides and
replaces them with new ones, thus ensuring the accurate replication of DNA.
In studying the structure and replication of DNA, make a note of one thing. The sequence of
bases along a strand is, as you will, learn, not random. Inherent in the sequence of nitrogen bases is all
the information determine the structure an function of an organism.
1.How does your body prevent errors in your DNA?
2.How can mistakes happen? (What causes errors?)
3.What can happen if an error is made?