DNA

Genetics is the study of how genes bring about traits in living things and how those characteristics are inherited.

Molecular Genetics

During the 1950s, an explosion of biological research occurred.

The knowledge generated during this period helped explain how genes function, and it gave rise to the science of molecular genetics. This science is based on the activity of deoxyribonucleic acid (DNA) and how this activity brings about the production of proteins in the cell.

DNA is made up of a series of units called nucleotides

The DNA Structure

Each nucleotide is made of three parts:

1) a sugar called deoxyribose (composed by 5 C)

2) a phosphate group (also called phosphoric acid)

3) a nitrogen-containing bases. The 4 nitrogen bases are adenine (A), guanine (G), thymine (T), and cytosine (C)

188 MHR • Genetic Continuity

For example, Figure 6.5 shows an adeninenucleotide. Therefore, in DNA there are fourpossible nucleotides, each containing one of fourbases. Note that adenine and guanine have adouble-ring structure. They belong to a group ofnucleotides called purines. Thymine and cytosinehave a single-ring structure. Single-ring nucleotidesare called pyrimidines.

Figure 6.5 One DNA nucleotide, the adenine nucleotide.

Further studies showed that nucleotides arejoined together to form long chains, much likebeads in a necklace. The phosphate group of onenucleotide is bonded to the deoxyribose sugar of anadjacent nucleotide, as shown in Figure 6.6. Thephosphate groups and deoxyribose molecules formthe backbone of the chain, and the nitrogen basesstick out like the teeth on a zipper. Erwin Chargaff,another scientist studying the chemical structure ofDNA, found that in any given species the amountsof adenine and thymine were virtually the same, aswere the amounts of guanine and cytosine. Thequestions still remained, however, of how DNAwas constructed and exactly how DNA carriedgenetic information.

Figure 6.6 Nucleotides are joined together in a long chain.

Discovering the Double HelixLike athletes, scientists can be very competitive. Asthe evidence mounted, scientists knew they werevery close to determining the structure of DNA.Many people were vying to be the first to make thisimportant discovery. The race was on! Here are thefacts the scientists knew at the time.

DNA is made of nucleotides.

Nucleotides are linked together in a string. Thesugar of one nucleotide is attached to thephosphate group of the next, and the nitrogenbases stick out from one side of this sugar-phosphate “backbone.”

In each DNA molecule, the number of adeninenucleotides equals the number of thyminenucleotides, and the number of cytosinenucleotides equals the number of guaninenucleotides.

If the nucleotides are strung in a straight line, a typical DNA molecule would be over a metrelong. Somehow, the DNA molecule must be compressed.

The work of Rosalind Franklin (see Figure 6.7)provided another crucial piece of evidence neededto solve the mystery of DNA’s structure. Franklin,along with Maurice Wilkins, the head of thelaboratory where she worked in King’s College,London, had perfected a technique that used X raysto photograph molecules. Although it takes anexpert eye to interpret these pictures (see Figure 6.8),some of Franklin’s photographs of DNA showedpatterns that indicated DNA was like a giant springor coil. Furthermore, the DNA molecule had aconstant diameter of 2 nm. It did not get wider insome parts and narrower in others.

Now that you know what DNA is made of and where DNA isfound, why do you think scientists called this chemicaldeoxyribonucleic acid?

L I N KWord

AT T

GC

PS

PS

PS

PS

PS

P S sugarphosphate

H HH

OO

O

O

O = P − O−

C C

C

OH HSugar(deoxyribose)

Nitrogen base

Phosphategroup

CH2

Figure 6.7 Rosalind Franklin,a British crystallographer,used X ray diffraction techniques to discover thedouble helix nature of theDNA molecule.

188 MHR • Genetic Continuity

For example, Figure 6.5 shows an adeninenucleotide. Therefore, in DNA there are fourpossible nucleotides, each containing one of fourbases. Note that adenine and guanine have adouble-ring structure. They belong to a group ofnucleotides called purines. Thymine and cytosinehave a single-ring structure. Single-ring nucleotidesare called pyrimidines.

Figure 6.5 One DNA nucleotide, the adenine nucleotide.

Further studies showed that nucleotides arejoined together to form long chains, much likebeads in a necklace. The phosphate group of onenucleotide is bonded to the deoxyribose sugar of anadjacent nucleotide, as shown in Figure 6.6. Thephosphate groups and deoxyribose molecules formthe backbone of the chain, and the nitrogen basesstick out like the teeth on a zipper. Erwin Chargaff,another scientist studying the chemical structure ofDNA, found that in any given species the amountsof adenine and thymine were virtually the same, aswere the amounts of guanine and cytosine. Thequestions still remained, however, of how DNAwas constructed and exactly how DNA carriedgenetic information.

Figure 6.6 Nucleotides are joined together in a long chain.

Discovering the Double HelixLike athletes, scientists can be very competitive. Asthe evidence mounted, scientists knew they werevery close to determining the structure of DNA.Many people were vying to be the first to make thisimportant discovery. The race was on! Here are thefacts the scientists knew at the time.

DNA is made of nucleotides.

Nucleotides are linked together in a string. Thesugar of one nucleotide is attached to thephosphate group of the next, and the nitrogenbases stick out from one side of this sugar-phosphate “backbone.”

In each DNA molecule, the number of adeninenucleotides equals the number of thyminenucleotides, and the number of cytosinenucleotides equals the number of guaninenucleotides.

If the nucleotides are strung in a straight line, a typical DNA molecule would be over a metrelong. Somehow, the DNA molecule must be compressed.

The work of Rosalind Franklin (see Figure 6.7)provided another crucial piece of evidence neededto solve the mystery of DNA’s structure. Franklin,along with Maurice Wilkins, the head of thelaboratory where she worked in King’s College,London, had perfected a technique that used X raysto photograph molecules. Although it takes anexpert eye to interpret these pictures (see Figure 6.8),some of Franklin’s photographs of DNA showedpatterns that indicated DNA was like a giant springor coil. Furthermore, the DNA molecule had aconstant diameter of 2 nm. It did not get wider insome parts and narrower in others.

Now that you know what DNA is made of and where DNA isfound, why do you think scientists called this chemicaldeoxyribonucleic acid?

L I N KWord

AT T

GC

PS

PS

PS

PS

PS

P S sugarphosphate

H HH

OO

O

O

O = P − O−

C C

C

OH HSugar(deoxyribose)

Nitrogen base

Phosphategroup

CH2

Figure 6.7 Rosalind Franklin,a British crystallographer,used X ray diffraction techniques to discover thedouble helix nature of theDNA molecule.

188 MHR • Genetic Continuity

For example, Figure 6.5 shows an adeninenucleotide. Therefore, in DNA there are fourpossible nucleotides, each containing one of fourbases. Note that adenine and guanine have adouble-ring structure. They belong to a group ofnucleotides called purines. Thymine and cytosinehave a single-ring structure. Single-ring nucleotidesare called pyrimidines.

Figure 6.5 One DNA nucleotide, the adenine nucleotide.

Further studies showed that nucleotides arejoined together to form long chains, much likebeads in a necklace. The phosphate group of onenucleotide is bonded to the deoxyribose sugar of anadjacent nucleotide, as shown in Figure 6.6. Thephosphate groups and deoxyribose molecules formthe backbone of the chain, and the nitrogen basesstick out like the teeth on a zipper. Erwin Chargaff,another scientist studying the chemical structure ofDNA, found that in any given species the amountsof adenine and thymine were virtually the same, aswere the amounts of guanine and cytosine. Thequestions still remained, however, of how DNAwas constructed and exactly how DNA carriedgenetic information.

Figure 6.6 Nucleotides are joined together in a long chain.

Discovering the Double HelixLike athletes, scientists can be very competitive. Asthe evidence mounted, scientists knew they werevery close to determining the structure of DNA.Many people were vying to be the first to make thisimportant discovery. The race was on! Here are thefacts the scientists knew at the time.

DNA is made of nucleotides.

Nucleotides are linked together in a string. Thesugar of one nucleotide is attached to thephosphate group of the next, and the nitrogenbases stick out from one side of this sugar-phosphate “backbone.”

In each DNA molecule, the number of adeninenucleotides equals the number of thyminenucleotides, and the number of cytosinenucleotides equals the number of guaninenucleotides.

If the nucleotides are strung in a straight line, a typical DNA molecule would be over a metrelong. Somehow, the DNA molecule must be compressed.

The work of Rosalind Franklin (see Figure 6.7)provided another crucial piece of evidence neededto solve the mystery of DNA’s structure. Franklin,along with Maurice Wilkins, the head of thelaboratory where she worked in King’s College,London, had perfected a technique that used X raysto photograph molecules. Although it takes anexpert eye to interpret these pictures (see Figure 6.8),some of Franklin’s photographs of DNA showedpatterns that indicated DNA was like a giant springor coil. Furthermore, the DNA molecule had aconstant diameter of 2 nm. It did not get wider insome parts and narrower in others.

Now that you know what DNA is made of and where DNA isfound, why do you think scientists called this chemicaldeoxyribonucleic acid?

L I N KWord

AT T

GC

PS

PS

PS

PS

PS

P S sugarphosphate

H HH

OO

O

O

O = P − O−

C C

C

OH HSugar(deoxyribose)

Nitrogen base

Phosphategroup

CH2

Figure 6.7 Rosalind Franklin,a British crystallographer,used X ray diffraction techniques to discover thedouble helix nature of theDNA molecule.

Each nitrogen base is attached to a sugar, and each sugar is then attached to a phosphate group to form a single nucleotide. Each nucleotide is named for the base it contains.

C

DNA is made up of a series of units called nucleotides

The DNA Structure

Therefore, in DNA there are four possible nucleotides, each containing one of four bases.

Note that adenine and guanine have a double-ring structure. They belong to a group of nucleotides called purines. Thymine and cytosine have a single-ring structure. Single-ring nucleotides are called pyrimidines

187DNA Structure and Replication • MHR

The outer coating of the bacteriophage is made of protein, and the inside of the bacteriophage carriesa core of DNA. Hershey and Chase devised a wayto mark the protein coat and the DNA usingradioactive sulfur (35S) and phosphorus (32P) sothey could follow what each component did as theviruses attacked bacteria cells. They found that theprotein coat of the bacteriophage remained outsidethe bacterial cell. However, the bacteriophage’sDNA moved into the bacterial cells and directedthem to create new bacteriophages (see Figure 6.3).The Hershey-Chase experiment was very importantin helping to confirm that it is DNA, not protein,that carries the genetic information.

NucleotidesWhile Hershey and Chase’s experiment determinedthe role of DNA in heredity, earlier work bybiochemist P.A. Levine had shown that DNA wasmade up of a series of units called nucleotides.Each nucleotide is made of three parts — a sugarcalled deoxyribose, a phosphate group (also calledphosphoric acid), and four nitrogen-containingbases. The nitrogen bases are adenine, guanine,thymine, and cytosine, which are abbreviated as A, G, T, and C, respectively. Figure 6.4 shows thefour nitrogen bases of DNA.

Levine found that each nitrogen base is attachedto a sugar, and each sugar is then attached to aphosphate group to form a single nucleotide. Eachnucleotide is named for the base it contains.

bacteriophage

protein coat

DNA

bacterium

Figure 6.3 Hershey-Chase experiment

Bacteriophage attacksbacterium and injectsDNA into bacterium.Protein coat of the bacteriophage remainsoutside.

A

Bacterium contains DNA of thebacteriophage with 32P.

B

Bacterium produces more bacteriophages. Since the proteincoat remained on the outside ofthe bacterium, this is evidencethat DNA carried the geneticmaterial into the bacterial cell.

C

Bacterium ruptures,releasing new bacteriophages.

D

Thymine (T)

Guanine (G)

Adenine (A)

Cytosine (C)

C

C N

N

N

N

NNN

NN

NH

N

NN

H

H

H

H

C C

C

CCC

CCC

CCC

C

C

C

C

O

O

O

O

H

HHH

H

H

H

H

H

H

H

CH3

Figure 6.4 The nitrogen bases of DNA are adenine, guanine, thymine, and cytosine.

Nucleotides are joined together trough hydrogen bonds to form the long DNA chains

188 MHR • Genetic Continuity

For example, Figure 6.5 shows an adeninenucleotide. Therefore, in DNA there are fourpossible nucleotides, each containing one of fourbases. Note that adenine and guanine have adouble-ring structure. They belong to a group ofnucleotides called purines. Thymine and cytosinehave a single-ring structure. Single-ring nucleotidesare called pyrimidines.

Figure 6.5 One DNA nucleotide, the adenine nucleotide.

Further studies showed that nucleotides arejoined together to form long chains, much likebeads in a necklace. The phosphate group of onenucleotide is bonded to the deoxyribose sugar of anadjacent nucleotide, as shown in Figure 6.6. Thephosphate groups and deoxyribose molecules formthe backbone of the chain, and the nitrogen basesstick out like the teeth on a zipper. Erwin Chargaff,another scientist studying the chemical structure ofDNA, found that in any given species the amountsof adenine and thymine were virtually the same, aswere the amounts of guanine and cytosine. Thequestions still remained, however, of how DNAwas constructed and exactly how DNA carriedgenetic information.

Figure 6.6 Nucleotides are joined together in a long chain.

Discovering the Double HelixLike athletes, scientists can be very competitive. Asthe evidence mounted, scientists knew they werevery close to determining the structure of DNA.Many people were vying to be the first to make thisimportant discovery. The race was on! Here are thefacts the scientists knew at the time.

DNA is made of nucleotides.

Nucleotides are linked together in a string. Thesugar of one nucleotide is attached to thephosphate group of the next, and the nitrogenbases stick out from one side of this sugar-phosphate “backbone.”

In each DNA molecule, the number of adeninenucleotides equals the number of thyminenucleotides, and the number of cytosinenucleotides equals the number of guaninenucleotides.

If the nucleotides are strung in a straight line, a typical DNA molecule would be over a metrelong. Somehow, the DNA molecule must be compressed.

The work of Rosalind Franklin (see Figure 6.7)provided another crucial piece of evidence neededto solve the mystery of DNA’s structure. Franklin,along with Maurice Wilkins, the head of thelaboratory where she worked in King’s College,London, had perfected a technique that used X raysto photograph molecules. Although it takes anexpert eye to interpret these pictures (see Figure 6.8),some of Franklin’s photographs of DNA showedpatterns that indicated DNA was like a giant springor coil. Furthermore, the DNA molecule had aconstant diameter of 2 nm. It did not get wider insome parts and narrower in others.

Now that you know what DNA is made of and where DNA isfound, why do you think scientists called this chemicaldeoxyribonucleic acid?

L I N KWord

AT T

GC

PS

PS

PS

PS

PS

P S sugarphosphate

H HH

OO

O

O

O = P − O−

C C

C

OH HSugar(deoxyribose)

Nitrogen base

Phosphategroup

CH2

Figure 6.7 Rosalind Franklin,a British crystallographer,used X ray diffraction techniques to discover thedouble helix nature of theDNA molecule.

1'

2'3'

4'

5'

The phosphate group of one nucleotide is bonded to the 3' C of the deoxyribose sugar of an adjacent nucleotide. The phosphate groups and deoxyribose molecules form the backbone of the DNA chain.

The Deoxyribose is a five-carbon sugar (carbohydrate). The carbons are numbered 1' (read “one prime”), 2', 3', 4', and 5' to distinguish from the the atoms of the nitrogen bases rings. The 5' carbon links to the phosphate group

Deoxyribose

BASE

P

Deoxyribose

BASE

P

Deoxyribose

BASE

P

3'

5'

3'

5'

The DNA Structure

1'

2'3'

4'

5'

The nitrogen bases are attached to the deoxyribose molecules, and the purine and pyrimidine bases are opposite one another on the two nucleotide chains.

Deoxyribose

BASE

P

Deoxyribose

BASE

P

Deoxyribose

BASE

P

3'

5'

3'

5'

Deoxyribose

BASE

P

Deoxyribose

BASE

P

Deoxyribose

BASE

P

The DNA Structure

1'

2'3'

4'

5'

The DNA Structure

Deoxyribose

BASE

P

Deoxyribose

BASE

P

Deoxyribose

BASE

P

3'

5'

3'

5'

Deoxyribose

BASE

P

Deoxyribose

BASE

P

Deoxyribose

BASE

P

5'

3'5'

3'

The nitrogen bases are attached to the deoxyribose molecules, and the purine and pyrimidine bases are opposite one another on the two nucleotide chains.

The DNA Structure

A

P

5'

C

P

T

P

G

P

P

P

P

P

A

G

T

C

5'3'

3'

The nitrogen bases are attached to the deoxyribose molecules, and the purine and pyrimidine bases are opposite one another on the two nucleotide chains.

As resulted from the studies of Rosalind Franklin and Maurice Wilkins and proposed by James Watson and Francis Crick in 1953, DNA consists of two long nucleotide chains.

The two nucleotide chains twist around one another to form a double helix, a shape resembling a spiral staircase. Weak chemical bonds (hydrogen bridge) between the chains hold the two chains of nucleotides to one another. Furthermore, the DNA molecule had a constant diameter of 2 nm

The DNA Double Helix Structure