Biology 12

THINKING LAB

DNA Deductions

Background

Erwin Chargaff discovered that although the nucleotide
composition of DNA varies from one species to another,
that composition always follows certain rules. While the
variation in nucleotides helps to explain the complexity of
life, the physical structure of the DNA itself can also help
certain organisms adapt to particular environments.

Imagine you are working with a research team sampling the
ocean floor near a hot vent that releases a steady stream of
hot water. The hot water has a temperature of about 45°C,
while the surrounding ocean has a temperature of 6°C.

Your team collects two samples of bacteria — one from the

mouth of the hot vent, and one from the ocean floor about

20 m away. When you return to the lab, you isolate the

DNA from these bacteria to determine their nucleotide

composition. The table shows the results of your test for

the adenine content of the DNA.

You Try It

Apply what you have learned about Chargaff’s findings and

DNA composition to solve the following problems.

1.Complete the table to determine the amounts of the

other nucleotides found in each DNA sample.

2.For each DNA sample, draw a linear stretch of DNA

about 15 nucleotides long, with a nucleotide

composition that corresponds to its data set. With a

dotted line, illustrate the hydrogen bonds between

complementary base pairs.

3.Considering the bonds between base pairs, which

of these DNA samples is most likely taken from the

bacteria collected at the mouth of the hot vent?

Explain your answer.

Nucleotide

Presence in DNA of

bacterial sample 1

(percent)

Presence in DNA of

bacterial sample 2

(percent)

adenine

cytosine

guanine

thymine

31 18

Chapter 7 Nucleic Acids: The Molecular Basis of Life • MHR 227

A

A

A

G

T

T

G C

T

C

P

S

P

S

P

S

P

S

P

S

P

S

P

S

P

S

P

S

S

5 ′ end

3 ′ end

3 ′ end

5 ′ end

hydrogen bond

Strand A Strand B
Ladder structure

P

P

P

T

S A S

deoxyribose

base

4 ′

4 ′

5 ′ end

1 ′

1 ′

1 ′

2 ′

2 ′

2 ′

3 ′

3 ′

3 ′ end

phosphate

One pair of bases

5 ′

5 ′

4 ′

O

C

C C

C

C

3 ′

5 ′

carbon numbering system

Figure 7.14The two strands of DNA have complementary base sequences.

Strand A has the nucleotide sequence TGTCA. How would the sequence of

strand B be written, according to convention?