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Chapter 2 PowerPoint PPT Presentation

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Chapter 2 . The Molecular Nature of Genes II How does DNA act as a genetic information carrier?. Page 24-35. 3’. 5’. 5’-3’. OH. 3’. 5’. 5’ end. 3’ end. 5’ TCA 3’. How does DNA serve as a genetic information carrier?. Polynucleotide sequence can carry complex information.

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Chapter 2

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Chapter 2

The Molecular Nature of Genes


How does DNA act as a genetic information carrier?

Page 24-35







5’ end

3’ end

5’ TCA 3’

How does DNA serve as a genetic information carrier?

Polynucleotide sequence can carry complex information

n=2, 16 different possible sequences (4x4)

n=3, 64 different possible sequences (4x4x4)

n=4, 256 different possible sequences (4x4x4x4)

n=5, 1024 different possible sequences (4x4x4x4x4)

n=6, 4096 different possible sequences (4x4x4x4x4x4)


n=y, 4y different possible sequences

Chargaff’s Rules

Erwin Chargaff observed in 1950 that the content of purines was always roughly equal to the content of pyrimidines.

Furthermore, the amounts of adenine and thymine were always roughly equal, as were the amounts of guanine and cytosine.

A=T, C=G, but what does this mean?

X-ray structural information

Rosaline Franklin, 1952

What does this interesting pattern mean?

The DNA is a simple, regular structure with repeated units



The helix must have the same width



From the X-ray diffraction pattern, Crick also deduced that DNA should be a double helix with the phosphate groups on the outside and the bases on the inside.

How do the helies fit together?

How are the nitrogen bases arranged?

In a moment of part insight and part luck, Waston realized that adenine could pair closely with thymine, and that guanine could pair closely with cytosine. Moreover, the A/T base pair was about the same width as a G/C pair.

More importantly, this “base pairing” agreed with and explained Chargaff’s observations: A=T and G=C

Hydrogen bond

Francis Crick agreed with Waston’s hypothesis. He also pointed out that because of certain bond angles and the proximity of the base pairs, the two helices had to run in opposite directions. The helices are antiparallel to one another.

Watson and Crick submitted a 900 word manuscript on their DNA structure prediction to Nature.

In their paper, they wrote: “It has not escaped our notice that the specific base pairing we have proposed immediately suggests a possible copying mechanism for the genetic material.”

This basically serves as a guideline on how DNA can be faithfully replicated or copied into the next generation.

Francis Crick

Maurice Wilkins

Rosalind Franklin

James Waston





Inclination of

base pair from



per turn


Pitch Å


B33.2~10 -1.2º

Z45.6 12 -9º

Although G=C and A=T are true for every organism, the amounts of G+C contents vary from organism to organism

Two DNA strands can be separated simply by heating, a process called DNA denaturation or DNA melting.

The temperature at which the DNA strands are half denatured is called the melting temperature, or Tm.

Tm of a DNA is largely determined by its G/C%.

The amount of DNA strand separation can be measured by the absorbance of the DNA solution at 260 nm light.

The process of reuniting the separated DNA strands is called annealing or renaturation

Temperature -- 25ºC below Tm.

DNA concentration -- the higher concentration, the better the annealing.

Renaturation time -- the longer the time, the better the annealing.


The process of annealing a DNA strand with a complementary or nearly complementary RNA strand or DNA strand from a different origin is called hybridization.

DNAs are of various sizes and shapes

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