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In 1953, James Watson and Francis Crick proposed the double helix as a

model for the structure of DNA (Fig. 2 A and B). Their discovery was based, in

part, on X-ray diffraction analysis performed by Rosalind Franklin in Maurice

Wilkins’ laboratory (Fig. 3).

The story is a fascinating one of a competitive race to the finish. Franklin’s

photographs are still considered to be some of the most beautiful X-ray

photographs ever taken of any molecule. Other scientific findings at the time

provided the necessary context for the proposed structure of DNA. These included

findings that will be discussed in more detail in later lectures:

1) the proposal by Linus Pauling in 1951 that proteins can adopt a coiled, helical

secondary structure.

2) the disproving of Levene’s tetranucleotide hypothesis by Erwin Chargaff by

showing that the four bases of DNA were not in a 1: 1: 1: 1 ratio. Figure 4

shows the essential features of the double helix model.

The ribbons represent the sugar–phosphate backbone chains of the two

strands of the DNA molecule, each following a right-handed helical path. The

horizontal bars represent hydrogen-bonded base pairs, each of which lies in a

plane perpendicular to the vertical axis. The four different kinds of DNA bases—

adenine, cytosine, guanine, and thymine—are represented here by four different

colors. Thus the double helix marked the starting point for fundamental

molecular biology.

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In his studies of the DNA molecule in 1951, Erwin Chargaff provided two

important empirical rules regarding the nitrogenous base content of most DNA

molecules. First, he noted that the total amount of pyrimidine nucleotides

(thymine T and cytosine C) was always equal to the total amount of purine

nucleotides (adenine A and guanine G). In other words, the amount of T+C is

always equal to the amount of A+G in a double-stranded DNA molecule. Second,

the amount of A always equals the amount of T and the amount of G always equals

the amount of C. ( A = T ), ( G = C ) and (A + G = T + C).

These facts must be taken into account in any structure for duplex DNA and

were quite important to Watson and Crick as they worked to establish the three-

dimensional structure of the DNA molecule base pair model, although Chargaff

himself did not realize the significance of his findings.

The two strands of a DNA molecule are complementary in sequence; that is,

the base A always base pairs with T, and C with G. In the DNA double helix,

the two strands are antiparallel, which means they run in opposite directions.

One Strand runs in the 5’ to 3’ direction (’ is pronounced “primer”) and the

second strand in the 3’ to 5’ direction. This nomenclature refers to carbon

atoms in the pentose sugar, numbers on to five (Fig.4 and 5).

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Figure 2. The DNA double helix. (A) James Watson (left) and Francis Crick are seen with their model

of part of a DNA molecule in 1953. (B) Watson and Crick’s structure suggested that an original DNA

molecule could be duplicated by copying each strand with the same proposed base-pairing scheme.

Figure 3. X-ray diffraction photograph of DNA. This image of the DNA double helix was obtained by Rosalind Franklin in

1953, the year in which Watson and Crick discovered DNA’s structure, aided by Franklin’s work. The image results from a

beam of X-rays being scattered onto a photographic plate by the crystalline DNA. Features about the structure of the DNA

can be determined from the pattern of spots and bands. The regularity of the pattern and the cross of bands indicate the

helical nature of DNA. The spacing between the bands at the top and bottom of the X-shape gave the spacing between

elements (base pairs) of the helix as 3.4 Å. The spacing between neighboring bands in the pattern gave the overall length of

one helical turn as 34 Å.

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Genetic Material Possesses Several Key Characteristics

Life is characterized by tremendous diversity, but the coding instructions for all

living organisms are written in the same genetic language—that of nucleic acids.

Even before nucleic acids were identified as the genetic material,

biologists recognized that whatever the nature of the genetic material, it must

possess four important characteristics:

1) The genetic material must contain complex information. First and

foremost, the genetic material must be capable of storing large amounts of

information—instructions for the traits and functions of an organism.

2) The genetic material must replicate faithfully. Every organism begins life

as a single cell. To produce a complex multicellular organism like yourself,

this single cell must undergo billions of cell divisions. At each cell division,

the genetic instructions must be accurately transmitted to descendant cells.

Figure 4: The DNA double helix with a sugar-phosphate backbone held in a double helix by pairing of the bases. The positions of the 5-carbons on the first two deoxyribose sugars are shown.

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And when organisms reproduce and pass genes to their progeny, the genetic

instructions must be copied with fidelity.

3) The genetic material must encode the phenotype. The genetic material (the

genotype) must have the capacity to be expressed as a phenotype—to code for

traits. The product of a gene is often a protein or an RNA molecule, so there

must be a mechanism for genetic instructions in the DNA to be copied into

RNAs and proteins.

4) The genetic material must have the capacity to vary. Genetic information

must have the ability to vary, because different species—and even individual

members of the same species—differ in their genetic makeup.

The genetic material must carry large amounts of information, replicate

faithfully, express its coding instructions as phenotypes, and have the capacity to

vary)).

Summary:

All Genetic Information Is Encoded in the Structure of DNA or RNA

Fig.: Many people have contributed to our understanding of the structure of DNA.

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The hereditary basis of every living organism is its genome, a long sequence of

DNA that provides the complete set of genetic information carried by the organism. The

genome includes chromosomal DNA as well as DNA in plasmids and (in eukaryotes)

organellar DNA as found in mitochondria and chloroplasts. Physically, the genome

may be divided into a number of different DNA molecules, or chromosomes. The

ultimate definition of a genome is the sequence of the DNA of each chromosome.

Functionally, the genome is divided into genes. Each gene is a sequence of DNA that

codes for at least one RNA or polypeptide as its final product.

This picture shows how DNA bases are stacked together along the

double helix. The atoms of the sugar–phosphate backbone are shown in

gray, and the ultraviolet-absorbing portions of DNA—the bases—are

shown in blue or magenta, depending on which side of the DNA strand

they are located.

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Some Terms of Molecular Biological definitions:

Genomics: The study of the structure and function of the genome.

Proteomics: The study of the proteome, i.e. the full complement of

proteins made by a cell. The term includes protein–protein and

protein–small molecule interactions as well as expression profiling.

Transcriptomics: The study of the transcriptome, i.e. all the RNA

molecules made by a cell, tissue or organism.

Metabolomics: The use of genome sequence analysis to determine

the capability of a cell, tissue or organism to synthesize small

molecules.

Bioinformatics: The branch of biology that deals with in silico

processing and analysis of DNA, RNA and protein sequence data,

i.e. The use of computer methods in studies of genomes.