The Structure of DNA

The Discovery of DNA

• Since DNA was identified as the hereditary material it was now time to discover its physical and chemical structure

The First Evidence

• Miescher discovered in the 1860s that DNA was slightly acidic and composed of large amounts of phosphorus and nitrogen

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• In the early 1900s Phoebus Levene isolated two types of nucleic acid:

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1) ribose nucleic acid because of five-carbon sugar called ribose

• Now known as ribonucleic acid (RNA)

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2) deoxyribose nucleic acid because of five-carbon sugar with one less oxygen

-Now known as deoxyribonucleic acid (DNA)

• In the 1920s, Phoebus Leven determined that DNA and RNA are composed of three main parts:

1) Sugar Molecule

2) Phosphate Group (negative charge)

3) Nitrogenous Bases

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• There are five nitrogenous bases:

Purines

• Adenine (found in DNA and RNA)
• Guanine (found in DNA and RNA)

Pyrimidines

• Cytosine (found in DNA and RNA)
• Thymine (found in ONLY DNA)
• Uracil (found in ONLY RNA)

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• Levene also proposed that nucleic acids are made of long chains of nucleotids

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The Nitrogenous Bases

Complementary Bases in DNA

• Erwin Chargaff (1949) analyzed DNA from many different organisms to determine the chemistry of DNA
• He found that there was variation in composition of nucleotides among different species
• Found that the total amount of purine equals pyrimidines in an organism
• Found that the proportion of Adenine equals Thymine and Guanine equals Cytosine
• This is known as Chargaff’s Rule

Complementary Bases in DNA

Structure of Nucleotides in DNA

• Chains of nucleotides are held together by phosphodiester bonds making DNA a polymer
• Each nucleotide includes a
• Deoxyribose sugar (five carbon cyclic ring)
• Phosphate Group
• Nitrogenouse Base (Adenine, Thymine, Guanine, and Cytosine)

Structure of Nucleotides in DNA

• Deoxyribose sugar structure
• 4 carbon and 1 oxygen molecule make the ring
• Fifth carbon extends out
• Numbered in a clockwise fashion

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• 1’ carbon is where the nitrogenous base attaches by a glycosyl bond (bond between a sugar and other organic molecule by an intervening nitrogen or oxygen atom)
• 2’ carbon has a Hydrogen atom
• 3’ carbon has a Hydroxyl group
• 4’ is where the fifth carbon is attached
• 5’ is where the phosphate group is attached by an ester bond

Next Step, A Model of DNA

• In 1953, scientists had two clues
• The chemical composition of DNA
• The proportions of Nitrogenous Bases

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• The next step was to determine the structure of DNA

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• In 1951, Linus Pauling discovered that proteins have helical structures (which was later noted to suggest helical shape for DNA by Crick)

Model of DNA

• Rosalind Franklin and Maurice Wilkins (1953) were using X-ray diffraction to determine the structure of DNA

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• Molecules are bombarded with a beam of X-rays that are reflected off atoms producing an image that is used to determine the three dimensional structure

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• Wilkins presented Watson and Crick with an informal look at Rosalind’s image

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• This image showed that DNA had a helical structure, 2 nm diameter and turns every 3.4 nm

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• She also found that DNA reacted with water and concluded that nitrogenous bases were located on the inside and sugar-phosphate backbone on the outside facing the watery nucleus of the cell

Model of DNA

• Using this information Watson and Crick built a model of DNA in 1953

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• In 1962 James Watson, Francis Crick and Maurice Wilkins were awarded the Nobel Prize in Physiology or Medicine

DNA: The Double Helix

• Watson and Crick proposed that DNA consists of two antiparallel strands of nucleotides

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• The bases are paired with each other, one on each strand ( Purine is always paired with Pyrimidine)

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• Complementary Base Pairing because Adenine is always paired with Thymine and Cytosine is always paired with Guanine
• Consistent with Chargaff and Franklin’s and Wilkins’ diameter of 2 nm

DNA: The Double Helix

• The bases are bonded together by hydrogen bonds
• Individual hydrogen bonds are very weak but when there are multiple they are strong which explains why DNA is so stable

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• Thymine (pyrimidine) could not pair with Guanine (Purine) because of the hydrogen bonds
• 2 bonds between Adenine and Thymine
• 3 bonds between Guanine and Cytosine

DNA: The Double Helix

• DNA turns in a clockwise direction (right-handed helix) which makes one turn every ten nucleotides
• Happens every 3.4 nm meaning each base is 0.34 nm apart

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• Deoxyribose sugar and phosphate group are on the perimeter of the molecule
• Acts as structural support

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• One strand runs in the 5’ to 3’ direction while the other runs in the 3’ to 5’ direction
• 3’ end has a hydroxyl group of the deoxyribose sugar
• 5’ end has a phosphate group

DNA: The Double Helix

Summary

• Two polynucleotide strands twist around each other to form a double helix
• Backbone has an alternating phosphate groups and sugar with the base attached to each sugar protruding inward at 0.34 nm apart

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• Two strands are complementary with Adenine paired with Thymine and Guanine paired with Cytosine, this is known as Complementary Base Pairing

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• Hydrogen bonds link each base pair with A & T having 2 bonds and C & G having three bonds

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• Two strands are antiparallel
• 5’ end lies across from 3’ end of the complementary strand
• 5’ end has phosphate group on the 5’ carbon and 3’ end has an OH group on 3’ carbon

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• The sequence is always written in the 5’ to 3’ direction