The Structure of DNA

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The Structure of DNA
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What Does DNA Stand For?

• DNA ? Deoxyribonucleic Acid
• Deoxyribo refers to the sugar found in DNA
• DNA is a nucleic acid- which is one of the four
  major organic compounds we studied earlier in the
  year.
• Nucleic acids are composed of smaller monomers
  put together called nucleotides.

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DNA

• DNA is often called the blueprint of life.
• In simple terms, DNA contains the instructions
  for making proteins within the cell.

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Why do we study DNA?

• We study DNA for many reasons, e.g.,
• its central importance to all life on Earth,
• medical benefits such as cures for diseases,
• better food crops.

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Chromosomes and DNA

• Our genes are on our chromosomes.
• Genes are instructions to build proteins.
• We all have the genes but each of our genes say
  different things.
• Thats what makes everyone unique!

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Discovery of DNA

• First isolated by Swiss scientists Friedrich
  Miescher (1869) from pus from used bandages.
• The function of DNA was unknown but later
  experiments proved that DNA was the genetic
  material.
• The next question to answer was to determine the
  structure of DNA.

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DNA Structure Discovered

• James Watson and Francis Crick solved the mystery
  of DNA by combining information from Maurice
  Wilkins and X-ray diffraction from Rosalind
  Franklin.
• Watson, Crick and Wilkins earned the noble prize
  in 1962.

• Watson and Crick 1953

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The Shape of the Molecule

• DNA is a very long polymer(many nucleotides).
• The basic shape is like a twisted ladder or
  zipper.
• This is called a double helix.

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The Double Helix Molecule

• The DNA double helix has two strands twisted
  together.
• (In the rest of this unit we will look at the
  structure of one strand.)

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One Strand of DNA

• The backbone of the molecule is alternating
  phosphate and deoxyribose, a sugar, parts.
• The teeth are nitrogenous bases.

phosphate
deoxyribose
bases
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Nucleotides
ATP

• One deoxyribose together with its phosphate and
  base make a nucleotide.

Nitrogenous base
O
Phosphate
C
C
C
Deoxyribose
O
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One Strand of DNA
nucleotide

• One strand of DNA is a polymer of nucleotides.
• One strand of DNA has many millions of
  nucleotides.

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Four nitrogenous bases
DNA has four different bases

• Cytosine C
• Thymine T
• Adenine A
• Guanine G

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Two Kinds of Bases in DNA
N

• Pyrimidines are single ring bases.
• Purines are double ring bases.

C
N
C
C
O
C
N
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Thymine and Cytosine are pyrimidines

• Thymine and cytosine each have one ring of carbon
  and nitrogen atoms.

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Adenine and Guanine are purines

• Adenine and guanine each have two rings of carbon
  and nitrogen atoms.

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Two Stranded DNA

• Remember, DNA has two strands that fit together
  something like a zipper.
• The teeth are the nitrogenous bases but why do
  they stick together?

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Hydrogen Bonds

• The bases attract each other because of hydrogen
  bonds.
• Hydrogen bonds are weak but there are millions
  and millions of them in a single molecule of
  DNA.
• (The bonds between cytosine and guanine are shown
  here.)

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Hydrogen Bonds, cont.

• When making hydrogen bonds, cytosine always pairs
  up with guanine,
• And adenine always pairs up with thymine.
• (Adenine and thymine are shown here.)

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Important

• Adenine and Thymine always join together
• A T
• Cytosine and Guanine always join together
• C G

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Chargaffs Rule

• The bases form the rungs on the DNA ladder by
  complementary pairing
• A T
• C G
• T A
• G C
• A T
• T A

Sugar-phosphate backbone
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DNA by the numbers

• Each cell has about 2 m of DNA.
• The average human has 75 trillion cells.
• The average human has enough DNA to go from the
  earth to the sun more than 400 times.
• DNA has a diameter of only 0.000000002 m.

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How do we contain all that DNA in each cell?

• In order for each cell to store all that DNA, it
  must be packed into structures called
  chromosomes.
• DNA gets wrapped around protein like string on a
  spool.
• Those spools of DNA will continue to condense to
  form chromosomes.

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