End Show Slide 1 of 21 Copyright Pearson Prentice Hall 12-2 Chromosomes and DNA Replication 12–2 The Structure of DNA
End Show Slide 1 of 21 Copyright Pearson Prentice Hall 12-2 Chromosomes and DNA Replication 12–2 The Structure of DNA.
Dec 16, 2015
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12-2 Chromosomes and DNA Replication
12–2 The Structure of DNA
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Lesson OverviewLesson Overview The Structure of DNAThe Structure of DNA
THINK ABOUT IT
The DNA molecule must somehow specify how to assemble enzymes, which are proteins needed to regulate the various functions of each cell.
What kind of structure could serve this purpose without varying from cell to cell?
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Nucleic Acids and Nucleotides
Recall (from Ch. 2):•Nucleic acids are macromolecules, containing H, O, N, C, and P, originally found in cell nuclei•There are 2 kinds of nucleic acids – DNA and RNA.•Nucleic acids are polymers. Each molecule is made of repeating units called monomers.
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The Components and Structure of DNA
•The monomer that makes up a nucleic acid molecule is called a nucleotide. Nucleotides are linked together to form long chains.
•In DNA, the nucleotide is made up of 3 parts:•a five-carbon sugar called deoxyribose. •a phosphate group (contains phosphorus and oxygen atoms)•a nitrogenous base (a base that contains the element nitrogen).
Nucleic Acids and Nucleotides
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nitrogen base
One
nucleotide
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The nucleotides in a strand of DNA are joined by covalent bonds formed between their sugar and phosphate groups.
DNA is a nucleic acid made up of nucleotides joined into long strands or chains by covalent bonds.
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The Components and Structure of DNA
There are four kinds of bases in in DNA:adenine (A)guanine (G) cytosine (C)Thymine (T)
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Adenine and guanine have 2 rings in their structure. They belong to a group of compounds known as purines.
Cytosine and thymine are known as pyrimidines. They have a one ring structure.
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The Components and Structure of DNA
The backbone of a DNA chain is formed by the sugar and phosphate groups of each nucleotide. The bases stick out sideways from the backbone.
The nucleotides can be joined together in any order. That means, any sequence of nitrogen bases is possible.
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The Components and Structure of DNA
Chargaff's Rules
Erwin Chargaff discovered that:•The percentages (or amounts) of guanine [G] and cytosine [C] bases are almost equal in any sample of DNA. •The percentages (or amounts) of adenine [A] and thymine [T] bases are almost equal in any sample of DNA.•[A] = [T] and [G] = [C] became known as Chargaff's Rule.
Chargaff's Rules.
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The Components and Structure of DNA
X-Ray Evidence Rosalind Franklin used X-ray diffraction to get information about the structure of DNA.
She aimed an X-ray beam at concentrated DNA samples and recorded the scattering pattern of the X-rays on film.
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Franklin’s X-ray pattern did not reveal the structure of DNA. It did carry some important clues, however.
The X-shaped pattern seen indicated that strands of DNA were twisted around one another in a shape known as a helix.
The X-rays suggested 2 strands were in the structure and the nitrogen bases were in the middle of the molecule.
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The Components and Structure of DNA
The Double Helix
The clues in Franklin’s X-ray pattern enabled Watson and Crick to build a model that explained the specific structure and properties of DNA.
Using clues from Franklin’s pattern, James Watson and Francis Crick built a 3-D model that explained how DNA carried information and could be copied.
Watson and Crick's model of DNA was a double helix, in which two strands were wound around each other.
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The Components and Structure of DNA
DNA Double Helix
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The Components and Structure of DNA
Watson and Crick discovered that hydrogen bonds form between bases of the two strands to hold the DNA molecule together. These bonds could form only between certain base pairs however—adenine to thymine (or thymine to adenine), and guanine to cytosine (or cytosine to guanine).
This principle is called base pairing. It explains Chargaff’s rules. For every thymine there had to be exactly one adenine and for every cytosine, there had to be exactly one guanine.
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Antiparallel Strands
In the double-helix model, the two strands of DNA are “antiparallel”—they run in opposite directions.
This arrangement enables the nitrogenous bases on both strands to come into contact at the center of the molecule.
It also allows each strand of the double helix to carry a sequence of nucleotides, arranged almost like letters in a four-letter alphabet.
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Hydrogen bonds
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