Describe the clues that helped scientists determine the structure of DNA.
Explain what the double-helix model shows about DNA.
Nucleotide Structure
Composition of DNA: DNA is composed of nucleotides that are joined into long strands or chains by covalent bonds.
Building Blocks of Nucleic Acids: Nucleic acids, such as DNA, are made up of smaller units called nucleotides.
Components of a Nucleotide:
Base: A nitrogenous base that can be one of the four types—adenine, guanine, cytosine, or thymine.
5-Carbon Sugar: A ribose or deoxyribose sugar, depending on the type of nucleic acid.
Phosphate Group: A phosphate group that connects to the sugar of the nucleotide.
Nitrogenous Bases
The four nitrogenous bases found in DNA are:
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
Structural Differences:
The structure of adenine and guanine (purines) differs from that of cytosine and thymine (pyrimidines).
Nucleic Acid Structure
In DNA, a covalent bond links nucleotides together to form a long chain.
Connection of Nucleotides: The sugar of one nucleotide attaches to the phosphate group of the next nucleotide in the chain.
Chargaff’s Rule
Discovery by Erwin Chargaff: He established that the percentages of adenine [A] and thymine [T] are almost equal in any sample of DNA, as are the percentages of cytosine [C] and guanine [G].
Mathematical Representation: This can be expressed as:
[A] = [T]
[C] = [G]
Franklin’s X-rays
Key Findings:
DNA exhibits a helical structure.
The molecule likely consists of two strands.
The nitrogenous bases are located near the center of the molecule.
Research Timeline: These experiments were conducted in the 1950s.
The Work of Watson and Crick
Structure Overview: DNA is described as a double helix, where two strands of nucleotide sequences are wound around each other.
Modeling: James Watson and Francis Crick constructed three-dimensional models of DNA using materials such as cardboard and wire to illustrate the helical structure.
The Double Helix: Antiparallel Strands
The two strands of a DNA molecule run in opposite directions, a feature known as antiparallel orientation. This means that one strand runs in a 5' to 3' direction, while the complementary strand runs in a 3' to 5' direction.
The Double Helix: Hydrogen Bonding
Bonding Mechanism:
The nucleotides within a strand of DNA are connected by strong covalent bonds.
The nucleotides between the two strands are held together by weaker hydrogen bonds.
The Double Helix: Base Pairing
Pairing Rules: The two strands of DNA are held together through hydrogen bonds between nitrogenous bases. The specific pairings are as follows:
Adenine (A) always pairs with Thymine (T) through two hydrogen bonds.
Guanine (G) always pairs with Cytosine (C) through three hydrogen bonds.