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Sec. 12.1, DNA: The Genetic Material

Main Idea—The discovery that DNA
is the genetic code involved many
experiments.
 DNA and RNA are nucleic acids.
 DNA stands for:
Deoxyribonucleic Acid
 RNA stands for: Ribonucleic
Acid
Background

In the early 1900’s, scientists did not know
which type of molecule was involved in
inheritance.
 They suspected it was DNA or protein.
Nucleic Acid Structure

Scientists focused on figuring out the structure
of DNA.
 Nucleotides are the subunits (smaller parts)
that make up nucleic acids.
Nucleic Acid Structure
 All
nucleotides are made of 3 parts:
 5-carbon Sugar
 Phosphate Group
 Nitrogen Base
DNA

DNA nucleotides contain:
 Deoxyribose Sugar
 Phosphate Group
 1of
4 Nitrogen Bases
 Adenine
 Guanine
 Cytosine
 Thymine
RNA

RNA nucleotides contain:
 Ribose Sugar
 Phosphate Group
 1 of 4 Nitrogen Bases:
 Adenine
 Guanine
 Cytosine
 Uracil (instead of thymine)
Chargaff’s Rule

The bases can only bond in certain ways:
 A can only bond with T
 G with C
 C with G
 T with A
Chargaff’s Rule


This is called
complementary base
pairing.
When RNA is being made
from DNA, the bonds are
the same, except U bonds
with A. There is no T in
RNA.
DNA Shape

In 1951, Franklin, Watson, and Crick used X-ray
diffraction to show that the shape of DNA is a double
helix.
 This looks like a twisted ladder or spiral staircase.
DNA Shape

DNA is a very long molecule made of 2 strands of
nucleotides running in opposite directions.
 The “sides of the ladder” are alternating sugars and
phosphates.
 The “rungs of the ladder” are the complementary
base pairs.
 The bases are held together by hydrogen bonds.