Chapter 12.1
Molecular Genetics
Do you like to read mystery novels or watch people
on television solve crimes? Detectives search for
clues that will help them solve the mystery.
Geneticists are detectives looking for clues in the
mystery of inheritance.
The Building Blocks of DNA
The heritable genetic
information of an organism is
stored in the molecule called
DEOXYRIBONUCLEIC ACID
(DNA).
The Building Blocks of DNA continued..
 Nucleotides-are the building blocks (the
monomers) of nucleic acid polymers.
 Only 4 types of nucleotides make up DNA.
 Notice that each nucleotide has 3 parts.
 1. A ring-shaped sugar called
DEOXYRIBOSE
 2. A phosphate group
 3. A nitrogenous base
The Building Blocks of DNA
continued….Pasting Activity…
Nitrogenous Bases
1. Thymine ( T )
2. Cytosine ( C )
3. Adenine ( A )
4. Guanine ( G )
DNA Strands
 Nucleotides are joined to one another
by covalent bonds that connect the
sugar of one nucleotide to the
phosphate group of the next.
 This repeating pattern of sugarphosphate-sugar phosphate is called a
sugar-phosphate “backbone”.
DNA’s Structure
 The Double Helix-DNA
forms a twisting shape
Watson and Crick
Complementary Base Pairs
 Watson and Crick realized that the
individual structures of the nitrogenous
bases determine very specific pairings
between the nucleotides of the two strands
of the double helix.
 A pairs with T
 G pairs with C
Why do almost all organisms have
the same genetic code?
 The genetic code is the set of rules by which a gene is
translated into a functional protein.
 Each gene consists of a specific sequence of nucleotides encoded
in a DNA strand; a correspondence between nucleotides, the
basic building blocks of genetic material, and amino acids, the
basic building blocks of proteins, must be established for genes
to be successfully translated into functional proteins.
 The correspondence between codons (3 base code of
DNA) and amino acids is nearly universal among all known
living organisms.
How does the universal genetic code
support the theory of evolution?
 Genes are carried throughout time.
 Natural Selection reuses genes and structures that have been
used in the past.
Complementary Base Pairs continued..
 Example problem:
 If six bases on one strand of a DNA
double helix are AGTCGG, what are
the six bases on the complementary
section of the other strand of DNA?
(when completed call me over to
check it)