From Gene to
The Bread Mold
• Two scientists named Beadle and Tatum
exposed bread mold to X-rays, creating
mutants that were unable to survive.
o The mold was not able to produce certain molecules it
needed to live.
o A specific enzyme that produced these molecules was
missing in the mold’s cells.
• Their hypothesis: “one-gene, one-enzyme”.
o This means that each gene in the mold’s DNA codes for a
different enzyme needed for life.
One Gene One Protein
• However, some proteins aren’t enzymes.
o Researchers later revised the hypothesis: one
• However, many proteins are composed of
several polypeptides, each of which has its
• Therefore, Beadle and Tatum’s hypothesis is
now restated as one gene–one polypeptide
• Transcription is the production of a molecule
of RNA based on a segment of DNA in the
o RNA is needed because it can leave the nucleus,
while DNA cannot.
o This molecule of RNA is called messenger RNA
• The overall cellular chain of command is this:
DNA RNA protein
The Genetic Code
• Proteins are made of amino acids.
• There are 20 amino acids, but there are only
four nucleotide bases in DNA
• This is possible because the four DNA bases (A,
T, C, G) can be arranged in total of 64
Codons: Triplets of Bases
• The flow of information from gene to protein is
based on a triplet code: a series of three-
• These triplets are the smallest units of uniform
length that can code for all the amino acids
o Example: The triplet A-G-T on a DNA strand
encodes for the amino acid “serine” to be added
to the polypeptide chain being formed.
DNA strand 3 5
mRNA 5 3
The DNA Code
• All 64 codons were deciphered by the mid-
• Each codon only translates into one type of
• Each amino acid has more than one codon
that may produce it.
• Codons must be read in the correct sequence
in order for the specified polypeptide to be
Second mRNA base
Third mRNA base (3 end)
Evolution of the Genetic
• The existence of DNA is evidence of the
theory of evolution.
• The genetic code is nearly universal, shared
by the simplest bacteria to the most complex
• Genes can be transcribed and translated
after being transplanted from one species to
• Translation is the synthesis of a polypeptide,
which occurs under the direction of mRNA
• Ribosomes are the sites of translation
• A cell translates an mRNA message into
protein with the help of transfer RNA (tRNA)
• Molecules of tRNA are not identical:
o Each carries a specific amino acid on one end
o Each has an anticodon on the other end; the
anticodon base-pairs with a complementary
codon on mRNA
5 Codons 3
• Ribosomes facilitate specific coupling of tRNA
anticodons with mRNA codons in protein
• Ribosomes are made of proteins and
ribosomal RNA (rRNA)
• Mutations are changes in the genetic material
of a cell or virus
• Point mutations are chemical changes in just
one base pair of a gene
• The change of a single nucleotide in a DNA
template strand leads to production of an
o Example: “The cat ate the rat” becomes “The cat are the rat.”
• A famous example is Sickle-cell anemia, which
is caused by a mutation of a single base pair
within a gene.
Wild-type hemoglobin DNA Mutant hemoglobin DNA
3 5 3 5
5 3 5 3
Normal hemoglobin Sickle-cell hemoglobin
• A base-pair substitution replaces one
nucleotide and its partner with another pair of
• Missense mutations still code for an amino
acid, but not necessarily the right amino acid
• Nonsense mutations change an amino acid
codon into a stop codon, nearly always
leading to a nonfunctional protein
• Missense mutations are more common
Amino end Carboxyl end
No effect on amino acid sequence
U instead of C
A instead of G
U instead of A
Insertions and Deletions
• Insertions and deletions are additions or losses
of nucleotide pairs in a gene
• These mutations have a disastrous effect on
the resulting protein, because the entire
sequence of amino acids is altered.
• The result is called a frameshift mutation,
because it shifts the entire way that the
ribosome translates the mRNA.
Example: “The cat ate the rat.” becomes
“The cta tet her at.” by deleting an “a” or
“The caa tat eth era t.” by inserting an “a”
mRNA 5 3
Amino end Carboxyl end
Base-pair insertion or deletion
Frameshift causing immediate nonsense
extensive missense Missing
Insertion or deletion of 3 nucleotides:
no frameshift but extra or missing amino acid
• Spontaneous mutations can occur during
DNA replication, recombination, or repair.
• Mutagens are physical or chemical agents
that can increase the likeliness of mutations.
o Ultraviolet radiation
o Nuclear radiation