From DNA to Protein
Proteins
 All proteins consist of polypeptide chains
• A linear sequence of amino acids
 Each chain corresponds to the nucleotide base
sequence of a gene
The Path From Genes to Proteins
1. Transcription
• Enzymes use the base sequence of a gene as a
template to make a strand of RNA
2. Translation
• Information in the RNA strand is decoded
(translated) into a sequence of amino acids
Prokaryotes and Eukaryotes
 In prokaryotic cells (no nucleus)
• Transcription and translation occur in cytoplasm
 In eukaryotic cells
• Genes are transcribed in the nucleus
• Resulting mRNA is translated in the cytoplasm
Key Concepts:
INTRODUCTION
 Life depends on enzymes and other proteins
 All proteins consist of polypeptide chains
 Chains are sequences of amino acids that
correspond to sequences of nucleotide bases in
DNA called genes
 The path leading from genes to proteins has two
steps: transcription and translation
Transcription: DNA to RNA
 Two DNA strands unwind in a specific region
 RNA polymerase assembles a strand of RNA
• Covalently bonds RNA nucleotides (adenine,
guanine, cytosine, uracil) according to the
nucleotide sequence of the exposed gene
Three Types of RNA
 Messenger RNA (mRNA)
• Carries protein-building codes from DNA to
ribosomes
 Ribosomal RNA (rRNA)
• Forms ribosomes (where polypeptide chains are
assembled)
 Transfer RNA (tRNA)
• Delivers amino acids to ribosomes
RNA and DNA Compared
RNA Base Pairing
Fig. 13.3, p.198
Fig. 13.3, p.198
Key Concepts:
TRANSCRIPTION
 During transcription, the two strands of the DNA
double helix are unwound in a gene region
 Exposed bases of one strand become the
template for assembling a single strand of RNA
(a transcript)
 Messenger RNA is the only type of RNA that
carries DNA’s protein-building instructions
The Genetic Code
 Messenger RNA (mRNA) carries DNA’s proteinbuilding information to ribosomes for translation
 mRNA’s genetic message is written in codons
• Sets of three nucleotides along mRNA strand
Genetic Code: RNA Triplets
From DNA to Polypeptide
tRNA and rRNA
Function in Translation
 Transfer RNA (tRNA)
• Anticodon binds to mRNA codon
• Also binds amino acid specified by codon
 Different tRNAs carry different amino acids
• tRNAs deliver free amino acids to ribosomes
during protein synthesis
tRNA
rRNA
 Ribosomal RNA (rRNA) and proteins make up
the two subunits of ribosomes
Three Stages of Translation
 mRNA-transcript information directs synthesis of
a polypeptide chain during translation
 Translation proceeds in three stages
• Initiation
• Elongation
• Termination
Initiation
 One initiator tRNA, two ribosomal subunits, and
one mRNA come together as an initiation
complex
Initiation
Elongation
 tRNAs deliver amino acids to the ribosome in the
order specified by mRNA codons
 Ribosomal rRNA catalyzes the formation of a
peptide bond between amino acids
Elongation
An initiator tRNA
carries the amino acid
methionine, so the first
amino acid of the new
polypeptide chain will be
methionine. A second
tRNA binds the second
codon of the mRNA
(here, that
codon is GUG, so the
tRNA that binds carries
the amino acid valine).
The first tRNA
is released and the
ribosome moves to
the next codon in the
mRNA. A third tRNA
binds to the third
codon of the mRNA
(here, that codon is
UUA, so the tRNA
carries the amino
acid leucine).
A peptide bond
forms between the
first two amino acids
(here, methionine
and valine).
A peptide bond
forms between the
second and third
amino acids (here,
valine and leucine).
The second RNA
is released and the
ribosome moves to
the next codon. A
fourth tRNA binds
the fourth mRNA
codon (here, that
codon is GGG, so
the tRNA carries
the amino acid
glycine).
Termination
Steps d and e are repeated over and over
until the ribosome encounters a STOP codon
in the mRNA. The mRNA transcript and the
new polypeptide chain are released from the
ribosome. The two ribosomal subunits
separate from each other. Translation is
now complete. Either the chain will join the
pool of proteins in the cytoplasm or it will
enter rough ER of the endomembrane system
(Section 4.8).
A peptide bond
forms between the
third and fourth
amino acids
(here, leucine
and glycine)
Termination
 Translation ends when RNA polymerase
encounters a STOP codon in mRNA
• New polypeptide chain and mRNA are released
• Ribosome subunits separate from each other
Termination
Key Concepts:
TRANSLATION
 During translation, amino acids become bonded
together into a polypeptide chain in a sequence
specified by base triplets in messenger RNA
 Transfer RNAs deliver amino acids one at a time
to ribosomes
 Ribosomal RNA catalyzes the formation of
peptide bonds between the amino acids
Mutated Genes and
Their Protein Products
 Mutations are permanent, small-scale changes
in the base sequence of a gene
 Common mutations include insertions, deletions,
and base-pair substitutions
Common Gene Mutations
Some Causes of Mutations
 Natural and synthetic chemicals
• Cigarette smoke
 Environmental agents
Assembly of RNA on unwound regions of
DNA molecule
Transcription
mRNA
processing
mRNA
rRNA
tRNA
proteins
mature mRNA
transcripts
Translation
At an intact
ribosome,
synthesis of
a polypeptide
chain at the
binding sites
for mRNA
and tRNAs
ribosomal
subunits
Convergence
of RNAs
mature
tRNA
cytoplasmic
pools of
amino acids,
ribosomal
subunits,
and tRNAs
Final protein
Fig. 13.11, p.206