Gene expression occurs in two steps: transcription and translation. In transcription, RNA polymerase copies information from DNA to mRNA. In translation, mRNA directs ribosomes to assemble a polypeptide chain from amino acids based on the genetic code. The central dogma of molecular biology states that DNA is transcribed into RNA which is then translated into protein.

1. Chapter 14
From DNA to Protein: Gene
Expression

2. Gene Expression
Path from a gene to a Gene expression occurs
in two steps:
phenotype
Transcription
“Central Dogma of Copies information from a
Molecular Biology” DNA sequence (a gene)
to a complementary
RNA sequence
DNA RNA Protein Translation
Converts RNA sequence
to amino acid sequence
of a polypeptide

3. RNA
RNA (ribonucleic acid)
differs from DNA:
• Usually one
polynucleotide
strand
• The sugar is ribose
• Contains uracil (U)
instead of thymine
(T)

4. RNA
Bases in RNA can pair with a single
strand of DNA, except that adenine
pairs with uracil instead of thymine.
Single-strand RNA can fold into complex
shapes by internal base pairing.

5. Three Types of RNA
Messenger RNA (mRNA)
Carries copy of a DNA sequence to site
of protein synthesis at the ribosome
Transfer RNA (tRNA)
Carries amino acids for polypeptide
assembly
Ribosomal RNA (rRNA)
Catalyzes peptide bonds and provides
structure

6. DNA RNA Protein

7. Transcription (DNA RNA)
Transcription components:
A DNA template for base pairings (one of
the two strands of DNA)
Nucleotides
(ATP,GTP,CTP,UTP) as substrates
RNA polymerase enzyme

8. RNA Polymerase
• Catalyze the
Synthesis of RNA
• Do NOT need a
primer to start
synthesizing RNA
• Do NOT have
proofreading ability

9. Phases of Transcription
Initiation
Elongation
Termination

10. Initiation
RNA Polymerase binds to the target DNA
at the promoter
Unwinds a ~10bp bubble of the DNA to
initiate transcription
Transcription begins at the initiation site

11. Promoter
Sequence of DNA where RNA
polymerase binds
Critical Regulation Point for the Flow of
Genetic Information in the Cell
Tells RNA polymerase “When, Where,
How Much”

12. Elongation
RNA polymerase unwinds DNA about ten
base pairs at a time; reads template in
3′ to 5′ direction.
The RNA transcript is antiparallel to the
DNA template strand, and adds
nucleotides to its 3′ end.
RNA polymerases do not proofread and
correct mistakes.

13. Elongation

14. Termination
Is specified by a specific DNA base
sequence (termination site).
Mechanisms of termination are complex
and varied.
For some genes the transcript falls away
from the DNA template and RNA
polymerase (others require another
protein to assist)

15. Following Transcription
The completed mRNA leaves the nucleus
through the nuclear pore
Must interact with a ribosome to initiate
translation
Information encoded on the mRNA is the
utilized to guide protein synthesis

16. Genetic Code
Specifies which amino acids will be used to
build a protein
Codon:
A sequence of three bases—each codon
specifies a particular amino acid.
Start codon: AUG
Initiation signal for translation.
Stop codons: UAA, UAG, UGA
Stop translation and polypeptide is released.

17. Genetic Code
20 amino acids
64 different
possible
codons
Codons are
redundant
Genetic Code is
Universal

18. Translation
Needs:
mRNA template
“charged tRNAs”
Ribosome

19. tRNAs
Transfer RNAs act as the adapter
between the mRNA and the growing
polypeptide
Must be “charged” or bound to a specific
amino acid before they can be used in
translation

20. tRNAs
Clover leaf secondary
structure
Two important sites:
Anticodon
Binds to mRNA
Amino Acid Attachment Site
Binds to the respective amino
acid

21. Ribosome
The workbench that holds mRNA and
charged tRNAs in the correct positions
to allow assembly of polypeptide chain.
Ribosomes are not specific, they can
make any type of protein.
Composed of a Large and Small Subunit

22. Large Subunit
Three tRNA binding sites:
A (amino acid) site binds
with anticodon of
charged tRNA
P (polypeptide) site is
where tRNA adds its
amino acid to the
growing chain
E (exit) site is where tRNA
sits before being
released from the
ribosome.

23. Small Subunit
Has a fidelity function:
When proper
binding occurs,
hydrogen bonds
form between the
base pairs.

24. Phases of Translation
• Initiation
• Elongation
• Termination

25. Initiation
Initiation Complex
Forms
1) mRNA
2) Charged tRNA
(bound to
methionine)
3) Small subunit
bound to the mRNA
at the start codon

26. Initiation
The large subunit then
joins the complex;
The charged tRNA is
now in the P site of
the large subunit.

27. Elongation
The second charged tRNA
enters the A site.
Large subunit catalyzes
two reactions:
• It breaks bond between
tRNA in P site and its
amino acid
• Peptide bond forms
between that amino
acid and the amino acid
on tRNA in the A site

28. Elongation
• The large subunit
has peptidyl
transferase activity
• Critical to growing
the polypeptide
chain

29. Elongation
When the first tRNA has
released its methionine,
it moves to the E site
and dissociates from
the ribosome—can then
become charged again.
Elongation occurs as the
steps are repeated,
assisted by proteins
called elongation
factors.

30. Termination
Translation ends when a
stop codon enters the A
site.
Stop codon binds a
protein release factor—
allows hydrolysis of
bond between
polypeptide chain and
tRNA on the P site.
Polypeptide chain
separates from the

31. Post-Translational
Modifications
Proteolysis: Cutting of a long
polypeptide chain into final products,
by proteases
Glycosylation: Addition of sugars to form
glycoproteins
Phosphorylation: Addition of phosphate
groups catalyzed by protein kinases—
charged phosphate groups change the
conformation
Leads to a mature and functional protein