Download From Gene to Protein

From Gene to Protein
A.P. Biology
Typical Gene
Structure
Regulatory
sites
Promoter
(RNA polymerase
binding site)
Start transcription
DNA strand
Stop transcription
RNA
•Contains ribose instead of deoxyribose
•Single stranded
•Nucleotide uracil replaces thymine
3 Types of RNA
• Messenger RNA = mRNA
-Carries DNA sequence to the
ribosome
• Ribosomal RNA = rRNA
-Component of ribosome
• Transfer RNA = tRNA
-Carries amino acid (anticodon)
to the codon on mRNA
Basic Principles of
Transcription and Translation
• Transcription
– Is the synthesis of RNA under the direction
of DNA
– Produces messenger RNA (mRNA)
• Translation
– Is the actual synthesis of a polypeptide,
which occurs under the direction of mRNA
– Occurs on ribosomes
•In prokaryotes
–Transcription and translation occur together
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
(a) Prokaryotic cell. In a cell lacking a nucleus, mRNA
produced by transcription is immediately translated
without additional processing.
•In eukaryotes
–RNA transcripts are modified before becoming true
mRNA
Nuclear
envelope
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
Ribosome
TRANSLATION
Polypeptide
Figure 17.3b
(b) Eukaryotic cell. The nucleus provides a separate
compartment for transcription. The original RNA
transcript, called pre-mRNA, is processed in various
ways before leaving the nucleus as mRNA.
•During transcription
–The gene determines the sequence of bases along the
length of an mRNA molecule
Gene 2
DNA
molecule
Gene 1
Gene 3
DNA strand 3
5
A C C A A A C C G A G T
(template)
TRANSCRIPTION
mRNA
5
U G G U U U G G C U C A
Codon
TRANSLATION
Protein
Figure 17.4
Trp
Amino acid
Phe
Gly
Ser
3
•In laboratory experiments
–Genes can be transcribed and translated after
being transplanted from one species to another
Molecular Components of
Transcription
• RNA synthesis
– Is catalyzed by RNA polymerase, which pries
the DNA strands apart and hooks together
the RNA nucleotides
– Follows the same base-pairing rules as DNA,
except that in RNA, uracil substitutes for
thymine
Synthesis of an RNA Transcript
• The stages of transcription are
– Initiation
– Elongation
– Termination
Promoter
Transcription unit
5
3
3
5
Start point
RNA polymerase
DNA
Initiation. After RNA polymerase binds to
the promoter, the DNA strands unwind, and
the polymerase initiates RNA synthesis at the
start point on the template strand.
1
5
3
Unwound
DNA
3
5
Template strand of
DNA
transcript
2 Elongation. The polymerase moves downstream, unwinding the
DNA and elongating the RNA transcript 5  3 . In the wake of
transcription, the DNA strands re-form a double helix.
Rewound
RNA
RNA
5
3
3
5
3
5
RNA
transcript
3 Termination. Eventually, the RNA
transcript is released, and the
polymerase detaches from the DNA.
5
3
3
5
5
Figure 17.7
Completed RNA
transcript
3
Non-template
strand of DNA
Elongation
RNA nucleotides
RNA
polymerase
A
T
C
C
A
A
3
3 end
U
5
A
E
G
C
A
T
A
G
G
T
T
Direction of transcription
(“downstream”)
5
Newly made
RNA
Template
strand of DNA
RNA Polymerase Binding and Initiation of Transcription
•Promoters signal the
initiation of RNA synthesis
1 Eukaryotic promoters
TRANSCRIPTION
DNA
RNA PROCESSING
Pre-mRNA
mRNA
Ribosome
TRANSLATION
Polypeptide
Promoter
5
3
3
5
T A T A A AA
AT AT T T
T
TATA box
•Transcription factors
Start point
Several transcription
factors
2
–Help eukaryotic RNA
polymerase recognize
promoter sequences
Template
DNA strand
Transcription
factors
5
3
3
5
3 Additional transcription
factors
RNA polymerase II
Transcription factors
–TATA box
5
3
3
5
5
RNA transcript
Figure 17.8
Transcription initiation complex
Transcription – Making a Copy of the DNA
1. RNA polymerase separates the DNA strands at a
promoter region on the DNA (TATA box)
2. RNA polymerase adds nucleotides in sequence to
mRNA
3. RNA polymerase falls off the DNA at a terminator
sequence on the DNA
Unwinding of the DNA
Helix by RNA Polymerase
Step #1
Transcription of mRNA
Complementary to DNA
Step #2
Alteration of mRNA Ends
•Each end of a pre-mRNA molecule is modified
–The 5 end receives a modified nucleotide cap
–The 3 end gets a poly-A tail
A modified guanine nucleotide
added to the 5 end
TRANSCRIPTION
RNA PROCESSING
50 to 250 adenine nucleotides
added to the 3 end
DNA
Pre-mRNA
5
mRNA
G
Protein-coding segment
Polyadenylation signal
3
P P P
AAUAAA
AAA…AAA
Ribosome
TRANSLATION
5 Cap
Polypeptide
5 UTR
Start codon
Stop codon
3 UTR
Poly-A tail
•RNA splicing
–Removes introns and joins exons
•Ribozymes
–Are catalytic RNA molecules that function as
enzymes and can splice RNA
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
5 Exon Intron
Pre-mRNA 5 Cap
30
31
1
Coding
segment
mRNA
Ribosome
Intron
Exon
Exon
3
Poly-A tail
104
105
146
Introns cut out and
exons spliced together
TRANSLATION
Polypeptide
mRNA 5 Cap
1
3 UTR
Poly-A tail
146
Introns - Intervening sequence = Junk DNA
Exons – Expressed sequence
3 UTR
mRNA Processing in Eukaryotic Cells
Molecular Components of Translation
•A cell translates an mRNA message into protein
–With the help of transfer RNA (tRNA)
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
•Molecules of tRNA are
not all identical
Amino
acids
Polypeptide
–Each carries a
specific amino acid on
one end
Ribosome
tRNA with
amino acid
attached
Gly
–Each has an
anticodon on the other
end
tRNA
Anticodon
A A A
U G G U U U G G C
Codons
5
mRNA
3
•A tRNA molecule
–Consists of a single RNA strand that is only
about 80 nucleotides long
–Is roughly L-shaped
3
A
C
C
A 5
C G
G C
C G
U G
U A
A U
A U
U C
UA
C A C AG
*
G
*
G U G U *
C
C
* *
U C
*
* G AG C
(a) Two-dimensional structure. The four base-paired regions and three
G C
U A
loops are characteristic of all tRNAs, as is the base sequence of the
* G
amino acid attachment site at the 3 end. The anticodon triplet is
A
A*
unique to each tRNA type. (The asterisks mark bases that have been
C
U
*
chemically modified, a characteristic of tRNA.)
A
G
A
Amino acid
attachment site
Figure 17.14a
Anticodon
C U C
G A G
A G *
*
G
A G G
Hydrogen
bonds
5
3
Amino acid
attachment site
Hydrogen
bonds
A AG
3
Anticodon
(b) Three-dimensional structure
5
Anticodon
(c) Symbol used
in this book
•A specific enzyme called an aminoacyl-tRNA
synthetase
–Joins each amino acid to the correct tRNA
•The ribosome
–Is part of the cellular machinery for translation,
polypeptide synthesis
–Facilitates the specific coupling of tRNA
anticodons with mRNA codons during protein
synthesis
•The ribosome has three binding sites for
tRNA
–The P site
P site (Peptidyl-tRNA binding site)
–The A site
–The E site
E site (Exit site)
mRNA binding site
A site (Aminoacyl-tRNA binding site)
Central Dogma in Biology
Gene Mutations:
Substitution, Insertion, & Deletion
Substitution
Insertion
Deletion
Point Mutation
Frameshift Mutation
Base Pair Substitution
• Missense mutation – altered codon codes
for a different amino acid
• Nonsense mutation – altered codon changed
to a stop codon (no amino acid delivered)
Chromosomal Mutations
Deletion
Duplication
Inversion
Translocation