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From Gene to Protein
A.P. Biology
Regulatory sites
Promoter(RNA polymerase binding site)
Start transcription
DNA strand
Stop transcription
Typical Gene Structure
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
Prokaryotic cell. In a cell lacking a nucleus, mRNAproduced by transcription is immediately translatedwithout additional processing.
(a)
TRANSLATION
TRANSCRIPTION DNA
mRNA
Ribosome
Polypeptide
•In prokaryotes
–Transcription and translation occur together
Figure 17.3b
Eukaryotic cell. The nucleus provides a separatecompartment for transcription. The original RNAtranscript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA.
(b)
TRANSCRIPTION
RNA PROCESSING
TRANSLATION
mRNA
DNA
Pre-mRNA
Polypeptide
Ribosome
Nuclearenvelope
•In eukaryotes
–RNA transcripts are modified before becoming true mRNA
•During transcription
–The gene determines the sequence of bases along the length of an mRNA molecule
Figure 17.4
DNAmolecule
Gene 1
Gene 2
Gene 3
DNA strand(template)
TRANSCRIPTION
mRNA
Protein
TRANSLATION
Amino acid
A C C A A A C C G A G T
U G G U U U G G C U C A
Trp Phe Gly Ser
Codon
3 5
35
•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
Figure 17.7
PromoterTranscription unit
RNA polymerase
Start point
53
35
35
53
53
35
53
35
5
5
Rewound
RNA
RNA
transcript
3
3Completed RNA transcript
Unwound
DNA
RNA
transcript
Template strand of DNA
DNA
1 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.
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.
3 Termination. Eventually, the RNA
transcript is released, and the
polymerase detaches from the DNA.
Elongation
RNApolymerase
Non-templatestrand of DNA
RNA nucleotides
3 end
C A E G C AA
U
T A G G T TA
AC
G
U
AT
CA
T C C A AT
T
GG
3
5
5
Newly madeRNA
Direction of transcription(“downstream”) Template
strand of DNA
RNA Polymerase Binding and Initiation of Transcription
•Promoters signal the initiation of RNA synthesis
•Transcription factors
–Help eukaryotic RNA polymerase recognize promoter sequences
–TATA boxFigure 17.8
TRANSCRIPTION
RNA PROCESSING
TRANSLATION
DNA
Pre-mRNA
mRNA
Ribosome
Polypeptide
T A T A AA A
A T A T T T T
TATA box Start point TemplateDNA strand
53
35
Transcriptionfactors
53
35
Promoter
53
355
RNA polymerase IITranscription factors
RNA transcript
Transcription initiation complex
Eukaryotic promoters1
Several transcriptionfactors
2
Additional transcriptionfactors
3
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
Transcription – Making a Copy of 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 tailA modified guanine nucleotideadded to the 5 end
50 to 250 adenine nucleotidesadded to the 3 end
Protein-coding segment Polyadenylation signal
Poly-A tail3 UTRStop codonStart codon
5 Cap 5 UTR
AAUAAA AAA…AAA
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide
G P P P
53
•RNA splicing
–Removes introns and joins exons
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide
5 CapExon Intron
1
5
30 31
Exon Intron
104 105 146
Exon 3Poly-A tail
Poly-A tail
Introns cut out andexons spliced together
Codingsegment
5 Cap1 146
3 UTR3 UTR
Pre-mRNA
mRNA
Introns - Intervening sequence = Junk DNA
Exons – Expressed sequence
•Ribozymes
–Are catalytic RNA molecules that function as enzymes and can splice RNA
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
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
Polypeptide
Aminoacids
tRNA withamino acidattached
Ribosome
tRNA
Anticodon
mRNA
Trp
Phe Gly
AG
C
A A A
CC
G
U G G U U U G G C
Codons5 3
•Molecules of tRNA are not all identical
–Each carries a specific amino acid on one end
–Each has an anticodon on the other end
•A tRNA molecule
–Consists of a single RNA strand that is only about 80 nucleotides long
–Is roughly L-shaped
Figure 17.14a
Two-dimensional structure. The four base-paired regions and three loops are characteristic of all tRNAs, as is the base sequence of the amino acid attachment site at the 3 end. The anticodon triplet is unique to each tRNA type. (The asterisks mark bases that have been chemically modified, a characteristic of tRNA.)
(a)
3
CCACGCUUAA
GACACCU*
GC
* *G U G U *CU
* G AGGU**A
*A
A GUC
AGACC*
C G A GA G G
G*
*GA
CUC*AUUUAGGCG5
Amino acidattachment site
Hydrogenbonds
Anticodon
A
(b) Three-dimensional structureSymbol used in this book
Amino acidattachment site
Hydrogen bonds
AnticodonAnticodon
A AG
53
3 5
(c)
•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
–The A site
–The E site
P site (Peptidyl-tRNA binding site)
A site (Aminoacyl-tRNA binding site)
E site (Exit site)
mRNA binding site
Central Dogma in Biology
Gene Mutations: Substitution, Insertion, & Deletion
InsertionDeletionSubstitution
Frameshift Mutation
Point 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