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