Gene Expression: From Gene to Protein Chapter 17 How does a single faulty gene result in the dramatic appearance of an albino animal? The albino deer has.

Gene Expression: From Gene to Protein

Chapter 17

How does a single faulty gene result in the dramatic appearance of an albino animal?The albino deer has a faulty version of a key protein, an enzyme required for pigment synthesis, and this protein is faulty because the gene that codes for it contains incorrect information.

Concept 17.1: Genes specify proteins via transcription and translation The information content of DNA is in the form

of specific sequences of nucleotides. The DNA inherited by an organism leads to

specific traits by dictating the synthesis of proteins and RNA molecules involved in protein synthesis.

Proteins are the links between genotype (what the genome says) and phenotype (what traits physically appear).

Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation.

Evidence from the Study of Metabolic Defects In 1902, British physician Archibald Garrod

first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions.

He thought symptoms of an inherited disease reflect an inability to synthesize a certain enzyme. “Inborn errors of metabolism.”

Linking genes to enzymes required understanding that cells synthesize and degrade molecules in a series of steps, a metabolic pathway.

Nutritional Mutants in Neurospora: Scientific Inquiry George Beadle and Edward Tatum exposed

bread mold to X-rays, creating mutants that were unable to survive on minimal media.

Using crosses, they and their coworkers identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine.

They developed a one gene–one enzyme hypothesis, which states that each gene dictates production of a specific enzyme.

The Products of Gene Expression: A Developing Story Some proteins aren’t enzymes, so researchers

later revised the hypothesis: one gene–one protein.

Many proteins are composed of several polypeptides, each of which has its own gene.

Therefore, Beadle and Tatum’s hypothesis is now restated as the one gene–one polypeptide hypothesis.

Note that it is common to refer to gene products as proteins rather than polypeptides.

Basic Principles of Transcription and Translation RNA is the bridge between genes and the

proteins for which they code. Transcription is the synthesis of RNA using

information in DNA. Transcription produces messenger RNA

(mRNA). Translation is the synthesis of a polypeptide,

using information in the mRNA. Ribosomes are the sites of translation.

Prokaryotes Eukaryotes Transcription and

translation occur in the cytoplasm.

In prokaryotes, translation of mRNA can begin before transcription has finished.

mRNA is not modified.

Transcription occurs in the nucleus.

Transcription directly produces pre-mRNA molecules.

Pre-mRNA transcripts are modified (before leaving nucleus) through RNA processing to yield the finished mRNA.

Translation occurs in the cytoplasm.

Figure 17.UN01 Central Dogma First dubbed by Francis Crick in 1956. Some exceptions to this rule have emerged

over the years (some enzymes produce DNA from RNA), but they have not invalidated this idea.

DNA RNA Protein

Figure 17.3

DNA

mRNARibosome

Polypeptide

TRANSCRIPTION

TRANSLATION

TRANSCRIPTION

TRANSLATION

Polypeptide

Ribosome

DNA

mRNA

Pre-mRNARNA PROCESSING

(a) Bacterial cell (b) Eukaryotic cell

Nuclearenvelope

Codons: Triplets of Nucleotides

The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words. This explains how 4 nucleotides can be translated

into 20 amino acids. The words of a gene are transcribed into

complementary nonoverlapping three-nucleotide words of mRNA.

These words are then translated into a chain of amino acids, forming a polypeptide.

Figure 17.4

DNAtemplatestrand

TRANSCRIPTION

mRNA

TRANSLATION

Protein

Amino acid

Codon

Trp Phe Gly

5

5

Ser

U U U U U3

3

53

G

G

G G C C

T

C

A

A

AAAAA

T T T T

T

G

G G G

C C C G GDNAmolecule

Gene 1

Gene 2

Gene 3

C C

During transcription, one of the two DNA strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript. The template strand is always the same strand for

a given gene. For other genes on the same DNA molecule, the

opposite strand may be the one that functions as the template.

An mRNA molecule is complementary to its DNA template because RNA nucleotides are assembled on the template according to base-pairing rules.

During translation, the mRNA base triplets, called codons, are written in the 5 to 3 direction.

Codons along an mRNA molecule are read by translation machinery in the 5 to 3 direction.

Each codon specifies the amino acid (one of 20) to be placed at the corresponding position along a polypeptide.

Cracking the Code All 64 codons were deciphered by the mid-

1960s. Of the 64 triplets, 61 code for amino acids; 3

triplets are “stop” signals to end translation. The genetic code is redundant (more than

one codon may specify a particular amino acid) but not ambiguous; no codon specifies more than one amino acid.

Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced.

Figure 17.5 Second mRNA base

Fir

st

mR

NA

base (

5

en

d o

f cod

on

)

Th

ird

mR

NA

base (

3

en

d o

f cod

on

)

UUU

UUC

UUA

CUU

CUC

CUA

CUG

Phe

Leu

Leu

Ile

UCU

UCC

UCA

UCG

Ser

CCU

CCC

CCA

CCG

UAU

UACTyr

Pro

Thr

UAA Stop

UAG Stop

UGA Stop

UGU

UGCCys

UGG Trp

GC

U

U

C

A

U

U

C

C

CA

U

A

A

A

G

G

His

Gln

Asn

Lys

Asp

CAU CGU

CAC

CAA

CAG

CGC

CGA

CGG

G

AUU

AUC

AUA

ACU

ACC

ACA

AAU

AAC

AAA

AGU

AGC

AGA

Arg

Ser

Arg

Gly

ACGAUG AAG AGG

GUU

GUC

GUA

GUG

GCU

GCC

GCA

GCG

GAU

GAC

GAA

GAG

Val Ala

GGU

GGC

GGA

GGGGlu

Gly

G

U

C

A

Met orstart

UUG

G

Evolution of the Genetic Code The genetic

code is nearly universal, shared by the simplest bacteria to the most complex animals.

Genes can be transcribed and translated after being transplanted from one species to another.

(a) Tobacco plantexpressing a firefly genejellyfish gene

(b) Pig expressing a