Chapter 17

From Gene to Protein

Central Dogma of Molecular Biology

DNA RNA Protein

Metabolic control

• Gene to protein relationship 1st proposed by

Garrod: inherited diseases reflect inability to

make a particular enzyme

“inborn errors of metabolism”

Beadle & Tatum’s experiment

• Studying Neurospora (fungus) supported

“one gene – one enzyme” & modified to

“one gene – one polypeptide”

• Nucleic acids & proteins are informational

polymers assembled from linear sequences of nucleotides & amino acids, respectively

Figure 17.1 Beadle and Tatum’s evidence for the one gene-one enzyme hypothesis

Figure 17.2 Overview: the roles of transcription and translation in the flow of genetic information

Figure 17.3 The triplet code

Transcription(mRNA synthesis)

• Catalyzed by RNA polymerase• (Initiation, Elongation, Termination)• DNA portion (one gene in length) unwinds,

unzips• Free RNA nucleotides pair up on 1 of the DNA

strands• Promoters signal initiation of transcription until terminator sequence is reached, then it breaks

off & DNA rejoins & rewinds• Modified, then mRNA travels to cytoplasm

Figure 17.6 The stages of transcription: initiation, elongation, and termination (Layer 1)

Figure 17.6 The stages of transcription: initiation, elongation, and termination

Figure 17.6 The stages of transcription: initiation, elongation, and termination

Figure 17.6 The stages of transcription: initiation, elongation, and termination (Layer 4)

Figure 17.6 The stages of transcription: elongation

Figure 17.7 The initiation of transcription at a eukaryotic promoter

Translation(Protein synthesis)

• (Initiation, Elongation, Termination)

• tRNA’s pick up specific AA’s based on anticodon & carry AA’s to mRNA attached to a ribosome

• ATP driven process catalyzed by many AA activating enzymes

• P and A sites of ribosome help hold mRNA & tRNA together; peptide bond forms, mRNA moves down the ribosome (made of protein & rRNA)

Figure 17.12 Translation: the basic concept

Figure 17.13a The structure of transfer RNA (tRNA)

Figure 17.13b The structure of transfer RNA (tRNA)

Figure 17.15 The anatomy of a functioning ribosome

Figure 17.17 The initiation of translation

Translation

• mRNA is freed from ribosome & AA chain

• Proteins functioning on membranes or exported from cell are synthesized on ribosomes on rough ER

Figure 17.18 The elongation cycle of translation

Figure 17.19 The termination of translation

Figure 17.20 Polyribosomes

Figure 17.21 The signal mechanism for targeting proteins to the ER

Figure 17.22 Coupled transcription and translation in bacteria

The Genetic Code

• Determined in early 1960’s

• codon – 3-base unit (triplet) that codes for 1 AA

• anticodon – 3-base unit on tRNA which is complementary to codon on mRNA

• All codons don’t code for AA (43 codons)

• Most genes are interrupted by introns – long noncoding regions

Figure 17.4 The dictionary of the genetic code

Eukaryotic RNA processing

• Removing (excising) introns & joining exons by RNA splicing triggered by sets of nucleotides at either end of intron

• Splicing catalyzed by small nuclear ribonucleoproteins (snRNP’s) consisting of small nuclear RNA (snRNA) & proteins operating within larger groups called spliceosomes

Figure 17.8 RNA processing; addition of the 5 cap and poly(A) tail

Figure 17.9 RNA processing: RNA splicing

Figure 17.10 The roles of snRNPs and spliceosomes in mRNA splicing

Eukaryotic RNA processing

• Eukaryotic mRNA receives a modified GTP cap at the 5’ end and a poly-A tail (stretch of nucleotides) at the 3’ end (protects from degradation & enhance translation)

• 30 – 200 adenine nucleotides (poly–A)

Figure 17.11 Correspondence between exons and protein domains

Mutations

1) base pair substitutions - point mutations

2) base pair insertions/deletions - frameshift

3) conditional mutations – harmful under certain

environmental conditions (high/low temp. if

temp. sensitive)

4) spontaneous mutations – may occur during

replication or repair

Figure 17.23 The molecular basis of sickle-cell disease: a point mutation

Figure 17.24 Categories and consequences of point mutations: Base-pair substitution

Figure 17.24 Categories and consequences of point mutations: Base-pair insertion or deletion

Figure 17.25 A summary of transcription and translation in a eukaryotic cell