UNIT 6

From DNA to Protein:

Gene Expression

PART 1 Hillis Textbook, CH 10

Discoveries…

Identification of a gene product as a protein began with a mutation.

A man named Garrod saw a disease phenotype— alkaptonuria—occurring in children who shared more alleles as first cousins.

A substance in their blood (HA) accumulated—was not catalyzed—and the gene for the enzyme was mutated.

Garrod correlated one gene to one enzyme.

Discoveries…

Phenylketonuria (PKU) is another genetic disease that involves this pathway.

The enzyme that converts phenylalanine to tyrosine is nonfunctional.

Untreated, it can lead to mental retardation, but is easily detected in newborns.

One gene leads to one protein?

Phenotypic expression of alkapatonuria and phenylketonuria led to the one gene–one protein hypothesis.

A mutant phenotype arises from a change in the protein’s amino acid sequence.

However, the one gene–one protein hypothesis proved too simple in studies of human mutations.

One gene leads to one polypeptide!

The gene–enzyme relationship has since

been revised to the one gene–one

polypeptide relationship.

Example: In hemoglobin, each polypeptide

chain is specified by a separate gene.

Other genes code for RNA but are not

translated to polypeptides; some genes

are involved in controlling other genes.

Gene expression is protein synthesis

Molecular biology is the study of nucleic acids and proteins, and often focuses on gene expression.

Gene expression to form a specific polypeptide occurs in two steps:

• Transcription—copies information from a DNA sequence (a gene) to a complementary RNA sequence

• Translation—converts RNA sequence to amino acid sequence of a polypeptide

RNA

Roles of three kinds of RNA in protein synthesis:

• Messenger RNA (mRNA) and transcription—carries copy of a DNA sequence to the site of protein synthesis at the ribosome

• Ribosomal RNA (rRNA) and translation—catalyzes peptide bonds between amino acids

• Transfer RNA (tRNA) mediates between mRNA and protein—carries amino acids for polypeptide assembly

What is the difference

between DNA and RNA?

Figure 10.3 From Gene to Protein

Transcription: Transcription—the formation of a specific RNA sequence from a specific DNA sequence

Requires some components:

• A DNA template for base pairings—one of the two strands of DNA

• Nucleoside triphosphates (ATP,GTP,CTP,UTP) as substrates

• An RNA polymerase enzyme

Takes three steps:

1. Initiation

2. Elongation

3. Termination

FIRST HALF of

protein synthesis, or

gene expression

RNA

Polymerases RNA polymerases catalyze synthesis of

RNA from the DNA template.

RNA polymerases are processive—a single

enzyme-template binding results in

polymerization of hundreds of RNA

bases.

Unlike DNA polymerases, RNA

polymerases do not need primers.

What do you know

about polymerases?

Initiation: Initiation requires a promoter—a special

sequence of DNA- at the transcription initiation site.

RNA polymerase binds to the promoter.

Promoter tells RNA polymerase two things:

1. Where to start transcription

2. Which strand of DNA to transcribe

Elongation: Elongation: RNA polymerase unwinds DNA about

13 base pairs at a time; reads template in 3′-to-5′ direction.

RNA polymerase adds nucleotides to the 3′ end of the new strand.

The first nucleotide in the new RNA forms its 5′ end and the RNA transcript is antiparallel to the DNA template strand.

Termination: Termination is specified by a specific DNA

base sequence.

Mechanisms of termination are complex and varied.

For some genes, the transcript falls away from the DNA template and RNA polymerase—for others a helper protein pulls it away.

INTRONS and EXONS:

Coding regions are sequences of a DNA

molecule that are expressed as proteins.

Eukaryotic genes may have noncoding

sequences—introns (intervening

regions).

The coding sequences are exons

(expressed regions).

Figure 10.6 Transcription of a Eukaryotic Gene (Part 1)

Introns and exons

appear in the primary

mRNA transcript—

pre-mRNA

Introns are removed

from the final mRNA.

Figure 10.6 Transcription of a Eukaryotic Gene (Part 2)

It’s called splicing:

RNA splicing removes introns and splices exons together.

Newly transcribed pre-mRNA is bound at ends by snRNPs—small nuclear ribonucleoprotein particles.

Besides the snRNPs, other proteins are added to form an RNA–protein complex, the spliceosome.

This complex cuts pre-mRNA, releases introns, and splices exons together to produce mature mRNA.

Figure 10.9 The Spliceosome: An RNA Splicing Machine

While the pre-mRNA is

in the nucleus it

undergoes two

processing steps:

A 5′ cap (or G cap) is

added to the 5′ end

A poly A tail is added to

the 3′ end.