Chapter 10:

Molecular Biology of

Gene Expression

Jones and Bartlett Publishers © 2005

Chapter 10.1- 10.3

• Details of the central dogma will be discussed in Chapter 10.

• For Exam 3, we will cover up through p. 408.

• Our main topic for the rest of this lecture will be transcription, but first we will review amino acids and polypeptide chains.

Structure of an amino acid

Structure of all 20 amino acids commonly found in proteins

Joining of amino acids by peptide bonds to create polypeptides

Three-dimensional structure of a monomeric protein

Location of a number of mutants in the trpA gene and the resulting amino acid replacements

Colinearity – the sequence of base pairs in DNA determines the amino acid sequence in a colinear manner.

Transcription

• Transcription is the synthesis of RNA from the DNA coding for the gene.

• (Remember the differences between DNA and RNA).– RNA uses the four ribonucleosides ATP, GTP

CTP and UTP.– RNA polymerase is used instead of DNA

polymerase.– A promoter is used for initiation of

transcription (not a primer, as with DNA).

A ribonucleoside differs from a deoxyribonucleoside in 2 ways

RNA is synthesized by the addition of new nucleotides to the 3’-OH end of a growing chain

Types of RNA polymerases

• Bacterial cells use only one RNA polymerase, with six polypeptides.

• The RNA polymerase holoenzyme contacts the promoter, then 35 nt of DNA.

• Processivity can be 10,000 nt for prokaryotes and 1,000,000 for eukaryotes.

Mechanism of transcription

1. Promoter recognition

2. Chain initiation

3. Chain elongation

4. Chain termination

RNA synthesis creates a bubble in the double-stranded DNA template

Note the 5` triphosphate group, and that only one strand is the template.

Comparison of a number of E. coli promoter sequences reveals two conserved sequences (at -10 and -35 nt)

The position in DNA where the RNA chain starts is called +1 site. DNA 5’ to the start site is called “upstream” sequence and DNA to

the 3’-side is called “downstream” sequence.

Rho-independent site for transcription termination generates RNA that can form a stable hairpin

followed by a number of U residues

During transcription, RNA is copied from only one DNA strand for any one gene