Download Biochemistry 6/e

Berg • Tymoczko • Stryer
Biochemistry
Sixth Edition
Chapter 4:
DNA, RNA, and the Flow of
Genetic Information
Copyright © 2007 by W. H. Freeman and Company
4.1
A nucleic acid consists of four kinds of bases
linked to a sugar phosphate backbone
Linear polimer
RNA and DNA differ in the sugar component
and one of the bases
Backbone of the nucleic acid
Purines and pyrimidines
3’ to 5’ phosphodiester bond
Greater stability of DNA than that of RNA
Nucleotides are monomeric units of nucleic acids
Nucleoside ( e.g. adenosine, deoxyadenosine)
N-glycosidic linkage
Nucleotide ( e.g. ATP, deoxyadenylate)
Length of a DNA molecule
4.2
A pair of nucleic acid chains with complementary sequences
can form a double-helical structure
replication
The double helix is stabilized by hydrogen bonds and
hydrophobic interactions
X-ray diffraction of DNA fiber – helical structure with two chains
Regular structure with different bases
Stability of the helix: hydrogen bonds, van der Waals interactions,
hydrophobic interactions
The double helix facilitates the accurate transmission
of hereditary information
Semiconservative replication
Meselson and Stahl experiment: grow E. coli in 15NH4Cl first then
in 14NH4Cl
Density-gradient equilibrium sedimentation
The double helix can be reversibly melted
melting temperature, Tm
hypochroism
melting and annealing
helicase
Some DNA molecules are circular and supercoiled
linear or circular
superhelix
relaxed or supercoiled
Single-stranded nucleic acids can adopt elaborate structures
stem-loop
More complex structures with nonstandard pairings
4.3
DNA is replicated by polymerases that take instructions
from templates
DNA polymerase catalyzes phosphodiester-bond formation
template, primer, dNTP
template-directed enzyme
high fidelity
The genes of some viruses are made of RNA
retrovirus
reverse transcriptase
4.4
Gene expression is the transformation of DNA information
into functional molecules
Several kinds of RNA play key roles in gene expression
tRNA, mRNA, rRNA
snRNA, scRNA, miRNA, siRNA
All cellular RNA is synthesized by RNA polymerases
template, rNTP, a divalent metal ion (Mg2+, Mn2+)
No primer, lower fidelity than DNA polymerases
RNA polymerases take instructions from DNA templates
coding strand, template strand
Transcription begins near promoter sites and ends
at terminator sites
promoter and terminator
Rho-independent terminator
Transfer RNA is the adaptor molecule in protein synthesis
an amino acid-attachment site and a template-recognition site
aminoacyl-tRNA
codon and anticodon
4.5
Amino acids are encoded by groups of three bases
starting from a fixed point
Genetic code: relation between base sequence and amino acid sequence
An amino acid is encoded by a codon
Nonoverlapping, no punctuation, and degenerate
Major features of the genetic code
Degeneracy minimizes the deleterious effects of mutations
Permitting DNA base composition to vary a wide range
Messenger RNA contains start and stop signals
for protein synthesis
Termination codons are read by release factors
fMet-tRNA recognizes AUG or GUG in prokaryotes
The genetic code is nearly universal
Mitochondria and ciliated protozoa
4.6
Most eukaryotic genes are mosaics of introns and exons
Intervening sequences or introns
RNA processing generates mature RNA
Colinear with the protein products
splicing
Many exons encode protein domains
Exon shuffling
Differential splicing