Download Liu-2-DNA and RNA

Carbohydrates
Definition: carbohydrates are aldehyde or ketone
compounds with multiple hydroxyl groups.
Function:
(1) serves as energy stores, fuels,and metabolic
intermediates.
(2) ribose and deoxyribose sugars form part of
the structure framework of RNA and DNA.
(3) polysaccharides are structural elements in the
cell walls of bacteria and plants. Cellulose, the main constituent of
plant cell walls, is one of the most abundant organic compounds
in the biosphere.
(4) carbohydrates are linked to many proteins
and lipids, where they play key roles in mediateng interactions
among cells and interactions between cells and other elements in
the cellular environment.
Complex carbonhydrate are formed by linkage of monosacchrides
Monosaccharides
O-glycosidic bonds
Oligosaccharides
O-Glycosidic bond: between the anomeric carbon atom of
glucose and the hydroxyl oxygen atom of methonol.
N-Glycosidic bond: anomeric carbon atom – nitrogen
atom of an amine
DNA, RNA, and the flow of genetic information
DNA
Transcription
RNA
Translation
Protein
A nucleic acid consists of four kinds of bases linked to a
sugar-phosphate backbone.
A nucleic acid consists of four kinds of bases linked to a
sugar-phosphate backbone.
A monomer unit
Backbones of DNA and RNA
Backbones of DNA and RNA
Four kinds of bases
G
A
C
U
T
(RNA only)
(DNA only)
Structure of DNA chain: the chain has a 5′ end,
which is usually attached to a phosphate, and a 3′
end, which is uaually a free hydroxyl group.
A nucleotide:
A DNA molecule must comprise many nucleotides to carry the
genetic information.
Electron
microscope of
part of the E. coli.
The E.coli genome is a single DNA molecule consisting of
two chains of 4.6 milion nucleotides.
The human genome comprises approximately
3 billion nucleotides, divided among 24
distinct DNA molecules: 22 autosomes, X and
Y sex chromosomes.
An indian muntjak: its genome
is as large as human genome
but is distributed on only 3
chromosomes.
A muntjak’s chromosomes
The double-helical structure of DNA facilitates the replication
of the genetic material
Watson and Crick, 1953
The double-helical structure of DNA facilitates
the replication of the genetic material
(10 nucleotides)
The features of Watson-Crick model of
DNA deduced from the diffraction
patterns are:
1. Two helical polynucleotide chains are
coiled around a common axis. The
chains run in opposite directions.
2. The sugar-phosphate backbones are
on the outside and, therefore, the
purine and pyrimidine base lie on
the inside of the helix.
3. The bases are nearly perpendicular
to the helix axis, and adjacent
bases are seperated by 3.4A. The
helical structure repeats every 34A,
so there are 10 bases per ture of
helix.
4. The diameter of the helix is 20 A.
The structures of the base pairs proposed by Watson and Crick
The double helix can be reversibly melted
Single –stranded DNA absorbs light
more effectively than does doublehelical DNA-----Hypochromism.
The absorbance of a DNA solution at
260nm increased when the double helix
is melted into single strands.
Some DNA molecules are circular and supercoiled
Supercoiled form
Relaxed form
Stem-loop structures may be formed from single-stranded
DNA and RNA molecules
Replication of DNA
•DNA : Deoxyribose Nucleic Acid
RNA: Ribose Nucleic Acid
• DNA
replication
DNA
transcription
RNA
translation
• body－organs－tissues－cells－molecules
Protein
mitosis
The double helix facilitates the accurate transmission of
hereditary information
● Semiconservative replication
DNA polymerase catalyzes phosphodiester-bond formation
○ DNA polymerases catalyze the step-by-step addition of deoxyribonucleotide
units to a DNA chain
○ Importantly, the new DNA chain is assembled directly on a preexisting DNA
template
The genes of some viruses are made of RNA
1. Genes in all cellular organisms are made of DNA.
2. For some viruses:
(1) A single RNA strand
(2) RNA→DNA→RNA.
RNA-dependent RNA
polymerase
RNA replication
Gene Expression:
Transcription: DNA →mRNA
Translation: mRNA → Protein
RNA polymerases take instructions from DNA template
RNA sequence is the precise complement of the DNA template sequence
Transcription begins near promoter sites
Transcription ends at terminator sites
●RNA polymerase proceeds along the DNA template,
transcribing one of its strands untill it reaches a terminator
sequence.
● The terminator sequence in E. coli encode a base-pair
hairpin in newly synthesized RNA molecule.
● Less is known about the termination of transcription in
eukaryotes.
Most eukaryotic genes are mosaics of introns and exons
RNA processing generates mature RNA
RNA processing generates mature RNA
5’ end: capping
1.Protect mRNA from the attack of RNase.
2. Associating with the cap-binding complex
of protein which facilitates the binding of
mRNA to ribosomes.
3’ end: addition of poly-A
Why ?
1. Prevent mRNA from the attack by RNAse, stablizing the mRNA.
2. Maintain the activity of mRNA as a template of translation.
All cellular RNA is synthesized by RNA polymerase
RNA polymerase requires the following components:
1. A template
2. Activated precursors. All four ribonucleoside
triphosphates ATP, GTP,UTP and CTP are required.
3. A divalent metal ion. Mg2+ or Mn2+.
Gene Translation: RNA → Protein
Several kinds of RNA play key roles in gene translation
1. Messemger RNA (mRNA) is the template for protein synthesis or
translation
2. Transfer RNA (tRNA) carries Amino acids in an activated form th
the ribosome for peptide-bond formation, in a sequence dictated
by the mRNA template.
3. Ribosomal RNA (rRNA), the major component of ribosomes, plays
both a catalytic and a structureal role in protein synthesis.
Gene Translation: RNA → Protein
The problem: How does a particular sequence of
nucleotides specify a particular sequence of amino acids?
The answer: by means of transfer RNA molecules, each
specific for one amino acid and for a particular triplet of
nucleotides in mRNA called a codon. The family of tRNA
molecules enables the codons in a mRNA molecule to be
translated into the sequence of amino acids in the protein.
Amino acid are encoded by groups of three bases
starting from a fixes point
Messenger RNA contains start and stop signals for protein synthesis
● In eukaryotes, the processes of transcription and translation are
separated both spatially and in time. Transcription of DNA into mRNA
occurs in the nucleus. Translation of mRNA into polypeptides occurs on
ribosomes.
Post-translational modification of protein
Glycosylation
Phosphorylation
Methylation
……….
Bonifacino JS and Glick BS, 2004
DNA damage and DNA repair
From molecular level:
• DNA damage = DNA mutation
• This mutation could happen in just one dNMP
residue, this is called point mutation.
spontaneous mutation
• DNA mutation is the molecular base for evolution and
differentiation.
• Evolution is built on continued DNA mutation.
Un-repairable damage
death
Programmed Cell Death
（apoptosis）
tadpole
frog
Genetics pioneer Dr. J. Craig Venter
Genetics pioneer Dr. J. Craig Venter
Scientists create artificial life
Genetics pioneer Dr. J. Craig Venter
Scientists create artificial life
This undated handout image provided by
the J. Craig Venter Institute shows
negatively stained transmission electron
micrographs of aggregated M. mycoides.
Scientists announced a bold step
Thursday in the enduring quest to create
artificial life. They've produced a living cell
powered by manmade DNA. (AP Photo/J.
Craig Venter Institute)
“Towards artificial
life: Cell powered by
man-made DNA”
“Artificial life:
time to debate implications”
“a scientific milestone”
“Church warns cell scientists
not to play God！”