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Ch.8 Nucleotides & Nucleic Acids, Nucleotides and Nucleosides, DNA…
Ch.8 Nucleotides & Nucleic Acids
Functions
Nucleic acids are polymers of nucleotides used for:
transmission of genetic info (mRNA)
processing of genetic information (ribozymes)
storage of genetic info (DNA)
protein synthesis (tRNA and rRNA)
Nucleotides are also used in the monomer form for cellular functions:
enzyme cofactors (NAD+)
signal transduction (cAMP)
energy for metabolism (ATP)
Nucleotides and Nucleosides
Nucleotide核苷酸 =
pentose
phosphate
nitrogeneous base
N-GIycosidic Bond
the pentose ring is attached to the nitrogenous
base via a N-glycosidic bond.
The bond is formed:
to position NI in pyrimidines
to position N9 in purines
This bond is quite stable toward hydrolysis, especially in pyrimidines.
Conformation around N-GIycosidic Bond
syn- : 朝內
anti- : 朝外
Anticonformation is found in normal B-DNA.
Nucleosid核苷 =
nitrogeneous base
pentose
嘧啶
2個N
嘌呤
4個N
Phosphate Group
Typically attached to 5' position
May be attached to other positions for specialized function
Negatively charged
Pentose
內凹 : 2' , 3'
Nomenclature:
Deoxyribonucleotides
Ribonucleotides
互變異構物(Tautomer)
B. 常見的互變異構物有：酮＆烯醇、醯胺＆亞胺酸、
:star: 內醯胺（lactam）＆內醯亞胺（lactim）
C. 就核苷酸而言，大多為內醯胺＆內醯亞胺的反應，
由oxo-和amino-兩種官能基進行互變
A. 原理：藉由質子轉移，讓單鍵和相鄰的雙鍵互換，使有機化合物官能基改變；而改變前後的異構物，稱為互變異構物
UV Absorption of Nucleobases
Absorption of UV light at 250—270 nm is due to
π→π＊ electronic transitions.
Excited states of common nucleobases decay rapidly
via radiationless transitions.
Minor Nucleosides in DNA
N6-Methyladenosine is common in bacteria but not found in eukaryotes.
Epigenetic marker:
way to mark own DNA so that cells can degrade foreign DNA (prokaryotes)
way to mark which genes should be active
(eukaryotes)
5-Methylcytosine is common in eukaryotes and is also found in bacteria.
Minor Nucleosides in RNA
Inosine肌苷 is sometimes found in the "wobble position" of the anticodon in tRNA.
provides richer genetic code
Pseudouridine ( ψ)假尿嘧啶核苷 is found widely in tRNA and rRNA.
may stabilize the structure of tRNA
may help in folding of rRNA
Polynucleotides
DNA backbone is fairly stable.
Hydrolysis accelerated by enzymes (DNAse)
RNA backbone is unstable.
In water, RNA lasts for a few years ;
In cells, mRNA is degraded in a few hours.
no branching or cross-links
Covalent bonds are formed via phosphodiester linkages.
negatively charged backbone
DNA & RNA
DNA Denaturation變性
be induced by :
high temperature
change in pH
Denaturation may be reversible: annealing.
Thermal DNA Denaturation (Melting)
The reversible thermal denaturation and annealing
form the basis for the polymerase chain reaction.
DNA denaturation is commonly monitored by UV
spectrophotometry at 260 nm.
Factors Affecting DNA Denaturation
The midpoint of melting (Tm) depends on base composition.
High CG increases Tm.
CG之間有3個氫鍵
AT-rich regions melt at a lower temperature than GC-rich regions.
Mechanism of Base-catalyzed RNA Hydrolysis鹼催化水解
Mixture Of 2'- and 3' -monophosphate derivatives
Complementarity of DNA Strands
Two chains are complementary.
Two chains run antiparallel.
coding
髮夾彎 :arrow_forward: 穩定
Complex Structures Are Stabilized by
Non-Watson- Crick Base-Pair Interactions
非 AT / CG配對
Molecular Mechanisms of
Spontaneous Mutagenesis致突變作用:
Deamination脫氨基
large number of residues
100 C :arrow_right: U events/day in a mammalian cell.
very slow reactions
Depurination脫嘌呤
N-glycosidic bond is hydrolyzed
Significant for purines:
10,000 purines lost/day in a mammalian cell
Molecular Mechanisms of Oxidative & Chemical Mutagenesis
Oxidative damage : hydroxylation
Chemical alkylation : methylation
Molecular Mechanisms of Radiation-Induced Mutagenesis
UV light
Ionizing radiation (x rays and y rays
