1 genes are dna. ex biochem c1-genes dna 2 1.1 introduction figure 1.2
TRANSCRIPT
Ex Biochem c1-genes DNA 3
1.5 Polynucleotide Chains
Nitrogenous Bases 鹼基 Linked to a Sugar–Phosphate Backbone
A nucleoside consists of a purine or pyrimidine base linked to position 1 of a pentose sugar.
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Transfection DNA can enter
eukaryotic cells and produce functional proteins Become part of the
genome DNA can also be
introduced into eggs by microinjection Become part of the
genome
Ex Biochem c1-genes DNA 5
Nucleic acid structure Positions on the ribose ring are described with a prime (′) to
distinguish them. The difference between DNA and RNA is in the group at the
2′ position of the sugar. DNA has a deoxyribose sugar (2′–H) RNA has a ribose sugar (2′–OH)
A nucleotide consists of a nucleoside linked to a phosphate group on either the 5′ or 3′ position of the (deoxy)ribose.
Successive (deoxy)ribose residues of a polynucleotide chain are joined by a phosphate group
Between the 3′ position of one sugar and the 5′ position of the next sugar
One end of the chain (left) has a free 5′ end The other end has a free 3′ end
Ex Biochem c1-genes DNA 6
Nucleosides
Nucleoside:Nucleoside: a compound that consists of D-ribose 核糖 or 2-deoxy-D-ribose 去氧核糖bonded to a nucleobase by a -N-glycosidic bond
anomericcarbon
a -N-glycosidicbond
HH
HH
OHOCH2
HO OH
O
O
HN
N
Uridine
-D-riboside
uracil
1'
2'3'
4'
5'
1
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Nucleotide
NucleotideNucleotide:: a nucleoside in which a molecule of phosphoric acid is esterified with an -OH of the monosaccharide, most commonly either the 3’-OH or the 5’-OH
5'
O-
O
O
H
H
OH
H
HOH
1'
-O-P-O-CH2
N
N N
N
NH2
3'
Adenosine 5'-monophosphate(5'-AMP)
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Nucleotides
Deoxythymidine 3’-monophosphate (3’-dTMP)
5'O
H
H
H
H
OH
1'
HOCH2
3'
HN
N
OCH3
O
P
O-
O O-
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DNA Structure
5'
O-
O NO
HN
O
H
H
H
H
O
H
-O-P-O-CH2
2'
1'
O
HH
OH
H
H
H
HN
N N
N
O
H2N
OCH3
O-
O=P O CH2
2'
1'
5'
3'
phosphorylated5' end
free 3' end
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Pyrimidine/Purine Bases
HN
NO
H
N
N
NH2
H
HN
N
H
CH3
Uracil (U)(in RNA)
Thymine (T)(DNA andsome RNA)
Cytosine (C)(DNA andsome RNA)
N
N
Pyrimidine
1
2
34
5
6
HN
N N
NO
HH2N
Guanine (G)(DNA and RNA)
N
N N
NNH2
HAdenine (A)
(DNA and RNA)
N
N N
N
HPurine
1
2
34
56 7
8
9
O O
O O
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Other Bases
Several “unusual” bases occur, principally but not exclusively, in transfer RNAs
HN
N
O
H5,6-Dihyro-
uracil
HN
N N
N
O
HHypo-
xanthine
N
N N
NN
H
N6-Dimethyl-adenine
CH3H3C
N
N
NH2
O
H5-Methyl-cytosine
CH3
O
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DNA Structure
Writing a DNA strand an abbreviated notation
even more abbreviated notations: pdApdCpdGpdT, or pdACGT, or ACGT
P P P
dA dC dG dT
OH 3'
5' P
3'
5'
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1.6 DNA Is a Double Helix
The B-form of DNA is a double helix consisting of two polynucleotide chains that run antiparallel.
The nitrogenous bases of each chain are flat purine or pyrimidine rings They face inward They pair with one another by hydrogen bonding
to form A-T or G-C pairs only
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The diameter of the double helix is 20 Å There is a complete turn
every 34 Å Ten base pairs per turn
The double helix forms: a major (wide) groove a minor (narrow) groove
Figure 1.10
Ex Biochem c1-genes DNA 191.7 DNA Replication Is Semiconservative
The Meselson–Stahl experiment used density labeling to prove that: The single polynucleotide strand is the unit of
DNA that is conserved during replication Each strand of a DNA duplex acts as a
template 模版 to synthesize a daughter strand.
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Figure 1.11: Base pairing accounts for specificity of replication.
DNA replication is semiconservative
Ex Biochem c1-genes DNA 22
Enzymes The enzymes that synthesize DNA are called DNA
polymerases (DNA 聚合脢 )
The enzymes that synthesize RNA are called RNA polymerases
Nucleases are enzymes that degrade nucleic acids They include DNAases and RNAases They can be divided into endonucleases and
exonucleases.
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Figure 1.14: Endonucleases attack internal bonds.
Figure 1.15: Exonucleases nibble from the ends.
Ex Biochem c1-genes DNA 241.9 Genetic Information Can Be Provided by DNA or RNA
Cellular genes are DNA Viruses and viroids may have
genomes of RNA
DNA is converted into RNA by transcription RNA may be converted into
DNA by reverse transcription
The translation of RNA into protein is unidirectional.
Figure 1.16
Ex Biochem c1-genes DNA 261.10 Nucleic Acids Hybridize by Base Pairing
Heating causes the two strands of a DNA duplex to separate.
The Tm is the midpoint of the temperature range for denaturation.
Complementary single strands can renature when the temperature is reduced.
Denaturation and renaturation/hybridization 雜交can occur with the combinations:
DNA–DNA DNA–RNA RNA–RNA
They can be intermolecular or intramolecular
Ex Biochem c1-genes DNA 281.11 Mutations Change the Sequence of DNA
All mutations 突變consist of changes in the sequence of DNA.
Mutations may: occur spontaneously be induced by
mutagens
Figure 1.22
Ex Biochem c1-genes DNA 29
1.12 Mutations May Affect Single Base Pairs or Longer Sequences
A point mutation changes a single base pair. Point mutations can be caused by:
the chemical conversion of one base into another mistakes that occur during replication
Insertions are the most common type of mutation They result from the movement of transposable
elements
Ex Biochem c1-genes DNA 30
A transition replaces a G-C base pair with an A-T base pair or vice versa.
Figure 1.23 Figure 1.24
Ex Biochem c1-genes DNA 311.13 The Effects of Mutations Can Be Reversed
Forward mutations inactivate a gene Back mutations (or revertants) reverse
their effects
Insertions can revert by deletion of the inserted material Deletions cannot revert
Suppression occurs when a mutation in a second gene bypasses the effect of mutation in the first gene.
Figure 1.25