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Гените са ДНК
Част 1
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1.1 Introduction
Figure 1.2
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1.2 DNA Is the Genetic Material of Bacteria
• Bacterial transformation provided the first proof that DNA is the genetic material of bacteria.
Figure 1.3
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• Genetic properties can be transferred from one bacterial strain to another– DNA is extracted from the first strain and added to
the second strain
Figure 1.4
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1.3 DNA Is the Genetic Material of Viruses
• Phage infection proved that DNA is the genetic material of viruses.
• When the DNA and protein components of bacteriophages are labeled with different radioactive isotopes:– Only the DNA is
transmitted to the progeny phages produced by infecting bacteria.
Figure 1.5
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1.4 DNA Is the Genetic Material of Animal Cells
• DNA can be used to introduce new genetic features into animal cells or whole animals.
• In some viruses, the genetic material is RNA.
Figure 1.6
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1.5 Polynucleotide Chains Have Nitrogenous Bases Linked to a Sugar–Phosphate Backbone
• A nucleoside consists of a purine or pyrimidine base linked to position 1 of a pentose sugar.
Figure 1.7
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• 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.
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• 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 (conventionally the left) has a free 5′ end – The other end has a free 3′ end
• DNA contains the four bases adenine, guanine, cytosine, and thymine– RNA has uracil instead of thymine
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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.
Figure 1.8
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• 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.
Figure 1.9
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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
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1.7 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 to synthesize a daughter strand.
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• The sequences of the daughter strands are determined by complementary base pairing with the separated parental strands.
Figure 1.11
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1.8 DNA Strands Separate at the Replication Fork
• Replication of DNA is undertaken by a complex of enzymes that:– separate the parental strands – synthesize the daughter strands
• The replication fork is the point at which the parental strands are separated.
Figure 1.13
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• The enzymes that synthesize DNA are called DNA polymerases
• 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.
Figure 1.14Figure 1.15
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1.9 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
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1.10 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 can occur with the combinations:
• DNA–DNA• DNA–RNA• RNA–RNA
– They can be intermolecular or intramolecular
Figure 1.20
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• The ability of two single-stranded nucleic acid preparations to hybridize is a measure of their complementarity.
Figure 1.21
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1.11 Mutations Change the Sequence of DNA
• All mutations consist of changes in the sequence of DNA.
• Mutations may:– occur
spontaneously – be induced by
mutagensFigure 1.22
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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
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• A transition replaces a G-C base pair with an A-T base pair or vice versa.
Figure 1.23 Figure 1.24
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• A transversion replaces a purine with a pyrimidine– such as changing A-T to T-A
• Insertions are the most common type of mutation– They result from the movement of transposable
elements
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1.13 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 theinserted 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
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1.14 Mutations Are Concentrated at Hotspots
• The frequency of mutation at any particular base pair is determined by statistical fluctuation…
• Except for hotspots– The frequency is increased by at least an order of
magnitude
Figure 1.26
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1.15 Many Hotspots Result from Modified Bases
• A common cause of hotspots is the modified base 5-methylcytosine– It is spontaneously deaminated to thymine
Figure 1.27
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1.16 Some Hereditary Agents Are Extremely Small
• Some very small hereditary agents do not code for protein– They consist of RNA or of protein that has
hereditary properties.
Figure 1.29