1 genes are dna. ex biochem c1-genes dna 2 1.1 introduction figure 1.2

1

Genes Are DNA

Ex Biochem c1-genes DNA 2

1.1 Introduction

Figure 1.2


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.


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


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


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


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)


Nucleotides

Deoxythymidine 3’-monophosphate (3’-dTMP)

5'O

H

H

H

H

OH

1'

HOCH2

3'

HN

N

OCH3

O

P

O-

O O-


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


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


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


Figure 1.07: A polynucleotide has a repeating structure.


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'


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


Figure 1.08: The double helix has constant width.


Figure 1.09: Flat base pairs connect the DNA strands.


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


DNA double helix

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.


Figure 1.11: Base pairing accounts for specificity of replication.

DNA replication is semiconservative


Semiconservative Replication


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.


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


Figure 1.18: Genomes vary greatly in size.

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


Figure 1.20: DNA can be denatured and renatured.

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


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


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

1 genes are dna. ex biochem c1-genes dna 2 1.1 introduction figure 1.2

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