Properties of Nucleic AcidsLecture 1 of

Introduction to Molecular Biology

生理所 蔡少正

The Discovery of DNAThe idea that genetic material is nucleic acid had its

roots in the discovery of transformation by Griffith in 1928.

In 1944, Avery and colleagues showed chemically that the isolated transforming principle is deoxyribonucleic acid (DNA).

The components of DNAA nucleic acid consists of a) nitrogen base, b)

pentose, and c) phosphate.

Two types of pentose are found in nucleic acids. They distinguish DNA (contains 2-deoxyribose) and RNA (contains ribose).

Pyrimidine & Purine

Two types of nitrogenous bases: pyrimidine (six-member ring) and purine (fused five-and six-member rings).

Building block for nucleic acid

A base linked to a sugar is called a nucleoside; when a phosphate group is added, the base-sugar-phosphate is called a nucleotide (building block for nucleic acid).

Nucleoside

Nucleotide

Polynucleotide chain

The nucleotides are linked together into a polynucleotide chain by a backbone consisting of an alternating series of sugar and phosphate residues.

The terminal nucleotide at one end of the chain has a free 5' group; the terminal nucleotide at the other end has a free 3' group.

Traditionally, we write nucleic acid sequences in the 5' to 3' direction, i.e. 5' terminus at the left and 3' terminus at the right.

Abbreviations

Bases, nucleosomes, and nucleotides have related names. Abbreviations of the form NMP stand for nucleoside monophosphate; "d" indicates the 2-deoxy form.

Base Nucleoside Nucleotide Abbreviation Abbr. II. RNA DNA

Adenine Adenosine Adenylic acid AMP dAMP A Guanine Guanosine Guanylic acid GMP dGMP G Cytosine Cytidine Cytidylic acid CMP dCMP C Thymine Thymidine Thymidylic acid dTMP T

Uracil Uridine Uridylic acid UMP U

AMP, ADP, and ATP

ADPATP

Adenosine

Secondary Structure of Nucleic Acid

Double Strand DNA

Watson and Crick proposed that the two polynucleotide chains in the double helix associate by hydrogen bonding between the nitrogenous bases. In their usual forms, G can hydrogen bond specifically only with C, while A can bond specifically only with T.

Complementary bases

These reactions are described as base pairing, and the paired bases (G with C, or A with T) are said to be complementary. The two polynucleotide chains run in opposite directions (antiparallel); therefore, one strand runs in the 5'-3' direction, while its partner runs 3'-5'.

DNA Denatured

DNA can be denatured and renatured. The mid point of the temperature range over which the strands of DNA separate is called the melting temperature (Tm). Tm depends on the proportion of G C base pair (GC content).

Forms of DNADNA can exist in several types of structure families.

Rotation per base pair is indicated as (+) for a right-handed duplex and (-) for a left handed duplex.

Type Base/Turn Length/turn Distance /2 bases RotationB 10 3.4 nm 0.34 nm +A 11 2.3 nm 0.21 nm +Z 12 4.5 nm 0.33 nm -

B DNA

B form vs. Z form DNA

A Nucleotide

helix conformation

Right-handed Left-handed

Base-pairing