Nucleic Acids

Nucleic acids are composed of chains of nucleotides. Each nucleotide is composed of a sugar (either ribose or deoxyribose), a phosphate (-PO4) group, and a purine or pyrimidine base. The nucleotides are joined into a DNA or RNA strand by a sugar-phosphate-linked backbone with the bases attached to and extending from the first carbon of the sugar group. The purine and pyrimidine bases are weakly basic ring molecules, which form N-glycosidic bonds with ribose or deoxyribose sugar. Purines are comprised of two rings, a six-member ring and a five-member ring (C5H4N4), while pyrimidines consist of a single six-member ring (C4H2N2). Purines (guanine, G, and adenine, A) pair with pyrimidines (cytosine, C, and thymine, T) via hydrogen bonds between two DNA molecules (Figure 1-1). The additional hydrogen bond that forms between G and C base pairing (i.e., three hydrogen bonds) dramatically enhances the strength of this interaction compared to the two hydrogen bonds present between A and T nucleotides. This hydrogen-bonding capacity between G:C and A:T forms a pivotal molecular interaction for all nucleic acids and assures the passage of genetic information during

Figure 1-1. DNA base pairing. DNA nucleotides are composed of three moieties (e.g., sugar, base, and phosphate groups). The bases are either purine (adenine and guanine) or pyrimidine (thymine and cytosine). Note the difference in hydrogen bonds between adenine and thymine base pairs, with two hydrogen bonds, compared to cytosine and guanine base pairs, with three hydrogen bonds. (Reprinted from Leonard D. Diagnostic Molecular Pathology, copyright 2003, with permission from Elsevier.)

Figure 1-1. DNA base pairing. DNA nucleotides are composed of three moieties (e.g., sugar, base, and phosphate groups). The bases are either purine (adenine and guanine) or pyrimidine (thymine and cytosine). Note the difference in hydrogen bonds between adenine and thymine base pairs, with two hydrogen bonds, compared to cytosine and guanine base pairs, with three hydrogen bonds. (Reprinted from Leonard D. Diagnostic Molecular Pathology, copyright 2003, with permission from Elsevier.)

DNA replication, RNA synthesis from DNA (transcription), and the transfer of genetic information from nucleic acids to the amino acids of proteins.

Amino Acids

Amino acids are the building blocks of proteins. Amino acids linked together via peptide bonds form large, complex molecules. Amino acids consist of an amino group (NH3), a carboxy group (COO-), an R group, and a central carbon atom. The R group can be a simple hydrogen, as found in glycine, or as complex as an imidazole ring, as found in histidine. Twenty different R groups exist; and determine whether an amino acid has a neutral, basic, or acidic charge (Table 1-2). The amino group of the polypeptides is considered the beginning of a protein (N-

Table 1-2. Amino Acids

Amino Acid Symbols

Amino Three Single

Acid Letter Letter R Group

Amino Three Single

Acid Letter Letter R Group

Alanine

ala

A

CH3—CH(NH2)—COOH

Arginine

arg

R

HN=C(NH2)—NH—(CH2)3—CH(NH2)—COOH

Asparagine

asn

N

H2N—CO—CH2—CH(NH2)—COOH

Aspartic acid

asp

D

HOOC—CH2—CH(NH2)—COOH

Cysteine

cys

C

HS—CH2—CH(NH2)—COOH

Glutamine

glu

Q

H2N—CO—(CH2)2—CH(NH2)—COOH

Glutamic acid

gln

E

HOOC—(CH2)2—CH(NH2)—COOH

Glycine

gly

G

NH2—CH2—COOH

Histidine

his

H

NH—CH=N—CH=C—CH2—CH(NH2)—COOH

Isoleucine

ile

I

CH3—CH2—CH(CH3)—CH(NH2)—COOH

Leucine

leu

L

(CH3)2—CH—CH2—CH(NH2)—COOH

Lysine

lys

K

H2N—(CH2)4—CH(NH2)—COOH

Methionine

met

M

CH3—S—(CH2)2—CH(NH2)—COOH

Phenylalanine

phe

F

Ph—CH2—CH(NH2)—COOH

Proline*

pro

P

NH—(CH2)3—CH—COOH

Serine

ser

S

HO—CH2—CH(NH2)—COOH

Threonine

thr

T

CH3—CH(OH)—CH(NH2)—COOH

Tryptophan

trp

W

Ph—NH—CH=C—CH2—CH(NH2)—COOH

Tyrosine

tyr

Y

HO—p—Ph—CH2—CH(NH2)—COOH

Valine

val

V

(CH3)2—CH—CH(NH2)—COOH

* Proline has a ring shape arising from the covalent bond formed between the amino group and the central carbon.

* Proline has a ring shape arising from the covalent bond formed between the amino group and the central carbon.

terminus), while the carboxyl group is at the opposite end, providing directionality to the protein.

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