The two β pleated sheets in each domain are held together by hydrophobic interactions and the conserved disulfide bond.  The three loops of each variable domain show considerable variation in length and amino acid sequence; these hyper variable regions  make up the antigen-binding site. 

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#Unit 3. Fundamental Processes

Chemical mutagens can be divided into two groups:

those that are mutagenic to both replicating and nonreplicating DNA, such as the alkylating agents and nitrous acid; and 

those that are mutagenic only to replicating DNA, such as base analogs— purines and pyrimidines with structures similar to the normal bases in DNA.

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#Unit 3. Fundamental Processes

The most frequent and important kind of hydrolytic damage is deamination of the base cytosine.

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#Unit 3. Fundamental Processes

Adenine and guanine are also subject to spontaneous deamination. Deamination converts adenine to hypoxanthine, which hydrogen-bonds to cytosine rather than to thymine; guanine is converted to xanthine,

which continues to pair with cytosine, although with only two hydrogen bonds.

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#Unit 3. Fundamental Processes

DNA also undergoes depurination by spontaneous hydrolysis of the N-glycosyl linkage, and this produces an abasic site (i.e., deoxyribose lacking a base) in the DNA.

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#Unit 3. Fundamental Processes

Common types of hydrolytic DNA damage.

Deamination of cytosine creates uracil

Depurination of guanine by hydrolysis creates apurinic deoxyribose

Deamination of 5-methylcytosine generates a natural base in DNA, thymine

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#Unit 3. Fundamental Processes

G modification. The figure shows specific sites on guanine that are vulnerable to damage by chemical treatment, such as alkylation or oxidation, and by radiation.