Recombination Signal Sequences Direct Recombination

The discovery of two closely related conserved sequences in variable-region germ-line DNA paved the way to fuller understanding of the mechanism of gene rearrangements. DNA sequencing studies revealed the presence of unique recombination signal sequences (RSSs) flanking each germ-line V, D, and J gene segment. One RSS is located 3' to each V gene segment, 5' to each J gene segment, and on both sides of each D gene segment. These sequences function as signals for the recombination process that rearranges the genes. Each RSS contains a conserved palindromic heptamer and a conserved AT-rich nonamer sequence separated by an intervening sequence of 12 or 23 base pairs (Figure 5-6a). The intervening 12- and 23-bp sequences correspond, respectively, to one and two turns of the DNA helix; for this reason the sequences are called one-turn recombination signal sequences and two-turn signal sequences.

The VK signal sequence has a one-turn spacer, and the JK signal sequence has a two-turn spacer. In X light-chain DNA, this order is reversed; that is, the VX signal sequence has a two-turn spacer, and the JX signal sequence has a one-turn spacer. In heavy-chain DNA, the signal sequences of the VH and JH gene segments have two-turn spacers, the signals on either side of the DH gene segment have one-turn spacers (Figure 5-6b). Signal sequences having a one-turn spacer can join only with sequences having a two-turn spacer (the so-called one-turn/two-turn joining rule). This joining rule ensures, for example, that a VL segment joins only to a JL segment and not to another VL segment; the rule likewise ensures that VH, DH, and JH segments join in proper order and that segments of the same type do not join each other.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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