Site-Specific Recombination Involves Breakage and Reunion

KEY CONCEPT
- Cleavages staggered by 7 bp are made in both attB and attP, and the ends are joined crosswise.
The att sites have distinct sequence requirements, and attP is much larger than attB. The function of attP requires a stretch of 240 bp, whereas the function of attB can be exercised by the 23-bp fragment extending from −11 to +11, in which there are only 4 bp on either side of the core. The disparity in their sizes suggests that attP and attB play different roles in the recombination, with attP providing additional information necessary to distinguish it from attB.
Does the reaction proceed by a concerted mechanism in which the strands in attP and attB are cut simultaneously and exchanged? Or,are the strands exc hanged one pair at a time, with the first exchange generating a Holliday junction and the second cycle of nicking and ligation occurring to release the structure? The alternatives are depicted in FIGURE 1.

FIGURE 1. Does recombination between attP and attB proceed by sequential exchange or concerted cutting?
The recombination reaction has been halted at intermediate stages by the use of “suicide substrates,” in which the core sequence is nicked. The presence of the nick interferes with the recombination process. This makes it possible to identify molecules in which recombination has commenced but has not been completed. The structures of these intermediates suggest that exchanges of single strands take place sequentially.
The model illustrated in FIGURE 2. shows that if attP and attB sites each suffer the same staggered cleavage, complementary single-stranded ends could be available for crosswise hybridization.
The distance between the lambda crossover points is 7 bp, and the reaction generates 3′–phosphate and 5′–OH ends. The reaction is shown for simplicity as generating overlapping single-stranded ends that anneal, but actually occurs by a process akin to the recombination event . The corresponding strands on each duplex are cut at the same position, the free 3′ ends
exchange between duplexes, the branch migrates for a distance of 7 bp along the region of homology, and then the structure is resolved by cutting the other pair of corresponding strands.

FIGURE 2. Staggered cleavages in the common core sequence of attP and attB allow crosswise reunion to generate reciprocal recombinant junctions.

اخر الاخبار

اخبار العتبة العباسية المقدسة

المجمَع العلمي يطلق الدورة التطويرية الخامسة لمعلمي التربية الإسلامية ومدرسيها في النجف الأشرف

خدمات إنسانية متنوّعة تقدّمها شعبة الشهداء والجرحى في مختلف المحافظات

قسم العلاقات العامة يطلق فعاليات مخيم البذرة الواعدة الخاص بأبناء المنتسبين

جامعة العميد تنظم سلسلة دورات تطويرية لملاكاتها الإدارية

المزيد

الآخبار الصحية

اختراق علمي.. دواء يؤخر ظهور السكري من النوع الأول لسنوات

هذا ما يحدث لجسمك إن أهملت نظافة فراشك

لماذا تعتبر النساء أكثر عرضة للإصابة بألزهايمر من الرجال؟

لماذا يحذر الخبراء من الإفراط في تناول الفراولة؟

المزيد

الآخبار التكنلوجية

السيارات الكهربائية أنظف بنسبة 73% من سيارات الوقود

سيارة لوسِد الكهربائية تدخل "غينيس" بأطول مدى في العالم

نظرية جديدة تنسف كل ما نعرفه عن تشكل أول قارة في الأرض

منذ 37 ألف عام.. دراسة تكشف "أقدم أمراض" أصابت البشر

المزيد

مواضيع ذات صلة

Mating Type Conversion in Saccharomyces cerevisiae

The Yeast Cell Cycle

Structure and Function of TFIIIA

Switching from Oocyte to Somatic 5S Synthesis

Genome Editing of Immune Effector Cells for Safer and More Potent Cancer Therapies (CAR-T Cells)

Therapeutic Genome Editing for Hematologic Diseases: β-Thalassemia

Therapeutic Genome Editing for Hematologic Diseases: Sickle Cell Disease

TFIIIA Binding to the Middle of its Gene as Well as to RNA

Biology of 5S RNA Synthesis in Xenopus

Entropy, a Basis for Lambda Repressor Inactivation

Induction from the Lysogenic State

The Need for and the Realization of Hair-Trigger Induction