The genetics of recombination / D.G. Catcheside.

  • Catcheside, D. G.
Date:
[1977]
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: The genetics of recombination / D.G. Catcheside. Source: Wellcome Collection.

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    7.2 Lambda bacteriophage 129 cohesive ends. Their sequences have been determined by synthesizing, with the aid of DNA polymerase, a labelled oligonucleotide com¬ plementary to each of the single stranded ends. Each proves to be a sequence of twelve nucleotides, predominantly G and C, that is com¬ plementary to the other (Fig. 7.4). g, G GG CGGCGACCT J | 3. Í 6' 12' M V с' 3' I I ] Ê li 5' rll CCCGCCGCTGGA -• 4-6x10^ nucleotide pairs ► Fig. 7.4 Lambda bacteriophage, illustrating the molecular structure of its DNA, particularly of the free ends of the infective phage. The genetic maps of lambda phage are illustrated in Fig. 7.7. The map of the lytic phage is circular, while that of the prophage is an arc of the larger circle of the E. coli chromosome. As a result of integration some genes, such as J and int, that are fairly close in the circular map of the lytic phage are widely separated in the prophage. In the lytic cycle the linear DNA, after injection, rapidly assumes a circular structure 17 /im long. Although united initially by hydrogen bonds, the nicks are soon sealed by a DNA ligase, already present in the bacterium, to form 'closed circles'. It reproduces in this form, commenc¬ ing at an initiation site located in the right half of the molecule near to the right end of the genetic map and proceeding at first to the left. When bacteria are infected with / phage having isotopically dense DNA, most of the closed circles quickly become isotopically hybrid. Later, closed circles without any atoms from the infecting phage are found. Re¬ plication of this sort is semiconservative. By 15 minutes after infection about twenty circles are present in each bacterium. 'Nicked circles' with a break (or perhaps several breaks) in one chain are also found ; they appear to be intermediates in the replication of closed circles. In electron micrographs of molecules in the first round of replication, most of those that can be followed completely have three loops joined at two points of branching (Fig. 7.5a). Two of the loops (a and a') are alike in length and are the replicated parts. The two segments a and b together equal the characteristic length of the / DNA molecule. Expressed as a fraction of this length the a and a' segments range from nearly zero to nearly unity. Other configurations observed could be regarded as derived from the common structure by breakage. The two branch points could be regarded respectively as the site of initiation and the growing point in replication, but in fact both are growing points. It is possible to analyse the structure by observing its molecular denaturation. Exposure of DNA to high pH or high tempera-