The causal sequence of meiosis
• . i . • pro6eeds"., and where crossing-over takes place. These prime wriables ..at6 ghe.eontaet-point, the th~e-lhnit of pairing and. the torsion in paired regions. Their action has been inferred from the comparison of pairing in ~dated forms witt{ variabie pairmg frequencies and ohiasma distribution ; from ~he pa.h-ing in hybrids, mutants and po]yploids; and h'om the ~ifieiaI !n}errnption of normal pairing. Suc]a comparisons have to be m~de ~vifla even ~ea te r accuracy and detgii if gee theory of meiotic p~mng.as to be ~ested and consolidated. The time has now arrived for an exac$ definitio~ of the individual aoti.ons and of the interactions of the t~{'egva.riables and of {Heir effect onohiasma formatim.~. k it}~iJ.y of pagfing variM;ion rest~Ith~g from genetic aad :from en~dr0nmental Souzces, i o an interspeeitie eo.mparison wibh varied environmental eoi~Ntigns,i.forms ~ahe most promising approach. ~,r,gt~igcvria., which h.a,s proved such Useful material in the discovery of ~he ele.me~lts o:[ mitosis and . . . . meiosis, is eminent.]y st~ita, ble :for further ana.].gsis} :[or the following rea.soJ.is:
Inversions inFritillaria
Summary1. In twenty-two species ofFritillaria no evidence of inversions has been found.2. In the following species, viz.F. dasyphylla 2x, F. dasyphylla 3x, F. pudica 3x, F. citrina andF. recurva 3x, the results of crossing-over in inversions have been observed.3. InversionA inF. dasyphylla 2x, located at, or near, the distal end of anS bivalent, has a frequency of about 70 per cent of one or two chiasmata in the inversion.4. The expected configurations for more than one chiasma proximal to the inversion are analysed.5. Chiasma formation in the bivalent carrying inversionA has been studied. The frequencies of the various configurations indicate a slight preponderance of comparate chiasmata.6. Cell-wall formation by acentric fragments and univalents has been studied inF. dasyphylla. The formation of a separate cell wall requires a position of the lagging unit satisfying the maximum repulsion from both daughter nuclei. The possession of a centromere by the micronucleus is not apparently indispensable.
Recent progress in engineering near-infrared persistent luminescence nanoprobes for time-resolved biosensing/bioimaging
Persistent luminescence nanoprobes (PLNPs) can remain luminescent after ceasing excitation. Due to the ultra-long decay time of persistent luminescence (PersL), autofluorescence interference can be efficiently eliminated by collecting PersL signal after autofluorescence decays completely, thus the imaging contrast and sensing sensitivity can be significantly improved. Since near-infrared (NIR) light shows reduced scattering and absorption coefficient in penetrating biological organs or tissues, near-infrared persistent luminescence nanoprobes (NIR PLNPs) possess deep tissue penetration and offer a bright prospect in the areas of in vivo biosensing/bioimaging. In this review, we firstly summarize the design of different types of NIR PLNPs for biosensing/bioimaging, such as transition metal ions-doped NIR PLNPs, lanthanide ions-doped NIR PLNPs, organic molecules-based NIR PLNPs, and semiconducting polymer self-assembled NIR PLNPs. Notably, organic molecules-based NIR PLNPs and semiconductor self-assembled NIR PLNPs, for the first time, were introduced to the review of PLNPs. Secondly, the effects of different types of charge carriers on NIR PersL and luminescence decay of NIR PLNPs are significantly emphasized so as to build up an in-depth understanding of their luminescence mechanism. It includes the regulation of valence band and conduction band of different host materials, alteration of defect types, depth and concentration changes caused by ion doping, effective radiation transitions and energy transfer generated by different luminescence centers. Given the design and potential of NIR PLNPs as long-lived luminescent materials, the current challenges and future perspective in this rapidly growing field are also discussed.