Mice that are deficient in either the Pms2 or Msh2 DNA mismatch repair genes have microsatellite instability and a predisposition to tumours. Interestingly, Pms2-deficient males display sterility associated with abnormal chromosome pairing in meiosis. Here mice deficient in another mismatch repair gene, Mlh1, possess not only microsatellite instability but are also infertile (both males and females). Mlh1-deficient spermatocytes exhibit high levels of prematurely separated chromosomes and arrest in first division meiosis. We also show that Mlh1 appears to localize to sites of crossing over on meiotic chromosomes. Together these findings suggest that Mlh1 is involved in DNA mismatch repair and meiotic crossing over.

Precise control over molecular movement is of fundamental and practical importance in physics, biology, and chemistry. At nanoscale, the peculiar functioning principles and the synthesis of individual molecular actuators and machines has been the subject of intense investigations and debates over the past 60 years. In this review, we focus on the design of collective motions that are achieved by integrating, in space and time, several or many of these individual mechanical units together. In particular, we provide an in-depth look at the intermolecular couplings used to physically connect a number of artificial mechanically active molecular units such as photochromic molecular switches, nanomachines based on mechanical bonds, molecular rotors, and light-powered rotary motors. We highlight the various functioning principles that can lead to their collective motion at various length scales. We also emphasize how their synchronized, or desynchronized, mechanical behavior can lead to emerging functional properties and to their implementation into new active devices and materials.

A. Sample preparationThe YBa 2 Cu 3 O 6.5 (YBCO) single crystal used here was grown by top-seeded solution growth using a Ba 3 Cu 5 O solvent [1]. The as-grown single crystal was first annealed at 700°C for 70 h with flowing oxygen and quenched down to room temperature. The La 1.85 Sr 0.15 CuO 4 (LSCO) single crystal was synthesised by a travelling-solvent-floating-zone method utilizing infrared radiation furnaces (Crystal system, FZ-T-4000) and annealed in oxygen gas under ambient pressure at 600°C for 7 days[2]. B. Femtosecond pump-probe set-upA detailed description of the set-up used is found in [3]. In a "pump-probe" experiment, a "pump" pulse excites the sample and the induced change in transmission or reflection of a delayed probe pulse monitors the relaxation behaviour. In the linear approximation ∆R R directly tracks the electronic relaxation processes, and the time constants obtained from fits of its dynamics are the characteristic times of the underlying relaxation processes. In our data, this approximation is justified by two essential characteristics: (i) the ∆R R amplitude is linear in the excitation intensity (see Figure 1a for LSCO), and (ii) the same decay times appear independently of the probe wavelength, only with different spectral weights of the individual components.In order to resolve the dynamics of fast processes very short pulses are necessary, since the instrumental response function is given by the cross correlation between the pump and probe pulses. We use sub-10 fs probe pulses from an ultrabroadband (covering a spectral range from 500 to 700 nm) non-collinear optical parametric amplifier (NOPA) and ∼15 fs pump pulses from a narrower band (wavelength tunable, in our case centred at 530 nm) NOPA. The seed pulses for the NOPAs and the amplified pulses are steered and focussed exclusively with reflecting optics to avoid pulse chirping.A schematic of the experimental apparatus is shown in Fig. 1. The laser source is a regeneratively amplified modelocked Ti:sapphire laser (Clark-MXR Model CPA-1), delivering pulses at 1 kHz repetition rate with 780 nm center wavelength, 150 fs duration, and 500 µJ energy. Both NOPAs are pumped by the second harmonic of the Ti:sapphire laser, which is generated in a 1-mm-thick lithium triborate crystal (LBO), cut for type-I phase matching in the XY plane (θ = 90°, ϕ = 31.68°, Shandong Newphotons).The ultrabroadband visible NOPA that generates the probe pulses has been described in detail before[4]; a schematic of it is shown in Fig. 2. The white light continuum seed pulses are generated by a small fraction of the fundamental