Which microscope provides image reconstruction from thousands of 2D projections?
Single-particle cryo-EM reconstructs three-dimensional density maps from thousands to millions of randomly oriented two-dimensional projections of vitrified molecules. Each particle image represents projection of electrostatic potential; orientation determination, classification, and averaging via algorithms yield 3D structure without tilting. This contrasts with AFM or SIM acquiring direct surface images. By combining many projection views computationally, cryo-EM overcomes low signal-to-noise of individual exposures and achieves atomic detail of ribosomes, ion channels, and viruses, facilitating mechanistic insights impossible with single projections alone in conventional electron microscopy.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.