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#initiation

3 public questions tagged with this topic.

Periodic Motion and SHM Basic Concepts

What causes the restoring force in a spring-mass system to initiate SHM?

**Periodic motion** repeats after fixed period T, x(t+T)=x(t), while oscillatory motion involves to-and-fro about equilibrium. SHM is special periodic motion where restoring force proportional to displacement, F = -k x, acceleration a = -ω² x, leading to sinusoidal displacement x = A cos(ωt + φ). The elasticity of the spring generates a restoring force ( F = -kx ) proportional to displacement, driving the oscillatory motion characteristic of SHM. Applying x = A cos(ωt + φ), v = -ωA sin(ωt + φ), a = -ω²x and E = ½kA² = ½mω²A², result Elasticity of the spring follows, reflecting SHM dependence o

Ref: NCERT > Physics Book > Oscillations > Periodic Motion and SHM Basic Concepts

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Translation Basics

Which ribosomal site first binds initiator tRNA?

Initiator methionyl-tRNA occupies P site first, not A site, during translation initiation. In bacteria, IF2 GTP delivers fMet-tRNAfMet directly to P site base-pairing with AUG positioned by Shine-Dalgarno interaction. In eukaryotes eIF2 GTP delivers Met-tRNAi to 40S P site within 43S complex. This positioning sets reading frame, allowing second aminoacyl-tRNA entry into A site for first peptide bond. P site initiation distinguishes from elongation where tRNAs enter A site, ensuring directionality, start site selection and preventing out-of-frame translation initiation events.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 7, P-site first binding of initiator tRNA during initiation

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Replication in Prokaryotes

Primase in E. coli is

Because DNA polymerases cannot start chains de novo, lagging strand requires frequent primers to initiate each Okazaki fragment. Function executed by DnaG primase, monomeric RNA polymerase synthesizing short 10 to 12 nucleotide RNA primers on SSB-coated single-stranded DNA template. DnaG interacts transiently with helicase DnaB at replication fork, each unwinding event triggering primer synthesis when exposed sequence recognized. Primers provide essential 3' OH for extension by Pol III. DnaA initiates origin melting, DnaB unwinds parental duplex, DnaC loads helicase, but DnaG uniquely provides

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: DnaG primase function at bacterial replication fork

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