Contraction of myofibrils within a muscle fiber begins when

calcium is released from the sarcoplasmic reticulum

Explanation:

Muscle contraction is the stimulation of sites that produce stress within muscle fibres.

Calcium ions bind to tropomyosin in inducing muscle fibre contraction, exposing active site for Crossbridge formation on actin. Then ATP binds to myosin, transferring the myosin to its high-energy state, freeing the myosin head from the active actin site. Triggering tension-generating sites within muscle fibres.

Hence, the contraction process is initiated with teh relaease of calcium from  sarcoplasmic reticulum.

you are certainly talking about this picture:

a: is the label of the white square, it is showing the  replication fork, formed by helicase (d) ==> so it is the site of dna separation and replication 

b: is a new dna brand that is following directly the replication fork, so it is continuous ==> built continuously from 5' to 3

c: dna polymerase cannot replicate dna on its own, it needs the of their proteins, and one of them plays the role of a clamp which fixes  the dna with the dna template and increases  the processivity of the enzyme. ==>   stops polymerase from falling off the  template.

d: is the enzyme responsible for forming the replication fork (helicase) ==> unwinds and separates dna into two strands

e: is the delayed strand, it is built  in the opposite sense of the helicase, so dna will be built in fragments ==> built  fragments from 5' to 3 joins

and finally f: is the enzyme that builds  the new dna strand (dna polymerase) ==>   adds nucleotides to make a new dna helix.

credit: gradman555p98x9p

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