Chemical mutagens can be divided into two groups:

those that are mutagenic to both replicating and nonreplicating DNA, such as the alkylating agents and nitrous acid; and 

those that are mutagenic only to replicating DNA, such as base analogs— purines and pyrimidines with structures similar to the normal bases in DNA.

TLS Online TPP Program

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#Unit 2. Cellular Organization

The CaM*-Ca2+ complex binds and activates MLC kinase, phosphorylates the myosin LC. 

TLS Online TPP Program

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#Unit 2. Cellular Organization

Phosphorylation of MLC 

unfolds the myosin II, active and can assemble into bipolar filaments for contraction

TLS Online TPP Program

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#Unit 2. Cellular Organization

When the Ca2+ levels drop:

Myosin LC is dephosphorylated by MLC phosphatase, causing muscle relaxation.

TLS Online TPP Program

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#Unit 2. Cellular Organization

Sarcomere

Within each skeletal muscle cell are many myofibrils consisting of a repeating array of a specialized structure

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#Unit 2. Cellular Organization

Each sarcomere contains two major types of filaments: 

Thick filaments (myosin II)

Thin filaments (actin and associated proteins)

TLS Online TPP Program

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#Unit 2. Cellular Organization

Z disk 

Thin actin filaments are assembled with their (+) ends embedded in a densely staining structure