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Question

Label the parts of the actin molecule and recognise the events 4 and 5 in the presence/absence of calcium.



Your Answer
A

1.Actin                  2.Tropomyosin      3.Troponin           4.Sites exposed          5.Sites Blocked

Your Answer
B

1.Troponin                 2.Actin             3.Tropomyosin      4.Sites Blocked         5.Sites Exposed

Correct Answer
C

1.Actin                 2.Tropomyosin      3.Troponin           4.Sites Blocked           5.Site Exposed

Your Answer
D

1.Tropomyosin         2.Actin                3.Troponin         4.Site Exposed              5.Site Blocked


Solution

The correct option is B

1.Actin                 2.Tropomyosin      3.Troponin           4.Sites Blocked           5.Site Exposed


Thin filaments consists of 3 proteins : Actin, tropomyosin and troponin.  
The backbone of the thin filaments is formed by actin molecules joined into 2 strands and twisted together like 2 intertwined strands of pearls.
Tropomyosin molecules are thread like proteins that lie end to end alongside the groove of the actin spiral. in this position tropomyosin covers the actin sides that bind with the cross bridges blocking their interaction that leads to muscle contraction. the other thin filament component troponin is a protein complex made up of 3 polypeptide units. 
The formation of a cross bridge is initiated when a calcium ion released from a sarcoplasmic reticulum bind to troponin. this binding causes troponin to change shape. with this the tropomyosin moves away from the myosin binding site on actin.

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