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Question Video: Identifying Regions in the Sarcomere Biology

The diagram provided shows the basic structure of a sarcomere. Which letter indicates the I band?

01:40

Video Transcript

The diagram provided shows the basic structure of the sarcomere. Which letter indicates the I band?

As we can see in this image of a sarcomere, it is made up of two main filaments: actin myofilaments, which are shown in red, and myosin myofilaments, which are shown in blue. Actin is a thinner filament made up of two strands of protein fibers twisted together. Myosin is a thicker filament and is a long rod-shaped fiber with globular heads that project outwards. The different letters in this diagram show us the different regions of the sarcomere. So, let’s go through each one by one to find out which is labeling the I band.

The region labeled V right in the middle of the sarcomere is called the M line. The M line is found within a region called the H zone, which has been labeled here with the letter X. The H zone contains myosin filaments only. When the sarcomere contracts, the actin filaments will be pulled closer towards the M line and the H zone will shorten. The region labeled with a Z is called the A band. The A band encompasses the H zone, but it also contains regions around its outer edges where actin and myosin overlap, which makes these regions appear slightly darker.

The length of a single Sarcomere is measured as the distance between two Z lines, which on this diagram were indicated by the letter W. As the only letter remaining is Y, we can tell that this is labeling the I band. The I band is a region on the sarcomere that only contains actin filaments, which makes it appear lighter in color and is the reason why it’s sometimes called the light band comparatively to the other regions of the sarcomere which will all contain some myosin filaments and will therefore appear darker. Therefore, the letter indicating the I band is Y.

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