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One of the mechanisms of transferring bacterial colonies (clones) from one agar plate to another is by using the replica plating technique.

Why would a scientist mark the original Petri dish, as well as the other dishes at “noon,” with a permanent marker before placing a sterile velvet on a block on top of the colonies?


Because scientists want to differentiate colonies on a plate from colonies on the sterile velvet block. They are two sets of colonies, and it is always important to mark the original plate.


Because scientists want to match the mark on the original plate with the sterile velvet on the block in order to have the same spatial pattern in all other plates. This way they can identify specific colonies with unique characteristics.


By marking the original plate at a specific site, colonies can have a point of reference, and they can have a direction as they spread and become confluent with each other.


Because the plate moves easily, marking it and clamping it at the site of the mark will allow for better transfer onto sterile velvet blocks.


Because the marked area is the location where a paper with a white background is usually attached, it facilitates the detection of colonies.

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