where do the microtubules of the spindle originate during mitosis in both plant and animal cells?
MTOC (microtubule organizing center)
During mitosis, microtubules of the spindle originate from a structure called the centrosome. The centrosome acts as the main microtubule organizing center (MTOC) in animal cells, while in plant cells, a similar function is performed by the spindle pole body (SPB). Both the centrosome and the SPB have a pair of cylindrical structures called centrioles.
To understand how microtubules of the spindle originate, let's break it down for both plant and animal cells:
1. Animal Cells:
- In animal cells, the centrosome is located near the nucleus. Prior to mitosis, the centrosome duplicates, resulting in two pairs of centrioles.
- As the cell enters mitosis, the two pairs of centrioles begin to move away from each other, positioning themselves at opposite poles of the cell.
- At each pole, the centrioles organize and nucleate the formation of microtubules, extending outwards in all directions. These microtubules form a radial array called the aster.
- Some of these microtubules elongate towards the center of the cell, forming the spindle fibers that will attach to the chromosomes during mitosis.
2. Plant Cells:
- Unlike animal cells, plant cells lack centrosomes and centrioles. Instead, they have SPBs, which serve as their MTOCs.
- The SPBs are embedded within the nuclear envelope, one at each pole. They are involved in the nucleation and organization of microtubules during mitosis.
- As the cell enters mitosis, the SPBs perform a similar function to animal cell centrosomes. They nucleate microtubules, and additional microtubules are elongated from them towards the center of the cell to form the spindle.
In summary, the microtubules of the spindle in both plant and animal cells originate from the centrosome/centrioles (in animals) or the spindle pole body (in plants). These structures act as the main centers for nucleating and organizing the formation of microtubules during mitosis.