Question

1- There are some cells in our body in which mitosis does not occur, and damaged cells cannot be healed. What are these cells? 2- Briefly explain the check-points of the cell cycle. 3- Write down the phases of mitosis in occurring order and explain each of them with figures. 4- Which part of the root is more suitable to observe mitosis? Mark the appropriate area of the root on the image below, and write down the reason. 5- Describe the formation of the cell-plate in plants.

Answer 1

1. Nerve cells. In nerve cells or neurons, mitosis doesn't occure. Once damaged, they cannot be healed.

2. There are three checkpoints in cell cycle:

• Near the end of G1(G1checkpoint)
• End of G2(G2 checkpoint)
• During metaphase(M checkpoint)

The G1 checkpoint determines whether all conditions are okay for the cell division to proceed. It checks whether DNA is damaged or not.

The G2 checkpoint checks whether the replication of DNA is coompleted and then if completed, whether the replicated DNA is damaged or not.

The M checkpoint determines whether all sister chromatids are correctly attached with the spindle fibres.

3.Mitosis consists of four phases.

• Prophase
• Metaphase
• Anaphase
• Telophase

During prophase, chromosomes gets duplicated with two sister chromatids attached to each other tightly at the centromere.

During metaphase,individual chromosomes line up at metaphase plate by the action of the spindle formed. And they are now ready to divide. Spindles are capturing each sister chromatids from opposite poles to drag them and break the connection in next phase.

During anaphase, sister chromatids seperate from each other and are pulled towards opposite ends of the cell via spindles.

During telophase, chromosomes are decondensing and spindles are disappearing and the cell is almost divided and cytokinesis takes place.

4. Apical meristem at the root tip just inside the root cap and surrounded by the root cap.

Here, the cells are undifferentiated, so it is capable of mitosis and cell division unlike other differentiated cells.

$1588514890031_image.png$

5.

The cell plate is formed in plants because they need cell wall. First, a phragmoplast is formed when golgi vesicles are transported on microtubules. The vessicles fuse and and coalesce from the center toward the cell walls. The structure thus formed is the cell plate. It grows as more vesicles fuse. And it grows till it merges with the cell walls at the periphery of the cell.