Mitosis in plant and animal cells

 

These micrographs show the various stages of mitosis in plant cells (onion root tips) and in animal cells (Ascaris ova). Remember, these are snapshots of cells at one point in a continuous process. The cells don't really stop and wait for us to take their pictures!

Interphase. This is usually the longest portion of the cell cycle, and is composed of three sections: 1) the time between the completion of the last mitosis and the onset of DNA synthesis G1), 2), DNA synthesis (S), and 3) the time between DNA synthesis and the onset of mitosis (G2). This is when the cell is most active, and genes are transcribed. The chromosomes are not condensed, and can't be distinguished from each other. The genes that encode the ribosomal RNA are gathered into one area of the nucleus, which stains more darkly than the rest. The first microscopists to see this dark area called it the nucleolus.

 

Interphase nucleus in an onion root tip.

 

Interphase nucleus of an Ascaris egg.

 

 


Prophase. In prophase, the chromosomes begin to condense into threadlike structures that become visible with a microscope. The nuclear membrane breaks up, and the spindle pole bodies (centrosomes in animal cells) duplicate and begin to move to the poles of the spindle. Spindle microtubules form, and the microtubules attach to the kinetochore (located at the centromere) of the chromosome. The beginning of prophase marks the onset of mitosis.

Prophase chromosomes in an onion cell.

Prophase chromosomes in an Ascaris egg.

 


Metaphase. Metaphase is much more prominent in fixed cell preparations than it is in real time. During prophase (and prometaphase, which can only be seen in live cells), the chromosomes are in constant motion. At metaphase, the chromosomes line up near the middle of the cell.

Metaphase chromosomes in onion root cells.

Metaphase chromosomes in an Ascaris egg. Note the prominent spindle poles.

 

 


Anaphase. Anaphase occurs in two parts: Anaphase A and Anaphase B. In Anaphase A, the centromeres of the sister chromatids separate, and the chromosomes begin to move apart as their spindle microtubules shorten. In Anaphase B, the spindle poles move further apart, dragging the chromosomes with them.

Anaphase A in onion cells.

Anaphase A in Ascaris eggs.

Anaphase B in onion cells.

Anaphase B in Ascaris eggs.

 

 


Telophase. When the chromosomes have reaached their furthest separation, they begin to decondense. The nuclear membrane reforms, and the mitotic spindle begins to disassociate. Often, a dense line of material forms at the midline of the cell. This is known as the telophasic bundle, and defines the point at which cytokinesis will occur. In plant cells, the telophasic bundle may help organize the cell plate. The end of telophase marks the end of mitosis.

Telophase in onion roots. Note the prominent telophasic bundle.

Telophase nuclei in Ascaris eggs. Note the beginning of nuclear division.

 

 


Cytokinesis. Cytokinesis (or cytoplasmic division) is not part of mitosis, since mitosis is defined as nuclear division, and some organisms can undergo several rounds of mitosis without dividing the cytoplasm. In plant cells, cytokinesis involves formation of a cell plate made of cellulose, which will become part of the cell wall. In animal cells, a ring of actin and myosin forms just inside the cell membrane, and this contracts to cause the division of the cytoplasm.