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SCI203A: Biology | Unit 3: Cell Biology | Lesson 24: Mitosis
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Begin: SCI203A: Biology | Unit 3: Cell Biology | Lesson 24: Mitosis
When a cell is ready to divide in two, it must first divide its nucleus in a process called mitosis.
The cell cycle has several phases.
Just as the entire organism has a life cycle from egg to larva to adult, each cell also has a cycle from interphase to mitosis to cytokinesis.
Interphase has 90% of Cell cycle. It consists of three separate phases
The Nucleus of the Cell divides into the identical nuclei, and a nucleus is created for each new cell.
The remainder of the cell divides, including the cytoplasm and cell membrane. In plants, something a bit different happens. The final result is 2 new identical body cells.
What does the chromosome structure look like?
There is a nuclear envelope with chromosomes in the middle. There are Sister chromatids and coiled DNA strands. Between, there is the centromere
Before the nucleus divides, the DNA copies itself and condenses into chromosomes.
Before a cell can divide into two cells, the DNA makes a copy of itself during interphase. The two DNA copies are joined together by a structure called a centromere.
Once the copying of DNA is complete, the long DNA strands condense into a thick X shape.
The condensed X shape is a chromosome. Each side of the X is a chromatid, and sometimes they are called sister chromatids. Sister chromatids are identical DNA copies of each other.
A cell's nucleus divides in two during mitosis, which occurs in four steps.
Once the chromosomes are copied and condensed, the cell leaves the GS phase and enters into mitosis. Mitosis is the part of cell division in which the nucleus divides into two identical nuclei. A special organelle used in mitosis, a centrosome, produces and organizes important proteins called spindle fibers.
Spindle fibers separate the sister chromatids from each other and pull them to opposite sides of the cell.
Spindle fibers are like fishing poles, catching and reeling in chromosomes. In animal cells, a pair of small structures called centrioles is found in the centrosome. Centrioles help organize the spindle fibers. Not all cells divide. Some never divide, and some may divide almost daily. Once formed, human nerve cells will not divide again. If damaged, they cannot be replaced. However, cells that line the human digestive tract may divide everyday.
Chromosomes move smoothly through the steps of mitosis. They are in the nucleus but have not condensed down into X-shaped chromosomes.
In this whitefish cell, it is difficult to see any specific chromosomes in the nucleus. The cell is preparing to divide but has not entered mitosis. During interphase, the cell will increase in size, copy all of its DNA, and build the organelles needed to complete cell division.
Prophase is the first step of mitosis.
The first step of mitosis is prophase. During prophase, three things happen: The condensed chromosomes are visible.
The membrane of the nucleus, or nuclear envelope, breaks apart.
The spindle fibers start to form between the centrosomes.
Whitefish cell in prophase
In this whitefish cell, the lumpy chromosomes are becoming visible in the nucleus. They begin to look speckled and splotchy. The membranes, spindle fibers, and centromeres are difficult to see. The centromeres are the dark pinkish areas on each side of the nucleus.
Metaphase is the second step of mitosis.
The prefix meta- means between or middle. In metaphase,
The chromosomes line up at the equator, or middle, of the cell.
The spindle fibers attach to the centromeres of the chromosomes, one on each side.
Anaphase is the third step of mitosis.
Chromatids become two separate sets of DNA during the third step of mitosis. During anaphase,
The spindle fibers pull at the centromeres, separating the sister chromatids.
Once separated, the sister chromatids are again called chromosomes.
The two identical sets of chromosomes are pulled by the spindle fibers to opposite poles of the cell.
This whitefish cell is in anaphase. The sister chromatids are being pulled apart at the centromeres, protecting the chromosomes' DNA code from damage.
Once the chromatids are pulled apart, they are moved to opposite sides of the cell. Anaphase, the third step in mitosis
Telophase is the fourth and final step of mitosis. Mitosis is now nearly complete. Remember, the two sets of chromosomes are identical to each other,
before mitosis began, the original DNA was copied. In telophase,
Two nuclear envelopes form around each set of chromosomes.
The chromosomes begin to unwind and lose their condensed shape.
The spindle fibers dissolve.
Cytokinesis results in two separate cells.
Cytokinesis is the process by which the remainder of the cell divides once mitosis is complete.
It is a separate process from mitosis, and is often described as a pinching-in of the cell membrane near both sides of the cell's equator. When the pinching-in is complete, two separate cells exist. In plants, rather than pinching-in, a cell plate forms. This cell plate eventually becomes a solid cell wall that separates the two new plant cells.
This whitefish cell is finishing cytokinesis.
Cytokinesis is a separate process from mitosis, but in many cells, it overlaps with telophase, the fourth and final step of mitosis.
During cytokinesis, the membrane pinches in and divides the cytoplasm between the new cells.
Review what you've learned about the cell cycle.
An easy way to remember the order of steps in mitosis is PMAT. The stepswprophase, metaphase, anaphase, and telophasewoccur in the order of the letters.
Finally, A cell nucleus divides in two during mitosis for the purpose of making a new, identical cell.
Cells spend most of their lives during interphase: growing, carrying out daily functions, and preparing to divide. Once the preparations are accomplished, which includes making a full copy of the DNA, the cell starts to divide. The first part of this process is called mitosis, where the nucleus of the cell divides in two.
The cytoplasm of the cell divides and completes making new cells in a process called cytokinesis. The result is two cells that are genetically identical to each other.
Mastery Map: The cell cycle includes three things, Interfase, mitosis, and cytokinesis.
End of SCI203A: Biology | Unit 3: Cell Biology | Lesson 24: Mitosis
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