Meiosis & Mitosis

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Meiosis & Mitosis

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Cell Cycle
1. G1-growth phase
2. S – synthesis phase (DNA replication)
3. G2-2nd growth phase (prepares for division)
Mitosis- asexual reproduction of somatic (body) cells for growth & repair
1. Prophase-chromosomes condense, nucleus disappears, spindles form
2. Metaphase-chromosomes align at equator
3. Anaphase-chromosomes pull apart from one another at centromere
4. Telephase-chromosomes move to opposite ends
5. Cytokinesis- Cytoplasm splits into 2 cells
Meiosis-sexual reproduction of germ cells for production of egg & sperm
1. oogenesis- produces 1 egg & 3 polar bodies
2. spermategenesis-produces 4 sperm

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Today we are going to review the basics of eukaryotic cell division which includes mitosis and meiosis.

So cells splitting can be referred to as cell division or cells replicating which means the same thing.

Cells follow a life cycle (much like a frog, butterfly or us) and that process is divided up into stages. Interphase is the longest and where a cell spends most of its time. within interphase a cell first grows (G1) like a newborn baby and just does what it is supposed to do as a cell. Eventually it might do on to double its information (S phase) much like the school years) and finally after that a cell may grow and develop parts to prepare for dividing (say like like marriage, buying a house).

Beyond this the cell could do a variety of things depending on the cell type. A neuron for example will usually go into G0 its resting stage. A body cell will do mitosis when needed for growing and repairing. And finally germ cells will perform meiosis when triggered to produce gametes.

So let’s take a look at what a cell needs to do in order to make an identical copy of itself. This first stage is prophase and pro means for or before. Here we have stuff that needs to happen before division. Chromosomes need to condense, that nuclear membrane needs to dissolve and spindle fibers are going to go fishing for chromosomes.

Next up is metaphase, where those chromosomes line up at the equator… meta means change and in this case the change is moving to the middle.

Ana means apart and by this we can see those chromosomes have been pulled apart and migrating away from one another.

And here we reach the end… and well that’s convenient because telo means end. And the chromosomes at this stage have reached the opposite ends of the cell and nuclei are going to begin reforming at each end. Beyond this the cytoplasm in the middle needs to split and we have 2 identical cells!!

But a select group of cell called germ cells take a different route. These cells are called germ cells and are destined to become egg or sperm. This process is part of sexual reproduction (however this part never makes the R rated movies. Oogenesis is for making eggs and usually only one a month is lavished upon and survives and spermatogenesis makes sperm. Of course fertilization is what most people know to be sexual reproduction but really it’s only half the story!

So in summary…. all cell have a life cycle and most cells are in interphase growing, copying DNA. SOme cell may cease to divide and enter G0 like nerve cells. Other body cells do mitosis, asexual reproduction to make exact copies ofr growth and repair. ANd finally our germ cells go through meiosis in order to make egg and sperm that may just meet up during fertilization.

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