Cell DivisionMeiosis and Mitosis

Meiosis• Meiosis was the creation of gametes (sperm/egg) that are haploid.

• Haploid meant that the gametes contain ½ the number of your chromosomes.

• Your cell goes through this type of division to be sure that you only pass along one copy of each chromosome to your child.

• When the sperm fertilizes the egg, the new baby is conceived.

• But how did you go from that one tiny cell to this huge hunk of stuff sitting in your chair?

Mitosis

• The process of making new cells is called mitosis.

• Basically, when you grow, you don’t just get bigger. What really happens is that you add new cells. This makes more of you and we see that as “growing”.

• Every cell in your body is a clone (or a copy) of the others.

• “If all of your cells are alike, why do we have brain cells and heart cells and stomach cells then?”

• We’ll answer that question later!!

So What is This Mitosis Thing?

• Mitosis is the division of cells.

• In meiosis, I wanted to pass my chromosomes on to my kid. In mitosis, I want to use the chromosomes for me to tell my cells what to do.

• This time, I want to keep all of my chromosomes, so they will make diploid cells. These new cells are copies of the original cells way back from when sperm and egg first got together.

Does it happen all at once?• No. All good things take time.

• In fact, your cell has a life cycle and just like you, it is

born, it lives, it reproduces and it dies.

• The part of a cell’s life when it grows and lives is called Interphase and it has 3 distinct parts:

• G1: The first growth stage happens after the cell divides into two. Here the cell grows as it goes through cellular respiration and other reactions. Think of this as birth through the teenage years.

• S: This second stage is called synthesis. This is where your cell makes a copy of your DNA. This will be passed on to the new cell. Think of this as the college years as you finish college and start to look for a life mate.

• G2: This second growth stage is when the cell replicates all of the other organelles so that the new cell will have everything it needs. Thinks of this as your 20s when you prepare a home for your new family.

Huh?• So in other words, once your cell has lived a little, it’s

time to start making little cells that look just like the original one. In fact, they are so like the original one, we call it cloning!

• Making a new cell is actually pretty easy. First you have to split the DNA into two groups. Then you split all of the other cell organelles. This is what mitosis does.

• This ensures that the new cell will have a copy of everything it needs to survive.

• Let’s go through the steps so you know what they are.

Prophase• This cell is in Prophase. It is the first phase

as the DNA divides into two groups.

• Notice that some weird things are happening.

i. The nucleus is being to break apart. The cell will need all of the cytoplasm space to split the DNA so the nucleus will break down the end of mitosis.

ii.The chromosomes are also getting smaller. At this point, we can see individual chromosomes

iii.The centrioles are beginning to produce spindle fibers – the lines that are coming from them.

Draw this picture in the box marked “Prophase”.

Metaphase• This cell is in Metaphase. This

is the second phase of mitosis.

• The centrioles have produced spindle fibers that are attached to the centromere (middle part) of the chromosome.

• The chromosomes are now lined up in the middle of the cell. This makes it very easy to pick out this phase of mitosis.

Draw this picture in the box marked “Metaphase”.

Anaphase

• This cell is in anaphase. It is the third phase of mitosis.

• The chromosomes are pulled to the poles or ends of the cell. This separates the chromosomes into the two new groups. Each cell must have an exact copy of bad things could happen.

Draw this picture in the box marked “Anaphase”.

Telophase

• This cell is in telophase. It is the fourth phase of mitosis.

• Since the DNA has been split, the nucleus will come back and now surrounds each group of DNA

• The cell will begin to split by pinching in in the middle of the cell.

Draw this picture in the box marked “Telophase”.

Cytokinesis• The cells finally split during

cytokinesis.• Some books put cytokinesis as part

of mitosis, that’s why there are 5 steps on your study guide.

• Cytokinesis is characterized by the cleavage furrow.

• The cell will eventually split completely, making two new daughter cells that have identical DNA.

Draw this picture in the box marked “Cytokinesis”.

When does my body do mitosis?

• Anytime that you need to make new cells, your body will make them through mitosis. Here are some examples:When you need to grow, you will add more cells by mitosis.

When you need to repair something like a cut, a broken bone or a strained muscle, you replace those cells by mitosis.

When you need to replace cells that have died.

So why do I have stomach cells and heart cells and brain cells and lung

cells?• Remember that each cell has the same copy of your DNA, so

there is a chance for a newly made cell to become any kind of cell in your body. (This will become important next quarter.)

• What makes your cells into skin cells or muscle cells are the proteins that the cells around them produce. So if you are making new cells in the cut in your skin, it knows to act like skin because of what the skin cells around it do. If the new cell was made in your brain, then it would act like a brain cell and so on.