Thursday, September 30, 2010

Sex cells......Meiosis!

SO....once in a while an organism needs to reproduce.

Some organisms can make copies of their own cells that will become separate organisms that live separate from their parent.  Reproduction without any other contribution is asexual.

However some organisms create special cells known as gametes that only have 1/2 the normal amount of chromosomes as other body cells in that organism's structure.  Sexual reproduction is a process that requires these gametes and there is a specialized process to create these particular cells known as meiosis.

Meiosis is cellular division that creates gametes aka sex cells aka reproductive cells aka in humans called sperm and eggs.

A cell going through meiosis only takes place in the gonads of a human.  Each cell that will form gametes goes through the following stages:

1. Interphase: before meiosis begins, the chromatin (DNA) must replicate just like in interphase prior to mitosis.

2. Meiosis I begins!

a. prophase 1

Chromosomes appear as they do in mitosis, except a very special event is taking place.  Homologous chromosomes (chromosomes that have similar gene loci  ex. both contain different versions of the same blood type gene) pair up in a tetrad.  The pairing is known as a tetrad because there are 4 individual chromatids in the grouping and "tetra" means 4.   Tetrads form so that they can exchange little pieces of DNA.

Imagine that you are back in elementary school and it's lunchtime.  You pull out your sandwich and sigh because you have turkey. AGAIN!  You peer into your friend's bag and become jealous because they have a peanut butter and grape jelly sandwich that sounds SO much better than yours.  After some quick negotiation, you trade your sandwich for theirs and everyone is happy.

This exchange of sandwiches is much like the exchange of DNA during crossing over.  The section of DNA that is exchanged between nonsister chromatids is from the same gene (i.e. eye color isn't exchanged for blood type).

Once crossing over has occurred, the chromosome head towards the center still in their tetrads.

b. metaphase 1

Each tetrad lines up on the center plate between the centrioles instead of lining up in a straight line as the chromosomes do in mitosis.

Each set of homologous pair remains together like holding hands with a buddy and standing in line.

c. anaphase 1

Because each homologous chromosome is pulled to an opposing side, each cell is on its way to being haploid.  Remember diploid means having both members of each set of homologous chromosomes.

d. telophase 1

Each resulting cell now has 1/2 the chromosomes (n) as the original cell (sometimes called the parent cell).
3. Meiosis II begins and proceeds pretty much just like mitosis.

a. prophase II

In each cell the single homolog prepares to separate into gametes

b. metaphase II

Each homolog lines up in a straight line along the metaphase plate and centrioles attach spindle fibers to the centromere in order to now separate each chromatid from their sister.

c. anaphase II

Each chromosome in each of the two cells is split from its sister chromatid and head toward opposing poles.

d. telophase II

The nucleus reforms around the 1/2 set of DNA and the cell prepares for cytokinesis.

At the end of this process, we have 4 gametes that each have 1/2 the set of chromosomes that were in the original cell as opposed to the 2 clone cells that mitosis produces.

To review, click on the link below to watch an animation that should make this process clear.

Meiosis YouTube Review Video

Be sure that you look at the differences and similarities between mitosis and meiosis.  Below I have posted some diagrams and other helpful tools to guide you in your review.
Nova has a great comparison website that is interactive and puts each phase side by side.  Click on the link below and enjoy!

Nova comparison website

Below is a link to a great comparison chart.  Click away my pretties!

Comparison chart

Watch this cute old man biology teacher as he helps you hash out the differences between the two processes!

Video comparison

I hope all this information is wrinkling into your brains!  See you in the a.m.!


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