Science Class- Feb 2-6

This week, we learned about meiosis.  Meiosis is reproduction for the sex cells.  These cells are haploid, and only have 50% of their DNA.  The other cells with 100% of DNA are called diploid.  We started to learn what happens during the phases, and how it is similar and different from mitosis.

The parent cell starts by replicating the DNA.  The cell then goes through meiosis I, and ends up with two cells with the right amount of DNA.  In mitosis, the chromosomes turn back to DNA when it is over.  In meiosis, the chromosomes stay as chromosomes.  Right after that, without replicating DNA, those cells go to meiosis II.  The cells start with 100% DNA, and then they split in half and only have 50% DNA.

Phases of meiosis:

meiosis I

prophase I- nucleus starts to break down, homologous chromosomes connect and the homologues cross over, centrioles move to the poles of the cell  

metaphase I- homologues align next to metaphase plane next to each other, and a spindle fiber attaches to each homologue

anaphase I- homologues are separated and go to opposite poles  

telophase I- nuclei begin to form back and cells begin to separate

cells separate and meiosis II starts

prophase II- ( no DNA replication, and chromosomes are still chromosomes ) spindle fibers attach to centromeres and nucleus begins to fade   

metaphase II- chromosomes line up on the metaphase plane

anaphase II- chromatids separate and move to opposite poles

telophase II- cells start to pull apart and nucleus begins to form

The cells then separate and the chromosomes turn back to chromatin.  The new cells have 50% of the DNA.  

During the meiosis I, it is the homologues that are splitting.  In meiosis II, the chromosomes are splitting ( like in mitosis ).