Question

Record the stages of meiosis in eukaryotic cells realizing that there are two cell divisions involved, each having their particular terms and characteristics. 2. list the key differences between mitosis and meiosis. 3. do prokaryotes have mitosis or meiosis? 4. crossover occurs between homologous pairs of chromosomes, but can it occur between two different chromosomes, say between #1 and #17? 5. was it an evolutionary necessity that meiosis evolved at the same time as sexual reproduction?

Answer 1

1.1
Meiosis I
The first meiotic division: diploid → haploid
    Prophase I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes join and occurs crossing over.
    Metaphase-I: the homologous chromosomes align in the middle of the cell. Spindle fibers from the centrosomes connect to the chromosomes.
    Anaphase -I: Spindle fibers contract and split the homologous chromosomes, moving them to opposite poles of the cell.
    Telophase -I: Chromosomes decondense; cell divides to form two haploid cells.

1.2 Meiosis II
The second division: separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)

     Prophase II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
    Metaphase-II:  the chromosomes align in the middle of the cell. Spindle fibers from the centrosomes connect to the chromosomes (at the centromere)
    Anaphase-II: Spindle fibers contract and split the sister chromatids, and moves them to opposite poles of the cell.
    Telophase-II: Chromosomes decondense,cells divides again to form another 2 haploid daughter cells. Final: 4 new cells.


2. The differences:
Mitosis:
- has 1 division per cycle
- one cell produces 2 new cells
- the genetic information in the mother-cell and the daughter-cells are the same. ( the number of chromosomes is also the same)
- it occurs in somatic cells
Meiosis:
- two divisions per cycle
- one cell when divides produces 4 new cells
- the new cells have different genetic information. mixes the genetic material from the parent cells
- the number of chromosomes of the daughter cells is half of the mother's.

3. Prokaryotic organisms don't divide through mitosis, they use a different process called binary fission. Only eukaryotic organisms, or those whose cells have a defined nuclei, undergo mitosis. Bacteria, for example, are prokaryotic organisms that use binary fission.

4.
 It can't occur. Cross over is the exchange of DNA between homologous chromosomes.  That will result in recombinant chromosomes during sexual reproduction. It can't occur on different chromosomes because they don't code for the same genes.

5. There are a lot of different theories about that, but it's mostly believed that meiosis must evolve before sexual reproduction. That's because The cell replicates their information first and then divides. Plus the cell does that even though it didn't recombine DNA with another organism (sexual reproduction).