Comparing Nuclear and Chromosomal Activities in Mitosis and Meiosis
| Mitosis | Meiosis |
| 1. Synapsis | no | yes |
| 2. Cross-over | no | yes |
| 3. When Centromeres Split | as anaphase begins | as anaphas II begins |
| 4. Anaphase | single stranded chromosomes move to poles | AI: double stranded chromosomes moves to poles
AII: single stranded chromosomes move to poles |
| 5. Number of Divisions | 1 | 2 |
| 6. Number of daughter cells | 2 | 4 |
| 7. Number of Chromosomes in daughter cells | same as parent | one half number in parent |
Meiosis
1. takes place in all sexually reproducing animals.
2. Occurs in an animals gonadal cells
3. Meiosis means 2 nuclear divisions - Meiosis I
and Meiosis II
4. There is a shortened Interphase II between the
two MI and MII stages [enough time to produce requisite materials
for cell to under the second round of meiosis. No new synthesis of new
DNA occurs here]
5. Thus, 4 cells result from 1 parent cell which
contain half the number of chromosomes, one from each homologous pair.
6. Cells with only 1 of each homologous pair
of chromosomes are called haploid (n)
7. The parent cells, with pairs of chromosomes homologous
chromosomes, are called diploid (2n)
8. The advantage of of meiosis is that it prevents
the chromosome number from doubling with every generation when fertilization
occurs.
| Question: | What would be the consequences in successive generations of offspring if the chromosome number was not reduced through meiosis? |
| Answer: | Increasing the numbers of chromosomes lead to abnormalities in mitotic cycles, leading to the death of the organism. |
| Meiosis Review |
| Meiosis I |
| Interphase I |
| 1. Cell activities in G1 are similar to those activities to those activities in G1 of mitosis' interphase. |
| 2. In G1 stage of Interphase 1 the chromosomes are single stranded. |
| 3. During S phase of Interphase I the DNA replicates |
| 4. As in mitosis, in G2 the cell prepares for meiosis by synthesizing proteins and enzymes necessary for nuclear division. |
| Prophase 1 |
| 1. Remember, Meiosis consists of 2 consecutive nuclear divisions called meiosis I and meiosis II. |
| 2. As the first division begins, the chromosomes coil and condense just as in mitosis. |
| 3. As the chromosomes begin to coil and condense, prophase I begins. Each chromosome is double stranded, made up of sister chromatids. |
| 4. Also, 2 pairs of centrioles are outside the nucleus. |
| 5. The nuclear membrane breaks down and the spindle fibers begins to form (as in mitosis) |
| 6. Early on in prophase I, each chromosome finds its homologue and pairs up in a tight association called synapsis. |
| 7. Because the chromosomes are double stranded, each pair is made up of four strands and is called a tetrad. |
| 8. So, for example, if you have a cell where the diploid number equals 4 [2n=4], then you have 2 tetrad complexes. |
| 9. While as a tetrad a genetic shuffling occurs known as cross-over. This phenomenon is when 2 identical genetic segments from the two non sister chromatids exchange places. |
| 10. Basically, a segment from 1 chromatid will break and exchange with the exact same segment on a non-sister chromatid in the tetrad. The cross-over site has a specific name called the chiasma (sing.. = chiasmata) |
| 11. Genes (traits) are often expressed in different or alternative forms called alleles. For example, the gene expressing a rose plant's flower color as either red or white. So, white and red are the representative alleles of the gene that produces a specific color of the rose's flower petals. [note: usually more than just 2 alleles for any given trait] |
| 12. Many examples of alleles in humans: hair color, eye color, etc. [try and think up more examples on your own] |
| 13. Late in prophase I, the tetrads begins to move to the cell's equator (soon to be called the metaphase plate) |
| Metaphase I |
| 1. The tetrads are now at the middle of the cell. |
| Anaphase I |
| 1. Here separation of each double stranded chromosome from its homologue occurs |
| 2. One homologue moves toward each pole. |
| 3. Remember, in anaphase I of meiosis, the chromosomes are double stranded, but in anaphase of mitosis the chromosomes are single stranded. |
| 4. key word for anaphase I : double stranded chromosome |
| Telophase I |
| 1. The double stranded DNA are now at the two poles. |
| 2. The nuclear membranes reform followed by cytokinesis |
| 3. if 2n = 4 originally, at this phase their are now 2 chromosomes in each nuclei. |
| 4. Furthermore, these 2 new nuclei are described as being haploid. |
| Interphase II |
| 1. Usually short |
| 2. There is little cell growth and no synthesis of DNA. All the machinery for a 2nd nuclear division is synthesized though. |
| Meiosis II |
| 1. The events that take place specifically in meiosis II are identical to mitosis. Remember, meiosis I is NOT similar to mitosis. |
| 2. Meiosis I results in 2 nuclei with half the number of chromosomes as the parent cell, but the the chromosomes are double stranded (made up of 2 chromatids) - just as they are at the beginning of mitosis. |
| 3. The events of meiosis II must change the double stranded chromosomes into single stranded chromosomes. |
| 4. As meiosis II begins, 2 new spindles begin to form, establishing the axes for the dispersal of chromosomes to each new nucleus. |
| Prophase II |
| 1. Here the centrioles separate. |
| 2. In each new cell the nuclear membrane breaks down, the nucleoli breaks down, and a new spindle forms. |
| 3. The new spindle forms at a right angle to the axis of the spindle in meiosis I and the chromosomes begin moving towards the cell's equator. |
| Metaphase II |
| 1. As the chromosomes reach the equator, prophase II ends and metaphase II begins. |
| Anaphase II |
| 1. As metaphase II ends, the chromosomes finally split apart during anaphase II |
| 2. Thus the sister chromatids are now separated into single chromosomes moving toward the opposite poles. |
| 3. key word: single chromosome. |
| Telophase II |
| 1. chromosomes have now arrived at the poles. |
| 2. Spindles break downs. Nucleoli reappear and the nuclear envelope forms around each bunch of chromosomes as the uncoil. |
| 3. Cytokinesis follows telophase II |
| . |
| . |
| . |
| Meiosis example with 2n=4 |
| What is the total number of nuclei and cells now present? four |
| How many chromosomes in each nuclei? two |
| How many cells were present when the entire process began? one |
| How many chromosomes were present per cell when the entire process began? four |
| How many cells formed by the meiotic division are genetically identical? none, remember cross-over reshuffles the genetic material when the cells are in tetrads of synapsis. |
| Final Word on Cross-over Event |
| Q: What advantage does the of cross-over confer to a cell? |
| A: Cross-over produces new combinations of alleles which lead to new genetic variation in the next generation. This variation is important to a population as it is the material that natural selection acts upon. |
| Q: Could there be any disadvantages to cross-over? |
| A: yes, cross-overs can also disrupt advantageous gene combinations which would then decrease the fitness of the organization. |
http://www.life.umd.edu/cbmg/faculty/wolniak/wolniakmitosis.html