Meiosis is a tricky concept. To help to you understand it, go through the following activities in order
Start by watching this video (You will use this to do this week’s class lesson)
Meiosis Video
Here are the objectives for this unit. lets take them one at a time
Be able to distinguish Diploid and haploid cells
Most of the cells in our body are diploid, they have 2 copies of each chromosome. Our diploid number is 46. Sperm and egg are haploid cells,they only have 1 copy. At the end of meiosis 1, each cell is also haploid. For example, in females, after meiosis 1 there is only a single X chromosome. Note that it has 2 sisters which will be attached until meiosis 2. In males, there will be an X or a Y (but not both) after meiosis 1. Again this chromosome will have 2 sisters.
2. Be able to determine whether a cell is in Mitosis, Meiosis 1 or Meiosis 2
To be able to do this, one first needs to be able to distinguish sisters and homologues as shown here
Sisters and Homologs in Mitosis
The Sisters are genetically identical (before recombination, which we will ignore for the moment)
The homologues contain the same genes but can contain different genetic information (for example A versus B blood type)
In M1, the homologues consist of 2 sisters which remain attached to each other. The homologues separate from each other in M1. Thus all M1 cells will show a row of 4 sisters as shown in the bottom of the figure. You will not see this in any other stage. Thus it is easy to distinguish M1 from other stages.
In M2, the sisters separate. The homologues have already separated. Thus one should never see both homolgoues in an M2 cell
Now how do we distinguish M1 from mitosis
(1) Mitotic cells contain all chromosomes that an individual has. Therefore
(A) For each chromosome they should show one homologue from their mother and one from their father. Meiotic cells show only the maternal or paternal chromosome, not both
(B) Because of (A) mitotic cells will always have an even number of chromosomes. Cells in M2 can have an odd number of chromosomes. During anaphase, a mitotic cell will have an even number of chromosomes on each side of the metaphase plate, an M2 cells can have an odd number on each side of the plate. (this is true for an organism such as humans for which the total number of chromosomes is an odd number X 2. (46 for humans).
(C) Recombination does not occur in mitosis. If you see a recombinant chromosome (partly one color and partly another), it must not be in mitosis.
(D) If X and Y chromosomes are present (This will be indicated) A mitotic cell can have both the X and the Y (if it is male) but a cell in M2 can only have the X or the Y.
Here is some practice for you
3. Be able to compare mitosis and meiosis in terms of processes and end results
The purpose of mitosis is to create cells that are genetically identical to the parent cells. Mitosis gives you cells that have 2 copies of each chromosome, just like the parent cells
The purpose of meiosis is to create cells that have 1 (not 2) copies of each chromosome so that after fertilization, the embryo will have 2 copies of each chromosome.
In both process, there is a preceding interphase which includes chromosome duplication.
Both processes have prophase, metaphase Anaphase and Telophase using the same cellular structures such as spindles and centrioles. Meiosis has 2 sets of these stages.
In order to assure that egg and sperm only get one of each chromosome, the homologues (members of a pair) line up during M1 and separate. During M1, crossing over occurs between homologues, this results in increased genetic variation
Crossing over does not occur in mitosis.
Chromosome Duplication (Before meiosis) Then Pairing of Chromosomes (Prophase of M1) Crossing Over (Prophase of M1) separation of Homologues (Anaphase of M1) separation of sisters (Anaphase of M2)
Think you know it? Try this?
Know how meiosis leads to genetic variation (Know how recombination and independent assortment can increase the variation in gametes
Independent Assortment: 4 different combinations can result from 2 pairs of chromosomes. In general the number of combinations is 2N where N is the number of pairs of chromosomes
