PLOIDY, WHAT IS IT? MITOSIS vs MEIOSIS

HOW MEIOSIS AND MITOSIS DIFFER

MITOSIS
 
1. Occurs in somatic cells and the stem cells of the germ cell line.
2. One cell division results in 2 new cells.
3. Each of the 2 new cells receives one complete set of each original pair of chromosomes in the dividing cell.
4. Each of the 2 new cells contains a diploid number (2c) of chromosomes and diploid (2n) genetic content.
 
MEIOSIS
 
1. Occurs only in gametocyte stages of the germ cells.
2. Two cell divisions result in 4 new cells.
3. Each of the 4 new cells receives only one chromosome of each original pair of chromosomes present in the primary gametocyte.
4. Each of the 4 new cells contains a haploid number (1c) of chromosomes and haploid (1n) genetic content.

 
 
PLOIDY, WHAT IS IT?


All of us have heard or seen the terms haploid and diploid during our education and reading in the biological sciences, but what do these terms mean and how do they relate to cells undergoing meiosis? The explanations are often varied and sometimes misleading. This is further complicated by the fact that teachers and authors may refer to ploidy in terms of either genetic content (n) or chromosome number (c) and, unfortunately, they sometimes get these two different and important concepts confused.
Any diploid organism is formed from the fusion of haploid gametes; i.e. an egg from the "mother" and a sperm from the "father." If we are talking about multicellular animals [many plants are polyploid] such as you and I, aside from our gametes that have completed meiosis and other cells that have replicated their DNA in preparation for cell division, nearly all cells of that organism are diploid in terms of both genetic content and chromosome number (an example of an exception to this statement is the mature mammalian erythrocyte which has lost its nucleus and thus has no chromosomes and no ploidy).
So, how do we define ploidy in terms of genetic content (n) in a diploid human cell? A human cell that is diploid in terms of genetic content contains 46 chromosomes that makeup 22 homologous pairs plus an ÏXÓ and a ÏYÓ chromosome (Ïsex chromosomesÓ) making-up a 23rd pair. When we say these cells are diploid (2n) in terms of genetic content we are referring to the fact that each homologous pair (including the ÏXÓ and ÏYÓ pair) of chromosomes consists on one maternally derived chromosome (a copy of one that was in the mother's egg) and one paternally derived chromosome (a copy of one that was in the father's sperm). There is no third party involved! We are looking at the fact that each pair has only maternal and paternal components. If we replicate these chromosomes in preparation for division we now have two maternal chromatids and two paternal chomatids. Each of these chromatids represents a complete chromosome, but, until the cell divides, the two maternally derived chromatids remain connected at the centromere as do those that are paternally derived. Thus, barring a spontaneously appearing mutation, we have added nothing new in terms of genetic content. We still have only maternal and paternal components and the cell is still considered to be 2n. The "2" signifies the fact that each pair of chromosomes is represented by only a maternal and a paternal component (no fraternals here), thus, the maximum number of different gene alleles that can exist at a given gene locus on a homologous pair of chromosomes is 2 (a simple example of this would be one dominant and one recessive allele). The "n" can be viewed as the number of homologous pairs of chromosomes; however, a more precise definition would be that "n" is the number of homologous pairs of gene loci on all chromosomes present in the cell.
Ploidy in terms of chromosome number (c) is altogether different. In this case we are simply concerned with the total number of chromosomes (amount of DNA) and not at all concerned about gene loci or whether the chromosomes are maternal or paternal. A human cell that is diploid in terms of chromosome number (2c) also contains 46 chromosomes that makeup 23 pairs. The "2" signifies the fact that each pair of chromosomes is represented by two chromosomes. The "c" is the total number of pairs of chromosomes in the cell. When this cell replicates its DNA in preparation for division it makes copies of every chromosome. Each of the replicated chromosomes consists of two chomatids that are connected at the centromere and, as above, each of these chromatids represents a complete chromosome. Thus, we have now doubled the number of chromosomes in the cell and ploidy in terms of chromosome number is 4c. The "4" signifies the fact that each pair of chromosomes is now represented by 4 chromosomes (i.e., 2 copies of each member of the original pair or chromosomes). The "c" is the total number of pairs of chromosomes in the cell.
If we now apply these facts to the process of meiosis, we can see how ploidy in terms of genetic content and chromosome number differs as meiosis occurs. Initially (before replication of the DNA of the chromosomes), the primary gametocyte is 2n (diploid in terms of genetic content) and 2c (diploid in terms of the number of chromosomes), as described for a diploid cell above. The primary gametocyte then replicates its chromosomes (its DNA). After this replication is complete, the cell is still 2n (no new gene alleles have been added), but since the total number of chromosomes has doubled (as described above) it is now 4c (a tetraploid number of chromosomes).
Next the first meiotic division takes place. At this time the replicated maternal and paternal chromosomes of each pair are separated from each other and segregated into one or the other of the two secondary gametocytes that are formed. As a result, each secondary gametocyte contains only one replicated chromosome (an original and a copy) of each original pair of chromosomes. Thus, we have reduced the ploidy in terms of genetic content to 1n (haploid genetically), since for each gene locus, only one of the two possible alleles is present. Since the total number of chromosomes (in the form of chromatids) in each secondary gametocyte is now 1/2 of what it was before the division, the ploidy in terms of chromosome number is now 2c. Those of you who are really thinking hard about this may have realized that cross-over could confound this picture for some gene loci; however, what we are discussing here is an "ideal" example and what happens in the case of the majority of gene loci of any homologous pair.
Next, each secondary gametocyte divides. At this time, the two chromatids of each replicated chromosome in the secondary gametocyte separate at the centromere and the resulting identical chromosomes (original and copy) from each replicant are segregated into each of the two cells that are formed. Thus, each new cell has either a paternal or a maternal chromosome (but not both) from each original homologous pair and a ploidy in terms of genetic content of 1n (still only one of two possible alleles for each gene locus). Since the total number of chromosomes has been reduced to 1/2 the number present in the secondary gametocyte, the ploidy in terms of chromosome number is now 1c (haploid number of chromosomes). So, the resulting gamete (or gametes in the case of the male) will be haploid in terms of both genetic content (1n) and chromosome number (1c).

The figure below summarizes the process of meiosis and the changes in ploidy that occur along the way. In this diagram, the dashed circle surrounding the chromosomes represents the boundary of the nuclear membrane (which is actually not present during division). Only two pairs of chromosomes are shown in this example; however, the ploidies given for each stage (e.g. 2n, 2c) in the meiotic cycle would be the same regardless of how many chromosome pairs are present in the original primary gametocyte since these number/letter combinations represent a formula, not an actual number. Remember, ploidy in terms of genetic content is represented by "n." Ploidy in terms of chromosome number (amount of DNA) is represented by "c."