The chromosome content of a skin cell that is about to form two new skin cells is represented in the diagram below. Which diagram represents the chromosomes that would be
found in the two new skin cells?
Answers
Answer:
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 chromosome