Expert: John Locke - 11/9/2009

Question
In mitosis, how many chromosomes do you have when you have finished replicating all of the DNA?

In meiosis, -How many chromosomes do you have at the end of the S stage?
-How many pairs of chromosomes do you have now?
-At the end of telophase, how many chromosomes are present in each cell?
- At the end of telophase, how many pairs of chromosomes are present in each cell?

So confusing, they are all blurring together. Having such a hard time absorbing all of this information, and establishing the similarities and differences in mitosis and meiosis.

Thanks in advance,
Shannon


Answer
Thanks for using AllExperts. Understanding the terminology related to mitosis/meiosis can be confusing and difficult, but a solid grasp of the subject is essential to basic biology. I will try my best to clarify the subject here:

First, some terminology. Chromosome refers to a single piece of coiled DNA with associated bound proteins and organizational elements. During mitosis, chromosomes duplicate and are joined at a centromere.

Homologous chromosomes are those that pair to create gametes during meiosis. Homologous chromosomes have the same gene loci and control for the same features. Humans have 23 pairs of homologous chromosomes.

Each half of the replicated chromosome is termed a chromatid. The number of chromatids in a cell is a multiple of the number of pairs of homologous chromosomes. Thus, the number of chromatids in human cells may be 23, 46, or 92. Gametes--which are haploid--contain 23 chromatids, representing one copy of each chromosome. Normal human cells not in mitosis contain 46 chromatids, while those cells that have duplicated their genetic material in anticipation of mitosis or meiosis have 92 chromatids, divided into 46 duplicated chromosomes. Confusingly, haploid cells at certain stages of meiosis also contain 46 chromatids, but these chromatids represent duplications of the same chromosome. They are not homologous chromosomes.

Now, to your questions:

1. After replication of the DNA in mitosis, the cell still contains 46 chromosomes. However, it contains 46 replicated chromosomes, with 92 individual chromatids.
2. At the end of S stage, the cell contains 46 chromosomes, replicated and jointed just as in mitosis. There are 92 individual chromatids.
3. At the end of telophase I, there are 23 chromosomes--the haploid number. Each of these chromosomes consists of two paired chromatids, meaning that there are 46 total chromatids.
4. At the end of telophase I, there are no longer any paired chromosomes. There is only one copy of each chromosome, consisting of two paired chromatids joined at the centromere. These don't comprise a pair of chromosomes, as discussed earlier, but duplications of the same chromosome.

Hopefully this helps. You may wish to check out the info at the site below, and let me know if you have any additional questions.

http://www.pbs.org/wgbh/nova/miracle/divide.html