DNA Structure and Replication

ID #1114

I'm a little confused about the concept of sister chromatids. They are identical copies of the same chromatin and there are two pairs of sister chromatids per chromosome, one from your mother and one from your father, and each new cell gets one chromatid from each of the two sister chromatids pairs right?

Yes, that's right. I'll try to lay this out a little clearer for everyone. The chromosome number in an organism is generally denoted as a whole number in front of the lowercase letter "n." The "n" identifies the number of different types of chromosomes in that organism, and the number in front of the "n" identifies how many of those chromosomes there are. Haploid organisms like E. coli have a single copy of their chromosome, so they have a 1n number, which we just shorten to n. And since they have only a single chromosome, n=1, which makes this a very simple equation (1n = 1, the total number of unique chromosomal molecules in an E. coli cell). A diploid organism has a maternal copy and a paternal copy of each chromosome, so they are 2n. In a hypothetical diploid eukaryotic organism that has three different chromosomes (1, 2 and 3), prior to DNA replication there are actually 6 chromosomes in the nucleus (1, 2 and 3 from the mother and 1, 2 and 3 from the father). In this case, 2n = 2(3) = 6 = the number of chromosomes. Now let's say this hypothetical organism is ready for replication. BOTH copies of chromosome 1 are going to be replicated, as well as both copies of chromosome 2 and chromosome 3. That's a total of 6 chromosomes, all getting copied. So at the end of replication, there are actually 2 identical copies of the maternal chromosome 1 and two identical copies of the paternal chromosome 1. The two double helices that represent maternal chromosome 1 are "stuck" together and are called sister chromatids. If they were physically separated from each other, they would (and will eventually) be considered separate chromosomes, but for simplicity we still consider a pair of sister chromatids held together a "chromosome." Likewise, the two new daughter helices that comprise the replicated paternal chromosome 1 are also a pair of sister chromatids. They are identical to each other, but NOT identical to the maternal copies of chromosome 1, although maternal and paternal copies have the same genes in roughly the same locations -- this is why we call them both "chromosome 1".

This is only a temporary situation, since you will move from replication into mitosis in a fairly short amount of time, and then those sister chromatids will be separated. But during that period of time between the END of replication and the division of one cell into two, there is actually a 4n amount of DNA in this hypothetical diploid cell. We started replication with 6 independent double helices, and end replication with 12, although you do have to recall that each pair of sister chromatids are stuck together. So technically, prior to mitosis you have 2 copies of maternal chromosome 1, 2 copies of paternal chromosome 1, 2 copies of maternal chromosome 2, 2 copies of paternal chromosome 2, 2 copies of maternal chromosome 3, and 2 copies of paternal chromosome 3. This is enough to divide equally, so each resulting daughter cell gets one copy of each chromosome, and maintains its diploid character.

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