The replication process for DNA is a fundamental part of our survival and the passing on of genetic material between different generations. While DNA replication and cell division are fascinating subjects to study on their own, they go slightly beyond the scope of this article, as we are solely interested in sister chromatids. If you wish to learn more about DNA replication or the intricacies of mitosis or meiosis, you can check out these other articles on Science ABC.
However, before we dive into sister chromatids, a brief review of cellular replication may be required for context.
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What Happens During DNA Replication?
Before a cell can divide, the genetic material inside the nucleus must be replicated, so that it can be passed along to the subsequent generations. As you likely know, DNA comes in the form of a double-helix and consists of nucleotides, which are made of a nitrogenous base, a phosphate group and a sugar group. There are four types of nitrogenous bases—adenine, thymine, cytosine and guanine.
When it comes time for the DNA to replicate, an enzyme called helicase comes along and “unzips” the double-helix, leaving behind two single strands—a leading strand and a lagging one. Once a primer passes along these strands and draws complementary nucleotides to the template strand, the strand is “checked” and then zipped back up with DNA ligase. This leaves two identical (in theory) strands of DNA, each of which has one “old” half and one “new” half. These can recompress into their chromatin forms (sister chromatids) prior to anaphase in both mitosis and meiosis.
What Are Sister Chromatids?
As mentioned above, sister chromatids are identical copies of a chromosome that has been unfurled and replicated, then bound to its partner with a centromere. During the interphase portion of the cell cycle—the time between replications—all of the chromosomes are duplicated in preparation for cell division. When these sister chromatids eventually separate, it is to ensure that both daughter cells end up with the correct number of chromosomes. That being said, while sister chromatids are present in both mitosis and meiosis, their behavior during these two cellular activities.
Sister Chromatids in Meiosis
Since sex cell replication (meiosis) is slightly different than your average cell division, the replication process is slightly different, as is the movement of sister chromatids. There are homologous chromosomes of duplicated sister chromatids in the first step of meiosis, and these homologous chromosomes separate and move to their individual daughter cells during anaphase 1, then separate during telophase I. However, in anaphase II, the sister chromatids are pulled apart by the spindle towards opposite centrosomes. Thus, the result of meiosis is four daughter cells with half the number of chromosomes (23), which is the desired amount for a sex cell.
Interestingly enough, the sister chromatids are also involved in one of the more important stages of meiosis—genetic recombination. It is during this crossing over and swapping of DNA chunks between homologous chromosomes and sister chromatids that so much of the variety and diversity of our genetic expression comes from.
Sister Chromatids in Mitosis
In mitosis—the cellular replication and division of a somatic cell—the chromosomes replicate into sister chromatids before prophase begins, at which point they migrate to the center of the cell. During anaphase, the spindles pull the sister chromatids apart and tug them towards opposing centrosomes. During telophase, the original cell divides into two daughter cells, each of which has a full set of 46 chromosomes, which are known as daughter chromosomes.
How Are Sister Chromatids and Homologous Chromosomes Different?
Many people confuse sister chromatids with homologous chromosomes, which is understandable, as they are both “pairs” of chromosomes, although they do come into play during different processes, and have a number of key differences.
First of all, sister chromatids carry identical genetic material, either from a maternal or paternal source, as they are intended to be perfect copies distributed into different daughter cells. Homologous chromosomes, on the other hand, contain both a paternal chromosomes and maternal chromosomes. Furthermore, the gene sequence of homologous chromosomes may not be identical, and different alleles of the same gene may be present, which is not the case in sister chromatids.
In terms of when these “pairs” appear during the cellular process, sister chromatids appear during interphase, after the DNA has replicated, whereas homologous chromosomes don’t appear until metaphase I (of meiosis I). The homologous chromosomes separate during anaphase I, whereas sister chromatids don’t separate until anaphase II. Homologous chromosomes are also not connected, despite existing near one another in “pairs”; sister chromatids are connected by their centromere, until they are pulled apart by the spindle fibers.
The primary difference of these two groupings is their genetic composition, which will be different in homologous chromosomes, but identical in sister chromatids—until recombination or crossover events occur.
A Final Word
These tiny replicated clumps of DNA may seem unimportant or easy to overlook, but they represent a critical step in the replication of every piece of genetic material that makes us unique. Even small errors in sister chromatid separation can result in serious chromosomal deficiencies and life-threatening diseases, so don’t take these tiny pairs for granted!