Cellular genetic material is organized into chromosomes. Before cell division, chromosomes duplicate to ensure each new cell receives a complete copy. This duplication forms sister chromatids: two identical copies of a single chromosome. A central question in understanding cellular reproduction is why these sister chromatids are always identical.
What Sister Chromatids Are
Sister chromatids are two exact copies of a single chromosome, joined at the centromere. They form after DNA replication. Before replication, a chromosome consists of a single DNA molecule. After replication, a chromosome has two identical DNA molecules, each a chromatid.
These paired structures remain attached at the centromere until cell division separates them. Specific proteins mediate the centromere connection, organizing the identical copies. This pairing ensures each new daughter cell receives a complete set of genetic information during division.
How Identical Copies Are Made
The identity of sister chromatids stems from the precise process of DNA replication, which precedes cell division. DNA replication is described as semi-conservative, meaning that each new DNA molecule produced consists of one original strand and one newly synthesized strand. This mechanism ensures that the genetic information is accurately passed down.
During replication, the double-stranded DNA molecule unwinds, and each original strand serves as a template for the synthesis of a new complementary strand. DNA polymerase adds new nucleotides to the growing DNA strand, following specific base-pairing rules. It synthesizes DNA at rates of about 50 nucleotides per second in human cells.
DNA polymerase also has a proofreading ability, which minimizes errors during replication. As it synthesizes the new strand, the enzyme can detect and correct incorrectly paired nucleotides. This proofreading mechanism significantly enhances the accuracy of DNA replication, reducing the error rate to approximately one mistake per 10 million base pairs. The combined action of semi-conservative replication and high-fidelity DNA polymerase ensures that the two resulting sister chromatids are virtually identical copies of the original chromosome.
The Purpose of Identical Sister Chromatids
The identical nature of sister chromatids maintains genetic stability across cell generations. During cell division, particularly mitosis, the primary goal is to produce two daughter cells that are genetically identical to the parent cell. Identical sister chromatids ensure each new cell receives a complete set of chromosomes upon separation.
This precise distribution is important for growth, tissue repair, and asexual reproduction. For instance, skin cells constantly divide to replace old or damaged cells, and each new skin cell needs the same genetic instructions to function correctly. Without identical sister chromatids, the genetic integrity of these new cells would be compromised, potentially leading to cellular dysfunction.
In sexual reproduction, the accurate separation of sister chromatids during meiosis II is important for forming gametes (sperm and egg cells) with the correct chromosome number. This process ensures that when gametes fuse, the resulting offspring receive the appropriate genetic complement. The identical nature of sister chromatids provides a reliable mechanism for distributing genetic information faithfully.
What Happens When Copies Differ
Despite accurate DNA replication mechanisms, errors can occur, leading to differences between sister chromatids. These changes, known as mutations, involve alterations in the DNA sequence. A mutation could arise if the DNA polymerase makes an error that is not corrected by its proofreading function or by subsequent repair mechanisms. Such alterations would mean the sister chromatids are no longer perfectly identical.
Beyond sequence changes, problems can also arise if sister chromatids fail to separate properly during cell division, a phenomenon called non-disjunction. This error results in one daughter cell receiving an extra copy of a chromosome, while another receives none. Conditions like Down syndrome, which involves an extra copy of chromosome 21, are often a result of non-disjunction events. The identity of sister chromatids and their precise segregation are important for proper cellular function and organismal health.