Cells contain genetic instructions encoded in deoxyribonucleic acid (DNA), organized into structures called chromosomes. Segments of DNA are known as genes, which carry the blueprints for various characteristics. This genetic information shapes an individual’s unique traits.
What Are Sister Chromatids?
Sister chromatids are two exact, identical copies of a single chromosome. They form when a cell prepares to divide, duplicating its entire genetic material through DNA replication. This results in two copies that remain attached at a constricted region known as the centromere, creating an X-shaped structure.
These paired structures ensure new cells receive a complete and accurate set of genetic instructions during cell division. As identical duplicates, sister chromatids allow for an organized separation, maintaining genetic consistency from one cell generation to the next.
What Are Alleles?
Alleles are different versions of a gene, a specific segment of DNA that provides instructions for a particular trait. For example, a gene might determine eye color, while different alleles of that gene could lead to brown, blue, or green eyes.
These variations in alleles contribute to individual diversity. An individual inherits two alleles for each gene, one from each biological parent. The combination of these alleles influences the observable characteristics, or traits, an organism expresses.
Identical Copies and Rare Changes
Sister chromatids typically possess the exact same alleles. They are generated through a precise duplication process where a single DNA molecule is copied to create two identical strands. Since one chromatid is a replicated version of the other, they inherently carry the same genetic sequence, including all genes and their specific allelic forms.
However, in extremely rare instances, sister chromatids might not be perfectly identical in their alleles. This can occur if a spontaneous mutation arises during DNA replication. While DNA replication is highly accurate, the enzymes responsible for copying DNA can occasionally make errors, incorporating an incorrect nucleotide. Such an error alters the DNA sequence on one newly formed chromatid, but not the other.
These spontaneous mutations are infrequent because cells have sophisticated proofreading and repair mechanisms that correct most errors made during replication. Despite these robust systems, a small percentage of errors can evade detection, leading to a new allele on one sister chromatid. This change represents a new genetic variation not present in the original DNA molecule.