The genetic material within the nucleus of a cell is organized into structures known as chromosomes. The entire length of DNA must be precisely packaged to fit inside the microscopic nucleus. This organization protects the integrity of the genome and ensures its accurate transmission during cell reproduction. Understanding the structure of chromosomes, particularly before and after they prepare for division, is fundamental to grasping how a cell successfully reproduces itself.
Defining the Basic Units: Chromosomes and Chromatids
A chromosome represents a single, continuous molecule of deoxyribonucleic acid (DNA), which carries genetic information. This DNA is wrapped tightly around proteins called histones, forming a complex known as chromatin. This chromatin condenses into the compact, thread-like structures we recognize as chromosomes, which become visible only when the cell is preparing to divide.
The term chromatid refers to one of the two identical halves of a chromosome that has been duplicated in preparation for cell division. A pair of these identical copies are known as sister chromatids, and they are joined together at a constricted region called the centromere. Before the DNA is copied, a chromosome consists of only a single DNA molecule, or a single chromatid.
The centromere holds the sister chromatids together and serves as the attachment point for the spindle fibers during cell division. The structure of a replicated chromosome, often depicted as an “X” shape, is defined by these two sister chromatids held together at this central point. The formation of this two-part structure is a direct consequence of the cell’s preparation for creating two new, genetically identical cells.
Chromosome Replication and the S Phase
The precise answer to how many chromatids are in each replicated chromosome is two, specifically two sister chromatids. This state is reached during the Synthesis phase (S phase) of the cell cycle. Before the S phase (G1 phase), each chromosome exists as a single, unreplicated structure containing one DNA molecule and therefore one chromatid.
The S phase ensures that when the cell divides, each resulting daughter cell receives a complete and identical copy of the genetic instruction set. A human cell starts the S phase with 46 chromosomes (46 chromatids total). During the S phase, the DNA is duplicated, creating an identical copy that remains attached.
The result is that each of the 46 chromosomes now consists of two sister chromatids, totaling 92 chromatids. Despite the doubling of DNA content, the count of chromosomes remains 46 because the two copies are physically linked by a single centromere.
The Dynamics of Chromatid Separation
The replicated chromosome maintains its X-shape throughout prophase and metaphase of mitosis. This highly condensed state facilitates the movement and alignment of the genetic material on the cell’s equatorial plane. The sister chromatids are held together by specialized proteins, including cohesins.
The dramatic change occurs during anaphase, where the sister chromatids finally separate. Special enzymes trigger the breakdown of the cohesin proteins holding the sister chromatids together at the centromere. This separation is driven by the pulling forces of the spindle fibers.
Once the centromere splits and the sister chromatids move to opposite sides of the dividing cell, a critical change in terminology occurs. Each former chromatid is now considered a full, individual chromosome. This mechanism ensures that the genetic material is equally distributed toward each pole of the cell.
Clarifying Chromosome vs. Chromatid Count
The relationship between chromosome and chromatid number can be a source of confusion because the definition of a “chromosome” changes depending on the stage of the cell cycle. A practical rule for determining the chromosome count is to count the number of centromeres present in the cell. As long as the two sister chromatids share a single centromere, they constitute a single chromosome.
Consider a human somatic cell, which begins the cell cycle in G1 phase with 46 chromosomes and 46 chromatids. This 46-chromosome, 92-chromatid count persists through the G2, prophase, and metaphase stages of mitosis.
During anaphase, when the 46 centromeres split, the number of chromosomes temporarily doubles to 92, even though the total amount of DNA remains the same. Each of the 92 separated chromatids is now an independent, single-chromatid chromosome moving toward a pole. Finally, after cytokinesis, each of the two new daughter cells returns to the starting state of 46 chromosomes, each with one chromatid.