Cell division is a fundamental biological process that allows organisms to grow, repair tissues, and reproduce. Among the different types of cell division, mitosis is a specific process where a single cell divides to produce two new cells that are genetically identical to itself. This ensures that the new cells receive a complete and accurate set of genetic instructions. Mitosis is a precisely regulated process that supports the continuity of life.
What Are Chromosomes
Chromosomes are structures found within the nucleus of eukaryotic cells, carrying the genetic information in the form of DNA. Each chromosome consists of a long, tightly coiled DNA molecule associated with proteins, primarily histones, which help package the DNA into a compact form. Most organisms have a characteristic number of chromosomes in their somatic (non-reproductive) cells, known as the diploid number, often represented as 2n. For instance, human somatic cells are diploid and contain 46 chromosomes, arranged in 23 pairs. Before a cell undergoes mitosis, each chromosome duplicates itself, resulting in two identical copies called sister chromatids. These sister chromatids remain joined at a constricted region called the centromere, and despite the duplication, they are still considered a single chromosome until they separate during cell division.
How Cells Divide Through Mitosis
Mitosis is a continuous process, but scientists divide it into several distinct stages. These stages include prophase, metaphase, anaphase, and telophase, followed by cytokinesis. During prophase, the duplicated chromosomes condense, becoming visible under a microscope, and the nuclear envelope, which encloses the nucleus, begins to break down.
As the cell progresses to metaphase, the condensed chromosomes align precisely along the cell’s equatorial plane, known as the metaphase plate. This alignment is facilitated by a structure called the mitotic spindle, composed of microtubules, which attach to the centromere of each chromosome.
Anaphase is characterized by the separation of the sister chromatids. The centromere of each duplicated chromosome divides, and the sister chromatids, now considered individual chromosomes, are pulled apart towards opposite poles of the cell by the shortening of the spindle microtubules.
Following anaphase, telophase begins as the separated chromosomes arrive at the poles and begin to decondense. During telophase, a new nuclear envelope forms around each set of chromosomes at the poles, creating two distinct nuclei within the single parent cell. The mitotic spindle disassembles, and the chromosomes return to their less condensed state. Finally, cytokinesis, the division of the cytoplasm, usually overlaps with telophase. A contractile ring forms and pinches the cell membrane inward, eventually dividing the parent cell into two separate daughter cells, each containing one of the newly formed nuclei and its own set of organelles.
Chromosome Number After Mitosis
After a cell completes the process of mitosis, the resulting daughter cells will have the exact same number of chromosomes as the original parent cell. Each duplicated chromosome, consisting of two sister chromatids, is counted as one chromosome until the chromatids separate. When these sister chromatids pull apart, each new daughter cell receives one complete set of these now-individual chromosomes. For example, in humans, a parent cell that begins mitosis with 46 chromosomes (2n) will produce two daughter cells, each also containing 46 chromosomes. This ensures that the genetic information is faithfully passed on from one cell generation to the next, maintaining the characteristic chromosome number of the species.
Why Mitosis Matters
Mitosis plays a fundamental role in the life of multicellular organisms. One of its primary functions is growth, as it increases the number of cells within an organism, leading to an increase in size and complexity from a single fertilized egg to a mature individual.
Beyond growth, mitosis is also important for tissue repair and regeneration. It continuously replaces old, worn-out, or damaged cells throughout an organism’s life, such as skin cells, blood cells, and cells lining the digestive tract. This constant renewal helps maintain the integrity and function of tissues and organs.
For some organisms, particularly single-celled eukaryotes like yeast, mitosis serves as a method of asexual reproduction. In these cases, a single parent cell divides to produce two genetically identical offspring. This allows for rapid population growth and efficient propagation without the need for a partner.