Mitosis is a fundamental biological process where a single parent cell divides to produce two identical daughter cells. This cell division allows organisms to grow, replace old or damaged cells, and sometimes reproduce asexually. Mitosis ensures that each new cell receives a complete and accurate set of genetic instructions.
Understanding Chromosomes
Chromosomes are thread-like structures located inside the nucleus of eukaryotic cells. They are composed of DNA tightly coiled around proteins called histones, which help package DNA. These structures carry genetic information in units called genes. In humans, cells contain 23 pairs of chromosomes, totaling 46. One set of 23 chromosomes is inherited from each parent, making human cells diploid.
Preparing for Mitosis
Before a cell can enter mitosis, it undergoes a preparatory phase called interphase. This phase is divided into three sub-phases: G1, S, and G2. The S phase, or synthesis phase, is when DNA replication occurs.
During S phase, each chromosome duplicates itself, creating two identical copies known as sister chromatids. Sister chromatids remain connected at a constricted region called the centromere. Even after DNA replication, the cell’s chromosome count remains unchanged because sister chromatids joined at the centromere are considered a single chromosome.
Chromosomes at Mitosis’s Start
At the beginning of mitosis, during prophase, the cell maintains the same number of chromosomes as before DNA replication. For humans, this means the cell contains 46 chromosomes. Each of these 46 chromosomes now consists of two identical sister chromatids, formed during the S phase. While the total number of DNA molecules has doubled (resulting in 92 chromatids), the count of individual chromosomes, defined by their centromeres, remains 46. This distinction is important for understanding how genetic material is distributed during cell division.
Importance of Chromosome Count
Maintaining the correct chromosome number is important for accurate cell division and organism health. During mitosis, precise segregation of duplicated chromosomes ensures each new daughter cell receives an identical and complete set of genetic material. Errors in chromosome number, known as aneuploidy, can have serious consequences. Such imbalances can lead to developmental issues, as seen in conditions like Down syndrome, which results from an extra copy of chromosome 21. Accurate chromosome distribution is a tightly regulated process that safeguards genetic stability.