By Emma Woodhouse – Updated Aug 30, 2022

Chromosomes

Chromosomes are compact, thread‑like structures that house DNA wrapped around histone proteins. In eukaryotes, they reside within the nucleus, whereas prokaryotic cells lack a membrane‑bound nucleus and carry their DNA freely in the cytoplasm.

All human somatic cells contain 46 chromosomes (the diploid count). Gametes—sperm and egg—carry 23 chromosomes (haploid). When fertilization occurs, the two haploid sets merge to restore the diploid number.

• The 22 non‑sex chromosomes are numbered 1 through 22 and appear as well‑defined structures under a microscope.
• Each pair of corresponding chromosomes from mother and father is called a homologous pair.
• During replication, a chromosome stays attached to its duplicate at a constriction called the centromere, giving rise to two sister chromatids.
• Chromosomes have a short "p" arm and a long "q" arm, extending from the centromere.

Cell Division

Cell division is the process by which a cell duplicates its contents and splits into new cells. There are two main types:

• Mitosis – the routine division that generates two genetically identical daughter cells, essential for growth and repair.
• Meiosis – a specialized division that produces four haploid gametes, crucial for sexual reproduction.

While mitosis results in a pair of diploid cells, meiosis involves two successive divisions that ultimately yield four cells, each with half the chromosome number.

DNA Replication in Mitosis

Before a cell enters mitosis, its DNA must be precisely copied. Each chromosome duplicates, forming sister chromatids linked at the centromere. During anaphase, these chromatids separate, ensuring each daughter cell inherits a complete set of chromosomes.

DNA Replication in Meiosis

Meiosis adds a layer of complexity. After the initial DNA replication (identical to mitosis), homologous chromosomes may exchange segments in a process called recombination or crossover. This genetic shuffling creates genetic diversity. The two rounds of division then separate the chromatids, producing four haploid cells with unique genetic combinations.