DNA, Chromosomes, Genes, and Cell Division

DNA and Chromosomes

DNA (deoxyribonucleic acid) is the biomolecule containing the genetic instructions for building proteins. Proteins, especially enzymes, regulate all life processes, including growth, tissue repair, and reproduction. DNA resides in the nucleus of eukaryotic cells as part of the chromosomes.

Chromosomes are composed of chromatin, a complex of DNA and proteins. When a cell is at rest, chromatin is dispersed within the nucleus, making chromosomes invisible. Before cell division, DNA replicates, and chromatin condenses into shorter, thicker fibers, forming visible chromosomes. Due to DNA replication, each chromosome comprises two identical arms called chromatids, joined by a centromere.

Karyotype

The number, shape, and size of chromosomes are unique to each species. A karyotype represents an individual's complete set of chromosomes, arranged in pairs by size and structure. Each pair consists of homologous chromosomes. The human karyotype has 23 pairs (46 total) of chromosomes. The 23rd pair determines sex and are called sex chromosomes. The remaining pairs are autosomes.

Genes and Genome

A gene is a DNA segment containing the information to produce a specific protein. Genes on the two chromatids of a homologous chromosome are identical. An organism's complete set of genes is its genome. All cells of an individual carry the same genes, but not all genes are active in every cell. For example, genes for eye color are active in iris cells but not in liver or muscle cells.

Homologous chromosomes contain corresponding genes at the same position that determine the same trait (e.g., eye color, hair type).

Cell Division and the Cell Cycle

Cell division is the process by which a cell divides into two identical daughter cells. In unicellular organisms, each division creates a new individual. In multicellular organisms, numerous divisions are necessary to develop a new individual from a single cell. Mitosis is the primary form of cell division in eukaryotes.

The cell cycle has two main phases: a longer period called interphase and a much shorter period of cell division.

Interphase

Interphase occupies most of a cell's life. During this growth phase:

• The cell enlarges its cytoplasm and increases the size and number of organelles.
• The cell performs its functions (e.g., absorbing nutrients, photosynthesis, producing and storing energy).
• Protein synthesis occurs.

Non-dividing cells (e.g., muscle, neurons) remain in interphase. Dividing cells enter a brief stage within interphase called the S phase, where DNA replication happens.

Cell Division

During cell division:

• Mitosis (nuclear division) occurs.
• Cytokinesis (cytoplasmic division) separates the two daughter cells.

Chromosomes and Histones

Chromosomes are visible only during cell division. During interphase, DNA, combined with proteins called histones, forms dispersed chromatin. Before cell division, DNA replicates during the S phase. As mitosis begins, chromatin condenses into visible chromosomes with identical arms due to DNA replication. This condensation ensures equal distribution of genetic material between daughter cells.