DNA Structure, Chromosomes, and Genetic Inheritance

The Structure and Function of DNA

DNA is a molecule that stores information about all the features inherited from parent to offspring and, therefore, is hereditary or genetic material. DNA is formed by the union of several smaller units called nucleotides. Nucleotides are joined one after the other, forming a chain. DNA is made up of two opposing lines joined together by nitrogenous bases that are paired. The string is wound in the form of a spiral, forming a double helix.

The sequence or order in which the nucleotides are arranged in the DNA chain constitutes the genetic information. DNA has a unique property among the molecules of living things: it can duplicate itself, i.e., generate identical copies of itself. This allows the transmission of genetic information to daughter cells when cells reproduce. The two chains of DNA are held together because the nucleotides have complementary nucleobases facing each other. When two nucleobases are complementary, they form a chemical bond that unites them.

Chromatin and Chromosome Formation

Nuclear DNA is associated with a group of histone proteins to form very long filaments called chromatin fibers. In a cell in interphase (meaning it is not dividing), chromatin fibers are scattered throughout the nucleus. However, when a cell prepares to divide, the nuclear membrane disintegrates and the fibers become coiled and dispersed throughout the cell.

Chromatin fibers, which were previously scattered, coil up and become much thicker and shorter. These structures are now called chromosomes. A chromosome is more or less long and has a narrowing called a centromere that divides it into two arms of the same or different lengths.

Diploid and Haploid Chromosome Numbers

• Diploid (2n): Two sets of homologous chromosomes.
• Haploid (n): A set of homologous chromosomes.
• Human count: n = 23, 2n = 46 (2 x 23).

When sexual reproduction occurs at fertilization, the set of sperm chromosomes (n = 23) joins with the ovum (n = 23). The resulting embryo has 2n = 46 chromosomes; thus, it returns to being diploid. This resets the typical number of chromosomes in the human species.