Final answer:
Chromosomes are the threadlike structures in the cell nucleus made of DNA and proteins, each human cell having 23 pairs of them. They carry the genetic instructions for building and maintaining an organism, and are crucial in cell division and heredity.
Step-by-step explanation:
Chromosomes are threadlike nuclear structures consisting of DNA and proteins that serve as repositories for genetic information in living organisms. Human cells typically contain 23 pairs of chromosomes, totaling 46 individual chromosomes. Each chromosome is comprised of DNA wound around histone proteins into a tightly coiled structure, housing the genetic blueprints of an organism. These blueprints, or genes, are sequences of DNA that instruct the assembly of amino acids into proteins, which perform various functions in the body.
The DNA itself is a double helix, resembling a molecular spiral staircase, where each step consists of paired nitrogen bases—adenine with thymine and guanine with cytosine—connected via hydrogen bonds. The human genome is an impressive structure, consisting of almost 22,000 genes distributed across the chromosomes and is unique for each individual, with the exception of identical twins. During cell division, the chromatin condenses into chromosomes, ensuring the safe and organized distribution of genetic material to the resulting daughter cells.
In sexual reproduction, the egg and the sperm each contribute 23 chromosomes, combining to form the complete set of 46, where genetic traits such as eye color, hair color, and others are controlled by specific genes and their variants, known as alleles.