Final answer:
The functional unit of heredity is a gene, a sequence of DNA that is responsible for inheriting traits from parents to offspring. Genes specify amino acid sequences in proteins, which are critical for cellular functions. Therefore, the correct option is B.
Step-by-step explanation:
The functional unit of heredity is a gene. A gene is a sequence of DNA that codes for a specific peptide or RNA molecule and is the physical and fundamental unit that gets passed from parent to offspring. Genes are composed of DNA and are linearly arranged on chromosomes. They specify the sequences of amino acids which are the building blocks of proteins. These proteins are, in turn, responsible for orchestrating nearly every function of the cell - thus, genes are essential to life. Regarding the review question to put the following units in order from the smallest to the largest: the order would be C. nitrogen base, D. nucleotide, E. codon, B. gene, A. chromosome. Nitrogen bases are part of nucleotides, which make up codons (a group of three nucleotides), and codons make up genes, which in turn are segments of chromosomes.