* DNA molecule: A DNA molecule contains the entire genetic blueprint of an organism, including many genes.

* Gene: A gene is a specific segment of DNA that provides instructions for building a particular protein or functional RNA molecule. These proteins and RNAs are responsible for the organism's traits.

* Chromosome: A chromosome is a long, thread-like structure made of DNA and proteins that carries many genes.

* Cell: A cell is the basic unit of life and contains the entire genome, including all chromosomes and genes.

In summary: A single gene contains the code for a specific protein or RNA, which directly influences a single trait.