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Life Science: Session 5

DNA, Chromosomes, and Genes

What are the differences among DNA, chromosomes, and genes?

DNA double helix

The genome of an organism — its pool of hereditary material— is composed of an organic molecule called deoxyribonucleic acid, or DNA. DNA is a double helix, a spiral-shaped molecule that can be compared to a zipper. The two outer lengths exist as a supportive structure, with the “teeth” consisting of bases on each side that interlock together as base pairs. The bases are molecular sub-units and there are of only four types— adenine (A), thymine (T), cytosine (C) and guanine (G). This four-letter “alphabet” forms the codes for all of an organism ’s traits.

In eukaryotic organisms, DNA is found in the nucleus and is organized in rod-like structures of varying size and shape called chromosomes. Chromosomes contain both DNA and protein, with the DNA being wrapped around the protein, which preserves its shape. Chromosomes exist in the nucleus in chromosome pairs, with the members of each pair containing information that codes for the same traits — like having two sets of hereditary recipe books containing the same recipes. The entire genome exists in each of an organism’s body cells (with some exceptions — like alternation of generations in plants). In human beings, for example, there are 46 chromosomes organized as 23 pairs.
Genes are lengths of DNA that carry information for a particular trait. Each gene occupies a specific position on a specific chromosome, with the same gene being found in the same place on each member of a chromosome pair. Each gene consists of a sequence of the bases A, T, C, and G. Genes give rise to proteins, which ultimately result in the expression of traits.

PTC taster
Subject tastes
PTC paper

Within a population of organisms, there can be variation in the information contained in one gene. These variants are called alleles, and they differ because their sequence of bases differs. For example, one gene dictates peoples’ ability to taste phenol thiocarbamide or PTC, as was featured in the video. One allele codes for the ability to taste PTC while another codes for the opposite. An individual’s combination of alleles for any given gene determines which traits are expressed. For many traits, there are numerous alleles, and multiple genes control many traits.


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