Too many pairs: DNA, chromatin, and chromosomes

Some facts:

• Molecules of DNA are double-stranded, each strand a perfect complementary copy of the other.
• Humans have 23 pairs of chromosomes.
• Chromosomes are made of DNA.
• We have two copies of each gene.

Where do these two copies of each gene come from — the complementary DNA? Or the pairs of chromosomes? In fact, what is the relationship between DNA and chromosomes? These questions have proven thorny for students in the past, so I'll try here to describe the two different ways in which genetic information is duplicated in a cell.

1. Complementary copies of DNA strands

Molecules of DNA duplicate the information they carry. Each base in a DNA strand is bonded to its complement on the opposite strand: A bonds to T, C bonds to G. It’s like a backup mechanism: if something happens to one part of the strand, the other half is there with the complement of the information. So that’s the first way in which genetic information is paired. And the important part about this pairing is that the pairs are exact complements of each other, like mirror images: the information is duplicated precisely and does not vary.

Part of a DNA strand, demonstrating complementary bases in matching colors

This double-stranded molecule of DNA, then, is wrapped up tightly around proteins called histones, and this set of spools of DNA and histones all together is a material called chromatin.

Chromatin: light blue/green strands of DNA wrapped around bright blue histones.

2. Two copies of each chromosome

Chromosomes are made out of chromatin. In a lot of the pictures of chromosomes in which you can see the chromatin that makes it up, the chromatin looks sort of like yarn woven into a sweater. That’s a reasonable way to think of chromosomes: big structures (big enough that we can see them with a not-too-powerful microscope) made from this yarn-like chromatin.

Cartoon of chromosome made of yarn-like chromatin. (Image by Magnus Manske at Wikipedia.)

 A particular gene is always on a particular chromosome in a given species. For example, the gene for oxytocin, OXT, is always on chromosome 24 in dogs and chromosome 20 in humans. So your chromosomes are very orderly, each one containing a specific set of genes.

Humans have 23 pairs of chromosomes. Dogs have 39 pairs. In fact, Wikipedia has a whole page devoted to the number of pairs of chromosomes in different species. We think of chromosomes as looking like big X. The X is actually the two separate chromosomes in a pair, stuck together during the process of cell division. Usually those two arms of the X are separate in the cell.

Image by JWSchmidt at Wikipedia

In a pair of chromosomes, one chromosome is made of chromatin from one (double) strand of DNA and proteins, which you got from your mother; and the other chromosome is made of chromatin from another (double) strand of DNA and proteins, which you got from your father. So you have, for example, two copies of chromosome 20, one from your mother and one from your father.

Human chromosomes: 23 pairs.

These pairs of chromosomes are the second way in which your genetic information is paired. But this is very different from the exact copy pairing of strands of DNA. Your version of the OXT gene on the chromosome you got from your mother may be slightly different from your version of the OXT gene on the chromosome you got from your father. Where her version had a G-C, his version may have an A-T. (Or it may be just the same.)

DNA differences on two different copies of the same chromosome (see the difference highlighted in blue).

So we have two versions of each gene, one on each chromosome in a matched pair. For a particular gene, we may have two identical copies (in which case we are “homozygous” for that gene) or we may have two different versions (in which case we are “heterozygous” for that gene).

So, yes, we essentially have four instances of each gene in each cell. That’s four instances, but a maximum of two versions of the gene (the doubled instances on a double strand of DNA are always identical complements; it’s only when you compare instances between chromosomes that you may see differences).

In the minds of geneticists, it’s the two versions in the pair of chromosomes which really count. That’s the pair that could differ, after all. And that’s why you'll hear that we have “two” copies of each gene, even though the gene is paired both in the DNA and again between chromosomes.