Human eggs and sperm each contain 23 ______, which are composed of long strands of ______?

The answer

Human eggs and sperm each contain 23 chromosomes, and those chromosomes are made of long strands of DNA. Both blanks describe the same structural hierarchy: DNA is the underlying molecule, and a chromosome is what you get when a very long DNA strand coils up tightly around packaging proteins called histones.

Eggs and sperm are called gametes, and gametes are haploid, meaning they carry only one copy of each chromosome (23 total) rather than the two copies (46 total) found in ordinary body cells. This halving happens during a special type of cell division called meiosis. It matters because when a sperm fertilizes an egg, the two sets combine: 23 + 23 = 46 chromosomes, restoring the full diploid number for the new individual.

Zooming in: gamete to chromosome to DNA

Think of it as three levels of magnification:

1. The gamete (egg or sperm) is a single cell. Inside its nucleus sit 23 chromosomes.
2. A chromosome is a discrete package of genetic material. If you unwound one, you would get a single continuous double-stranded DNA molecule.
3. DNA itself is the famous double helix: two sugar-phosphate backbones twisted around each other, joined by paired bases (A-T and G-C). The sequence of these bases is the genetic code.

So the sentence is describing a nesting relationship. Chromosomes are the storage-and-transport form; DNA is the information molecule inside them.

Why other common answers are wrong

• "23 genes" is incorrect. A single chromosome carries hundreds to thousands of genes, so 23 genes would be far too few to run a human. Genes are segments of DNA, not the countable units in a gamete.
• "46 chromosomes" is the diploid number for body cells (somatic cells), not gametes. Writing 46 would ignore the whole point of meiosis and would give a fertilized egg 92 chromosomes.
• "Composed of proteins" for the second blank is a trap. Proteins (histones) help package DNA, but the long strands themselves are DNA. Chromosomes are DNA plus protein, but the informational "strands" are DNA.
• "RNA" is also wrong for the second blank. RNA is a working copy used to build proteins; the stable, heritable strands stored in chromosomes are DNA.

The bigger picture

The haploid/diploid distinction is the foundation of sexual reproduction. Because each parent contributes exactly half of the chromosomes, offspring inherit a genuinely new combination of genes, which is a major source of genetic variation. If gametes were diploid, chromosome number would double every generation, so the reduction to 23 during meiosis is not a quirk but a necessity for keeping our species at a stable 46 chromosomes.