A chromosome is a tightly packed bundle of DNA and proteins that carries genes , the instructions that tell cells how to grow, function, and reproduce. Humans usually have 46 chromosomes in most body cells, arranged as 23 pairs inside the cell nucleus.

What a chromosome is

  • Chromosomes are long DNA molecules wrapped around special proteins (histones) so that the genetic material can fit inside the tiny space of a cell nucleus.
  • Each chromosome contains many genes, which are segments of DNA that code for specific traits or functions in the body.

How many humans have

  • Most human body cells (like skin, muscle, and nerve cells) are diploid, meaning they have 23 pairs of chromosomes for a total of 46.
  • The first 22 pairs are autosomes, and the 23rd pair are sex chromosomes: typically XX in females and XY in males.

A quick extra detail

  • Sperm and egg cells are different: they are haploid and carry just 23 single chromosomes each, so that when they join at fertilization, the new cell has 46.
  • Some genetic conditions happen when there is an extra or missing chromosome, such as Down syndrome, which usually involves an extra copy of chromosome 21 (47 chromosomes instead of 46).