When a cell divides in two, one of its main jobs is to make sure that each of the two new cells gets a full, perfect copy of genetic material. Mistakes during copying, or unequal division of the genetic material between cells, can lead to cells that are unhealthy or nonfunctional (and even to diseases such as cancer). But what exactly is this genetic material, and how does it behave over the course of a cell division?
DNA and Genomes
DNA (deoxyribonucleic acid) is the genetic material of living organisms. In humans, DNA is found in almost all the cells of the body and provides the instructions they need to grow, function, and respond to their environment. When a cell of the body divides, it will pass on a copy of its DNA to each of its daughter cells. DNA is also passed on at the at the level of organisms, with the DNA in sperm and egg cells combining to form a new organism that has genetic material from both its parents. Physically speaking, DNA is a long string of paired chemical units (nucleotides) that come in four different types, and it carries information organized into units called genes. Genes typically provide instructions for making proteins, which give cells and organisms their functional characteristics.
In eukaryotes such as plants and animals, the great majority of DNA is found in the nucleus and is called nuclear DNA. Mitochondria, organelles that harvest energy for the cell, contain their own mitochondrial DNA, and chloroplasts, organelles that carry out photosynthesis in plant cells, also have chloroplast DNA. The amounts of DNA found in mitochondria and chloroplasts are much smaller than the amount found in the nucleus. In bacteria and other prokaryotes, most of the DNA is found in a central region of the cell called the nucleoid, which functions similarly to a nucleus but is not surrounded by a membrane.
A cell’s set of DNA is called its genome. Since all of the cells in an organism (with a few exceptions) contain the same DNA, you can also say that an organism has its own genome, and since the members of a species typically have similar genomes, you can also describe the genome of a species. In general, when people refer to the human genome, or any other eukaryotic genome, they mean the set of DNA found in the nucleus (that is, the nuclear genome). Mitochondria and chloroplasts are considered to have their own separate genomes.
Each species has its own characteristic number of chromosomes. Humans, for instance, have 46 chromosomes in a typical body cell, while dogs have 78. Like many species of animals and plants, humans are diploid (2n), meaning that most of their chromosomes come in matched sets known as homologous pairs. Thus, the 46 chromosomes of a human cell are organized into 23 pairs, and the two members of each pair are said to be homologues of one another (with the slight exception of the X and Y chromosomes; see below).
Human sperm and eggs, which have only one homologous chromosome from each pair, are said to be haploid (1n). When a sperm and egg fuse, their genetic material combines to form one complete, diploid set of chromosomes. So, for each homologous pair of chromosomes in your genome, one of the homologues comes from your mom and the other from your dad.
The two chromosomes in a homologous pair are generally very similar to one another. They’re the same size and shape, and have the same pattern of light and dark bands, as you can see in the human karyotype (image of the chromosomes) shown above. Bands appear when the chromosomes are stained with a dye, and the dark bands mark more compacted DNA (usually, with fewer genes), while the light bands mark less compacted DNA (usually, with more genes). Most importantly, the two homologues in a pair carry the same type of genetic information. For instance, there is a gene found near the bottom of chromosome 15 that affects eye color. A person might have the blue version, or allele, of this gene on one homologue, but the brown version on the other. Both homologues have the same type of gene in the same place, but they can (and often do!) have different versions of genes.
In humans, the X and Y chromosomes determine a person’s biological sex, with XX for female and XY for male. While the two X chromosomes in a woman’s cells are genuinely homologous, the X and Y chromosomes of a man’s cells are not. They differ in size and shape, with the X being much larger than the Y, and contain different mostly different genes (although they do have small regions of similarity). The X and Y chromosomes are known as sex chromosomes, while the other 44 human chromosomes are called autosomes.