IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
Why isn’t DNA perfect? Why are there mutations?
Plants and animals are able to grow thanks to DNA code that gives their cells instruc-
tions on how to build tissues, nerves, and other internal structures. But DNA doesn’t
always work properly—errors occur in the replication process. These are called
mutations, but how and why do they occur?
DNA relies on complex chemical reactions to copy itself, and because there are so many atoms involved,
the process isn’t 100 percent accurate. But DNA has an amazing ability to correct most of the errors that
The way DNA provides instructions for an organ-
ism’s growth and reproduction, copies itself, and
combines with other DNA to make new plants and
animals with a unique genetic code is one of the
most amazing aspects of biology.
DNA is essentially a molecular code made up of
around 440 million sets of instructions. It forms
a distinct “double helix” spiral shape, like a ladder
twisted around itself. Each rung of the ladder
provides vital information for the growth of the
organism to which the DNA belongs.
Plants and animals grow by dividing and rep-
licating their cells. Life that reproduces sexually
starts with just two cells—an egg and a sperm—
but by the time the life form is mature, it will
consist of trillions of cells. Each one has been
assembled according to the instructions in the
life form’s DNA.
Humans are eukaryotes, because our cells
have a nucleus in the middle. Eukaryote means
“good nut” or “good kernel” in Greek. This
nucleus contains our genetic material, which is
a mix of DNA and other molecules that, when
combined, are called chromosomes.
When your body needs to replace material
(we replace many—but not all—of our cells
about every 10 years), cells will grow and split
into two new cells in a process called mitosis.
At one stage of cell division, the DNA in the
nucleus makes a copy of itself for the new cell.
Even though DNA is a molecule, it’s a huge one,
consisting of more than 15 billion atoms. So it
can be forgiven for not making an exact copy
However, because creating a new cell de-
pends on the copy being correct, DNA can actu-
ally “proofread” itself. If it detects mismatches
in the new strand, it will undo the work and redo it. It’s an
amazing process that makes complex life possible.
But the proofreading system itself isn’t perfect, either,
and at the end of cell division, minor errors can creep
through. Many of these errors have no eect, but some can
be signiﬁcant enough to change the way a new cell forms.
These are called mutations.
In the vast majority of cases, a mutation will kill the
new cell. The body ﬂushes the cell and tries again. Total
amount of time, material, and energy wasted? Very little.
Sometimes, though, the mutation doesn’t kill the new
cell. The worst-case scenario is a new kind of cell that
divides and divides again, out of control in the body, im-
pacting on important tissues and organs. This is cancer.
A more signiﬁcant kind of mutation is one that occurs
in a sex cell like a sperm or an egg. These cells carry only
half the number of chromosomes as a normal cell, because
they will build a new set of chromosomes when the egg is
joined with a sperm. This kind of mutation can be passed
on to the embryo.
Mutations are not inherently good or bad. Some of
them will make a child sick, as in the genes that cause
cystic ﬁbrosis or type 1 diabetes. Others are beneﬁcial, like
the European mutation that gives people lactose tolerance
and the ability to drink cow’s milk. Or they might just
change the way we look, like giving us blue eyes.
Over very long time periods, mutations build up, and
the organism changes and becomes a new species. This is
the essence of evolution.
Free nucleotides are attracted
to their complementary bases
A strand separates
A piece of DNA
strands are formed