The New Childrens Encyclopedia

Genes and DNA

Unless you are an identical twin, your body is built

according to a unique set of biological instructions,

your own genetic code. These instructions, or genes,

are present in all cells and are passed from parent

to offspring.

DNA FOR DUMMIES

Genes are made of a substance called Deoxyribose

Nucleic Acid, or DNA. DNA is a very long molecule,

found packed up tightly in the chromosomes in the

nucleus of every cell.

 DNA has two chains, linked

by molecules called bases, which

always pair up the same way.

Guanine pairs with cytosine;

thymine with adenosine. The

order spells out the genetic code.

IN THE GENES

If a person inherits two different

genes from their parents, often one

will dominate the other. For

example, the gene for brown eyes

overrides the one for blue eyes.

+=

 CHROMOSOMES Human

beings normally have 46 chromosomes

in every cell, except gametes (egg and

sperm cells), which have just 23.

 CELL All living things

are made of cells. When cells

divide, the nucleus divides,

too, and the genetic message

is duplicated in each

new cell.

DNA

Chromosomes

Nucleus

Cell membrane

SCIENCE

TIMELINE OF MEDICINE

There are about 30,000 genes

in the human genetic code. Some

appear just once and others repeat

many times. About 99 percent of

human genes are identical to

chimpanzee genes. Even more

amazingly, you share about

75 percent of your genes

with a dog!

7

/^7 

1859

Charles Darwin’s

book Origin of

Species outlines

the importance of

inherited traits in

evolution.

Gregor Mendel’s

experiments on pea

plants prove the

existence of genes.

Friedrich Miescher

extracts DNA from

cells. He calls it

“nuclein.”

James Watson and Francis

Crick discover the structure of

DNA and show

how it can

copy itself.

1860 1869 1953