HUMAN BIOLOGY

416 Chapter 21

21.9

DNA sequencing
Determining the order of
nucleotides in DNA, typically
using a supercomputer.

genomics The study of
whole genomes.

Human Genome Project
(HGP) A research project
that determined the approxi-
mate number of human
genes and the order of
nucleotides in them.

What is DNa sequeNciNg?

• DNA sequencing determines the order of nucleotides in a DNA
  fragment.


• Researchers who study genomes use the information from DNA
  sequencing, among other tools.

taKe-home messaGe

mapping the

human Genome

“sequencing” Dna

F i g u r e 21.15 Animated! The printout from a DNA sequencing

machine matches colors with DNA nucleotides. (© Cengage Learning)

F i g u r e 21.16 Supercomputers are used in human genome

research. These gene-sequencing computers are at Celera

Genomics in Maryland. Many new genetic tests make use of

Celera’s database.

Printout

of DNA

sequence:

T C C A T G G A C C A

Tek Image/Science Source

Volker Steger/SPL/Science Source

n DNA sequencing provides useful information about genes,

including their size, location on chromosomes, and the

order of their nucleotides.

To study how a particular gene func-

tions, what kind of mutations occur

in it, and how it interacts with other

genes, it’s useful to have informa-

tion such as where the gene is on its

chromosome, how many nucleotides

are in it, and the order of the nucleo-

tides. DNA sequencing gives this

sort of information. Powerful super-

computers now can sequence DNA

with astonishing speed.

The sequencing method uses stan-

dard and modified versions of the

four nucleotides—A, T, G, and C. Each modified version has

been attached to another molecule that fluoresces (lights up)

a preselected color during the sequencing process. All the

nucleotides are mixed with millions of copies of the DNA

for which the sequence is to be determined, along with a

primer and DNA polymerase. Next, a series of chemical

steps produce a “soup” containing millions of copies of DNA

fragments, each one tagged with a fluorescing molecule.

More processing produces fragments in which each nucleo-

tide in the “mystery” base sequence fluoresces. After several

more steps, the computer program

interprets the information from all

the “marked” nucleotides in the

sample and assembles the original

DNA’s sequence (Figure 21.15).

DNA sequencing is a major tool

in the field of genomics, the study

of genomes. We will now look at the

accomplishments and challenges of

the multinational effort called the

Human Genome Project.

n the human Genome Project provided valuable insights into

the genetic basis of many disorders and diseases.

Thanks to the DNA sequencing of the Human Genome

Project (HGP), we now know that the human genome con-

sists of about 3.2 billion nucleotide bases—As, Ts, Gs, and

Cs (Figure 21.16). The bases are subdivided into roughly

21,500 genes, which provide instructions to build and

operate the body. When the HGP was completed in 2000,

researchers only knew where different genes were located

in the genome. Ongoing studies are gradually identifying

what traits particular genes encode.

One early discovery to come out of the HGP was that

only about 1.5 percent of human DNA is devoted to the

protein-coding parts of our genes, the exons. A challenge

biologists now face is figuring out what noncoding gene

regions do.

It also turns out that our DNA is sprinkled with SNPs

(“snips”). Each SNP (for single nucleotide polymorphism) is

a change in one nucleotide in a sequence. It appears there

are around 1.4 million SNPs in the human genome. Many

result in different gene alleles—the different versions of a

gene that encode slightly different traits.

mapping shows where genes are located

Unraveling the human genome has major implications

for human biology and medicine, in part because genome

sequencing can identify where specific genes are located on

chromosomes. For instance, we know that certain genes on

chromosome 21 are responsible for early-onset Alzheim-

er’s disease, some forms of epilepsy, and one type of

21.8

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