HUMAN BIOLOGY

DNa, GeNes, aND BioteChNoloGy 409

exons Nucleotide

sequences in mRNA that

carry the protein-building

instructions of DNA.

introns Sections of mRNA

that do not carry DNA’s

protein-building instructions.

regulatory proteins Pro-

teins that speed up or stop

gene transcription.

hoW is DNa coNverteD iNto rNa?

• The process of transcription converts DNA into RNA. A
  sequence of bases in one strand of a DNA molecule is the
  template for assembling a strand of RNA. Transcription is
  the first step in protein synthesis.


• Transcription occurs in the nucleus. Before leaving the nucleus,
  new RNA transcripts are modified into their final form.


• Gene transcription can be turned on or off to produce
  specialized cell structures or functions.

taKe-home messaGe

are essential to any cell’s structure

and functioning. Yet each type of

cell also uses a small subset of genes

in specialized ways. For example,

every cell carries the genes for hemo-

globin, but only the precursors of

red blood cells activate those genes.

Each cell determines which genes

are active and which gene products

appear, when, and in what amounts.

Some genes might be switched on

and off throughout a person’s life. Others might be turned

on only in certain cells and only at certain times.

Genes are regulated by molecules that interact with

DNA, RNA, or other substances. For example, regulatory

proteins speed up or halt transcription. Some also may

bind with noncoding DNA sequences and in this way trig-

ger or shut down the transcription of a neighboring gene.

For example, this is how steroid hormones, such as cortisol,

estrogen, and testosterone, act.

As RNA polymerase moves along the DNA, it joins

nucleotides together (Figure 21.7). When it reaches a

termination sequence of bases, the RNA strand—called a

transcript—is released, but it is not yet finished. It must

be modified before its protein-building instructions can

be used. For example, when many human genes are

transcribed, this “pre-mRNA” contains sections called

introns—in some cases including as many as 100,000

nucleotides! Introns may be a sort of genetic gibberish.

Researchers have not discovered any that code for proteins.

All new mRNA transcripts also contain regions called

exons. Unlike introns, exons are the nucleotide sequences

that carry DNA’s protein-building instructions. Before an

mRNA leaves the nucleus, its introns are snipped out and

its exons are spliced together. Now the mRNA is ready to

enter the cell cytoplasm and be translated into a protein.

Some genetic disorders, such as the blood disorder thalas-

semia (Chapter 8) arise when the splicing step goes awry.

Gene transcription can be turned on or off

Most cells of your body carry the same genes. Many of

those genes carry instructions for making proteins that

DNA RNA

sugar Deoxyribose Ribose

bases Adenine, cytosine Adenine, cytosine,

guanine, thymine guanine, uracil

Table 21.1 DNA and RNA

A RNA polymerase binds to a promoter in the DNA.
The binding positions the polymerase near a gene in
the DNA.

In most cases, the nucleotide sequence of the gene
occurs on only one of the two strands of DNA. Only
the complementary strand will be translated into RNA.

B The polymerase begins to move along the DNA and unwind it.

As it does, it links RNA nucleotides into a strand of RNA in the order

specifi ed by the base sequence of the DNA.

The DNA double helix winds up again after the polymerase passes.

The structure of the “opened” DNA molecule at the transcription site

is called a transcription bubble, due to its appearance.

forming RNA

transcript

DNA template

unwinding

DNA template

winding up

gene region

RNA polymerase, the enzyme

that catalyzes transcription

C The base sequence of the new RNA strand is

complementary to the base sequence of its DNA

template—the gene.

G

A

A

G

transcription site

growing RNA transcript

D At the end of the gene region, the last stretch of the

new transcript unwinds and detaches from the DNA template.

A U G C C U G A G G A G A A G

C The base sequence of the new RNA strand is

complementary to the base sequence of its DNA

template—the gene.

G

A

A

G

transcription site

growing RNA transcript

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