HUMAN BIOLOGY

408 Chapter 21

tRNA a nucleic acid chain that can bring a

(transfer RNA) specific amino acid to the mRNA/

ribosome complex during the transla-

tion step of protein synthesis

Only mRNA eventually is translated into a protein.

The other two types of RNA operate in translation, the

second stage of protein synthesis, which is described in

Section 21.6.

in transcription, Dna is decoded into rna

In transcription, a strand of RNA is assembled on a DNA

template according to the base-pairing rules:

Transcription takes place in the cell nucleus, but it isn’t

the same as DNA replication. Only the gene that is being

transcribed serves as the template, not the whole DNA

strand, and the enzymes involved are RNA polymerases.

Also, transcription makes a single-stranded molecule, not

one with two strands.

Transcription starts at a promoter, a sequence of bases

that signals the start of a gene. As transcription starts, a

nucleotide “cap” is added to the beginning of the mRNA

for pro tection. This capped end (designated 5’) is also

where the mRNA will bind to a ribosome when the time

comes for translation.

DNA

DNA

base pairing in

DNA replication

RNA

DNA

base pairing in

transcription

GCAT

CGTA

GCAU

CGTA

© Cengage Learning

n two processes, called transcription and translation, convert

the information in DNA into proteins. the first step,

transcription, converts DNA into RNA.

n Link to Primary protein structure 2.11

The path from genes to proteins

involves two processes, transcrip-

tion and translation. In both, mol-

ecules of ribonucleic acid, or RNA,

have major roles. Most often, RNA

consists of a single strand. Structur-

ally, it is much like a strand of DNA.

Its nucleotides each consist of a sugar

(ribose), a phosphate group, and a

nitrogen- containing base. However,

its bases are adenine, cytosine, gua-

nine, and uracil, not thymine. Table

21.1 summarizes these differences.

Like the thymine in DNA, the uracil

in RNA base-pairs with adenine.

In transcription, molecules of

RNA are assembled on DNA tem-

plates in the nucleus. Genes are transcribed into three

types of RNA:

rRNA a nucleic acid chain that combines

(ribosomal RNA) with proteins to form a ribosome, a

structure on which a polypeptide

chain is assembled

mRNA a sequence of nucleotides that carries

(messenger RNA) instructions for building proteins to

ribosomes in the cell cytoplasm

Figure 21.7 Animated! In transcription, an mRNA molecule is assembled on a DNA template. A A gene in part of a DNA double

helix. The base sequence of one of the two nucleotide strands is used as the template. B–D Transcribing that gene results in a molecule

of mRNA. (© Cengage Learning)

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

21.3

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

Dna into rna: the First step in making proteins

mRNA Messenger RNA; its
sequence of nucleotides is
a DNA-based template for
building proteins.

RNA polymerases
Enzymes that catalyze tran-
scription of DNA to RNA.

rRNA Ribosomal RNA; it
combines with proteins to
form ribosomes.

transcription Assembly of
RNA with a base sequence
that corresponds to the
sequence of the DNA region
where it was assembled.

tRNA Transfer RNA; it picks
up amino acids and pairs
them with mRNAs.

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).