HUMAN BIOLOGY

DNa, GeNes, aND BioteChNoloGy 411

What Do rrNas aND trNas Do?

• rRNAs and proteins form subunits that combine into ribosomes.


• Different kinds of tRNAs pick up amino acids that are used to
  build polypeptide chains.


• Different tRNAs bind specific codons in mRNA. In this way amino
  acids line up in the order specified by mRNA.


• Ribosomes are temporary structures where proteins are
  assembled.

taKe-home messaGe

anticodon A nucleotide

triplet that can base-pair

with codons. The matching

of codons and anticodons

lines up amino acids in the

proper order in a new poly-

peptide chain.

trna and rrna

F i g u r e 21.10 Animated! Ribosomes consist of small and large

subunits formed by rRNA. Chains of amino acids—the primary

structure of proteins, called polypeptide chains—are assembled

on part of the small subunit. Newly forming chains move through

a tunnel in the large subunit. (© Cengage Learning)

Figure 21.9 Amino acids “hook up” with tRNAs. The model

above represents a tRNA that carries the amino acid tryptophan.

Each tRNA’s anticodon is complementary to an mRNA codon.

Each also carries the amino acid that is specified by the codon.

(© Cengage Learning)

A C C

trp

anticodon

amino acid

attachment site

tunnel

small ribosome

subunit

large ribosome

subunit

assembled

+ ribosome

rrnas are ribosome building blocks

Our discussion of cell organelles in Section 3.2 introduced

you to ribosomes. In a cell’s cytoplasm, tRNAs come into

contact with mRNAs at binding sites on the surfaces of

ribosomes. Each ribosome has two subunits, as you can

see in Figure 21.10. The subunits are

built in the nucleus from rRNA and

proteins; then they are shipped to

the cytoplasm. There they will com-

bine into ribosomes. These tempo-

rary structures are the workbenches

where proteins are assembled dur-

ing translation, as described in the

next section.

n transfer RNA and ribosomal RNA carry out the protein-

building instructions of mRNA.

n Links to Primary protein structure 2.11, the parts of

a eukaryotic cell 3.2

trnas match amino acids

with mrna “code words”

A cell’s cytoplasm contains amino acids and tRNA

molecules. Each tRNA has a “hook” site where it can

attach to a specific amino acid. As shown in Figure 21.9, a

tRNA also has an anticodon, a nucleotide triplet that can

base-pair with codons. When a series of tRNAs bind to a

series of codons, the matching up of codons and anticodons

automatically lines up the amino acids attached to tRNAs

in the order specified by mRNA.

A cell has more than sixty kinds of codons but fewer

kinds of tRNAs. The needed match-ups occur anyway.

Remember that by the base-pairing rules, adenine pairs

with uracil, and cytosine with guanine. However, for

codon–anticodon interactions, the rules loosen up for the

third base. For example, only two tRNAs are needed to

hook onto the codons CCU, CCC, CCA, and CCG, which all

have instructions for making the amino acid proline.

21.5

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).