More Related Content More from Amo Oliverio (20) 1. © Cengage Learning 2015
9.3 What Roles Do mRNA, rRNA, and
tRNA Play During Translation?
• The messenger: mRNA
– Codon: an mRNA nucleotide base triplet that codes for an
amino acid (or stop signal)
– There are a total of sixty-four mRNA codons that constitute the
genetic code
– The sequence of bases in a triplet determines which amino acid
the codon specifies
• Example: UUU codes for the amino acid phenylalanine, and UUA
codes for leucine
1
3. © Cengage Learning 2015
The Messenger: mRNA (cont’d.)
• Codons occur one after another along the length of an
mRNA
• When an mRNA is translated, the order of its codons
determines the order of amino acids in the resulting
polypeptide
3
4. © Cengage Learning 2015
The Messenger: mRNA (cont’d.)
4
a gene
region
in DNA
mRNA
protein
codon
translation
codon
transcription
codon
5. © Cengage Learning 2015
The Messenger: mRNA (cont’d.)
• With a few exceptions, twenty naturally occurring amino
acids are encoded by the genetic code
– Some amino acids are specified by more than one codon
• Example: the amino acid tyrosine is specified by two codons: UAA and
UAC
5
6. © Cengage Learning 2015
The Messenger: mRNA (cont’d.)
• Some codons signal the beginning and end of a protein-
coding sequence
– The first AUG in an mRNA: signal to start translation
– UAA, UAG, and UGA: signals that stop translation
• The genetic code is highly conserved
6
7. © Cengage Learning 2015
The Translators: rRNA and tRNA
• Ribosomes interact with transfer RNAs (tRNAs) to
translate the sequence of codons in an mRNA into a
polypeptide
7
8. © Cengage Learning 2015
The Translators: rRNA and tRNA (cont’d.)
• A ribosome has two subunits, one large and one small
that consist mainly of rRNA
– During translation, a large and a small ribosomal subunit
converge as an intact ribosome on an mRNA
• rRNA catalyzes formation of a peptide bond between
amino acids as they are delivered to the ribosome
8
9. © Cengage Learning 2015
The Translators: rRNA and tRNA (cont’d.)
9
large subunit small subunit intact ribosome
=+
10. © Cengage Learning 2015
The Translators: rRNA and tRNA (cont’d.)
• Each tRNA has two attachment sites:
– Anticodon: a triplet of nucleotides that base-pairs with an
mRNA codon
– The other attachment site binds to an amino acid (as specified
by the codon)
10
11. © Cengage Learning 2015
The Translators: rRNA and tRNA (cont’d.)
11
anticodon
amino acid
attachment site
12. © Cengage Learning 2015
The Translators: rRNA and tRNA (cont’d.)
• During translation, tRNAs deliver amino acids to a
ribosome
– One after the next in the order specified by the codons in an
mRNA
• As the amino acids are delivered, the ribosome joins them
via peptide bonds into a new polypeptide
12
Editor's Notes Figure 9.6 The genetic code. Figure 9.7 Example of the correspondence between DNA, RNA, and protein. A gene region in a strand of chromosomal DNA is transcribed into an mRNA, and the codons of the mRNA specify a chain of amino acids—a protein. Figure 9.8 {Animated} Ribosome structure. Each intact ribosome consists of a large and a small subunit. The structural protein components of the two subunits are shown in green; the catalytic rRNA components, in brown. Figure 9.9 tRNA structure.
A Icon and model of the tRNA that carries the amino acid tryptophan. Each tRNA’s anticodon is complementary to an mRNA codon. Each also carries the amino acid specified by that codon. Figure 9.9 tRNA structure.
B During translation, tRNAs dock at an intact ribosome (for clarity, only the small subunit is shown, in tan). Here, the anticodons of two tRNAs have basepaired with complementary codons on an mRNA (red). The amino acids they carry are not shown, for clarity.
