Download Objectives • Describe the process of DNA transcription. • Explain

Objectives
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Describe the process of DNA transcription.
Explain how an RNA message is edited.
Describe how RNA is translated to a protein.
Summarize protein synthesis.
Key Terms
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messenger RNA (mRNA)
RNA polymerase
intron
exon
RNA splicing
transfer RNA (tRNA)
anticodon
ribosomal RNA (rRNA)
You have just learned how genetic information flows from DNA to RNA to protein. A sequence of DNA base
triplets is transcribed into RNA codons, which are translated into a sequence of amino acids that form a
polypeptide. In this section, you will learn the details of the mechanisms of transcription and translation.
Transcription: DNA to RNA
There are three types of ribonucleic acids (RNAs) involved in making proteins from the instructions carried in
genes. Starting with transcription, the RNA molecule called messenger RNA (mRNA) is transcribed from a
DNA template. The transcription process resembles replication of a DNA strand. However, in transcription,
only one of the DNA strands serves as a template for the newly forming mRNA molecule. The two DNA
strands first separate at the place where transcription will start. Then RNA bases pair with complementary DNA
bases (Figure 11-14).
Figure 11-14
During transcription, RNA nucleotides base-pair one by one with DNA
nucleotides on one of the DNA strands (called the template strand). RNA
polymerase links the RNA nucleotides together.
The base-pairing during transcription is the same as when DNA replicates, except that RNA has uracil instead
of thymine: the base U in RNA pairs with A in DNA. A transcription enzyme called RNA polymerase links the
RNA nucleotides together. In the transcription of a gene, specific sequences of DNA nucleotides tell the RNA
polymerase where to begin and end the transcribing process.
Editing the RNA Message
In prokaryotic cells, the mRNA transcribed from a gene directly serves as the messenger molecule that is
translated into a protein. But this is not the case in eukaryotic cells. In a eukaryotic cell, the RNA transcribed in
the nucleus is modified or processed before it leaves the nucleus as mRNA to be translated.
The initial RNA transcripts have stretches of noncoding nucleotides that interrupt nucleotide sequences that
actually code for amino acids. It is as if nonsense groups of letters were randomly scattered in an otherwise
normal document. Such internal noncoding regions are called introns, and are found in most plant and animal
genes. (Many researchers are now trying to determine the functions of introns and how they evolved.) The
coding regions of the RNA transcript—the parts of a gene that remain in the mRNA and will be translated, or
"expressed"—are called exons. Before the RNA leaves the nucleus, the introns are removed and the exons are
joined together, producing an mRNA molecule with a continuous coding sequence (Figure 11-15). This process
is called RNA splicing. With splicing completed, the "final draft" of eukaryotic mRNA is ready for translation.
Figure 11-15
In eukaryotes, the RNA transcript is edited
before it leaves the nucleus. Introns are
removed and the exons are spliced together
before the "final draft" transcript moves into
the cytoplasm where it gets translated.
Translation: RNA to Protein
Translating the nucleic acid language to the protein language is an elaborate process. Like other cellular
processes, the translation of mRNA requires enzymes and sources of chemical energy such as ATP. The main
players in the mRNA translation process are ribosomes and another kind of RNA called transfer RNA.
The Players Translating one language into another language requires an interpreter. Some person or device must
recognize the words of one language and convert them into the other. For the cell, that interpreter is transfer
RNA. Transfer RNA (tRNA) translates the three-letter codons of mRNA to the amino acids that make up
proteins (Figure 11-16).
Figure 11-16
During translation,
tRNAs transport and
match amino acids to
their appropriate
codons on the
mRNA transcript.
One end of the tRNA
attaches to an amino
acid. At the other
end, a triplet of bases
called the anticodon
matches to the
complementary
mRNA codon.
To perform this task, a tRNA molecule must (1) become bound to the appropriate amino acid and (2) recognize
the appropriate codon in the mRNA. The unique structure of tRNA molecules enables them to perform both
functions. There is a different version of tRNA molecule that matches each codon.
At one end of the folded tRNA molecule is a specific triplet of bases called an anticodon. The three bases of the
anticodon are complementary to a specific codon in the mRNA. During translation, the anticodon on tRNA
recognizes a particular codon on mRNA by using base-pairing rules. At the other end of the tRNA molecule is a
site where a particular amino acid can attach. An enzyme specific for each amino acid recognizes both a tRNA
and its amino acid partner and links the two together, using energy from ATP.
The ribosome, an organelle to which you were introduced in Chapter 6, coordinates the functioning of mRNA
and tRNA. The ribosome consists of two parts or subunits, each of which is made up of proteins and a
considerable amount of yet another kind of RNA, ribosomal RNA (rRNA). A complete ribosome has a binding
site for mRNA on its small subunit and two binding sites for tRNA on its large subunit (Figure 11-17). The
subunits of the ribosome act like a vise, holding the mRNA and tRNA molecules close together.
Figure 11-17
Ribosomes bring mRNA and
tRNAs together during translation.
Each ribosome has an attachment
site for an mRNA transcript, and
two sites for tRNAs.
One of the tRNA-binding sites, the "P" site, holds the tRNA carrying the growing polypeptide chain. The other
site, the "A" site, holds a tRNA carrying the next amino acid to be added to the chain. (An easy way to
remember which site is which is that "P" stands for "polypeptide" while "A" stands for "amino acid.") The
ribosome connects the newly arrived amino acid to the growing polypeptide chain.
The Process The first step in translation brings together all the pieces needed during translation: the mRNA, the
first tRNA with its attached amino acid, and the two subunits of a ribosome. The start codon AUG dictates
where translation will begin, as shown in Figure 11-17.
Next, amino acids are added one by one to the growing chain of amino acids. Each addition occurs in a threestep process (Figure 11-18). This lengthening process continues until the ribosome reaches a stop codon—
UAA, UAG, or UGA. Remember that the three stop codons do not code for amino acids. When a new amino
acid fails to arrive at the "A" site, the translation stops. The completed polypeptide, which is typically several
hundred amino acids long, is set free by hydrolysis from the tRNA. A single ribosome can make an averagesized polypeptide in less than a minute. The whole process of translation is summarized in Figure 11-19.
Figure 11-18
During translation, the ribosome adds amino acids to the polypeptide chain. The ribosome moves down the
transcript, codon by codon, until translation is completed.
Figure 11-19
Translation begins with the attachment of a ribosome and the first tRNA to a
"start" (AUG) codon. The ribosome then moves along the mRNA transcript. The
polypeptide elongates as an amino acid is added for each codon. When the
ribosome arrives at a "stop" codon, the completed polypeptide is released.
Review of Protein Synthesis
What is the overall significance of transcription and translation? In learning these processes, you now
understand how genes are responsible for the polypeptides and proteins that make up the structures and perform
the activities of cells. Put more broadly, this is the way that genotype relates to phenotype. The chain of
command originates with the information in the DNA of a gene. The DNA serves as a template, dictating
transcription of a complementary strand of mRNA. In turn, mRNA specifies the sequence of amino acids in a
polypeptide built with the assistance of tRNA and the rRNA of a ribosome. Finally, the proteins that form from
the polypeptides determine the appearance and functioning of the cell and of the whole organism.
Concept Check 11.5
1. What kind of nucleic acid is made during transcription?
2. How do introns and exons relate to RNA splicing?
3. List the three RNA types involved in transcription and translation, and describe the role of each.
4. Briefly describe the steps of protein synthesis.