biology

Three prime untranslated region

Three prime untranslated region is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Three prime untranslated region rather than just read about it. In short: In molecular genetics, the three prime untranslated region (3′ UTR) is the section of messenger RNA (mRNA) that immediately follows the translation termination codon. The 3′ UTR often contains regulatory regions that post-transcriptionally influence gene expression.

Key takeaways

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Reference excerpt

In molecular genetics, the three prime untranslated region (3′ UTR) is the section of messenger RNA (mRNA) that immediately follows the translation termination codon. The 3′ UTR often contains regulatory regions that post-transcriptionally influence gene expression. During gene expression, an mRNA molecule is transcribed from the DNA sequence and is later translated into a protein. Several regions of the mRNA molecule are not translated into a protein including the 5′ cap, 5′ untranslated region, 3′ untranslated region and poly(A) tail. Regulatory regions within the 3′ untranslated region can influence polyadenylation, translation efficiency, localization, and stability of the mRNA. The 3′ UTR contains binding sites for both regulatory proteins and microRNAs (miRNAs). By binding to specific sites within the 3′ UTR, miRNAs can decrease gene expression of various mRNAs by either inhibiting translation or directly causing degradation of the transcript. The 3′ UTR also has silencer regions which bind to repressor proteins and will inhibit the expression of the mRNA. Many 3′ UTRs also contain AU-rich elements (AREs). Proteins bind AREs to affect the stability or decay rate of transcripts in a localized manner or affect translation initiation. Furthermore, the 3′ UTR contains the sequence AAUAAA that directs addition of several hundred adenine residues called the poly(A) tail to the end of the mRNA transcript. Poly(A) binding protein (PABP) binds to this tail, contributing to regulation of mRNA translation, stability, and export. For example, poly(A) tail bound PABP interacts with proteins associated with the 5′ end of the transcript, causing a circularization of the mRNA that promotes translation. The 3′ UTR can also contain sequences that attract proteins to associate the mRNA with the cytoskeleton, transport it to or from the cell nucleus, or perform other types of localization. In addition to sequences within the 3′ UTR, the physical characteristics of the region, including its length and secondary structure, contribute to translation regulation. These diverse mechanisms of gene regulation ensure that the correct genes are expressed in the correct cells at the appropriate times.

Physical characteristics The 3′ UTR of mRNA has a great variety of regulatory functions that are controlled by the physical characteristics of the region. One such characteristic is the length of the 3′ UTR, which in the mammalian genome has considerable variation. This region of the mRNA transcript can range from 60 nucleotides to about 4000. On average the length for the 3′ UTR in humans is approximately 800 nucleotides, while the average length of 5′ UTRs is only about 200 nucleotides. The length of the 3′ UTR is significant since longer 3′ UTRs are associated with lower levels of gene expression. One possible explanation for this phenomenon is that longer regions have a higher probability of possessing more miRNA binding sites that have the ability to inhibit translation. In addition to length, the nucleotide composition also differs significantly between the 5′ and the 3′ UTR. The mean G+C percentage of the 5′ UTR in warm-blooded vertebrates is about 60% as compared to only 45% for 3′ UTRs. This is important because an inverse correlation has been observed between the G+C% of 5′ and 3′ UTRs and their corresponding lengths. The UTRs that are GC-poor tend to be longer than those located in GC-rich genomic regions. Sequences within the 3′ UTR also have the ability to degrade or stabilize the mRNA transcript. Modifications that control a transcript's stability allow expression of a gene to be rapidly controlled without altering translation rates. One group of elements in the 3′ UTR that can help destabilize an mRNA transcript are the AU-rich elements (AREs). These elements range in size from 50 to 150 base pairs and generally contain multiple copies of the pentanucleotide AUUUA. Early studies indicated that AREs can vary in sequence and fall into three main classes that differ in the number and arrangement of motifs. Another set of elements that is present in both the 5′ and the 3′ UTR are iron response elements (IREs). The IRE is a stem-loop structure within the untranslated regions of mRNAs that encode proteins involved in cellular iron metabolism. The mRNA transcript containing this element is either degraded or stabilized depending upon the binding of specific proteins and the intracellular iron concentrations.

The 3′ UTR also contains sequences that signal additions to be made, either to the transcript itself or to the product of translation. For example, there are two different polyadenylation signals present within the 3′ UTR that signal the addition of the poly(A) tail. These signals initiate the synthesis of the poly(A) tail at a defined length of about 250 base pairs. The primary signal used is the nuclear polyadenylation signal (PAS) with the sequence AAUAAA located toward the end of the 3′ UTR. However, during early development cytoplasmic polyadenylation can occur instead and regulate the translational activation of maternal mRNAs. The element that controls this process is called the CPE which is AU-rich and located in the 3′ UTR as well. The CPE generally has the structure UUUUUUAU and is usually within 100 base pairs of the nuclear PAS. Another specific addition signaled by the 3′ UTR is the incorporation of selenocysteine at UGA codons of mRNAs encoding selenoproteins. Normally the UGA codon encodes for a stop of translation, but in this case a conserved stem-loop structure called the selenocysteine insertion sequence (SECIS) causes for the insertion of selenocysteine instead.

Role in gene expression The 3′ untranslated region plays a crucial role in gene expression by influencing the localization, stability, export, and translation efficiency of an mRNA. It contains various sequences that are involved in gene expression, including microRNA response elements (MREs), AU-rich elements (AREs), and the poly(A) tail. In addition, the structural characteristics of the 3′ UTR as well as its use of alternative polyadenylation play a role in gene expression.

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Illustrations

Three prime untranslated region: The flow of information within a cell. DNA is first transcribed into RNA, which is subsequently translated into protein. (See Central dogma of molecular biology.) — The flow of information within a cell. DNA is first transcribed into RNA, which is subsequently translated into protein. (See Central dogma of molecular biology.)

Worked examples

Example 1 — a first encounter with Three prime untranslated region

Start with the simplest possible case. Write down what Three prime untranslated region claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Three prime untranslated region before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Three prime untranslated region ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Three prime untranslated region

In research: Three prime untranslated region appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Three prime untranslated region in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Three prime untranslated region is common in secondary-school and first-year university syllabi. It links to neighbouring topics Gene expression, RNA, so understanding it makes those chapters shorter.
In everyday life: Look for Three prime untranslated region outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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Frequently asked questions

What is Three prime untranslated region in simple terms?

Why does Three prime untranslated region matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Three prime untranslated region?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

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