biology

Minor spliceosome

Minor spliceosome 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 Minor spliceosome rather than just read about it. In short: The minor spliceosome is a ribonucleoprotein complex that catalyses the removal (splicing) of an atypical class of spliceosomal introns (U12-type) from messenger RNAs in some clades of eukaryotes. This process is called noncanonical splicing, as opposed to U2-dependent canonical splicing.

Key takeaways

Minor spliceosome belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Minor spliceosome to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Minor spliceosome from memory before moving on to harder problems.

Reference excerpt

The minor spliceosome is a ribonucleoprotein complex that catalyses the removal (splicing) of an atypical class of spliceosomal introns (U12-type) from messenger RNAs in some clades of eukaryotes. This process is called noncanonical splicing, as opposed to U2-dependent canonical splicing. U12-type introns represent less than 1% of all introns in human cells. However they are found in genes performing essential cellular functions.

Early evidence

A notable feature of eukaryotic nuclear pre-mRNA introns is the relatively high level of conservation of the primary sequences of 5' and 3' splice sites over a great range of organisms. Between 1989 and 1991, several groups reported four independent examples of introns with a splice site that differed from the common intron:

Cartilage matrix protein (CMP/MATN1) gene in humans and chickens Proliferating cell nucleolar protein P120 (NOL1) gene in humans Mouse Rep3 gene, presumably involved in DNA repair Drosophila prospero gene that encodes for a homeobox protein In 1991 by comparing the intron sequences of P120 and CMP genes, IJ Jackson reported the existence of ATATCC (5') and YYCAC (3') splice sites in these introns. The finding indicated a possible novel splicing mechanism. In 1994, S.L. Hall and R.A. Padgett compared the primary sequence of all reports on the four genes mentioned above. The results suggested a new type of introns with ATATCCTT 5' splice sites and YCCAC 3' splice sites and an almost invariant TCCTTAAC sequence near the 3' end of the introns (so called 3' upstream element). A search for small nuclear RNA sequences that are complementary to these splice sites suggested U12 snRNA (matches the 3' sequence) and U11 snRNA (matches the 5' sequence) as being putative factors involved in splicing of this new type of introns. In all these four genes, the pre-mRNA contains other introns whose sequences conform to those of major class introns. Neither the size nor the position of the AT–AC intron within the host gene is conserved. In 1996, Woan-Yuh Tarn and Joan A. Steitz described an in vitro system that splices a pre-mRNA substrate containing an AT–AC intron derived from the human P120 gene. Psoralen cross-linking confirms the base-pairing interaction predicted by Hall and Padgett between the branch site of the pre-mRNA substrate and U12 RNA. Native gel electrophoresis reveals that U11, U12, and U5 snRNPs assemble onto the P120 pre-mRNA to form splicing complexes.

Structure of U12-type introns Although originally referred to as AT-AC introns, not all these introns are delimited by AT-AC dinucleotides. Some of them have GT-AG or AT-AG ends, at least. Thus, it is more correct to speak about the splicing machinery which is used to process them, differentiating between U2-type (canonical or major) and U12-type (non-canonical or minor). The main determinants for distinguishing U2- and U12-type introns are 5' splice site and branch site sequences.

The minor spliceosome consists of U11, U12, U4atac, and U6atac, together with U5 and an unknown number of non-snRNP proteins. The U11, U12 and U4atac/U6atac snRNPs are functional analogs of the U1, U2 and U4/U6 snRNPs in the major spliceosome. Although the minor U4atac and U6atac snRNAs are functional analogs of U4 and U6, respectively, they share only limited sequence homology (c. 40%). Furthermore, the sequence of U11 in comparison with U1, as well as U12 compared with U2, are completely unrelated. Despite this fact, the minor U11, U12, U4atac and U6atac snRNAs can be folded into structures similar to U1, U2, U4 and U6, respectively.

Location of minor spliceosomal activity The location of spliceosomal activity for the minor class spliceosome is regarded by most experts to be in the nucleus. However, a single paper has claimed that the minor spliceosome is active in the cytosol. The data presented within this paper are not fully accepted within the field and directly contradict numerous other papers.

Evolution Like the major spliceosome, the minor spliceosome had an early origin: several of its characteristic constituents are present in representative organisms from all eukaryotic supergroups for which there is any substantial genome sequence information. In addition, functionally important sequence elements contained within U12-type introns and snRNAs are highly conserved during evolution.

Illustrations

Worked examples

Example 1 — a first encounter with Minor spliceosome

Start with the simplest possible case. Write down what Minor spliceosome 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 Minor spliceosome 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 Minor spliceosome 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 Minor spliceosome

In research: Minor spliceosome 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 Minor spliceosome 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: Minor spliceosome is common in secondary-school and first-year university syllabi. It links to neighbouring topics Gene expression, Organelles, RNA, so understanding it makes those chapters shorter.
In everyday life: Look for Minor spliceosome 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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How to study Minor spliceosome in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Minor spliceosome means in your own words.
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Work through the three examples above with pen and paper.
Explain Minor spliceosome out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Minor spliceosome in simple terms?

Why does Minor spliceosome 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 Minor spliceosome?

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.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Minor spliceosome.