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Chapter 15: Problem 48

You sequence a gene of interest and isolate the matching mRNA. You find that the mRNA is considerably shorter than the DNA sequence. Why is that? a. There was an experimental mistake. The mRNA should have the same length as the gene. b. The mRNA should be longer than the DNA sequence because the promoter is also transcribed. c. The processed mRNA is shorter because introns were removed. d. The mRNA is shorter because the signal sequence to cross the nuclear membrane was removed.

Short Answer

Expert verified
c. The processed mRNA is shorter because introns were removed.

Step by step solution

01

- Understand the DNA and mRNA Structure

DNA contains both coding regions (exons) and non-coding regions (introns). When a gene is transcribed into mRNA, the resulting pre-mRNA includes both exons and introns.
02

- Learn About mRNA Processing

Before mRNA can be translated into a protein, it undergoes processing. One crucial step of mRNA processing is the removal of introns through a process called splicing.
03

- Compare Pre-mRNA and Processed mRNA

After splicing, the introns are removed, leaving only the exons in the processed mRNA. This results in the processed mRNA being considerably shorter than the original DNA sequence that includes introns.
04

- Evaluate the Given Choices

Choice (a): Incorrect - Experimental mistakes can't generally account for the consistent observation that mRNA is shorter. Choice (b): Incorrect - Only exons are translated; the promoter is not part of the transcribed region in mRNA. Choice (c): Correct - This correctly identifies that mRNA is shorter because the introns have been removed during processing. Choice (d): Incorrect - The signal sequence for nuclear export does not account for the size difference.
05

- Conclusion

The correct reason why the mRNA is considerably shorter than the DNA sequence is due to the removal of introns during mRNA processing.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

DNA and mRNA structure
Understanding the structure of DNA and mRNA is key to solving why processed mRNA is shorter. DNA is a double-stranded molecule that holds our genetic code. It consists of coding regions called exons and non-coding regions called introns. When a gene is transcribed, the produced mRNA initially includes both these exons and introns. However, raw mRNA (also called pre-mRNA) must undergo further processing to become a mature mRNA molecule that cells can use for protein synthesis. This structure sets the stage for understanding the changes mRNA undergoes before it can be effectively translated into proteins.
mRNA splicing
One of the crucial steps in mRNA processing is splicing. This process removes the non-coding introns from the pre-mRNA. Specialized enzymes called spliceosomes carry out this task. The spliceosome cuts out the introns and stitches the exons together, creating a shorter, cohesive mRNA strand. This mature mRNA contains only the genetic sequences required to produce proteins. Without splicing, the mRNA would retain introns and result in a non-functional protein. Thus, mRNA splicing is essential for proper gene expression and protein creation.
introns and exons
Introns and exons are the two main components within a gene's DNA sequence. Introns are the non-coding sections that do not contribute to the final protein structure. Conversely, exons are the coding sections that remain in the mature mRNA and are translated into proteins. During gene expression, the entire DNA sequence of a gene, including both introns and exons, is transcribed into pre-mRNA. The next step is splicing, where introns are removed, and exons are joined together to form the mature mRNA. This spliced mRNA is then shorter than the original DNA sequence, consisting only of exons. Understanding the roles of introns and exons is vital for comprehending how mRNA processing affects the final length of the mRNA.

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Most popular questions from this chapter

Only a fraction of DNA encodes proteins. The noncoding portion of a gene is referred to as the intron. The intron fraction depends upon the gene. Introns are rare in prokaryotic and mitochondrial DNA; in human nuclear DNA, this fraction is about 95%. The intron is transcribed into mRNA, but this noncoding mRNA is edited out before translation of the coding portion, or exon, of a gene. The edited exon segments are then spliced together by a spliceosome, a very large and complex collection of RNAs and proteins. Although introns do not encode proteins, they have functions. In particular, they amplify expression of the exon, although the mechanism is unknown. When introns are very long, which is common among mammalian genes with roles in development, they can significantly extend the time required to complete transcription. Analysis of genes common to different plant and animal species shows many shared intronic positions and base sequences, although in some organisms, such as yeast, many introns have been deleted. Because introns do not encode proteins, mutations can remain silent and accumulate. A. As described above, introns are ancestral remnants that are replicated because they do not disadvantage the organism. Consider the claim that introns are 鈥渏unk DNA.鈥� Evaluate the claim with supporting evidence. B. Introns may be retained during transcription. Explain how the retention of a transcribed intron between two transcribed exons within a gene could do the following: 鈥� block expression of one polypeptide sequence 鈥� increase expression of a polypeptide 鈥� alter the polypeptide expressed

What would happen if the 5鈥� methyl guanosine was not added to an mRNA? a. The transcript would degrade when the mRNA moves out of the nucleus to the cytoplasm. b. The mRNA molecule would stabilize and start the process of translation within the nucleus of the cell. c. The mRNA molecule would move out of the nucleus and create more copies of the mRNA molecule. d. The mRNA molecule would not be able to add the poly-A tail on its strand at the 5鈥� end.

Which molecule in the central dogma can be compared to a disposable photocopy of a book kept on reserve in the library? a. DNA b. mRNA c. Protein d. tRNA

What is the flow of information for the synthesis of proteins according to the central dogma? a. DNA to mRNA to protein b. DNA to mRNA to tRNA to protein c. DNA to protein to mRNA to protein d. mRNA to DNA to mRNA to protein

What is often the first amino acid added to a polypeptide chain? a. adenine b. leucine c. methionine d. thymine
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