Problem 5 - Biology: The Dynamic Science

Question

A cDNA and a cloned fragment of genomic DNA share sequences from a mouse gene. What differences do you expect to see between the cDNA and genomic DNA sequences? a. None; they should be identical. b. The genomic DNA might have an intron or introns. c. The genomic DNA might have promoter sequences. d. The genomic DNA might have a poly(A) tail. e. Both b and c are correct.

Step-by-Step Solution

Answer

The correct answer is e. Both b and c are correct; the genomic DNA might have intron(s) and promoter sequences that the cDNA does not.

Step 1: Understanding cDNA and Genomic DNA

Complementary DNA (cDNA) is made from messenger RNA (mRNA) in a process that is reverse of the usual transcription. cDNA represents the coding regions of the gene, also known as exons, with introns (non-coding regions) removed during the process of RNA splicing before mRNA is used as the template. Genomic DNA, on the other hand, includes both exons and introns, as well as regulatory regions such as promoters.

Step 2: Comparing Sequences of cDNA and Genomic DNA

When comparing cDNA to genomic DNA, it should be noted that cDNA is complementary to mRNA and therefore lacks introns and regulatory regions, but genomic DNA contains the entire gene, comprising introns, exons, and regulatory regions such as promoters.

Step 3: Identifying Differences

Since cDNA is created from mRNA which has already been processed to remove introns and does not contain promoter regions, the differences between cDNA and genomic DNA are that genomic DNA may contain intron(s) (option b) and promoter sequences (option c). A poly(A) tail is added to mRNA after transcription and is not a region encoded by genomic DNA, excluding option d as a difference.

Key Concepts

Introns and Exons

Introns and exons are fundamental components that differentiate genomic DNA from cDNA. Genomic DNA is the complete sequence of DNA in an organism, containing both exons, which are sequences that code for proteins, and introns, the non-coding sequences that are spliced out during mRNA processing.

When a gene is expressed, the entire gene is transcribed, including both exons and introns. However, before the mRNA molecule can be translated into a protein, the introns must be removed through a process called RNA splicing. The exons are then joined together to form the final, mature mRNA transcript. Because cDNA is synthesized from mature mRNA, it only contains exons and lacks the introns that are found in genomic DNA.

mRNA Processing

mRNA processing is a critical step in gene expression that transforms the primary RNA transcript into mature mRNA. This process involves a series of modifications including the addition of a 5' cap, the splicing out of introns, and the addition of a poly(A) tail at the 3' end.

This processing not only protects the mRNA from degradation but also plays a critical role in its transportation and localization within the cell, and in its translation efficiency. The cDNA reflects the sequence of this processed, mature mRNA and hence, will not contain introns or other regulatory sequences that are removed during mRNA processing.

Gene Regulatory Sequences

Gene regulatory sequences are key elements in genomic DNA that control gene expression. They are not translated into proteins but are crucial for the regulation of when, where, and how much of a gene product is made.

The main types of regulatory sequences include promoters, enhancers, and silencers. Promoters are sequences located at the beginning of a gene that signal RNA polymerase where to start transcription. Because gene regulatory sequences do not exist in the mRNA, and thus are not present in cDNA, they help to distinguish genomic DNA from cDNA.

RNA Splicing

RNA splicing is the process where introns are removed and exons are connected to generate the mature mRNA transcript ready for translation. This process is orchestrated by a molecular machine known as the spliceosome.

RNA splicing is a crucial step because it can also produce multiple mRNA variants from a single gene through a process called alternative splicing, which contributes to the diversity of proteins that a single gene can produce. Since cDNA is reverse-transcribed from the spliced mRNA, it encapsulates only the exons rearranged as they are in the mature mRNA sequence, lacking the introns characteristic for the genomic DNA.