Understanding the Size of cDNA from Mature Stimulin mRNA

The size of complementary DNA (cDNA) made from mature stimulin mRNA can be calculated based on the structure of the gene, including the exons, introns, and untranslated regions (UTRs). Understanding the relationship between gene structure and cDNA size is crucial for molecular biology research, especially in gene cloning and expression studies.

Structure of the Stimulin Gene

The hypothetical stimulin gene contains the following components:

• Two exons encoding a protein of 100 amino acids.
• Intron of 100 base pairs (bp) starting after the codon for the 10th amino acid.
• 5′ Untranslated Region (UTR) of 70 nucleotides.
• 3′ Untranslated Region (UTR) of 30 nucleotides.

How to Calculate the Size of Stimulin cDNA

1. Exon Contribution

• A protein of 100 amino acids is encoded by 300 nucleotides (since each amino acid is encoded by a codon of 3 nucleotides).
• The two exons contribute a total of 300 bp to the cDNA.

2. Intron Exclusion

• Introns are removed during RNA splicing.
• Therefore, the 100 bp intron will not be present in the cDNA.

3. Contribution of Untranslated Regions (UTRs)

• The 5′ UTR contributes 70 nucleotides.
• The 3′ UTR contributes 30 nucleotides.
• Total UTR contribution = 70 + 30 = 100 bp.

4. Total cDNA Size Calculation

$$\text{300 bp (exons)}+\text{70 bp (5’ UTR)}+\text{30 bp (3’ UTR)}=400\text{bp}$$

Therefore, the correct answer is 400 bp.

Why Introns Are Absent in cDNA

• During transcription, the primary RNA transcript includes both exons and introns.
• Intron sequences are removed during RNA splicing, leaving only exons in the mature mRNA.
• Reverse transcription of mRNA into cDNA results in a product that lacks introns.

Significance of cDNA in Molecular Biology

1. Gene Expression Studies

• cDNA is used to study gene expression patterns under different conditions.
• It provides a direct representation of actively transcribed genes.

2. Cloning and Protein Production

• cDNA can be inserted into vectors for recombinant protein production.
• It allows the synthesis of eukaryotic proteins in bacterial systems without splicing machinery.

3. Functional Genomics

• cDNA libraries are constructed to analyze gene functions.
• Researchers can identify and characterize novel genes from cDNA sequences.

Challenges in cDNA Synthesis

1  Low mRNA concentration may limit the yield of cDNA.
2  Reverse transcriptase errors can introduce mutations.
3  Contamination from genomic DNA can interfere with accurate cDNA synthesis.

How to Improve cDNA Quality

1  Use high-quality RNA extraction methods.
2  Employ RNase inhibitors to prevent RNA degradation.
3  Optimize reverse transcription conditions to enhance efficiency.
4  Confirm cDNA integrity using gel electrophoresis or quantitative PCR.

Conclusion

The size of cDNA from mature stimulin mRNA is 400 bp. This size is derived from the total exon length (300 bp) and the combined length of the 5′ and 3′ UTRs (100 bp). Understanding gene structure and mRNA processing is essential for molecular biology research. For expert guidance in molecular biology techniques and competitive exam preparation, join Let’s Talk Academy — the leading institute for CSIR NET Life Science, IIT JAM, GATE Biotechnology, and DBT JRF.

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FAQs

Q1. Why are introns absent in cDNA?
Introns are removed during RNA splicing, so they are not present in mature mRNA or cDNA.

Q2. How can cDNA be used in gene expression studies?
cDNA represents the expressed genes, making it useful for analyzing gene activity in different tissues or conditions.

Q3. What is the typical size of cDNA from a gene coding 200 amino acids?
For 200 amino acids, the coding sequence would be 600 bp, plus the length of UTRs if included.

This article was written with guidance from Let’s Talk Academy, a top coaching institute for life sciences and biotechnology competitive exams.