Genomic DNA from a mouse is isolated, fragmented, and denatured into single strands. It is then mixed with mRNA isolated from the cytoplasm of mouse cells. The image represents an electron micrograph result showing the hybridization of single-stranded DNA and mRNA.
Which nucleic acid is indicated by the 'a' pointer? Justify your answer. 

Verified step by step guidance

1. Understand the context of the problem: The question involves hybridization of single-stranded DNA (ssDNA) and mRNA. Hybridization occurs when complementary nucleic acid strands bind to each other, forming a DNA-RNA hybrid.
2. Analyze the experimental setup: Genomic DNA is fragmented and denatured into single strands, while mRNA is isolated from the cytoplasm. The hybridization process allows mRNA to bind to complementary sequences in the ssDNA.
3. Interpret the electron micrograph: The 'a' pointer likely indicates a specific nucleic acid structure. In such experiments, regions of ssDNA that do not hybridize with mRNA remain as loops, while hybridized regions form double-stranded DNA-RNA hybrids.
4. Justify the identity of the nucleic acid: If the 'a' pointer is marking a loop, it represents single-stranded DNA (ssDNA) that does not have a complementary sequence in the mRNA. This occurs because introns (non-coding regions) in the genomic DNA are absent in the mRNA, which only contains exons (coding regions).
5. Conclude the reasoning: The 'a' pointer indicates single-stranded DNA (ssDNA) because it corresponds to intronic regions that are not transcribed into mRNA and therefore cannot hybridize with the mRNA.

Key Concepts

Hybridization

Hybridization refers to the process where complementary nucleic acid strands bind together to form a double-stranded structure. In this context, single-stranded DNA from the mouse genome can hybridize with mRNA, which is complementary to the DNA sequence that was transcribed. This process is crucial for understanding how genetic information is expressed and can be visualized in experiments like the one described.

mRNA (Messenger RNA)

mRNA, or messenger RNA, is a type of RNA that carries genetic information from DNA to the ribosome, where proteins are synthesized. It is synthesized during transcription and serves as a template for translation. In the context of the question, the mRNA isolated from mouse cells is essential for understanding which specific sequences of DNA are being expressed in the cells.

Electron Micrography

Electron micrography is a technique that uses a beam of electrons to create high-resolution images of biological specimens. This method allows researchers to visualize the hybridization of nucleic acids at a molecular level. In the question, the electron micrograph provides visual evidence of the interaction between the single-stranded DNA and mRNA, helping to identify the nucleic acid indicated by the 'a' pointer.