B. cDNA.
A gene probe that is produced by analyzing the amino acid sequence of all or part of a protein and chemically constructing a DNA sequence corresponding to the deduced gene is called a cDNA (complementary DNA) probe.
cDNA probes are generated through a process known as reverse transcription, which involves the synthesis of DNA from an RNA template. The RNA used as the template is typically mRNA (messenger RNA) that has been transcribed from a gene of interest. Reverse transcriptase, an enzyme, synthesizes a complementary DNA strand (cDNA) using the mRNA template. This cDNA represents the coding sequence of the gene and is complementary to the mRNA molecule. The cDNA can then be chemically constructed based on the amino acid sequence deduced from the protein sequence, allowing for the generation of a specific DNA probe.
Option A, mRNA probe, refers to a probe directly derived from mRNA molecules without undergoing the process of reverse transcription. mRNA probes can be used to detect the presence or expression of specific mRNA molecules.
Option C, synthetic probe, is a general term that can refer to any probe that is chemically synthesized, including cDNA probes. However, the term cDNA specifically denotes the process of constructing the probe based on the deduced gene sequence.
Option D, cloned probe, typically refers to a probe derived from a cloned DNA fragment or gene. Cloning involves the isolation and replication of a specific DNA sequence in a host organism, often using plasmid vectors. Cloned probes can be generated from genomic DNA or cDNA, depending on the source of the DNA fragment.
Therefore, the correct answer is B. cDNA.
