Analyzing Statements about Molecular Biology Techniques

The question asks to identify the true statement among the given options which describe various molecular biology techniques. Let's examine each statement carefully.

Nick Translation and DNA Labeling

The first statement says that Nick translation cannot be used for radio-labeling DNA fragments for use in Southern hybridization. Nick translation is a technique widely used to label DNA probes. It involves creating nicks (single-strand breaks) in the DNA backbone, followed by the activity of DNA polymerase I which replaces nucleotides downstream of the nick. If labeled nucleotides (like radioactive ones) are included in the reaction, the newly synthesized DNA strand becomes labeled. These labeled DNA fragments serve as probes in hybridization experiments like Southern blotting, where they bind to complementary DNA sequences on a membrane, allowing detection. Therefore, Nick translation can be used for radio-labeling DNA for Southern hybridization. This statement is FALSE.

RACE Technique for cDNA Ends

The second statement suggests that RACE can be used to obtain sequence information of the ends of a cDNA. RACE stands for Rapid Amplification of cDNA Ends. This technique is specifically designed to obtain the full-length sequence of a cDNA clone, particularly the 5' and 3' untranslated regions (UTRs) and the regions near the translation start and stop codons, which are often missing or incomplete in standard cDNA libraries. 5' RACE is used to determine the sequence of the 5' end of a cDNA, and 3' RACE is used for the 3' end. By amplifying these ends, researchers can obtain the complete sequence of the mRNA molecule that the cDNA was reverse-transcribed from. This statement is TRUE.

2-D Gel Electrophoresis for Proteins

The third statement describes the order of separation in 2-D gel electrophoresis for proteins. 2-D gel electrophoresis separates proteins based on two properties. The first dimension is typically Isoelectric Focusing (IEF), which separates proteins based on their isoelectric point (pI), the pH at which a protein has no net electrical charge. After IEF, the gel strip is placed on an SDS-PAGE gel, where proteins are separated in the second dimension based on their size (molecular weight). So, proteins are separated first by pI and then by size, not the other way around. This statement is FALSE.

AFLP Markers and Genotyping

The fourth statement claims that AFLP markers are typically used to screen heterozygotes from homozygotes. AFLP (Amplified Fragment Length Polymorphism) is a molecular marker technique that detects polymorphisms based on the presence or absence of restriction enzyme digestion sites and subsequent selective amplification of restriction fragments. While AFLP markers are useful for DNA fingerprinting, genetic mapping, and diversity studies, they are generally considered dominant markers because they typically detect the presence of a specific amplified fragment. In the simplest analysis, it is difficult to distinguish between a heterozygote (carrying one copy of the allele producing the band) and a homozygote (carrying two copies) if the marker is dominant (presence of the band is scored). Codominant markers like SSRs (Simple Sequence Repeats) or SNPs (Single Nucleotide Polymorphisms) are more commonly and directly used for routine screening of heterozygotes and homozygotes as they can distinguish between different allelic states. Therefore, AFLP markers are not typically the primary choice specifically for screening heterozygotes from homozygotes compared to other marker types. This statement is generally FALSE.

Based on the analysis of each statement, only the second statement, regarding the use of RACE to obtain sequence information of cDNA ends, is true.