Quiz 17: Non-Coding Rnas


Similarities : • Both involve crossing over and alignment of chromatids. • Both involve breakage of analogues segments of chromatids from the points of crossing over. • Both occur in eukaryotic chromosomes. Differences : • When crossing over takes place between sister-chromatids, the recombinant formed consists of the same combination of alleles as in the parent chromosomes because both are genetically identical to each other. But in case of recombination between homologous chromatids, the recombinants formed consists of new combination of alleles as compared to that of the parent chromosomes because the parent chromatids are genetically similar but not genetically identical to each other. • The proteins involved in both the processes are different. • Heteroduplex will be formed in case of recombination between homologs but it is not so in the case of sister chromatid exchangE.The proteins involved in both the processes would not be same because if it would be so then the recombinants would have been formed in both the processes. Since in case of sister chromatid exchange, they do not form any recombinant due to the same genes of sister chromatids, but in case of recombination involving homologs the recombinant is formed. Thus, we conclude that the proteins involved in both the processes would not be the samE.

In harlequin staining technique, one pair of sister chromatid of the eukaryotic chromosome is allowed to replicate with a nucleotide analog BrdU (5-bromodeoxyuridine) which binds to different types of stains with a different degree of attachment as compared to that of normal chromosomes. The sister chromatids after one round of replication forms two pairs of sister chromatids with one strand of normal chromosome and one strand of BrdU treated chromosome in both the pairs. These chromosomes are then again treated with BrdU in second round. Then it leads to the formation of four pairs of sister chromatids or we can say two chromosomes. In each chromosome, there were two sister chromatids out of which one pair of sister chromatids consist of both the strands that were BrdU treated while the other pair consist of one strand of normal chromosome while the other strand of BrdU treated chromosomE.When these two times BrdU treated chromosomes were stained with two dyes i.e. Hoechst 33258 and Giemsa. Thus, because of the presence of one complete pair of BrdU treated sister chromatids in each chromosome, one pair of sister chromatid in each chromosome will stain very brightly as compared to the other pair of sister chromatid (other pair of sister chromatid consists of only strand that is Brdu treated, thus fluoresce less brightly).

Similarities between homologous recombination, site specific recombination and transposition are as follows: • In all these mechanisms changes occur in the genome due to either crossing over or introduction of different segment in the DNA (deoxyribonucleic acid) strands. • All the mechanisms may cause mutations in the genomE.• All the mechanisms require certain elements to carry out its process. It may be proteins as in the case homologous recombination and site specific recombination while it may be enzymes in case of transposition. Differences between homologous recombination, site-specific recombination and transposition are as follows: • In homologous recombination, the two chromosomes are similar to each other; in site specific recombination it is not essential for the chromosomes to be similar. Thus, it can be least similar while in the case of transposition the segment of DNA may get transferred to the same chromosome at different site or in the different chromosome that may or may not be similar (homologous) to each other. • Homologous recombination occurs mainly during meiosis to produce reproductive cells. Site specific recombination is employed in various cellular processes thus it acts as a basis for the development of engineering tools. While transposition is not beneficial in humans because it is the main cause of certain cancers and other diseases.