Molecular Biology and Genetics

A triple-stranded DNA is a structure of DNA in which three oligonucleotides wind around each other and form a triple helix. One strand binds to a B-form DNA double helix through Hoogsteen or reversed Hoogsteen hydrogen bonds. The triple helix is a partial binding to one of the strands, where the other strand maintains the Watson-Crick pairing. The formation of triple helix is favoured when there are stretches of oligopurines/pyrimidines in a DNA.

The antiparallel nature of DNA molecule is primarily due to structure of deoxyribose sugar (it lacks an oxygen at 2') that causes the difference in chemical polarity and the directionalism of phosphodiester bonds makes the polarity zero.

The deoxyribose sugar has an 'H' instead of 'OH' at 2' position. The nitrogenous bases binds to the 1' position of the sugars by glycosidic bonds. Silencing of the 1' position will hinder the glycosidic bond formation and thereby disrupting the complete structure.

In accordance with Chargaff's rule, (A+T)/(G+C) = 1.

Initially we have 2 strands with radioactive thymidine. Going by the concept of replication and the statement provided that the replications were carried out in non-radioactive medium, there would be 2 molecules of DNA having radioactive thymidine.

In double-stranded DNA, adenine pairs with thymine (A=T) and guanine pairs with cytosine (G=C). If thymine is 14%, adenine is also 14% (total A+T=28%). Therefore G+C must be 72%, so cytosine alone is 36%.

The deoxyribose sugar has an 'H' instead of 'OH' at 2' position. The nitrogenous bases bind to the 1' position of the sugars by glycosidic bonds. Silencing of the 1' position will hinder the glycosidic bond formation and thereby disrupting the complete structure.

One turn of the helix in a B-form DNA is approximately 3.4 nm. 

In eukaryotes, there is a set of positively charged, basic proteins called histones. Histones are rich in the basic amino acid residues lysines and arginines. In a typical nucleus, some region of chromatin are loosely packed (and stains light) and are referred to as euchromatin. The chromatin that is more densely packed and stains dark are called as Heterochromatin.

The two chains have anti-parallel polarity. It means, if one chain has the polarity 5'→3', the other has 3'→5'. The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm and there are roughly 10 bp in each turn. Consequently, the distance between a bp in a helix is approximately equal to 0.34 nm.

The International Nucleotide Sequence Database Collaboration (INSDC) is a joint effort to collect and disseminate databases containing DNA and RNA sequences. It consists of the following databases: DNA Data Bank of Japan (Japan), GenBank (USA) and the European Nucleotide Archive (UK).

Correct answer is (3).

Genome composition is used to describe the make up of contents of a haploid genome, which should include genome size, proportions of non-repetitive DNA and repetitive DNA in details. By comparing the genome compositions between genomes, scientists can better understand the evolutionary history of a given genome. In prokaryotes, most of the genome (85-90%) is non-repetitive DNA, which means coding DNA mainly forms it, while non-coding regions only take a small part. On the contrary, eukaryotes have the feature of exon-intron organization of protein coding genes; the variation of repetitive DNA content in eukaryotes is also extremely high. When referring to mammals and plants, the major part of genome is composed by repetitive DNA.

A locked nucleic acid (LNA), often referred to as inaccessible RNA, is a modified RNA nucleotide. The ribose moiety of an LNA nucleotide is modified with an extra bridge connecting the 2' oxygen and 4' carbon.

Flow cytometry allows separation of nucleic acids on the basis of size and density.