Griffith used two strains of Streptococcus pneumoniae bacteria which infect mice – a type III-S (smooth) and type II-R (rough) strain. Griffith's experiment was conducted in 1928 by Frederick Griffith, one of the first experiments suggesting that bacteria are capable of transferring genetic information through a process known as transformation.
DNA MOLECULE SUGAR PHOSPHATE BACKBONE AND NUCLEOTIDES FREE
In the aqueous environment of the cell, the conjugated π bonds of nucleotide bases align perpendicular to the axis of the DNA molecule, minimizing their interaction with the solvation shell and therefore, the Gibbs free energy. The DNA double helix is stabilized primarily by two forces: hydrogen bonds between nucleotides and base-stacking interactions among the aromatic bases. One major difference between DNA and RNA is the sugar, with the 2-deoxyribose in DNA being replaced by the alternative pentose sugar ribose in RNA. The asymmetric ends of DNA strands are called the 5′ (five prime) and 3′ (three prime) ends, with the 5' end having a terminal phosphate group and the 3' end a terminal hydroxyl group. In a double helix the direction of the nucleotides in one strand is opposite to their direction in the other strand: the strands are antiparallel. These asymmetric bonds mean a strand of DNA has a direction. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings. The sugar in DNA is 2-deoxyribose, which is a pentose (five-carbon) sugar. The backbone of the DNA strand is made from alternating phosphate and sugar residues. If multiple nucleotides are linked together, as in DNA, this polymer is called a polynucleotide. A base linked to a sugar is called a nucleoside and a base linked to a sugar and one or more phosphate groups is called a nucleotide. The nucleotide repeats contain both the segment of the backbone of the molecule, which holds the chain together, and a base, which interacts with the other DNA strand in the helix.
These two long strands entwine like vines, in the shape of a double helix. In living organisms, DNA does not usually exist as a single molecule, but instead as a pair of molecules that are held tightly together. For instance, the largest human chromosome, chromosome number 1, is approximately 220 million base pairs long. Although each individual repeating unit is very small, DNA polymers can be very large molecules containing millions of nucleotides. According to another study, when measured in a particular solution, the DNA chain measured 22 to 26 Ångströms wide (2.2 to 2.6 nanometres), and one nucleotide unit measured 3.3 Å (0.33 nm) long.
Watson and Francis Crick, the structure of DNA of all species comprises two helical chains each coiled round the same axis, and each with a pitch of 34 Ångströms (3.4 nanometres) and a radius of 10 Ångströms (1.0 nanometres). DNA is a long polymer made from repeating units called nucleotides.
0 kommentar(er)
