13. Understanding DNA

  Today, to start off Unit 3, we learned about the structure and functions of DNA. DNA is the genetic material that transfers to a cell, and it is composed of nucleotides. Each nucleotide contains a nitrogen base, a 5-carbon sugar, and a phosphate group. 
  According to Chargaff's rule (discovered by Edwin Chargaff in 1947), he found out that the 4 types of bases are A (adenine), C (Cytosine), G (Guanine), and T (Thymine), and A=T, C=G. Also, Chargaff claimed that the composition of DNA is different between species, which add more evidence to prove that the DNA is the genetic material. 
  In early 1950s, Rosalin Franklin made a x-ray diffraction data on the structure of DNA. James D. Watson and Francis Crick indirectly obtained her data, and concluded that the structure of DNA is a double helix. The phosphate group and 5-carbon sugar are the backbones. The nitrogen bases became the "steps"  and they face toward the center of the helix. 
  The finding of Chargaff and the structure of DNA demonstrated the nature of base pair. Adenine always pairs with Thymine, and Cytosine always pairs with Guanine. This explains why the number of Adenine is always equal to that of Thymine, and the number of Cytosine is equal to that of Guanine. Adenine and Guanine are purines, which consist of two ring structures. Thymine and Cytosine are pyramidines which have one ring structures. By combining a purine structure with a pyramidine structure together, the distance between two sugar-phosphate backbones are always constant, thus maintaining a uniform structure. 

A summary of everything above... (just in case if you're bored)

  The activity we did today was to create a double-helical structure of DNA. We were given 4 pieces of paper with the drawing of 10 bases, scissors and tapes. We need to cut the paper in half, and cut along the edges of the nitrogen bases so that the bases can connect to each other, and become 5 base pairs with short pieces of tape attached. Later on, we taped all 4 pieces of paper together from the edges, and twisted the long piece of paper to create a helix shape. Finally we taped both ends of the paper. 

So this is what we got at the end... kindergarten style lol

  Finally we learned about the DNA replication. DNA replication occurs when the eukaryotic cell cycle is in the Synthesis phase. Both strands of the double helix can serve as the template for replication. 
  The process starts as an enzyme, DNA helicase, breaks the hydrogen bonds connecting the two parts of the parental strand and creates a replication fork. the final structure would be two branch strands (leading (5-Carbon) strand and lagging (3-carbon) strand) with exposed nitrogen bases, that allow themselves to be read by DNA polymerase. This enzyme is in charge of building complementary DNA strands. 

- Andy Liu '15