Explain the structure of DNA.
DNA is made of sugars, phosphates, and nitrogen bases and is a large polymer made of nucleotide monomers. This ladder-like structure has two sugar phosphate backbones connected by hydrogen bonds formed by the nucleotide bases, which are facing inwards toward each other. The backbone in the DNA model is represented by a blue pipe cleaner, with pink beads to represent the phosphate. The white pipe cleaners facing inwards represent the hydrogen bonds that are formed. The bases involved in the bonds are the purines, Adenine and Guanine, and the pyrimidines, Thymine and Cytosine. Complimentary base pairing insures that these bases can only bond with their respective pair. Only purines with purines, and pyrimidines with pyrimidines. The DNA backbones are antiparallel as they alternate “sugar-phosphate” in opposite directions; one strand having “sugar-phosphate-sugar-phosphate” while the other strand has “phosphate-sugar-phosphate-sugar”.
How does this activity help model the structure of DNA? What changes could we make to improve the accuracy of this model? Be detailed and constructive.
This activity really helped me get a visual image of the structure of DNA, the function of each part and why it is all important. This activity was great at demonstrating the bonds between the bases, as we used different coloured beads to represent the different bases. However, I believe there could be a better way to show the antiparallel strands as looking at the photo without knowing, it would be hard to notice the opposite directions.
When does DNA replication occur?
DNA replication occurs when a cell divides to repair, replace, or grow another cell.
Name and describe the 3 steps involved in DNA replication. Why does the process occur differently on the “leading” and “lagging” strands?
The initial step in DNA replication is called Unwinding. In this step, the DNA, unfolds from its helix shape, back to a flat ladder shape. Then, a DNA helicase enzyme moves along the base pairs splitting the hydrogen bonds. The helicase enzyme is demonstrated in the photos as a watermelon candy.
The secondary step is called Complimentary base pairing. In this step the unbounded nucleotides begin to form hydrogen bonds with their pairs on the single DNA strand. DNA polymerase is what facilitates this step, and is represented by a blue big foot in the photos.
The final step is called Joining. In this step the nucleotides on the new strand for, bonds to create a new double stranded DNA. DNA ligase acts as a glue to attach the nucleotides of the forming backbones to each other.
As the two strands are separating from one another, they are read differently due to their antiparallel structure. One strand is referred to as the “leading strand” as it is read continuously from bottom to top. Where as, the other strand is referred to as the “lagging strand” as it is read from top down and must be read in fragments, instead of a continuous read. In the picture the leading strand is on the left, and the lagging strand is on the right.
The model today wasn’t a great fit for the process we were exploring. What did you do to model the complimentary base pairing and joining of adjacent nucleotides steps of DNA replication? In what ways was this activity well suited to showing this process? In what ways was it inaccurate?
To demonstrate the complimentary base pairing we used the blue Bigfoot candies that represent DNA polymerase and pointed the big toe in the direction the strand pairs. To demonstrate the joining of nucleotides, we used a red candy which represents DNA ligase. This activity was beneficial in that the pipe cleaners could demonstrate the majority of the steps as they are flexible and can be bonded and unbonded. However, demonstrating the differences between the leading and lagging strands was difficult to show. As the lagging strand is read from top to bottom, it forced us to have to cover some of the strand under a piece of paper.
