Nucleic Acids and Nucleotides Quiz

Cells make two types of nucleic acids: RNA and DNA. DNA is a polymer of the four nucleotides A, C, G, and T with the sugar deoxyribose, while RNA is a polymer of the four nucleotides A, C, G, and U with the sugar ribose.

Nucleotides can be either purines (A and G), which have a double ring base, or pyrimidines (C, T, and U), which have a single ring base.

The Watson-Crick Model of DNA has four main features: double helix with sugar-phosphate backbone outside, antiparallel strands, strict base-pairing of nucleotides (A:T; C:G), and two strands held together by hydrogen bonding between bases.

DNA is copied using semi-conservative replication - each parental strand from a double helix is used as a template to make a daughter strand along its entire length. This results in two double helices, each one having an “old” DNA strand paired with a “new” DNA strand.

DNA replication involves a number of proteins that are required to unwind the double helix and make new, complementary strands. These proteins include Helicase, Topoisomerase, Single Strand Binding Protein, Primase, DNA polymerase III, DNA pol I, and Ligase.

DNA replication starts at an origin of replication, where origin recognition proteins bind and separate the DNA strands.

A replication bubble consists of two replication forks that move away from each other in opposite directions as helicase and topoisomerase work together to unwind the parental DNA. As the replication bubble expands, the new daughter strands are replicated inside the bubble using the parental strands as templates. Primase makes a short RNA primer to provide a pre-existing 3'-end. DNA Polymerase 3 extends each primer by adding DNA nucleotides to the 3'-end, and will continue until it "runs into" another daughter strand. DNA polymerase removes the RNA of the primer and replaces it with DNA. Ligase adds a phosphodiester bond between the 3'- and 5'-ends of two fragments, making a single daughter strand. On the leading strand, the 3'-end of the daughter DNA is towards the fork, so DNA polymerase 3 follows helicase and makes one long daughter strand. On the lagging strand, the 3'-end of the daughter DNA is away from the fork, so DNA polymerase 3 goes the "wrong way." As single-stranded DNA is exposed by the expanding bubble/fork, primase will start the entire process over again (repeatedly), creating DNA fragments.