5.2:
DNA as a Genetic Template
The Watson-Crick model, proposed in 1953, elucidated two structural features of the DNA molecule that provide a basis for heredity: its double-stranded nature and the four nucleotide bases.
This model proposed that DNA is made up of two strands of nucleotides that twist around each other to form a right-handed helix.
These strands are anti-parallel in nature – meaning the 3’ end of one strand faces the 5’ end of the other strand.
Each strand of DNA contains a sequence of nucleotides that is exactly complementary to its partner strand – enabling each strand to act as a template for its partner.
Thus, a cell can replicate its DNA before cell division by separating two strands of DNA and producing two new strands of DNA that are exactly complementary to each other.
Additionally, the Watson-Crick model posited that base-pairing takes place between a purine – either adenine or guanine – and a pyrimidine – either cytosine or thymine.
Adenine, guanine, cytosine, and thymine comprise the four-letter nucleotide ‘alphabet.’
We now know that when these nucleotide letters are strung together into a three-letter codon during translation, they can code for one of the 20 amino acids – much like letters in an alphabet can be arranged to spell out words.
These amino acid ‘words’ are linked into sentences, forming chains that fold into different proteins.
Different permutations of codons can result in different genes – much like how different combinations of words and sentences result in different books.
The genome is the entirety of the information contained within an organism’s genes and encompasses all of the RNA molecules and proteins that an organism will produce over its lifetime.
Differences between genomes result in genotypically and phenotypically distinct organisms and species.
5.2:
DNA as a Genetic Template
Two structural features of the DNA molecule provide a basis for the mechanisms of heredity: the four nucleotide bases and its double-stranded nature. The Watson-Crick model of double-helical DNA structure, proposed in 1952, drew heavily upon the X-ray crystallography work of researchers Rosalind Franklin and Maurice Wilkins. Watson, Crick, and Wilkins jointly received the Nobel Prize in Physiology or Medicine for their work in 1962. Franklin was, controversially, excluded from the prize for reasons that are still debated.
The Watson-Crick model of DNA, very simply, proposed that DNA is made up of two strands of nucleotides that twist around each other to form a right-handed helix and that nucleotide pairing takes place between a purine and a pyrimidine.
The two DNA strands are antiparallel, meaning that the 3’ end of one strand faces the 5’ end of the other. This allows each strand to act as a template for its partner during DNA replication, producing two new strands of DNA that are exactly complementary to each other. However, whether or not DNA replication occurred in this fashion was not clear.
The Meselson and Stahl Experiment
Meselson and Stahl grew E. coli for several generations in a medium containing a “heavy” isotope of Nitrogen, 15N. Over time, the heavy Nitrogen was incorporated into the nitrogenous nucleotide bases and, thus, into the DNA. After this, the E. coli was placed into a medium containing a different isotope of Nitrogen, 14N, and grown for several more generations. After each generation, a DNA sample was isolated from some of the cells, loaded into a gradient, and centrifuged at high speeds. In a gradient, the DNA will separate according to its buoyant density (i.e. the density within the gradient where the DNA will float).
In the 15N medium, a single band of high density was observed in the lower portion of the centrifuge tube. Immediately after the bacteria were transferred to the “lighter” media, this single band shifted upwards in the column, indicating a lower density. However, subsequent generations resulted in two bands: one corresponding to the 14N density and another in an intermediate location. This result could only be explained by a semi-conservative mode of replication, thus validating the Watson-Crick model.
Suggested Reading
- Watson, James D., and Francis HC Crick. "Molecular structure of nucleic acids." Nature 171.4356 (1953): 737-738.
- Meselson M, Stahl FW. THE REPLICATION OF DNA IN ESCHERICHIA COLI. Proc Natl Acad Sci U S A. 1958;44(7):671–682. doi:10.1073/pnas.44.7.671