Human: DNA AND MOLECULAR GENETICS
Table of Contents
The physical carrier of inheritance | The structure
of DNA | DNA Replication
The physical carrier of inheritance
of genetics, scientists still had not determined that DNA, and not protein,
was the hereditary material. However, during this time a great many
genetic discoveries were made and the link between genetics and evolution
was made.
Meischer in 1869 isolated DNA from fish sperm and the pus of open wounds.
Since it came from nuclei, Meischer named this new chemical, nuclein.
Subsequently the name was changed to
DNA was then found in the
the 1920s, biochemist P.A. Levene analyzed the components of the DNA
molecule. He found it contained four nitrogenous bases:
and a phosphate group. He concluded that the basic unit (
also attached to the sugar. He (unfortunately) also erroneously concluded
that the proportions of bases were equal and that there was a tetranucleotide
that was the repeating structure of the molecule. The nucleotide, however,
remains as the fundemantal unit (monomer) of the nucleic acid polymer.
There are four nucleotides: those with cytosine (C), those with guanine
(G), those with adenine (A), and those with thymine (T).
are three phosphates instead of the single phosphate found in the normal nucleotide. Images from Purves et al.,
(http://www.sinauer.com/) and WH Freeman (
used with permission.
the early 1900s, the study of genetics began in earnest: the link between
Mendel's work and that of cell biologists resulted in the chromosomal
theory of inheritance; Garrod proposed the link between genes and "inborn
errors of metabolism"; and the question was formed: what is a gene?
The answer came from the study of a deadly infectious disease: pneumonia.
During the 1920s Frederick Griffith studied the difference between a
disease-causing strain of the pneumonia causing bacteria (Streptococcus peumoniae) and a strain that
did not cause pneumonia. The pneumonia-causing strain (the S strain)
was surrounded by a capsule. The other strain (the R strain) did not
have a capsule and also did not cause pneumonia. Frederick Griffith
(1928) was able to induce a nonpathogenic strain of the bacterium
to a transforming facto
that caused the non-pathogenic bacteria to become pathogenic. Griffith
injected the different strains of bacteria into mice. The S strain killed
the mice; the R strain did not. He further noted that if heat killed
S strain was injected into a mouse, it did not cause pneumonia. When
he combined heat-killed S with Live R and injected the mixture into
a mouse (remember neither alone will kill the mouse) that the mouse
developed pneumonia and died. Bacteria recovered from the mouse had
a capsule and killed other mice when injected into them!
Hypotheses:
1. The dead S strain had been reanimated/resurrected.
2. The Live R had been transformed into Live S by some "transforming
factor".
Further experiments led Griffith to conclude that number 2 was correct.
Griffith's experiment and concluded the transforming factor was DNA.
Their evidence was strong but not totally conclusive. The then-current
favorite for the hereditary material was protein; DNA was not considered by many scientists to
be a strong candidate.
breakthrough in the quest to determine the hereditary material came
from the work of Max Delbruck and Salvador Luria in the 1940s.
and Luria worked with were those attacking Escherichia coli, a bacterium found in
human intestines. Bacteriophages consist of protein coats covering DNA.
Bacteriophages infect a cell by injecting DNA into the host cell. This
viral DNA then "disappears" while taking over the bacterial
machinery and beginning to make new virus instead of new bacteria. After
25 minutes the host cell bursts, releasing hundreds of new bacteriophage.
Phages have DNA and protein, making them ideal to resolve the nature
of the hereditary material.
Structure of a bacteriophage virus.
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
the link to view details of their experiment) conducted a series of
experiments to determine whether protein or DNA was the hereditary material.
By labeling the DNA and protein with different (and mutually exclusive)
radioisotopes, they would be able to determine which chemical (DNA or
protein) was getting into the bacteria. Such material must be the hereditary
material (Griffith's transforming agent). Since DNA contains Phosphorous
(P) but no Sulfur (S), they tagged the DNA with radioactive Phosphorous-32.
Conversely, protein lacks P but does have S, thus it could be tagged
with radioactive Sulfur-35. Hershey and Chase found that the radioactive
S remained outside the cell while the radioactive P was found inside
the cell, indicating that DNA was the physical carrier of heredity.
Diagrams illlustrating the Hershey and Chase
experiment that supported DNA as the hereditary material while it also
showed protein was NOT the hereditary material. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
The Structure of DNA
of life, concluding that the amount of
by the Hershey-Chase experiments, but how DNA served as genes was not
yet certain. DNA must carry information from parent cell to daughter
cell. It must contain information for replicating itself. It must be
chemically stable, relatively unchanging. However, it must be capable
of mutational change. Without mutations there would be no process of
evolution.
scientists were interested in deciphering the structure of DNA, among
them were Francis Crick, James Watson, Rosalind Franklin, and Maurice
Wilkens. Watson and Crick gathered all available data in an attempt
to develop a model of DNA structure. Franklin took
The data known at the time was that DNA was a long molecule, proteins
were helically coiled (as determined by the work of Linus Pauling),
Chargaff's base data, and the x-ray diffraction data of Franklin and
Wilkens.
Ball and stick model of DNA. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
X-ray diffraction photograph of the DNA double
helix. Image from the Internet.
James Watson (L) and Francis Crick (R), and
the model they built of the structure of DNA. Image from the
Internet.
the sides of the helix (like the sides of a twisted ladder). The strands
are complementary (deduced by Watson and Crick from Chargaff's data, A pairs with T
and C pairs with G, the pairs held together by
ring pyrimidine. Purines are Adenine (A) and Guanine (G). We have encountered
Thymine (T). The bases are complementary, with A on one side of the
molecule you only get T on the other side, similarly with G and C. If
we know the base sequence of one strand we know its complement.
Rendering of two complementary bases on a DNA
molecule. Image prepared using MacMolecule.
The ribbon model of DNA. Image from
Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
DNA Replication
its structure. What remained was to determine how DNA copied its information
and how that was expressed in the phenotype. Matthew Meselson and Franklin W. Stahl
designed an experiment to determine the method of DNA replication. Three
models of replication were considered likely.
1. Conservative replication would somehow produce an entirely new DNA strand during replication.
Conservative model of DNA replication.
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
of the parental DNA along with an entirely new complementary strand.
In other words the new DNA would consist of one new and one old strand
of DNA. The existing strands would serve as complementary templates
for the new strand.
The semiconservative model of DNA structure.
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
3. Dispersive replication involved the breaking
of the parental strands during replication, and somehow, a reassembly
of molecules that were a mix of old and new fragments on each strand
of DNA.
The dispersive replication model of DNA replication.
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
heavy nitrogen (Nitrogen-15 as opposed to the more common, but lighter
molecular weight isotope, Nitrogen-14). The first generation of bacteria
was grown on a medium where the sole source of N was Nitrogen-15. The
bacteria were then transferred to a medium with light (Nitrogen-14)
medium. Watson and Crick had predicted that DNA replication was semi-conservative.
If it was, then the DNA produced by bacteria grown on light medium would
be intermediate between heavy and light. It was.
replication involves a great many building blocks, enzymes and a great
deal of ATP energy (remember that after the
occurring in a cell once per (cell) generation, DNA replication in humans
occurs at a rate of 50 nucleotides per second, 500/second in prokaryotes.
Nucleotides have to be assembled and available in the nucleus, along
with energy to make bonds between nucleotides.
have opened the molecule, an area known as the replication bubble forms
(always initiated at a certain set of nucleotides, the origin of replication).
New nucleotides are placed in the fork and link to the corresponding
parental nucleotide already there (A with T, C with G). Prokaryotes
open a single replication bubble, while eukaryotes have multiple bubbles.
The entire length of the DNA molecule is replicated as the bubbles meet.
The roles of DNA polymerases in DNA replication.
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
Since the DNA strands are antiparallel, and replication proceeds in
thje 5' to 3' direction on EACH strand, one strand will form a continuous
copy, while the other will form a series of short Okazaki fragments.
Growth of replication forlks as DNA is replicated
base by base. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
Links
- A Structure for Deoxyribose Nucleic Acid Text of the original
paper that Watson and Crick published in 1953. - The DNA Learning Center (Cold Spring Harbor Laboratory)
This site has several excellent animations (Shockwave enhanced) as well
as information about their favorite molecule, DNA. - Access Excellence (Genentech) A resource with graphics
and other materials to augment molecular genetics. - Primer on Molecular Genetics (Department of Energy)
- Tutorial on EUKARYOTIC DNA TRANSCRIPTION (UC Davis)
- Glossary (DOE) Terms peculiar to molecular genetics.
- Gene Zine A very good beginning on how genes work.
- The Genome Database (Johns Hopkins University) Search
databases for a specific gene. - Genome Machine (University of Washington) A clickable
map connecting to the GDB (above).
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