UNRAVELLING THE STRUCTURE OF DNA QUESTIONS DNA MODEL
An important milestone in the history of genetics was the
purification of DNA by Oswald Avery and his co-workers in 1945.
2. Name the structure / substance that carries genetic material.
The realisation by the scientific community that the principle carrier
of genetic information and the blueprint for life was DNA led to a
3. What do you understand by “blueprint”
number of questions being asked. One of the most crucial of these
4. Describe the technique that was used to gather
being, that which was concerned with its molecular structure.
In 1951, a few years later, two scientists, namely, James Watson, an
Constructing DNA and RNA molecules
American biologist and Francis Crick an English physicist,
collaborated on solving the DNA puzzle about its molecular
Study all the information about nucleic acids gained by you thus
structure. Although they did little of the original research, they
far, construct a model of DNA using cardboard or any other
tackled the problem with brilliance, perception, and a great deal of
energy. The existing data from experiments on DNA at that time fascinated them. Erwin Chargaff showed that any model of DNA structure would have to contain deoxyribose sugar, phosphate,
purines and pyrimidines arranged in a way that would provide variation and a simple way of copying itself. Watson and Crick
spent long hours constructing models with cardboard cut-outs, and
kept alert for any and every bit of information that might give them
an edge. Two English biophysicists, namely, Maurice Wilkins and
Rosalind Franklin, had been painstakingly collecting data on X-ray crystallographs of DNA for several hours. With this technique, molecules of DNA bombarded by X-rays produce a photographic
image that can predict the three dimensional structure of the
molecule. After being allowed to view certain of Wilkin's and Franklin's data privately, Watson and Crick noticed an unmistakable pattern: The molecule appeared to be a double helix. Gradually the pieces of the puzzle fell into place, and a final model
was assembled a model that explained all of the qualities of DNA,
including how it is copied. Although Watson and Crick were credited for their clarity of their solution, it must be emphasized that their successes was due to the tremendous efforts of a number of
other scientists. This historical discovery has shown that the tools of physics and chemistry have very useful applications in the biological sciences. In addition, this discovery set the scene for numerous research in all areas of molecular genetics. PROCESS OF DNA REPLICATION
Since the discovery of the double helix, an extensive body of
DNA replication occurs before cell division.
biochemical, microscopic, and crystallographic analysis has left
The series of simplified diagrams below will explain
little doubt that the model first proposed by Watson and Crick is
correct. A new technique using scanning tunnelling microscopy produces three-dimensional images of DNA magnified two million times. Two new strands template Hydrogen bond
D N A m o l e c u l e template Original Original Strand 1____ Strand ____
Several free nucleotides with two extra phosphates
are found in the nucleus. The extra phosphates
activate the nucleotides The bases of the activated
nucleotides pair up with their complementary base on each of the original DNA strand.
wo new molecules of DNA
From the series of steps above it can be The sugar and the phosphates of the new
Seen that the process of DNA replication begins
nucleotides are then linked by an enzyme controlled
with one double stranded molecule and ends with two
reaction. This enzyme is DNA polymerase. DNA
double stranded molecules of DNA. Each of these
polymerase will only link an incoming nucleotide to
strand contains one strand from the original molecule
the growing new chain if it is complementary to the
plus a new complementary strand assembled from free
base on the old strand. Hence the chance of a
nucleotides in the nucleus. Each new double stranded
molecule is identical to the original strand.
Study the micrographs of human red blood corpuscles to answer the questions that follow.
1. Normal haemoglobin is called haemoglobin A, while
that which causes sickle cells is called haemoglobin S. Phenylalanine (AAA) Serine (AGA) Tyrosine (AUA) Cysteine (ACA)
1.1 As pointed out earlier, haemoglobin is a protein. How
Phenylalanine Tyrosine Cysteine
is it possible for two forms of the same protein to be
Tryptophan
1.2 Study a part of (first eight) two amino acid sequences
Histidine Arginine
of beta haemoglobin. One is that of a normal type of
Histidine Arginine
haemoglobin while the other is for the mutated form.
Glutamine Arginine val-his-leu-thr-pro-glu-glu-lys ------ normal Glutamine Arginine Isoleucine Threoline Asparagine val-his-leu-thr-pro-val-glu-lys ------ mutated
Threoline Asparagine Isoleucine
1.2.1 What do you understand by a mutation?
Isoleucine Threoline Arginine
1.2.2 List the difference/s between the two forms of
Methionine (start) Threoline Arginine Aspartate
1.2.3 Suggest why this mutation might have occurred.
Aspartate
1.2.4 Explain why genetic codes have to be precise. Glutamate Glutamate
Psicothema 2008. Vol. 20, nº 4, pp. 812-817 Chronic sildenafil (Viagra) administration reduces anxiety in intact and castrated male rats Abdel A. Solís, José A. Bethancourt* and Gabrielle B. Britton*Universidad de Salamanca and * Instituto de Investigaciones Científicas y Servicios de Alta TecnologíaEpidemiological research indicates that sildenafil (Viagra) abuse is associated with i
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UNRAVELLING THE STRUCTURE OF DNA