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Molecular cells & protein synthesis.cdr

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

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