Реферат: Extracting DNA From The Bacterium Escherichia Coli
Extracting DNA From The Bacterium Escherichia Coli Essay, Research Paper
Deoxyribonucleic acid is contained in all cells. The structure of DNA makes gene transmission
possible. Since genes are segments of DNA, DNA must be able to make exact copies of itself to enable the
next generation of cells to receive the same genes. The DNA molecule looks like a twisted ladder. Each
“side” is a chain of alternating phosphate and deoxyribose sugar molecules. The “steps” are formed by
bonded pairs of purine-pyrimidine bases. DNA contains four such bases the purines adenine (A) and guanine
(G) and the pyrimidines cytosine (C) and thymine (T).
The RNA molecule, markedly similar to DNA, usually consists of a single chain. The RNA chain
contains ribose sugars instead of deoxyribose. In RNA, the pyrimidine uracil (U) replaces the thymine of
DNA and RNA are made up of basic units called nucleotides. In DNA, each of these is composed of a
phosphate, a deoxyribose sugar, and either A, T, G, or C. RNA nucleotides consist of a phosphate, a
ribose sugar, and either A, U, G, or C.
Nucleotide chains in DNA wind around one another to form a complete twist, or gyre, every ten
nucleotides along the molecule. The two chains are held fast by hydrogen bonds linking A to T and C to G
A always pairs with T (or with U in RNA); C always pairs with G. Sequences of the paired bases are the
foundation of the genetic code. Thus, a portion of a double-stranded DNA molecule might read: A-T C-G G-C
T-A G-C C-G A-T. When “unzipped,” the left strand would read: ACGTGCA; the right strand: TGCACGT.
DNA is the “master molecule” of the cell. It directs the synthesis of RNA. When RNA is being
transcribed, or copied, from an unzipped segment of DNA, RNA nucleotides temporarily pair their bases
with those of the DNA strand. In the preceding example, the left hand portion of DNA would transcribe a
strand of RNA with the base sequence: UGCACGU.
Genes and Protein Synthesis
A genetic code guides the assembly of proteins. The code ensures that each protein is built from
the proper sequence of amino acids.
Genes transmit their protein-building instructions by transcribing a special type of RNA called
messenger RNA (mRNA). This leaves the cell nucleus and moves to structures in the cytoplasm called
ribosomes, where protein synthesis takes place.
Cell biologists believe that DNA also builds a type of RNA called transfer RNA, which floats
freely through the cell cytoplasm. Each tRNA molecule links with a specific amino acid. When needed for
protein synthesis, the amino acids are borne by tRNA to a ribosome.
The Genetic Code
Experimental evidence indicates that the genetic code is a “triplet” code; that is, each series
of three nucleotides along the DNA molecule orders where a particular amino acid should be placed in a
growing protein molecule. Three-nucleotide units on an mRNA strand for example UUU, UUG, and GUU are
called codons. The codons, transcribed from DNA, are strung out in a sequence to form mRNA.
According to the triplet theory, tRNA contains anticodons, nucleotide triplets that pair their
bases with mRNA codons. Thus, AAA is the anticodon for UUU. When a codon specifies a particular amino
acid during protein synthesis, the tRNA molecule with the anticodon delivers the needed amino acid to the
bonding site on the ribosome.
The genetic code consists of 64 codons. However, since these codons order only some 20 amino
acids, most, if not all, of the amino acids can be ordered by more than one of them. For example, the
mRNA codons UGU and UGC both order cysteine. Because mRNA is a reverse copy of DNA the genetic code for
cysteine is ACA or ACG. Some codons may act only to signal a halt to protein synthesis. Since code
transmission from DNA to mRNA is extremely precise, any error in the code affects protein synthesis. If
the error is serious enough, it eventually affects some body trait or feature. In this study, DNA was
extracted from the bacterium Escherichia coli and then some of its physical properties were studied.
Methods & Materials
The DNA extraction was completed in one session of study. First, 5.0 ml. of E.coli suspension
medium was placed in a measuring cup and added to the tube of freeze-dried E.coli. The tube was then
capped very tightly and shook gently until the bacteria went into the suspension. Then, 1.0 ml. of sodium
dodecyl sulfate (SDS) was added to the E.coli suspension. The tube was then capped and was rotated gently
for over a period of five minutes. The suspension became more viscous as the bacteria was lysed. (Sodium
dodecyl sulfate (SDS), a detergent used in laundry products, removes the lipids from E.coli cell walls.
When the cell walls are damaged, the cells lyse, releasing the contents of into the E.cole suspension
medium.) The tube was placed for 30 minutes in a hot water bath preheated to 60-65 degrees C. The lysate
was removed from the water bath and was allowed to cool until it reached room temperature. With a pipet,
the cold ethanol was added to the spooling tube, while the spo!
oling rod was lowered into the E.coli suspension.
The spooling rod was slowly rotated in a continuous, clockwise direction. Fibers of the DNA came out of
the solution and attached to the glass rod, and the rod was turned a few more minutes until a visible
mass of DNA attached itself to it. The spooling rod was removed and immersed in 95% ethanol.
Results (see specimen)
Along with DNA, enzymes and many other proteins are present in the lysate. Enzymes harmful to DNA
are inactivated by heating the suspension to 60-65 degrees C,
a temperature that degrades proteins but not DNA. DNA must be heated to about 80 degrees C before it
denatures. DNA is also protected by sodium citrate, which was in the E.coli suspension medium. The
citrate ion is a resembling agent having a strong relationship for magnesium ions. These ions are
essential to the activity of DNAase, the enzyme that degrades DNA. DNA is soluble in water, but is
insoluble in ethanol. Thus, is the reason why the DNA fibers come out of the solution and attach to the
rod. DNA was then soaked in ethanol to stabilize it. Once the DNA was dried, it appeared white and
stringy, and there also was a considerable mass of DNA for so small of a sampling of bacteria. Thus
proving, that there is a great quantity of DNA in E.coli cells. It had great assymetry. Its length was
tremendous compared to its width. The length of the DNA molecule makes it very susceptible to splitting.
The DNA fibers can be fractured very easily, and because of its length, solutions!
of DNA are noticeable viscous.