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 ...view middle of the document...
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...