BEHAVIORAL OBJECTIVES
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1. To understand protein synthesis as is accomplished by m-RNA, T-RNA and ribosomes.
The DNA unzippers and forms new halves in the process of duplication. When acted upon by certain chemical stimuli a portion of the DNA will unzipper and the order of the bases adenine, guanine, cytosine and thymine is copied in complement by a material known as RNA (messenger) The RNA substitutes uracil for thymine, is a single strand and has a ribose sugar.
The Ability of the RNA to pick up the sequence of bases by forming the complement and then carrying it to ribosomes so that amino acids and proteins can be assembled is known as protein synthesis.
The sequence of three of the DNA bases are picked up at a time and these are called a codon and the process of receiving the code from DNA by RNA is called transcription.
The messenger RNA on the ribosome receives the necessary amino acids from the cytoplasm by a transfer-RNA which attracts the acids from the cytoplasm that are necessary for the polypeptide synthesis coded for by M-RNA.
The m-RNA has a start codon and a stop codon so that the DNA can be transcribed for only certain order of bases.
It is important to remember that each amino acid which is a peptide to form a polypeptide chain consists of the purines and pyrimidine bases in a specific order with only three for any amino acid/peptide formation. Most amino acids can be formed by several different combinations of three bases (codon). The codons are formed by translation from RNA after the codes have been transcribed from DNA.
The significance of knowing how and what proteins can be synthesized is that the proteins and enzymes that are synthesized cause cells to differentiate and specialize giving organisms their respective characteristics.
Genes or paired aloles along the chromosomes are the sequence of bases in DNA which determine the proteins which determine the traits or characteristics.
In 1961 Crick and Brenner performed genetic experiments for the discovery of ‘codons’ and the code was cracked in 1966 by Marshall Nirenberg and H. Gobind Khorana.
All possible mRNA combinations were tried, yielding a genetic dictionary for the translation of RNA into proteins. All proteins begin with the amino acid methionine, its codon (AUG) represents the start signal and UAA, UAG and UGA are stop signals.
FOR BETTER STUDENT UNDERSTANDING STRESS THESE CONCEPTS
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1. From Procaryotes to Eucaryotes
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It is thought that all organisms living now on earth derive from one single primordial cell born several billion years ago . . . . The family resemblances between all organisms seem too strong to be explained in any other way. One important landmark along this evolutionary road occurred about 1.5 billion years ago, when there was a transition from small cells with a relatively simple internal structure—
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2. There are over 200 different types of specialized cells in the organisms . . . different cell types are transcribed from different sets of genes.
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3. Coiling of the DNA helical structure facilitates transcription because the coils permit the exposure of 10 bases. It creates superhelical tension.
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4. There is a tight packaging of DNA with histones to form chromatin. Histones become packaged to form the beaded structures known as nucleosomes.
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5. A phenomenon known as cell memory figures into the cell differentiation so that certain portions of the chromosomes are opened for transcription.
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6. Substances regulate and control
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a. transcription
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b. processing
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c. transport
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d. translation
At this point in the study students should be allowed visitation to a laboratory where an electron microscope is available or/and electronmicrographs of chromosomes should be available or copied from texts.
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karyotypes of chromosomes
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beaded appearance due to coiling and thickening due to histones
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twisting, crossing and splicing of chromosomes
(figure available in print form)
(figure available in print form)
(figure available in print form)
Many changes have been made to the Crick central dogma and it is now known that the flow of genetic information is not one-way. The molecular genetic system (DNA-RNA-Protein) is dynamic; elements exert feedback regulation in ways that could not have been originally known. The genetic material is ‘plastic’ so that a single ‘gene’ may, through rearrangement and biochemical editing, be responsible for the production of several different proteins.
(figure available in print form)
THE MECHANISM FOR PLASMID UPTAKE BY E. coli
Work with E. coli has shown that the bacterial cells can take on the DNA of other organisms and then the bacteria replicates that DNA as its own.
By treatment with certain solutions the E. coli become ‘competent’ or able to take on the DNA which is transported into the bacteria. The mechanism for transport is called a plasmid.
The acidic phosphates of the DNA helix are negatively charged. Similarly these make up the bacterial membranes and permit zones of adhesion.
(figure available in print form)
GEL ELECTROPHORESIS USED TO DETECT RESTRICTION FRAGMENTS
In 1970, Daniel Nathans introduced gel electrophoresis as a means to separate DNA restriction fragments. Gel electrophoresis means, literally, to carry with electricity. When placed within an electrical field, DNA molecules are attracted toward the positive pole and repelled from the negative pole. The process is used to determine the size in basepairs of DNA restriction fragments which have been enzymatically ligated.
Below—Right: This micrograph shows that lambda DNA material can be fragmented by the enzymes EcoRI, and HindII. Fragments of smaller size travel further during electrophoresis.
(figure available in print form)
Below—Left: The charged fragments of DNA make it possible for the binding of reversed charged DNA fragments.
(figure available in print form)
ENZYMES ARE THE MECHANISM BY WHICH BASEPAIRS ARE ACTIVATED OR MANY BASEPAIRS ARE ACTIVATED AS A GENE TO PRODUCE A PROTEIN
Enzymes are produced or synthesized with many amino acids
which are coded for from the DNA-RNA-protein sequence. Certain enzymes can cleave the DNA chain and these are called restriction endonucleases. Some of the endo’s cut clearly through the double helix and others such as the EcoRI cut each strand off-center in the recognition site, at two to four nucleotides apart leaving exposed ends. The latter ‘sticky ends’ are useful in making recombinant DNA molecules.
(figure available in print form)
