Before one can understand the basis of evolution, it’s necessary to become acquainted with its building blocks. These include the two nucleic acids, DNA and RNA, and the proteins synthesized by their mutual interplay. In terms of what we know in modern genetics, it’s the information encoded in DNA – specifically in terms of instructions for protein biosynthesis – that enables differentiation between a man, a mouse, a maggot and a mold. In other words, the information transmitted from one species’ generation to the next is primarily in its capacity to synthesize its own particular proteins.
DNA or deoxyribonucleic acid consists of two strands of what we call polynucleotides. In the case of DNA there are four bases: A (adenine), C (cytosine), G (guanine), and T (thymine) that are paired in two mutually exclusive ways[1]. That is, A always goes with T, while G always pairs with C.
One possible explanation for the preferential pairing arrangement is the presence of multiple hydrogen bonds between bases. The arrangement itself is evidently crucial to the encoding of information. In this sense, the way the bases are arranged in DNA forming various ‘messages’ is precisely analogous to the way this sentence encodes structured written information. Change the order of the letters in the words, the order of the words themselves – or both, and noise emerges.
On this basis DNA can be said to be an informational molecule. Also on this basis, as we shall see, if the information content alters – and can be replicated in its alteration, there is the possibility of molecular evolution. This is in fact the phase at which evolution commences, since it directly affects the instructions for protein synthesis.
RNA
Like DNA, RNA is a polymer, but much simpler. This is because the latter’s ribose sugar molecules are smaller than DNA’s deoxyribose sugar molecules. So there are fewer monomer units in all to make polymer chains. At the same time, while there is only one type of DNA, there are three types of RNA:
- Messenger RNA or mRNA: really a complementary copy of a DNA segment that conveys information from cell nucleus to cytoplasm. Once there it acts as a template for synthesis of a protein molecule.
- Transfer RNA or tRNA: as the name implies, it couples information in the nucleic acid base sequence to the amino acid base sequence in synthesized proteins.
- Ribosomal RNA or rRNA: forms the ribosomes of cells, from which proteins are made.
As we can see from the preceding, proteins are synthesized in living cells by a transcription (coding) process from RNA molecules, themselves transcribed from DNA. A simple diagram is appended to help to illuminate the process.
What are proteins? They are complex molecules composed mainly of carbon, hydrogen, oxygen and nitrogen. That is, four elements generated within the innards of massive stars. Like the nucleic acids they’re polymers but of amino acids rather than nucleotides, ribose, deoxyribose sugars. Further, they’re linked by peptide linkages, rather than hydrogen bonds.
Some of the twenty or so amino acids found in proteins, with their abbreviations, include: valine (Val); leucine (Leu); threonine (Thr); alanine (Ala); and glycine (Gly). On the surface, given what we know about polymers – as long chains of molecules, one might expect proteins to be similar long chains. In fact, proteins undergo at least a threefold coiling-folding process. In the first one, the polypeptides are coiled to a long spiral called the alpha-helix. The structure resulting resembles a long cylinder. Another folding yields a spherical form.
Molecular evolution at the level of DNA, RNA, and proteins is precipitated by one or more of the following:
- Base substitution, whereby one amino acid (nitrogenous base) is replaced by another. This is called point mutation.
- Deletion of a group of bases.
- Addition of a group of bases, e.g. if there are multiples of three, one or more amino acids may be deleted from the protein.
- Inversion, or removal of a segment of DNA accompanied by reinsertion elsewhere but in inverted order. Related to inversion in a way is translocation, wherein a portion of DNA is attached to a homologous chromosome.
In each of the molecular cases above, the alteration results in subtle changes in the coding of information, or the transfer of information. These molecular changes therefore act as sources of genetic variation, and ultimately evolution if the change is reinforced by natural selection .
And, if this is true, it should also be feasible to trace back species lineages using comparisons at the molecular level, viz. different segments of chromosomes for suspected related species.
Yunis and Prakash, in their groundbreaking work for example, have shown a remarkable homology between chimp and human chromosomes, when heterochromatin is excluded. This includes no less than thirteen ‘presumably identical chromosome pairs’.[2] They also show that for the common ape-human ancestor, from which both chimps and modern humans arose, eighteen chromosome pairs were similar to Homo sapiens, and fifteen to the chimpanzee. They also found that chromosome 17 in the chimp differs by a ‘pericentric inversion’ from that of man.
[1] RNA has the same bases except for T (thymine) now replaced by U (uracil).
[2] Yunis, J. and Prakash, O: 1982, The Origin of Man: A Chromosomal Pictorial Legacy, in Science, Vol. 215, p. 1525. The chromosomes are: 3, 6 to 8, 10, 11, 13, 14, 19 to 22 and XY.
DNA or deoxyribonucleic acid consists of two strands of what we call polynucleotides. In the case of DNA there are four bases: A (adenine), C (cytosine), G (guanine), and T (thymine) that are paired in two mutually exclusive ways[1]. That is, A always goes with T, while G always pairs with C.
One possible explanation for the preferential pairing arrangement is the presence of multiple hydrogen bonds between bases. The arrangement itself is evidently crucial to the encoding of information. In this sense, the way the bases are arranged in DNA forming various ‘messages’ is precisely analogous to the way this sentence encodes structured written information. Change the order of the letters in the words, the order of the words themselves – or both, and noise emerges.
On this basis DNA can be said to be an informational molecule. Also on this basis, as we shall see, if the information content alters – and can be replicated in its alteration, there is the possibility of molecular evolution. This is in fact the phase at which evolution commences, since it directly affects the instructions for protein synthesis.
RNA
Like DNA, RNA is a polymer, but much simpler. This is because the latter’s ribose sugar molecules are smaller than DNA’s deoxyribose sugar molecules. So there are fewer monomer units in all to make polymer chains. At the same time, while there is only one type of DNA, there are three types of RNA:
- Messenger RNA or mRNA: really a complementary copy of a DNA segment that conveys information from cell nucleus to cytoplasm. Once there it acts as a template for synthesis of a protein molecule.
- Transfer RNA or tRNA: as the name implies, it couples information in the nucleic acid base sequence to the amino acid base sequence in synthesized proteins.
- Ribosomal RNA or rRNA: forms the ribosomes of cells, from which proteins are made.
As we can see from the preceding, proteins are synthesized in living cells by a transcription (coding) process from RNA molecules, themselves transcribed from DNA. A simple diagram is appended to help to illuminate the process.
What are proteins? They are complex molecules composed mainly of carbon, hydrogen, oxygen and nitrogen. That is, four elements generated within the innards of massive stars. Like the nucleic acids they’re polymers but of amino acids rather than nucleotides, ribose, deoxyribose sugars. Further, they’re linked by peptide linkages, rather than hydrogen bonds.
Some of the twenty or so amino acids found in proteins, with their abbreviations, include: valine (Val); leucine (Leu); threonine (Thr); alanine (Ala); and glycine (Gly). On the surface, given what we know about polymers – as long chains of molecules, one might expect proteins to be similar long chains. In fact, proteins undergo at least a threefold coiling-folding process. In the first one, the polypeptides are coiled to a long spiral called the alpha-helix. The structure resulting resembles a long cylinder. Another folding yields a spherical form.
Molecular evolution at the level of DNA, RNA, and proteins is precipitated by one or more of the following:
- Base substitution, whereby one amino acid (nitrogenous base) is replaced by another. This is called point mutation.
- Deletion of a group of bases.
- Addition of a group of bases, e.g. if there are multiples of three, one or more amino acids may be deleted from the protein.
- Inversion, or removal of a segment of DNA accompanied by reinsertion elsewhere but in inverted order. Related to inversion in a way is translocation, wherein a portion of DNA is attached to a homologous chromosome.
In each of the molecular cases above, the alteration results in subtle changes in the coding of information, or the transfer of information. These molecular changes therefore act as sources of genetic variation, and ultimately evolution if the change is reinforced by natural selection .
And, if this is true, it should also be feasible to trace back species lineages using comparisons at the molecular level, viz. different segments of chromosomes for suspected related species.
Yunis and Prakash, in their groundbreaking work for example, have shown a remarkable homology between chimp and human chromosomes, when heterochromatin is excluded. This includes no less than thirteen ‘presumably identical chromosome pairs’.[2] They also show that for the common ape-human ancestor, from which both chimps and modern humans arose, eighteen chromosome pairs were similar to Homo sapiens, and fifteen to the chimpanzee. They also found that chromosome 17 in the chimp differs by a ‘pericentric inversion’ from that of man.
[1] RNA has the same bases except for T (thymine) now replaced by U (uracil).
[2] Yunis, J. and Prakash, O: 1982, The Origin of Man: A Chromosomal Pictorial Legacy, in Science, Vol. 215, p. 1525. The chromosomes are: 3, 6 to 8, 10, 11, 13, 14, 19 to 22 and XY.
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