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DNA and Evolution
WHAT IS DNA?
DNA, or deoxyribonucleic acid, is a molecule found in the nuclei of cells. DNA contains genes, the building blocks of all organisms.
THE STRUCTURE OF DNA
The most important function of DNA is its ability to replicate itself repeatedly. DNA must be copied when new cells are formed, when genetic material is passed from parents to offspring, and when coding for RNA (ribonucleic acid) to make proteins. The structure of DNA – a double helix – allows DNA to be copied successfully many times over with very few errors.
DNA’s double helix (which looks like a twisted ladder) is made of units called nucleotides. Each nucleotide consists of a phosphate, sugar, and base. The phosphates and sugars form the sides of the ladder, while the bases form the rung. A base from a nucleotide on one side of the ladder will chemically bond with a nucleotide from the other side, forming the rung. Certain bases always pair together; adenine always pairs with thymine and guanine always pairs with cytosine. These four bases have different chemical structures, causing them to pair in this specific manner.
When DNA replicates, the bonds between bases break and the DNA “unzips” itself. New nucleotides are joined to either side of the broken ladder by the work of DNA polymerase, an enzyme. Enzymes are proteins that mediate and initiate chemical reactions. When the polymerase has traveled the entire length of the DNA, it will have formed two new ladders from the original single ladder. Now the DNA has been perfectly copied from one strand into two. Some enzymes will even “proofread” the DNA to try to catch any errors!
When errors do occur during copying, mutations arise. Some mutations are beneficial, and some are not. If the mutations occur in sex cells, they can also be passed from parents to offspring. The existence of random mutations is essential for evolution theory. Populations will naturally vary; some individuals may have certain mutations while others do not. Those with beneficial mutations may be more likely to survive and produce offspring, passing their mutation to some of their offspring. Those with detrimental mutations may not be less likely to survive and produce offspring. (IMAGE from http://evolution.berkeley.edu/evosite/ evo101/images/dna-mutation.gif demonstrating a mutation)
Several scientists were responsible for the eventual discovery of DNA’s structure. Erwin Chargaff and his colleagues noticed in the mid-20th century that the amount of adenine always equaled the amount of thymine and that the same was true of guanine and cytosine. Rosalind Franklind and Maurice Wilkins performed X-ray crystallography of DNA, and the resulting image suggesting a helical shape. By putting these pieces of information together, Francis Crick and James Watson developed the double-helix model of DNA.
HOW DOES DNA PRODUCE PROTEINS?
The other major feature of DNA is its ability to make proteins. Proteins provide structure for our bones and other tissues, transport materials like iron throughout our bodies, help materials move from one cell to another, function as hormones that regulate our body’s functions, act as enzymes in chemical reactions, and fight diseases in the form of antibodies. In short, proteins are among the most important cells in the body.
A protein, at its most basic level, is a chain of amino acids. Amino acids are made from codons, sequences of three nucleotides in the DNA. There are 20 amino acids found in humans. While some amino acids can be made from more than one codon, each codon can only produce one amino acid. This feature of amino acids is called redundancy.
The process of making proteins is complicated, but to summarize here, the making of proteins begins with the unzipping of DNA, as if it were going to copy itself into two DNA strands. Instead, the nucleotides join the now open side of the ladder to form mRNA, or messenger RNA. This molecular has a slightly different chemical structure than DNA, allowing it to take the genetic code from the nucleus to the cytoplasm. In the cytoplasm, mRNA will bind with a structure called a ribosome. In the ribosome, each codon of mRNA is matched with the amino acid for which the codon codes. As the codons are read in sequence, the amino acids are also assembled in the same order, forming a protein. In this way, different sections of DNA can eventually make different kinds of proteins.
An older theory of genetics maintains the principle of “one gene, one protein.” However, modern genetics has discovered that oftentimes, proteins are determined by the coordinated activities of several genes.
WHAT IS A GENE?
A gene is a section of DNA responsible for a certain trait. Often, these traits are physical like the color of our hair or the length of our toes. However, genes can also produce subtler traits, like whether we have a propensity to develop cancer or what our blood type is. We inherit our genes from our biological parents.
Gregor Mendel, a monk living in the 19th century, was the first scientist to describe our modern understanding of genes. He noticed that different pea plants had different characteristics, and he rigorously bred them to see how offspring inherited the traits of their parents.
Many earlier scientists thought that offspring were a “blending” of their parents; you can see in yourself that you may look sort of like your mom and sort of like your dad. However, if you look closer, you may notice that you are not just a blending of their features. Instead, some parts of you (like your eye color) may be more similar to one parent than the other. Mendel noticed this in pea plants, too.
He realized that genes came in versions, producing different traits. For example, one gene codes for color in pea plant flowers. It may produce a purple or white color; these different versions are called alleles. It might help to think of them as “flavors” of ice cream – whether strawberry or vanilla, it’s still ice cream. However, unlike when you mix strawberry and vanilla ice cream, when Mendel bred a purple and white pea plant together, he did not get light-purple offspring. Instead, he got all purple plants. He realized that the purple allele was dominant over the recessive white allele. If a plant inherited a purple allele from one of its parents, it would be purple. It could only be white if it inherited two white alleles, one from each of its parents.
In this way, Mendel discovered several important principles of inheritance. First, an offspring inherited exactly half of its genetic material from each parent. Second, genes came in alleles and some alleles could be dominant over other alleles. Finally, he also noticed that traits independently assorted – just because a plant was purple did not also mean it had yellow seeds. These traits were inherited from different genes. You can already see how important these principles are to evolution. Traits can be inherited from parent to offspring, and the natural occurrence of different alleles creates variation within a population.
A Punnett Square is a model used by scientists to demonstrate this kind of inheritance. The genotypes – the genetic codes – of the parents are on the sides of the square. They each have two alleles – one from each of their own parents. One allele is selected from each parent and the resulting genotypes are then combined (like a multiplication table). These show the possible genotypes of a single offspring. Since genes independently assort each time parents procreate, each offspring has a possibility of being one of the four genotypes produced. A dominant allele is always written in capital letters, and a recessive allele is always written in lowercase. To determine the phenotype, or physical trait, of the possible offspring, just look at the genotype. If there is a capital letter, even just one, the offspring will have a dominant phenotype. If it has two recessive alleles, it will bear the recessive phenotype.
This was the earliest form of genetics, which is still called Mendelian (or Classical) genetics. In reality, scientists have discovered that genes are much more complicated. Some traits require the combined action of multiple genes, like hair color. Others have more than two alleles, like blood type, which has three alleles – A, B, and O. To make matters more complicated, the A and B alleles of blood are codominant. An individual who inherits an A from one parent and a B from another has AB blood type. Some genes are regulated by other genes, and some genes will not function if a mutation is present.
DO MY GENES DETERMINE WHO I AM?
The short answer is, yes, our genes determine our bodies. They provide the biological information that makes us who we are. Although future developments in science and medicine may allow us to change parts of ourselves, right now we cannot change our genetic code. For example, we cannot change the genes that give us our natural hair color. Instead, if we want to change our hair color, we would have to dye it. The same is true for many disorders and diseases that have a genetic origin; we cannot change them once we inherit them from our parents.
Genes may also determine certain parts of our personalities. Research has demonstrated that genes may relate to our sexuality, the development of addictions, how our moods change, and many other elements of human psychology. However, if you know identical twins, you may already realize how difficult these studies are. Even with the same genetic code, identical twins often form distinct personalities. A lot remains to be learned in this field.
Finally, although earlier theories of genetic determination maintained that all human features were determined by genes, modern scientists understand that environment also plays a role in forming many of our physical traits, personality characteristics, and illnesses. Additionally, epigenetic effects may cause genes to turn off or on, downregulate, or upregulate. Changing how a gene is expressed will change the trait produced, even if it does not change the basic DNA sequence of the gene.
Nash JM. 1998. The Personality Genes. Time. Monday April 27.
Stanford C, Allen JS, and Anton SC. 2009. Biological Anthropology: The Natural History of Humankind. Upper Saddle River, New Jersey: Pearson Prentice Hall.