DNA and Protein Synthesis
We’ve obviously been learning about genetics. But, in the last chapter, we learned about Mendel and his genetics experiments on pea plants. What do the two have to do with each other? This is a very important slide that demonstrates how these concepts are linked to one another. Here is another Punnett square in which two pea plants with identical genotypes and phenotypes are crossed. Each plant has yellow seeds in a green pod, and each plant is heterozygous. Remember that this means that both plants have each allele for pod color and seed color, but that the dominant allele is expressed. Breeding identical plants together like this, most would expect that the offspring should be identical to their parents, but genetics does not work that way. During meiosis, the top pea plant will produce a gamete with either big G or little g combined with either big Y or little y. Each has an equal chance of being produced resulting in four possible combinations of genes in the gametes: GY, Gy, gY, and gy. The same applies for the plant on the left of the Punnett square. Now, when these gametes are combined together in all possible ways to produce zygotes, the resulting baby plants will have the genotypes shown on the Punnett square, and a 9:3:3:1 ratio of phenotypes. Nine will have green pods and yellow seeds like the parents, three will have green pods and green seeds, three will have yellow pods and yellow seeds, and one will have yellow pods and green seeds (the exact OPPOSITE of what the parents had!). Notice that having one particular color of pea pod had nothing to do with the color of the seed. This is known as the law of independent assortment. Notice also that the rules of genetics and the process of meiosis produces plentiful variation—the raw material for natural selection to act on.
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Copyright ©2013 W.W. Norton, Inc.
Law of Independent Assortment
But, those are pea plants. What about humans? Here is an example that applies more to you and I. Suppose the gene for hair color is on the small chromosome and the gene for eye color is on the large chromosome. The blue allele on the small chromosome represents blond hair and on the large chromosome represents blue eyes. The red allele on the small chromosome represents brown hair and on the large chromosome brown eyes. Both parents are heterozygous and, we’ll say for argument sake, brown eyes and hair are dominant, meaning that both parents have brown hair and brown eyes. The parent on the left, we’ll call a female, produces four eggs through meiosis. Because of the law of independent assortment, the alleles for hair and eye color are independent from one another, producing two eggs that pass on the alleles for brown hair and brown eyes and two eggs with the alleles for blond hair and blue eyes. Just as likely is what happens with the male. He produces four sperm cells, two with brown eyes and blond hair and two with blue eyes and brown hair. Choose one egg and one sperm and combine them together. What do you get? Now choose another? Notice that there will be a mixture of these features. Some of the offspring will have blond hair and brown eyes; some will have brown hair and blue eyes. Keep in mind that these combinations were not present in the parents.
Some of you may be saying that hair color and eye color are NOT independent; that they do seem to be present together (brown with brown; blond with blue). You are of course right. One of the reasons for this is that some of the genes that code for these traits are in fact on the same chromosome. The bottom image shows how genes close to one another on the same chromosome will not follow the law of independent assortment and will instead by linked to one another. This is called linkage.
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Copyright ©2013 W.W. Norton, Inc.
DNA and Protein Synthesis
But, how does DNA cause a seed to be yellow or hair to be brown? When we talk about DNA as a code, what do we actually mean? Besides replicating itself, the other critically important thing DNA does is to code for proteins. Proteins are what bodies are made of. There are seven types of proteins described here. Some, called enzymes, help with chemical reactions, such as the protein lactase that helps break down the lactose sugar in milk. Others are structural proteins, like the keratin that makes up our hair and nails, or the collagen that helps make up bone—these are shown in this image to the right. There are gas transport proteins like hemoglobin, which transports oxygen throughout the body. Antibodies, which help fight diseases, are proteins. Hormones like insulin, which helps regulate the metabolism of sugar and fats in the body are proteins. Muscles are comprised of the mechanical proteins actin and myosin. Finally, protein can be of the nutrient-form, like ovalbumin, which is found in egg whites. Proteins are critical for the normal functioning of an organism. So, how does DNA code for these proteins? First, it is important to recognize that proteins are made of amino acids. There are 20 different kinds of amino acids; 12 of these humans can manufacture; the other 8 have to be eaten and are therefore called essential amino acids. These 20 different amino acids can combine together into chains of various lengths and different properties. These properties are what makes a protein like keratin different from a protein like hemoglobin.
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Copyright ©2013 W.W. Norton, Inc.
Transcription and Translation
- Transcription
- DNA transcribed into mRNA in the nucleus of the cell
- Translation
- mRNA translated into amino acid chain at the ribosomes
Protein synthesis, or the process by which a DNA code is turned into a chain of amino acids, occurs in cells. First the DNA code is read by enzymes, producing a molecule called messenger RNA. This process, in which messenger RNA is created from a DNA code is called transcription. The messenger RNA then leaves the nucleus of the cell and enters the cytoplasm. It binds to ribosomes, which are organelles that facilitate the translation of messenger RNA into a chain of amino acids, which ultimately form a protein. Let’s look in more detail how this actually happens.
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Copyright ©2013 W.W. Norton, Inc.
Transcription
Just like during DNA replication, the DNA is unwound, or unzipped, by enzymes. However, unlike replication, only one of the strands of DNA is used during transcription. Also, unlike replication, only a specific section of the DNA is unwound; this region is called a gene. The unwound DNA strand serves as a template for making a single-stranded molecule of messenger RNA. RNA is very similar to DNA, but instead of using A, G, C, and T as base pairs, RNA uses A, G, C, and U. U stands for Uracil and it binds to adenine (A), just like thymine (T) does in DNA. As is shown here, if the gene has the sequence TACTC, the messenger RNA molecule will be AUGAG and so on. Once the gene is fully transcribed, the messenger RNA molecule leaves the nucleus and finds ribosomes in the cytoplasm of the cell.
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Copyright ©2013 W.W. Norton, Inc.
Translation
Once messenger RNA binds to a ribosome, translation of the code into amino acids can begin. This process occurs in threes. Three nucleotides, called a codon, are read by the ribosome. These are “read” by matching a complementary anticodon to the codon. For instance, if the messenger RNA codon is AUG, then the anticodon has to be UAC since those are the three nucleotides that are complementary to the codon. Importantly, these anticodons are attached to a specific amino acid, in this case methionine, in a structure called a transfer RNA (tRNA). The next three nucleotides in the codon are AGU, which match with the anticodon UCA, which is attached to the amino acid serine. This goes on and one, in groups of three, until the last codon (UAG) , which is the stop sequence. The amino acid chain is then released into the cytoplasm. The amino acid chain folds into a three-dimensional structure, or bonds with other 3-D proteins, which give these proteins their specific properties. What we have described here happens in only a small percentage of the human genome. In fact, only about 5% of the total genome is composed of structural genes that code for proteins, or regulatory genes that turn genes on and off. We will turn to these genes next.
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Copyright ©2013 W.W. Norton, Inc.
Regulatory Genes
- On/off switches for genes
- Marfan syndrome
- Chicken teeth
- Human hair
- Lactose intolerance or persistence
Regulatory genes can be thought of as on/off switches, or, perhaps more accurately, dimmer switches. Regulatory genes determine if a gene is on or off, and can regulate the amount of protein produced, and when. For instance, if the genes controlling connective tissue growth are left on a bit longer during development, what can result are longer, thinner fingers as is shown in this image. This is characteristic of a disease called Marfan syndrome. Regulatory genes have also allowed us to understand major evolutionary events. For instance, paleontological evidence demonstrates that modern birds evolved from a group of feathered dinosaurs. But, anyone who has visited a science museum knows that dinosaurs have teeth. Birds do not. Where did their teeth go? Scientists have recently discovered that birds still have the structural genes to make teeth. But, the regulatory genes controlling those structural genes have been turned off. A similar thing has happened with human body hair. Humans have less body hair than other primates. We still have the genes for full body hair coverage, but these genes have been down-regulated. Similarly, all baby mammals have the ability to digest milk. This is because they produce the enzyme lactase, which breaks down lactose. However, these genes are turned off in most adult mammals. However, some humans have lactose persistence, meaning that the genes are not turned off and they can continue to digest milk as adults. Those who retain the typical mammalian condition of losing lactase production into adulthood are said to be lactose intolerant. Notice that natural selection can act upon the products of structural genes, but can also operate on the products of variation in regulatory genes. In fact, research on human and chimpanzee genomes have discovered that while our structural genes are very similar, there are important differences in those regulatory genes.
