Module 7 Lesson 4 - 3
Lesson 4 — Changes in the Genetic Code
Causes of Mutations
Read pages 644 - 645
Some mutations are spontaneous and occur naturally in the cell. One cause of spontaneous mutation is incorrect base pairing by DNA polymerase during DNA replication.
Factors in our environment can cause mutations; these mutations are referred to as induced mutations. The environmental factors that increase the rate of mutation are called mutagens. The two categories of mutagens that
cause induced mutations are physical and chemical.
Likely, you have been exposed to the physical mutagen, x-rays, if you have ever broken a bone or visited the dentist's office. Another common physical mutagen is ultraviolet (UV) rays.
Some chemical mutagens resemble DNA nucleotides and insert themselves
into the DNA sequence, resulting in mispairing of bases. Examples of chemical mutagens include nitrites used as food preservatives, gasoline fumes, and cigarette smoke.
Learn more about physical and chemical mutagens by reading pages 644 and 645 of your textbook.
Cancer
Cancer is a disease resulting from continuous exposure to cancer-causing factors called carcinogens. As mutations begin to accumulate in cells, the cells may lose their ability to control cell division. For example, if the gene promoting growth is mutated, cell division may proliferate. In other instances, genes that determine programmed cell death could be suppressed by the mutation.
Although cancer-causing mutations can be inherited, prolonged exposure to carcinogens such as physical and chemical mutagens and some viruses have been linked to cancer.
Evolutionary Relationships between Species
Read pages 646 - 647
In the 1960s, to study how various birds were related, biologists looked at their anatomical similarities and differences. The similarity of species has interested scientists for a long time. For example, human DNA and chimpanzee DNA are 98% similar.
The genetic similarity between two humans is more than 99.99%. This knowledge can be used to construct a phylogenetic tree that shows the evolutionary relationships among species. Today, scientists can compare the DNA of ancient plants, animals,
and even bacteria with the DNA of modern organisms to look at the ancestry of modern organisms, the movement of populations through time, the evolution of particular disease-causing bacteria, and the way that ecosystems respond to climate change.
Read more about this on page 647 of your textbook.
Are all DNAs located in the nucleus?
mtDNA is located in a cell's mitochondria and cpDNA is located in plant cell's chloroplasts. Mitochondrial and chloroplast DNA have their own DNA that is replicated, transcribed, and translated independently from the DNA in the nucleus of the cell.
Mitochondria are organelles found in the cytoplasm outside the cell's nucleus and serve as the source of energy for the cell. Typically, when fertilization occurs, the nuclear DNA of the zygote is a combination of the two parents' nuclear DNA. When the zygote forms, the cytoplasm and all cytoplasmic organelles are donated by the ovum (egg). This means that the mitochondrial DNA is identical to the mtDNA of the mother.
When you go back generations in your family tree, you will see that many men and women contributed to your nuclear DNA, but only one woman contributed to the mtDNA. Your mtDNA is a copy of your mother's, which is copied from her mother, and so on. Mutations occur in mtDNA over time but at a much slower rate. This mutation rate can help scientists deduce ancestry. The more similar the mtDNA between people, the closer they are related. The more dissimilar the mtDNA, the more mutations must have occurred, which indicates more time must have elapsed on the evolutionary path.
Another type of DNA found outside the nucleus is the chloroplast DNA in plants. Chloroplasts conduct photosynthesis and convert light energy into glucose that can be used by the plant cells. Both mitochondrial and chloroplast DNA analysis suggest that they evolved from bacteria more than 1 billion years ago. Some scientists hypothesize that anaerobic cells containing nuclei (eukaryotes) engulfed early aerobic mitochondrial and chloroplast cells when oxygen began accumulating in Earth's atmosphere. This theory is called the endosymbiont theory.
Both mitochondrial and chloroplast DNAs evolve very slowly. Scientists have sequenced chloroplast DNAs from liverwort and tobacco plants and found that their genomes are almost identical. This indicates that the chloroplast genome has not evolved for the past several hundred million years from the time when liverwort and tobacco diverged from each other.
Scientists have determined that mitochondria and chloroplasts have their own separate genetic systems, but other organelles in the cell do not. By continuing to analyze the mitochondrial and chloroplast DNA, scientists are gaining insight into the relationship between species and into the process of evolution.
Read pages 646 to 647 in your textbook to learn more about these special forms of DNA.