Chapter 14 Summary
Mutations:
Most people think that mutations are horrible things that only have negative effects. However, everything that lives needs mutations to occur, so there can be variation. Once there is variation, natural selection can continue to occur and the organsims can evolve throughout time. A mutation is a small change in the DNA structure that alters a particular gene. Mutations can cause two basic things to a gene. It can change the base sequence of the gene, or it could add or remove nucleotides from a gene. There are several different types of mutations. A point mutation occurs when a single amino acid base pair of the gene gets effected. A silent mutation changes the nucleotide sequence of a gene, but does not alter the amino acid sequence. A missense mutation changes a single amino acid in a polypeptide sequence. This mutation may not effect the function of the gene,as long as the substitute amino acid is has a chemistry similar to the original amino acid. Sickle Cell disease is an example of a Missense mutation. A nonsense mutation involves a change from a normal codon to a stop or termination codon. This causes translation to be terminated earlier than expected, producing a shorter polypeptide. Lastly, a Frame Shift Mutation involves the addition or deletion of nucleotides that are not in multiples of three nucleotides. Such a large change in polypeptide structure is likely to inhibit protein function. However, most mutations occur randomly. The Lederbergs and their replica plating experiment proved this fact.
DNA Repair:
If mutations can occur at random, how does a cell stop a random mutation from effecting the cell. Cells contain DNA repair systems that fix DNA mutations. The DNA repair system contains many proteins that aid in the repair mechanism. The first step is for specialized proteins to detect an error in the DNA structure. Second, more specialized proteins repair the abnormalities. Direct Repair is when enzymes modify the base sequence if the gene, for it to be repaired. However, sometimes the DNA sequence has to be removed and re-synthesized. This is called Base Excision and Nucleotide Excision Repair. Methyl - Directed Mismatch Repair occurs when the DNA defect is a base pair mismatch in the DNA. The mismatch is recognized, and a strand of DNA in the region is removed. The complementary strand is used to synthesize a normal strand of DNA.
Cancer:
Cancer is a disease caused by uncontrolled cell division. Carcinogens are environmental agents that increase the likelihood of cancer. UV rays and certain chemicals in cigarretes are example of carcinogens. Cancer is a multi-step process, the earlier you spot it the easier it is to treat. Cancer starts out in one cell, and its lineage daughter cells undergo a series of mutations that causes the cells to grow abnormally. These cells form a tumor. A tumor starts out as a benign tumor. Benign tumors don't spread throughout the body, however more mutations occur and the tumor starts to spread and goes into a malignant stage. At this point the person has cancer. The tumor effects neighboring tissues and begins to spread. If left untreated the individual will die. Oncogenes and Tumor suppressor genes are very important when it comes to this cancer topic. Oncogenes over stimulate everything, thus creating uncontrolled cell division, causing cancer. Tumor suppressor genes are the opposite. They encode for proteins to prevent cancer. Usually cancer occurs when a mutation inhibits the Tumor suppressor gene's function. Gene amplification, Chromosome Translocation, and Retroviral Insertion all create cancer. Gene Amplification is when there is an increase int he number of Proto-Onco Genes. Than once those proto-onco genes change to oncogenes, cancer starts. Chromosome Translocation is when a chromosome breaks and gets attached to the wrong chromosomal segment. The abnormal fusion creates an Chimeric Gene whose functional over activity leads to Luekemia. Lastly, Retroviral Insertion is when viruses convert proto-onco genes to onco genes. During the viral reproduction cycle. This causes an over expression of proto-onco genes causing cancer. During the Cell Cycle at the G1, G2, and M phase there are check point proteins that check the integrity of the DNA sequence making sure there are no mutations being passed on. P53 is a very popular example. It is a checkpoint protein at the G1 phase and will completely inhibit the cell cycle if there is any DNA damage. P53 can either fix the damage. But if it is too severe, it will signal other proteins to come and dispose of the mutated cell.
This is a blood cell that became a sickle cell because of a missense mutation. In a missense mutation, the amino acid can be replaced so the gene doesn't lose its entire function. So thats what happens with these sickle cells. They got a substitute amino acid, and they kept their function as a red blood cell. But they are altered.
This Young lady has a somatic mutation. Since it is Somatic this mutation cannot be passed on to her offspring. I also know this is somatic because for somatic mutations a patch of cells are effected. And a patch of her hair is white and not pigmented black.
http://www.ncbi.nlm.nih.gov/pubmed/21294243 - In this Pub Med article, it is talking about a point mutation that causes adult lukeimia. The gene is called RUNX1 and the mutation occurs in that gene. The scientists are test whether or not this point mutated gene is also the cause of pediatric Lukeimia patients. After the tests, i think they found that the mutated RUNX gene was in some of the children's luekmia cells.