Skip to content Main Navigation Search. Dictionary Articles Tutorials Biology Forum. Gene Regulation in Eukaryotes Learn about the general structure of a eukaryotic gene, the transcription factors, and post-transcriptional regulation Hormone Production Hormones are chemical messengers produced by specialized glands and they were produced by switching on the genes designe.. Consciousness and Behavior Human consciousness and behavior are an interesting topic since they are determined and controlled by the brain.
Each nucleotide is labeled with a letter representing the identity of the nitrogenous base. Three-nucleotide units, labeled below the letters as codons, are enclosed in brackets from left to right. An arrow points from the codon to a colored sphere representing the corresponding amino acid coded for by the three-nucleotide-long mRNA sequence. Each amino acid is identified by its three-letter abbreviation.
Because there are 24 nucleotides, there are eight codons, each containing three nucleotides. The first codon, AUG, codes for the amino acid methionine, represented by a beige circle. The following codon, AAA, codes for the amino acid lysine, represented by a gray circle. The codon CUU codes for the amino acid leucine, represented by a brown circle. The last three codons all have the nucleotide sequence AUG, and therefore all code for the amino acid methionine, represented by a beige circle.
The final amino acid sequence coded for by the given mRNA sequence is methionine-lysine-leucine-arginine-arginine-methionine-methionine-methionine. Mutations can arise in cells of all types as a result of a variety of factors, including chance. In fact, some of the mutations discussed above are the result of spontaneous events during replication, and they are thus known as spontaneous mutations.
Slippage of the DNA template strand and subsequent insertion of an extra nucleotide is one example of a spontaneous mutation; excess flexibility of the DNA strand and the subsequent mispairing of bases is another. Environmental exposure to certain chemicals, ultraviolet radiation, or other external factors can also cause DNA to change.
These external agents of genetic change are called mutagens. Exposure to mutagens often causes alterations in the molecular structure of nucleotides, ultimately causing substitutions, insertions, and deletions in the DNA sequence. Mutations are a source of genetic diversity in populations, and, as mentioned previously, they can have widely varying individual effects.
In some cases, mutations prove beneficial to an organism by making it better able to adapt to environmental factors. In other situations, mutations are harmful to an organism — for instance, they might lead to increased susceptibility to illness or disease.
In still other circumstances, mutations are neutral, proving neither beneficial nor detrimental outcomes to an organism. Thus, it is safe to say that the ultimate effects of mutations are as widely varied as the types of mutations themselves. This page appears in the following eBook. Aa Aa Aa. Where do mutations occur? Germ-line mutations occur in gametes or in cells that eventually produce gametes. In contrast with somatic mutations, germ-line mutations are passed on to an organism's progeny.
As a result, future generations of organisms will carry the mutation in all of their cells both somatic and germ-line. What kinds of mutations exist? Base substitution. Base substitutions are the simplest type of gene-level mutation, and they involve the swapping of one nucleotide for another during DNA replication.
For example, during replication, a thymine nucleotide might be inserted in place of a guanine nucleotide. With base substitution mutations, only a single nucleotide within a gene sequence is changed, so only one codon is affected Figure 1. Figure 1: Only a single codon in the gene sequence is changed in base substitution mutation.
The nitrogenous bases are paired so that blue and orange nucleotides are complementary and red and green nucleotides are complementary. However, the 5 th nucleotide from the right on both the bottom and top strand form a mismatched pair: an orange nucleotide pairs with a red nucleotide. This mismatched pair is highlighted in cyan. The sugar molecules of each individual nucleotide in the chain are connected to adjacent sugar molecules, which are represented by gray horizontal cylinders.
The nitrogenous bases hang down from the sugar molecules and look like vertical bars that are twice as long and half as wide as the gray cylinders; the bases are either blue, red, green, or orange. A second chain of 12 nucleotides forms the second DNA strand below the upper template strand; this strand is labeled the replicating strand in the lower right. Here, the nitrogenous bases point upward from the sugar-phosphate chain, nearly meeting the ends of the nitrogenous bases from the upper strand.
Since protein-coding DNA is divided into codons three bases long, insertions and deletions can alter a gene so that its message is no longer correctly parsed. These changes are called frameshifts. In frameshifts, a similar error occurs at the DNA level, causing the codons to be parsed incorrectly. There are other types of mutations as well, but this short list should give you an idea of the possibilities.
DNA: The molecular basis of mutations. When that happens, that change in DNA sequence or amino acid sequence, or both, could have no effect on the protein, in which case the substitution is benign and has no functional effect.
In other cases, if it changes the function of the protein, then it will be observed as either a functional polymorphism, something which increases the effectiveness of the protein product, and therefore would be evolutionarily selected for, or is bad--deleterious--in which case the person might die early and get evolutionarily selected against.
Substitutions which lead to mutations, which lead to a deleterious outcome, that is the organism having difficulty with living or dying early, those we call mutations, but they're the result of a certain kind of a substitution.
0コメント