is a human genetic disease that results from a single base difference within the coding region for the β-globin section of hemoglobin, causing a single amino acid change that changes hemoglobin's physical properties. sickle-cell versions of hemoglobin stick to themselves, stacking to form fibers that distort the shape of red blood cells carrying the protein. these sickle-shaped cells no longer flow smoothly through blood vessels, having a tendency to clog or degrade, causing the medical problems associated with this disease. some dna sequences are transcribed into rna but are not translated into protein products—such rna molecules are called non-coding rna. in some cases, these products fold into structures which are involved in critical cell functions (e.g. ribosomal rna and transfer rna). rna can also have regulatory effects through hybridization interactions with other rna molecules (such as microrna). the genetic code is a dictionary that matches together the amino acid and nucleotide sequences called add-ons. 64 genetic codons are there in which every codon has 3 bases. in 64 codons,20 amino acids are coded by 61 codons which are found in proteins and 3 codons don’t code for any amino acid. there are different types of codons. the codons that code for amino acids are called sense codons and the codons that code for protein synthesis are called signal codons. signal codons are of two types which are terminating codons and initiating codons. uaa uag uga is termed as termination codons or also called nonsense codons. aug is called an initiation codon used to code for the first amino acids in every protein. during the translation process the t-rna base sequence pairs with the codon of m rna which is known as an anticodon. the difference between codon and anticodon is that codon is present not only in dna but in rna, whereas anticodon is present only in rna but not in dna. codons will be directed from 5’ end to 3’ end in the same way anticodons are directed in the opposite way i.e., 3’ end to 5’ end. in some t rna molecules, anticodons must pair with more than one codon. the arrangement of codons is sequence manner based while an arrangement of anticodons is discretely present in cells with amino acids. nature and nurture siamese cats have a temperature-sensitive pigment-production mutation. although genes contain all the information an organism uses to function, the environment plays an important role in determining the ultimate phenotypes an organism displays. the phrase "nature and nurture" refers to this complementary relationship. the phenotype of an organism depends on the interaction of genes and the environment. an interesting example is the coat coloration of the siamese cat. in this case, the body temperature of the cat plays the role of the environment. the cat's genes code for dark hair, thus the hair-producing cells in the cat make cellular proteins resulting in dark hair. but these dark hair-producing proteins are sensitive to temperature (i.e. have a mutation causing temperature-sensitivity) and denature in higher-temperature environments, failing to produce dark-hair pigment in areas where the cat has a higher body temperature. in a low-temperature environment, however, the protein's structure is stable and produces dark-hair pigment normally. the protein remains functional in areas of skin that are colder—such as its legs, ears, tail, and face—so the cat has dark hair at its extremities. environment plays a major role in effects of the human genetic disease phenylketonuria. the mutation that causes phenylketonuria disrupts the ability of the body to break down the amino acid phenylalanine, causing a toxic build-up of an intermediate molecule that, in turn, causes severe symptoms of progressive intellectual disability and seizures. however, if someone with the phenylketonuria mutation follows a strict diet that avoids this amino acid, they remain normal and healthy. a common method for determining how genes and environment ("nature and nurture") contribute to a phenotype involves studying identical and fraternal twins, or other siblings of multiple births. identical siblings are genetically the same since they come from the same zygote. meanwhile, fraternal twins are as genetically different from one another as normal siblings. by comparing how often a certain disorder occurs in a pair of identical twins to how often it occurs in a pair of fraternal twins, scientists can determine whether that disorder is caused by genetic or postnatal environmental factors. one famous example involved the study of the genain quadruplets, who were identical quadruplets all diagnosed with schizophrenia. gene regulation the genome of a given organism contains thousands of genes, but not all these genes need to be active at any given moment. a gene is expressed when it is being transcribed into mrna and there exist many cellular methods of controlling the expression of genes such that proteins are produced only when needed by the cell. transcription factors are regulatory proteins that bind to dna, either promoting or inhibiting the transcription of a gene. within the genome of escherichia coli bacteria, for example, there exists a series of genes necessary for the synthesis of the amino acid tryptophan. however, when tryptophan is already available to the cell, these genes for tryptophan synthesis are no longer needed. the presence of tryptophan directly affects the activity of the genes—tryptophan molecules bind to the tryptophan repressor (a transcription factor), changing the repressor's structure such that the repressor binds to the genes. the tryptophan repressor blocks the transcription and expression of the genes, thereby creating negative feedback regulation of the tryptophan synthesis process. transcription factors bind to dna, influencing the transcription of associated genes. differences in gene expression are especially clear within multicellular organisms, where cells all contain the same genome but have very different structures and behaviors due to the expression of different sets of genes. all the cells in a multicellular organism derive from a single cell, differentiating into variant cell types in response to external and intercellular signals and gradually establishing different patterns of gene expression to create different behaviors. as no single gene is responsible for the development of structures within multicellular organisms, these patterns arise from the complex interactions between many cells. within eukaryotes, there exist structural features of chromatin that influence the transcription of genes, often in the form of modifications to dna and chromatin that are stably inherited by daughter cells. these features are called "epigenetic" because they exist "on top" of the dna sequence and retain inheritance from one cell generation to the next. because of epigenetic features, different cell types grown within the same medium can retain very different properties. although epigenetic features are generally dynamic over the course of development, some, like the phenomenon of paramutation, have multigenerational inheritance and exist as rare exceptions to the general rule of dna as the basis for inheritance. genetic change mutations main article: mutation gene duplication allows diversification by providing redundancy: one gene can mutate and lose its original function without harming the organism. during the process of dna replication, errors occasionally occur in the polymerization of the second strand. these errors, called mutations, can affect the phenotype of an organism, especially if they occur within the protein coding sequence of a gene. error rates are usually very low—1 error in every 10–100 million bases—due to the "proofreading" ability of dna polymerases. processes that increase the rate of changes in dna are called mutagenic: mutagenic chemicals promote errors in dna replication, often by interfering with the structure of base-pairing, while uv radiation induces mutations by causing damage to the dna structure. chemical damage to dna occurs naturally as well and cells use dna repair mechanisms to repair mismatches and breaks. the repair does not, however, always restore the original sequence. a particularly important source of dna damages appears to be reactive oxygen species produced by cellular aerobic respiration, and these can lead to mutations. in organisms that use chromosomal crossover to exchange dna and recombine genes, errors in alignment during meiosis can also cause mutations. errors in crossover are especially likely when similar sequences cause partner chromosomes to adopt a mistaken alignment; this makes some regions in genomes more prone to mutating in this way. these errors create large structural changes in dna sequence—duplications, inversions, deletions of entire regions—or the accidental exchange of whole parts of sequences between different chromosomes, chromosomal translocation. genetic mutation a highly pathogenic, more or less permanent alteration to the genetic code (genome) of a virus or a cell in a living creature that can be handed down to the progeny of the original cell or virus. a somatic mutation is a change in a multicellular organism's dna that can progress to progeny by dna replication. alterations can happen as a result of subjection to electromagnetic spectrum with high intensity (such as x-rays, ultraviolet light), mishaps that can place during the normal chemical transactions of dna, most frequently during replication. the bulk of variations are expected to be harmful because they are random changes, however, certain mutations might be helpful in certain circumstances. types of mutations mutations and perhaps other gene changes can be inherited or acquired. an inherited gene mutation is one that, as its name suggests, is passed down from one parent to the next. as a result, it can be found in the very first cell that develops into a human after the egg cell and sperm cell have mated. because it began in the first cell, which gave rise to all the other cells in the body, this modification is present in every cell in the body and can be passed on to the next generation. because the cells that give rise to eggs and sperm are known as germ cells, also known as a hereditary alteration. a gene mutation that is acquired does not come from the parent. as opposed to that, it emerges at some point in a person's life. acquired mutations start in one cell and spread to any subsequent cells that develop from that cell. because this mutation does not affect sperm or egg cells, it cannot be passed down to a person's offspring. somatic mutation or spontaneous mutation are other names for this kind of mutation.this is a diagram showing mutations in an rna sequence. figure (1) is a normal rna sequence, consisting of 4 codons. figure (2) shows a missense, single point, non silent mutation. figures (3 and 4) both show frameshift mutations, which is why they are grouped together. figure 3 shows a deletion of the second base pair in the second codon. figure 4 shows an insertion in the third base pair of the second codon. figure (5) shows a repeat expansion, where an entire codon is duplicated. natural selection and evolution mutations alter an organism's genotype and occasionally this causes different phenotypes to appear. most mutations have little effect on an organism's phenotype, health, or reproductive fitness. mutations that do have an effect are usually detrimental, but occasionally some can be beneficial. studies in the fly drosophila melanogaster suggest that if a mutation changes a protein produced by a gene, about 70 percent of these mutations are harmful with the remainder being either neutral or weakly beneficial. an evolutionary tree of eukaryotic organisms, constructed by the comparison of several orthologous gene sequences population genetics studies the distribution of genetic differences within populations and how these distributions change over time. changes in the frequency of an allele in a population are mainly influenced by natural selection, where a given allele provides a selective or reproductive advantage to the organism, as well as other factors such as mutation, genetic drift, genetic hitchhiking, artificial selection and migration. over many generations, the genomes of organisms can change significantly, resulting in evolution. in the process called adaptation, selection for beneficial mutations can cause a species to evolve into forms better able to survive in their environment. new species are formed through the process of speciation, often caused by geographical separations that prevent populations from exchanging genes with each other. by comparing the homology between different species' genomes, it is possible to calculate the evolutionary distance between them and when they may have diverged. genetic comparisons are generally considered a more accurate method of characterizing the relatedness between species than the comparison of phenotypic characteristics. the evolutionary distances between species can be used to form evolutionary trees; these trees represent the common descent