These DNA strands are often extremely long; the largest human chromosome, for example, is about million base pairs in length. While haploid organisms have only one copy of each chromosome, most animals and many plants are diploid, containing two of each chromosome and thus two copies of every gene.
Many species have so-called sex chromosomes that determine the gender of each organism. In evolution, this chromosome has lost most of its content and also most of its genes, while the X chromosome is similar to the other chromosomes and contains many genes. The X and Y chromosomes form a strongly heterogeneous pair.
When cells divide, their full genome is copied and each daughter cell inherits one copy. This process, called mitosis, is the simplest form of reproduction and is the basis for asexual reproduction.
Asexual reproduction can also occur in multicellular organisms, producing offspring that inherit their genome from a single parent. Offspring that are genetically identical to their parents are called clones. Eukaryotic organisms often use sexual reproduction to generate offspring that contain a mixture of genetic material inherited from two different parents. The process of sexual reproduction alternates between forms that contain single copies of the genome haploid and double copies diploid.
Diploid organisms form haploids by dividing, without replicating their DNA, to create daughter cells that randomly inherit one of each pair of chromosomes. Most animals and many plants are diploid for most of their lifespan, with the haploid form reduced to single cell gametes such as sperm or eggs.
Some bacteria can undergo conjugation, transferring a small circular piece of DNA to another bacterium. The diploid nature of chromosomes allows for genes on different chromosomes to assort independently or be separated from their homologous pair during sexual reproduction wherein haploid gametes are formed.
In this way new combinations of genes can occur in the offspring of a mating pair. Genes on the same chromosome would theoretically never recombine. However, they do, via the cellular process of chromosomal crossover.
During crossover, chromosomes exchange stretches of DNA, effectively shuffling the gene alleles between the chromosomes. The first cytological demonstration of crossing over was performed by Harriet Creighton and Barbara McClintock in Their research and experiments on corn provided cytological evidence for the genetic theory that linked genes on paired chromosomes do in fact exchange places from one homolog to the other.
The probability of chromosomal crossover occurring between two given points on the chromosome is related to the distance between the points. For an arbitrarily long distance, the probability of crossover is high enough that the inheritance of the genes is effectively uncorrelated. The amounts of linkage between a series of genes can be combined to form a linear linkage map that roughly describes the arrangement of the genes along the chromosome.
Genes generally express their functional effect through the production of proteins, which are complex molecules responsible for most functions in the cell. Proteins are made up of one or more polypeptide chains, each of which is composed of a sequence of amino acids, and the DNA sequence of a gene through an RNA intermediate is used to produce a specific amino acid sequence.
This process begins with the production of an RNA molecule with a sequence matching the gene's DNA sequence, a process called transcription. This messenger RNA molecule is then used to produce a corresponding amino acid sequence through a process called translation. Each group of three nucleotides in the sequence, called a codon, corresponds either to one of the twenty possible amino acids in a protein or an instruction to end the amino acid sequence; this correspondence is called the genetic code.
The specific sequence of amino acids results in a unique three-dimensional structure for that protein, and the three-dimensional structures of proteins are related to their functions. Proteins can bind to other proteins and simple molecules, sometimes acting as enzymes by facilitating chemical reactions within the bound molecules without changing the structure of the protein itself.
Protein structure is dynamic; the protein hemoglobin bends into slightly different forms as it facilitates the capture, transport, and release of oxygen molecules within mammalian blood. A single nucleotide difference within DNA can cause a change in the amino acid sequence of a protein. Because protein structures are the result of their amino acid sequences, some changes can dramatically change the properties of a protein by destabilizing the structure or changing the surface of the protein in a way that changes its interaction with other proteins and molecules.
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. In some cases, these products fold into structures which are involved in critical cell functions e. 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. 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. 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.
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.
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. 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.
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. 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.
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— million bases—due to the 'proofreading' ability of DNA polymerases. The repair does not, however, always restore the original sequence.
In organisms that use chromosomal crossover to exchange DNA and recombine genes, errors in alignment during meiosis can also cause mutations.
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.
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.
Population genetics studies the distribution of genetic differences within populations and how these distributions change over time.
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.
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 and divergence of species over time, although they do not show the transfer of genetic material between unrelated species known as horizontal gene transfer and most common in bacteria. Although geneticists originally studied inheritance in a wide range of organisms, researchers began to specialize in studying the genetics of a particular subset of organisms.
The fact that significant research already existed for a given organism would encourage new researchers to choose it for further study, and so eventually a few model organisms became the basis for most genetics research. Organisms were chosen, in part, for convenience—short generation times and easy genetic manipulation made some organisms popular genetics research tools. Widely used model organisms include the gut bacterium Escherichia coli , the plant Arabidopsis thaliana , baker's yeast Saccharomyces cerevisiae , the nematode Caenorhabditis elegans , the common fruit fly Drosophila melanogaster , and the common house mouse Mus musculus.
Medical genetics seeks to understand how genetic variation relates to human health and disease. At the population level, researchers take advantage of Mendelian randomization to look for locations in the genome that are associated with diseases, a method especially useful for multigenic traits not clearly defined by a single gene. In addition to studying genetic diseases, the increased availability of genotyping methods has led to the field of pharmacogenetics: the study of how genotype can affect drug responses.
Individuals differ in their inherited tendency to develop cancer, [90] and cancer is a genetic disease. Mutations occasionally occur within cells in the body as they divide. Although these mutations will not be inherited by any offspring, they can affect the behavior of cells, sometimes causing them to grow and divide more frequently. There are biological mechanisms that attempt to stop this process; signals are given to inappropriately dividing cells that should trigger cell death, but sometimes additional mutations occur that cause cells to ignore these messages.
An internal process of natural selection occurs within the body and eventually mutations accumulate within cells to promote their own growth, creating a cancerous tumor that grows and invades various tissues of the body.
Normally, a cell divides only in response to signals called growth factors and stops growing once in contact with surrounding cells and in response to growth-inhibitory signals.
It usually then divides a limited number of times and dies, staying within the epithelium where it is unable to migrate to other organs.
To become a cancer cell, a cell has to accumulate mutations in a number of genes three to seven. A cancer cell can divide without growth factor and ignores inhibitory signals. Also, it is immortal and can grow indefinitely, even after it makes contact with neighboring cells.
It may escape from the epithelium and ultimately from the primary tumor. Then, the escaped cell can cross the endothelium of a blood vessel and get transported by the bloodstream to colonize a new organ, forming deadly metastasis. Although there are some genetic predispositions in a small fraction of cancers, the major fraction is due to a set of new genetic mutations that originally appear and accumulate in one or a small number of cells that will divide to form the tumor and are not transmitted to the progeny somatic mutations.
The most frequent mutations are a loss of function of p53 protein, a tumor suppressor, or in the p53 pathway, and gain of function mutations in the Ras proteins, or in other oncogenes. DNA can be manipulated in the laboratory. Restriction enzymes are commonly used enzymes that cut DNA at specific sequences, producing predictable fragments of DNA. The use of ligation enzymes allows DNA fragments to be connected.
By binding 'ligating' fragments of DNA together from different sources, researchers can create recombinant DNA, the DNA often associated with genetically modified organisms. In the process known as molecular cloning, researchers can amplify the DNA fragments by inserting plasmids into bacteria and then culturing them on plates of agar to isolate clones of bacteria cells—'cloning' can also refer to the various means of creating cloned 'clonal' organisms.
DNA sequencing, one of the most fundamental technologies developed to study genetics, allows researchers to determine the sequence of nucleotides in DNA fragments. The technique of chain-termination sequencing, developed in by a team led by Frederick Sanger, is still routinely used to sequence DNA fragments. As sequencing has become less expensive, researchers have sequenced the genomes of many organisms using a process called genome assembly, which utilizes computational tools to stitch together sequences from many different fragments.
Next-generation sequencing or high-throughput sequencing came about due to the ever-increasing demand for low-cost sequencing. These sequencing technologies allow the production of potentially millions of sequences concurrently.
Genomics can also be considered a subfield of bioinformatics, which uses computational approaches to analyze large sets of biological data. It is my hope that you will find my approach helpful to you in teaching this course successfully, as have so many colleagues who have used past editions. Modern Coverage.
The field of genetics has grown rapidly in recent years. In creating this text I have worked with experts in the field to ensure that we present these exciting developments with the highest degree of accuracy. The book covers all major areas of genetics, balancing classical and molecular aspects to give students an integrated view of genetic principles. The classical genetics material tends to be abstract and more intuitive, while the molecular genetics material is more factual and conceptual.
Teaching genetics, therefore, requires teaching these two styles, as well as conveying the necessary information. The modern coverage reflects this. The molecular material, which is the material that changes most rapidly in genetics, is current and presented at a suitable level for students. Enhanced for this edition is the coverage of genomics, the analysis of the information contained within complete genomes of organisms. Experimental Approach.
Research is the foundation of our present knowledge of genetics. The presentation of experiments throughout iGenetics allows students to learn about the formulation and study of scientific questions in a way that will be of value in their study of genetics and, more generally, in all areas of science.
The amount of information that students must learn is constantly growing, making it crucial that students not simply memorize facts, but rather learn how to learn. In my classroom and in this text I emphasize basic principles, but I place them in the meaningful context of classic and modern experiments. Thus, in observing the process of science, students learn for themselves the type of critical thinking that leads to the formulation of hypotheses and experimental questions and, thence, to the generation of new knowledge.
Classic Principles.
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