Genome Organization In Prokaryotes And Eukaryotes Ppt To Pdf
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Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce protein as the end product.
gene expression in prokaryotes ppt
Gene , unit of hereditary information that occupies a fixed position locus on a chromosome. Genes achieve their effects by directing the synthesis of proteins.
In eukaryotes such as animals , plants , and fungi , genes are contained within the cell nucleus. The mitochondria in animals and the chloroplasts in plants also contain small subsets of genes distinct from the genes found in the nucleus. In prokaryotes organisms lacking a distinct nucleus, such as bacteria , genes are contained in a single chromosome that is free-floating in the cell cytoplasm.
Many bacteria also contain plasmids —extrachromosomal genetic elements with a small number of genes. For example, whereas the human genome contains an estimated 20, to 25, genes, the genome of the bacterium Escherichia coli OH7 houses precisely 5, genes.
Arabidopsis thaliana —the first plant for which a complete genomic sequence was recovered—has roughly 25, genes; its genome is one of the smallest known to plants. Among extant independently replicating organisms, the bacterium Mycoplasma genitalium has the fewest number of genes, just A brief treatment of genes follows.
For full treatment, see heredity. Genes are composed of deoxyribonucleic acid DNA , except in some viruses , which have genes consisting of a closely related compound called ribonucleic acid RNA. A DNA molecule is composed of two chains of nucleotides that wind about each other to resemble a twisted ladder.
The sides of the ladder are made up of sugars and phosphates, and the rungs are formed by bonded pairs of nitrogenous bases. These bases are adenine A , guanine G , cytosine C , and thymine T.
An A on one chain bonds to a T on the other thus forming an A—T ladder rung ; similarly, a C on one chain bonds to a G on the other. If the bonds between the bases are broken, the two chains unwind, and free nucleotides within the cell attach themselves to the exposed bases of the now-separated chains. The free nucleotides line up along each chain according to the base-pairing rule—A bonds to T, C bonds to G. This process results in the creation of two identical DNA molecules from one original and is the method by which hereditary information is passed from one generation of cells to the next.
The sequence of bases along a strand of DNA determines the genetic code. When the product of a particular gene is needed, the portion of the DNA molecule that contains that gene will split. Through the process of transcription, a strand of RNA with bases complementary to those of the gene is created from the free nucleotides in the cell.
This single chain of RNA, called messenger RNA mRNA , then passes to the organelles called ribosomes , where the process of translation , or protein synthesis, takes place. Each set of three nucleotides codes for one amino acid. The series of amino acids built according to the sequence of nucleotides forms a polypeptide chain ; all proteins are made from one or more linked polypeptide chains.
Experiments conducted in the s indicated one gene being responsible for the assembly of one enzyme , or one polypeptide chain. This is known as the one gene—one enzyme hypothesis. However, since this discovery, it has been realized that not all genes encode an enzyme and that some enzymes are made up of several short polypeptides encoded by two or more genes.
Experiments have shown that many of the genes within the cells of organisms are inactive much or even all of the time. Thus, at any time, in both eukaryotes and prokaryotes, it seems that a gene can be switched on or off. The regulation of genes between eukaryotes and prokaryotes differs in important ways.
The process by which genes are activated and deactivated in bacteria is well characterized. Bacteria have three types of genes: structural, operator, and regulator. Structural genes code for the synthesis of specific polypeptides. Operator genes contain the code necessary to begin the process of transcribing the DNA message of one or more structural genes into mRNA. Thus, structural genes are linked to an operator gene in a functional unit called an operon.
Ultimately, the activity of the operon is controlled by a regulator gene , which produces a small protein molecule called a repressor. The repressor binds to the operator gene and prevents it from initiating the synthesis of the protein called for by the operon. The presence or absence of certain repressor molecules determines whether the operon is off or on. As mentioned, this model applies to bacteria. The genes of eukaryotes, which do not have operons, are regulated independently.
The series of events associated with gene expression in higher organisms involves multiple levels of regulation and is often influenced by the presence or absence of molecules called transcription factors. These factors influence the fundamental level of gene control, which is the rate of transcription, and may function as activators or enhancers. Specific transcription factors regulate the production of RNA from genes at certain times and in certain types of cells.
Transcription factors often bind to the promoter, or regulatory region, found in the genes of higher organisms. Following transcription, introns noncoding nucleotide sequences are excised from the primary transcript through processes known as editing and splicing.
The result of these processes is a functional strand of mRNA. For most genes this is a routine step in the production of mRNA, but in some genes there are multiple ways to splice the primary transcript, resulting in different mRNAs, which in turn result in different proteins. Some genes also are controlled at the translational and posttranslational levels. Mutations occur when the number or order of bases in a gene is disrupted.
Nucleotides can be deleted, doubled, rearranged, or replaced, each alteration having a particular effect. Mutation generally has little or no effect, but, when it does alter an organism, the change may be lethal or cause disease. A beneficial mutation will rise in frequency within a population until it becomes the norm. For more information on the influence of genetic mutations in humans and other organisms, see human genetic disease and evolution.
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Genes are made up of promoter regions and alternating regions of introns noncoding sequences and exons coding sequences. The production of a functional protein involves the transcription of the gene from DNA into RNA, the removal of introns and splicing together of exons, the translation of the spliced RNA sequences into a chain of amino acids, and the posttranslational modification of the protein molecule.
Britannica Quiz. Genes and Alleles: Fact or Fiction? Genes are an important part of who you are—but how much do you actually know about them? Find out in this quiz. Learn what defines an organism and about two candidates for the title of world's smallest organism, the bacteria Carsonella ruddii and Mycoplasma genitalium.
Get a Britannica Premium subscription and gain access to exclusive content. Subscribe Now. Model of the operon and its relation to the regulator gene. Learn More in these related Britannica articles:. As has been discussed, each individual in a sexually reproducing species inherits two alleles for each gene, one from each parent.
Furthermore, when such an individual forms sex cells, each of the resultant gametes receives one member of each allelic pair. The formation of…. But the nature of the genes themselves remained a mystery, as did the mechanism by which they exert their influence.
Molecular genetics—the study of the structure and function of genes at the molecular level—provided answers to these fundamental questions. The gene pool is the sum total of all the genes and combinations of genes that occur in a population of organisms of the same species.
It can be described by citing the frequencies of the alternative genetic…. History at your fingertips. Sign up here to see what happened On This Day , every day in your inbox! Email address. By signing up, you agree to our Privacy Notice. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox.
Gene , unit of hereditary information that occupies a fixed position locus on a chromosome. Genes achieve their effects by directing the synthesis of proteins. In eukaryotes such as animals , plants , and fungi , genes are contained within the cell nucleus. The mitochondria in animals and the chloroplasts in plants also contain small subsets of genes distinct from the genes found in the nucleus. In prokaryotes organisms lacking a distinct nucleus, such as bacteria , genes are contained in a single chromosome that is free-floating in the cell cytoplasm.
Overview: Eukaryotic gene regulation
Epigenetics Ppt. Epigenetic regulation of liver specific genes in the mouse -. Epigenetics is a hot topic in medical research, especially in the quest for therapies targeted to specific types of tumors and other genetic illnesses.
A model of prokaryotic gene … Let us make an in-depth study of the gene expression regulation. In this process, the information flows from genes to proteins. Hence, the rest of this article deals exclusively with prokaryotes. After reading this article you will learn about 1.
Although humans contain a thousand times more DNA than do bacteria, the best estimates are that humans have only about 20 times more genes than do the bacteria. This means that the vast majority of eukaryotic DNA is apparently nonfunctional.
The sequence of bases within a DNA molecule represents the genetic information of the cell. Segments of DNA molecules are called genes , and individual genes contain the instructional code necessary for synthesizing various proteins, enzymes, or stable RNA molecules. The full collection of genes that a cell contains within its genome is called its genotype. However, a cell does not express all of its genes simultaneously. Instead, it turns on expresses or turns off certain genes when necessary. Genes that are always expressed are known as constitutive genes ; some constitutive genes are known as housekeeping genes because they are necessary for the basic functions of the cell.
Plant nuclear genome sizes are large and widely varied. Maeve's Office Hours. Genetics part 1 — Sexual and asexual reproduction A human has approximately 24 genes in total. A chromosome contains hundreds to thousands of genes.
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