Genetic fine structure

About genetic fine structure

1. Genetic Fine Structure

2. Contents: Introduction History of genes Physical Structure Genes DNA genes RNA genes Functional structure of a gene The Structure Analysis of Eukaryotes Introns Classification of Introns The Structure analysis gene in Prokaryotes Conclusion

3. Introduction A gene is a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions and/or other functional sequence regions. The physical development and phenotypic of organisms can be thought of as a product of genes interacting with each other and with the environment. The structure of the gene analyzed at the level of the smallest units of recombination and mutation (nucleotides ) is known as genetic fine structure. Figure 1: This stylistic diagram shows a gene in relation to the double helix structure of DNA and to a Chromosome (right).

4. In cells, genes consist of a long strand of DNA that contains a promoter, which controls the activity of a gene, and coding and non-coding sequence. Coding sequence determines what the gene produces, while non-coding sequence can regulate the conditions of gene expression. When a gene is active, the coding and non-coding sequence is copied in a process called transcription, producing an RNA copy of the gene's information. This RNA can then direct the synthesis of proteins via the genetic code.

5. The genes of eukaryotic organisms can contain regions called introns that are removed from the messenger RNA in a process called splicing. In prokaryotes( bacteria and archaea , introns are less common and genes often contain a single uninterrupted stretch of DNA, called a cistron , that codes for a product. Prokaryotic genes are often arranged in groups called operons with promoter and operator sequences that regulate transcription of a single long RNA.

6. History of genes The existence of genes was first suggested by Gregor Mendel (1822-1884), who, in the 1860s, studied inheritance in pea plants. Wilhelm Johannsen abbreviated this term to "gene" ("gen" in Danish and German) two decades later. Morgan's introduction of the fruit fly to genetics revolutionized it because the fly's rapid life cycle and minute size enabled the scale of experimentation to be markedly increased. By 1957 Seymour Benzer had used this system to make an estimate of the likelihood of crossing-over between two mutants one DNA base apart in the bacteriophage T4 to be 1 in 10,000.

7. DNA genes The vast majority of living organisms encode their genes in long strands of DNA. DNA consists of a chain made from four types of nucleotide subunits: adenine, cytosine, guanine, and thymine (Figure 2 ). Each nucleotide subunit consists of three components: a phosphate group, a deoxyribose sugar ring, and a nucleobase. The most common form of DNA in a cell is in a double helix structure, in which two individual DNA strands twist around each other in a right-handed spiral. Physical Structure Genes Figure 2: The chemical structure of a four-base fragment of a DNA double helix.

8. RNA genes RNA is an intermediate product in the process of manufacturing proteins from genes . However , for other gene sequences, the RNA molecules are the actual functional products. For example, RNAs known as ribozymes are capable of enzymatic function, and miRNAs have a regulatory role. The DNA sequences from which such RNAs are transcribed are known as RNA genes.  

9. Functional structure of a gene All genes have regulatory regions in addition to regions that explicitly code for a protein or RNA product . A regulatory region shared by almost all genes is known as the promoter (Figure 3 ). which provides a position that is recognized by the transcription machinery when a gene is about to be transcribed and expressed . Although promoter regions have a consensus sequence that is the most common sequence at this position, some genes have "strong" promoters that bind the transcription machinery well, and others have "weak" promoters that bind poorly. Figure 3: Diagram of the "typical" eukaryotic protein-coding gene. Promoters and enhancers determine what portions of the DNA will be transcribed into the precursor mRNA (pre-mRNA). The pre-mRNA is then spliced into messenger RNA (mRNA) which is later translated into protein .

10. The Structure Analysis of Eukaryotes Introns Introns are common in eukaryotic pre-mRNA, but in prokaryotes they are only found in tRNA and rRNA. Introns , which are non-coding sections of a gene that are removed, are the opposite of exons which remain in the mRNA sequence after processing. The number and length of introns varies widely among species, and among genes within the same species.

11. Classification of Introns Some introns, such as Group I and Group II introns, are actually ribozymes that are capable of catalyzing their own splicing out of a primary RNA transcript. Four classes of introns are known to exist: Group I intron Group II intron Group III intron Nuclear introns Sometimes group III introns are also identified as group II introns, because of their similarity in structure and function . Nuclear or spliceosomal introns are spliced by the spliceosome and a series of snRNAs (small nuclear RNAs)

12. The Structure analysis gene in Prokaryotes Molecular classification subdivides prokaryotes into two domains: bacteria and archaea that, with eukaryotes, form the three main branches of the tree of life . Archaea and bacteria are generally similar in size and shape, although a few archaea actually show unusual patterns (such as the flat and square  shaped cells of Haloquadratum walsbyi ). Despite this visual similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes, notably the enzymes involved in transcription and translation processes. Being prokaryotes the simplest forms of life, they represent an excellent case  study to determine the molecular basis of such responses.

13. Figure 4: Structure of prokaryotic genes Operator: a DNA segment to which a transcription factor binds to regulate the gene expression

14. Actually , in a prokaryotic perspective, appropriate reactions to external stimuli invariably involve alterations in the gene expression levels. A great deal of information contained in the prokaryotic genome is dedicated to maintaining the basic infrastructure of the cell, such as its ability in: - building and replicating the DNA (no more than 32 genes) - synthesizing proteins (between 100 and 150 genes) - obtaining and storing energy (at least 30 genes) The prokaryotic genomes have a very high gene density: on average, the protein  coding genes occupy 85% of the genome . In addition, the prokaryotic genes are not interrupted by introns and are sometimes organized in transcriptional polycistronic units (leading information related to several genes), called operons.

15. Conclusion Fine structure genetics encompasses a set of tools used to examine not just the mutations within an entire genome, but can be isolated to either specific pathways or regions of the genome . Ultimately, this more focused lens can lead to a more nuanced and interactive view of the function of a gene.

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