Eukaryotic and prokaryotic gene structure and How does the organization of genetic material differ in prokaryotic and eukaryotic organisms? Gene Regulation EVOLUTION OF GENE REGULATION

Abstract 
 
Genes consist of multiple sequence elements that together encode the functional product and regulate its expression. Despite their fundamental importance, there are few freely available diagrams of gene structure. Presented here are two figures that summarise the different structures found in eukaryotic and prokaryotic genes. Common gene structural elements are colour-coded by their function in regulation, transcription, or translation.

Introduction

 Gene structure

 Genes contain the information necessary for living cells to survive and reproduce
[2] In most organisms, genes are made of DNA, where the particular DNA sequence determines the function of the gene.
A gene is transcribed (copied) from DNA into RNA, which can either be non-coding (ncRNA) with a direct function, or an intermediate messenger (mRNA) that is then translated into protein.

genetic material differ in prokaryotic and eukaryotic organisms?


The DNA of prokaryotes is much more compact because it contains much less non-coding DNA in and between the genes compared to eukaryotes. In prokaryotes genes can be transcribed together into one mRNA, these groups of genes are called operons. In eukaryotes most of the DNA does not code for a protein.

Prokaryotes have one circular strand of DNA whereas eukaryotes have several strands of linear DNA.

Explanation:

Prokaryotes

 Prokaryotes are single celled organisms without membrane enclosed organelles (specialized compartments/structures in the cell). Therefore the DNA resides in the cytoplasm. Prokaryotes have double stranded DNA molecules clustered into a so called nucleoid. Next to this chromosomal DNA, prokaryotes often also have small circular pieces of DNA with only a small amount of genes, these are called plasmids and can replicate independent of the chromosomal DNA.

Eukaryotes

              Eukaryotes have a specialized membrane enclosed organelle that contains the DNA, this is called the nucleus. Each nucleus contains multiple linear molecules of double stranded DNA, organized into 23 pairs of chromosomes

Prokaryotes

 The DNA of prokaryotes is much more compact because it contains much less non-coding DNA in and between the genes compared to eukaryotes. In prokaryotes genes can be transcribed together into one mRNA, these groups of genes are called operons.

Eukaryotes 

in eukaryotes most of the DNA does not code for a protein. It was once termed 'junk DNA' but we know now that it has some important regulatory functions. In eukaryotes there are no operons, each gene is transcribed separately into its own mRNA.

In both

In both eu- and prokaryotes the DNA molecules are condensed with the aid of different proteins. In eukaryotes the DNA is wrapped around proteins called histones. In prokaryotes the HU-protein fulfills this task

2   How is gene expression different in eukaryotes and prokaryotes?

Prokaryotic

 transcription and translation occur simultaneously in the cytoplasm, and regulation occurs at the transcriptional level. Eukaryotic gene expression is regulated during transcription and RNA processing, which take place in the nucleus, and during protein translation, which takes place in the cytoplasm.

Prokaryotic and Eukaryotic Gene Regulation

To understand how gene expression is regulated, we must first understand how a gene codes for a functional protein in a cell. The process occurs in both prokaryotic and eukaryotic cells, just in slightly different manners.

Prokaryotic organisms are single-celled organisms that lack a cell nucleus, and their DNA therefore floats freely in the cell cytoplasm. To synthesize a protein, the processes of transcription and translation occur almost simultaneously. When the resulting protein is no longer needed, transcription stops. As a result, the primary method to control what type of protein and how much of each protein is expressed in a prokaryotic cell is the regulation of DNA transcription. All of the subsequent steps occur automatically. When more protein is required, more transcription occurs. Therefore, in prokaryotic cells, the control of gene expression is mostly at the transcriptional level.

Eukaryotic cells, in contrast, have intracellular organelles that add to their complexity. In eukaryotic cells, the DNA is contained inside the cell’s nucleus and there it is transcribed into RNA. The newly synthesized RNA is then transported out of the nucleus into the cytoplasm, where ribosomes translate the RNA into protein. The processes of transcription and translation are physically separated by the nuclear membrane; transcription occurs only within the nucleus, and translation occurs only outside the nucleus in the cytoplasm. The regulation of gene expression can occur at all stages of the process (Figure 1). Regulation may occur when the DNA is uncoiled and loosened from nucleosomes to bind transcription factors (epigenetic level), when the RNA is transcribed (transcriptional level), when the RNA is processed and exported to the cytoplasm after it is transcribed (post-transcriptional level), when the RNA is translated into protein (translational level), or after the protein has been made (post-translational level).

Figure 1. Prokaryotic transcription and translation occur simultaneously in the cytoplasm, and regulation occurs at the transcriptional level. Eukaryotic gene expression is regulated during transcription and RNA processing, which take place in the nucleus, and during protein translation, which takes place in the cytoplasm. Further regulation may occur through post-translational modifications of proteins.

The differences in the regulation of gene expression between prokaryotes and eukaryotes are summarized in Table 1. The regulation of gene expression is discussed in detail in subsequent modules.

Table 1. Differences in the Regulation of Gene Expression of Prokaryotic and Eukaryotic Organisms
Prokaryotic organisms	Eukaryotic organisms
Lack nucleus	Contain nucleus
DNA is found in the cytoplasm	DNA is confined to the nuclear compartment
RNA transcription and protein formation occur almost simultaneously	RNA transcription occurs prior to protein formation, and it takes place in the nucleus. Translation of RNA to protein occurs in the cytoplasm.
Gene expression is regulated primarily at the transcriptional level	Gene expression is regulated at many levels (epigenetic, transcriptional, nuclear shuttling, post-transcriptional, translational, and post-translational) Gene Expression in Prokaryotes  Prokaryotes, by contrast, possess a very simple chromosomal arrangement. Most of the DNA within prokaryotic organisms is housed within a single, circular chromosome. In some instances, a secondary chromosome, known as a plasmid, may exist. Prokaryotes act as if by default all the genes are on. The DNA is interpreted word by word and it tells the ribosomes to make everything it says. So to control genes the cell selectively turns off parts it doesn't need at certain times  An operon is a gene regulatory feature unique to prokaryotes. It consists of a group of related genes that must be transcribed in sequence. A "promoter" region lay at the front of the operon, which, when a transcription enzyme binds to it, bends the DNA in a way that makes it easier for transcription factors to access the genes within the operon. Although there are many kinds of operons in prokaryotes, the lac operon is the best understood and most widely used to teach bacterial gene regulation. Previously, the presence of a secondary "plasmid" chromosome in many prokaryotes, which is much smaller than the primary circular chromosome, was discussed. In terms of evolution, the plasmid serves a very important purpose for the prokaryotic genome. Due to their simplicity, prokaryotes cannot engage in sexual reproduction. You might imagine the inherent genetic dilemma in a population of organisms that are only capable of dividing asexually, through binary fission, as bacteria do. Because every individual within a population of bacterial cells is so closely related genetically EVOLUTION OF GENE REGULATION Prokaryotic cells can only regulate gene expression by controlling the amount of transcription. As eukaryotic cells evolved, the complexity of the control of gene expression increased. For example, with the evolution of eukaryotic cells came compartmentalization of important cellular components and cellular processes. A nuclear region that contains the DNA was formed. Transcription and translation were physically separated into two different cellular compartments. It therefore became possible to control gene expression by regulating transcription in the nucleus, and also by controlling the RNA levels and protein translation present outside the nucleus. Some cellular processes arose from the need of the organism to defend itself. Cellular processes such as gene silencing developed to protect the cell from viral or parasitic infections. If the cell could quickly shut off gene expression for a short period of time, it would be able to survive an infection when other organisms could not. Therefore, the organism evolved a new process that helped it survive, and it was able to pass this new development to offspring.