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I know that there are two most important directions of genetic information transfer in living organisms: DNA->DNA and DNA->RNA. The first is replication, and the second is transcription. I wonder if there is a reason for this choice of directions. According to this article, all other directions are possible. Why do we use DNA for example? RNA is capable of self-replication since it happens in viruses. And why do we use RNA, not DNA, as messenger molecules? Is it just an accident or is it possible to explain why this is the right way of doing it?
DNA is more chemically stable than RNA, which makes it ideal for long-term storage. RNA viruses like HIV have a short lifespan and must replicate to survive, which is why they can get by with a less chemically stable genome.
RNA is a useful format to transcribe since it has multiple forms and functions (e.g. rRNA, mRNA, tRNA, siRNA, snRNA, miRNA, etc.). RNA can sometimes function like a protein in which it carries out cellular actions without needing to be translated. It has been hypothesized that RNA were the first molecules as precursors to life since they can function for both storage and action. The theory is that RNA was the first molecule but was then able to be translated into proteins (which were more variable/useful) and able to be stored as DNA (which was more stable as a storage medium).
If you had a complex life form which used only DNA or RNA, it would have no way to tell transcribed mXNA from genomic gXNA. This would cause problems during cellar replication, as you could also replicate your mXNA along with your gXNA. It would also cause problems repairing breaks in your gXNA, as you would run the risk of including mXNA during the repair process.
Therefore it seems advantageous to have a storage system for information which is not currently being translated into protein (i.e., DNA), as compared to just having RNA.
On the other hand a completely DNA organism would need RNA for functional ribosomes anyway. If RNA is being used for ribosomes, using it for mRNA as well to avoid confusion with genomic DNA seems advantageous.
In order to test this hypothesis, you would need to create a completely RNA/DNA based lifeform and investigate its properties. Short of starting life from scratch as RNA based and monitoring its evolution over a few million years, a conclusive proof as to why things are the way they are as opposed to being down to an evolutionary historical accident is difficult to obtain!
E3. The RNA World
- Contributed by Henry Jakubowski
- Professor (Chemistry) at College of St. Benedict/St. John's University
Given that RNA expresses catalytic activities and can carry genetic information (some viruses have ds and ss RNA as their genome), it has been suggested that early life might have been based on RNA. DNA would evolve later as a more secure carrier of genetic information. An inspection of chemical properties of DNA, RNA, and proteins shows them to have attributes needed for their expressed function. Let's examine each for structural features that might be important for function.
a. Why does DNA lack a 2' OH group (found in RNA), which has been replaced with a hydrogen? This required the evolutionary creation of a new enzyme, ribonucleotide reductase, to catalyze the replacement of the OH in a ribonucleotide monomer to form the deoxyribonucleotide form. One possible explanation if offered in the figure below. DNA, the main carrier of genetic information, must be an extremely stable molecules. An OH present on C'2 could act as a nucleophile and attack the proximal P in the phosphodiester bond, leading to a nucleophilic substitution reaction and potential cleavage of the link. RNA, an intermediary molecule, whose concentration (at least as mRNA) should rise and fall based on the need for a potential transcript, should be more labile to such hydrolysis.
b. Why do both DNA and RNA contain a phosphodiester link between adjacent monomers instead of more "traditional" links such as carboxylic acid esters, amides, or anhydrides? One possible explanation is given below. Nucleophilic attack on the sp3 hybridized P in a phosphodiester is much more difficult than for a more open sp2 hybridized carboxylic acid derivative. In addition, the negative charge on the O in the phosphodiester link would decrease the likelihood of a nucleophilic attack. The negative charges on both strands in ds-DNA probably helps keep the strands separated allowing the traditional base pairing and double stranded helical structure observed.
c. Why is DNA found as a repetitive double-stranded helix but RNA is usually found as a single stranded molecule which can form complicated tertiary structures with some ds-RNA motifs?
Another reason for the absence of the 2' OH in DNA is that it allows the deoxyribose ring in DNA to pucker in just the right way to sterically allow extended ds-DNA helices (B type). The pucker in deoxyribose and ribose can be visualized by visualizing a single plane in the sugar ring defined by the ring atoms C1', O and C4'. If a ring atom is pointing in the same direction as the C4'-C5' bond, the ring atom is defined as endo. If it is pointing in the opposite direction, it is defined as exo (see Jmol below). In the most common form of double-stranded DNA, B-DNA, which is the iconic extended double helix you know so well, C2' is in the endo form. It can also adopt the C3' endo form, leading to the formation of another less common helix, more open ds-A helix. In contrast, steric interference prevents ribose in RNA from adopting the 2'endo conformation, and allows only the 3'endo form, precluding the occurrences of extended ds-B-RNA helices but allowing more open, A-type helices.
The figure below shows another comparison between the A-RNA and B-DNA double helices
after Zhou et al Nature Structural and Molecular Biology. doi:10.1038/nsmb.3270
d. What about the molecular dynamics of A-RNA and B-DNA?
The information above suggests that the sugar ring of DNA is conformationally more flexible than the ribose ring of RNA. This can clearly be inferred from the observation that dsDNA can adopt B and A forms, which requires a switch from the 2' endo in the B form to the 3'endo form in the A form. The smaller H on the 2'C would offer less steric interference with such flexibility. The rigidity in ribose is associated with a smaller 5'O to 3'O distance in RNA leading to a compression of the nucleotides into a helix with a smaller number of base pairs/turn.
The increased flexibility in DNA allows rotation around the C1'-N glycosidic bond connecting the deoxyribose and base in DNA, allowing different orientations of AT and GC base pairs with each other. The normal "anti" orientation allows "Watson-Crick" (WC) base pairing between AT and GC base pairs while the altered rotation allows "Hoogsteen" (Hoog) base pairs. The different orientations for an AT base pair are shown below.
Hoogsteen base pairs can be found in distorted dsDNA structures (caused by protein:DNA interactions) but also in normal B-DNA. The figures below shown Watson-Crick and Hoogsteen AT base pairs in the MATa2 homeodomain:DNA complex (pdb 1K61). Note that the dA base in the Hoogsteen base pair is rotated syn (with respect to the deoxyribose ring) instead of the usual anti, allowing the Hoogsteen base pair.
Studies (Zhou et al, 2016) show that Watson-Crick (WC) and Hoogsteen (HG) base pairs in B-DNA are in a dynamic equilibrium with the equilibrium greatly favoring the WC form. In a DNA:protein complex, the WC <----> HG equilibrium can actually favor the WG form for AT and GC + forms (in the latter, the C is protonated) when those base pairs are also involved in protein recognition. They can also occur more frequently in damaged DNA. In contrast, molecular dynamic studies show that the HG base pairs A-U and GC + are strongly disfavored in ds A-RNA.
One type of DNA damage is methylation on N1-adenosine and N1-guanosine. This modification prevents normal Watson-Crick base pairing but for DNA, these modified bases can still engage in Hoogsteen base pairing, preserving the overall structure of dsDNA and its ability to stably carry genetic information. This same methylation occur normally in post-transcriptional modified RNA. Hence, N1 adenosine and N1 guanosine methylation prevents any type of base pairing in the modified RNA. These properties make DNA a better carrier of molecular information and offers another way to regulate the structural and functional properties of RNA.
Why do scientists call DNA the "blueprint of life?"
Ever since Watson and Crick and probably even before then, scientists held an immense curiosity concerning the shape and function of nucleic acids that contain the code for life. Duke University&rsquos Hashim M. Al-Hashimi and his team of researchers have a rich history of studying the structure of DNA and RNA, and previous findings indicate DNA&rsquos flexibility as the quality making it superior.
"There is an amazing complexity built into these simple beautiful structures, whole new layers or dimensions that we have been blinded to because we didn't have the tools to see them, until now," said Al-Hashimi, lead author of the recent Duke study published in Nature Structural and Molecular Biology.
The traditional double helix of DNA, portrayed in images and depictions of the nucleic acid for generations, is what scientists believes keeps the genome stable and strong, protecting against things like cancer and aging. But can&rsquot RNA form a double helix as well? It can, but adapting to this formation makes RNA rigid, fragile, and &ldquounaccomodating&rdquo to nucleotide binding.
In the past, Al-Hashimi&rsquos research led him to discover the change in structure DNA goes through when dealing with so-called &ldquochemical insults&rdquo - being bound by a protein or receiving damage to its traditional structure. DNA responds to changes by &ldquocontorting itself into different shapes to absorb chemical damage to the [nucleotides].&rdquo Once DNA is able to shed the bound protein or repair any damage, it reverts back to the traditional, Watson and Crick-style double helix.
In his most recent study, Al-Hashimi and his team searched for changes in RNA nucleotide binding pairs as a response to similar chemical insults, expecting a reaction like that of DNA. They used a sophisticated imaging technology called NMR relaxation dispersion to observe changes in individual guanine and adenine bases, which make the infamous &ldquospiraling steps&rdquo of two model double helices: DNA and RNA.
Surprisingly, there was &ldquono detectable movement&rdquo of the base pairs in RNA, while previous studies had clocked DNA bases moving in response to protein binding or chemical damage from the traditional double helix by at least one percent. To confirm, the researchers continued testing more RNA molecules under several different conditions. Still no movement of bases.
Finally, the researchers manually altered the formation of RNA into the structure observed in DNA in response to chemical insult. What they saw next seemed to completely explain why DNA is charged with holding the genetic code. After being altered, RNA base pairs couldn&rsquot reconnect, and the RNA strands fell apart at the site of alteration.
What is the cause of this key structural difference between RNA and DNA? Scientists believe it&rsquos because RNA&rsquos double helix structure is more &ldquocompressed&rdquo than that of DNA. Scientists also define the difference as a case of &ldquoA-form&rdquo (RNA) versus &ldquoB-form&rdquo (DNA). It is this difference that scientists believe adds an extra &ldquodimension&rdquo to DNA&rsquos structure, believed to add a higher level of functionality that allows it to adequately carry genetic information.
Source: Duke University
Huiqing Zhou, Duke University
Which of the following is not found within DNA?
B. phosphodiester bonds
C. complementary base pairing
D. amino acids
If 30% of the bases within a DNA molecule are adenine, what is the percentage of thymine?
Which of the following statements about base pairing in DNA is incorrect?
A. Purines always base pairs with pyrimidines.
B. Adenine binds to guanine.
C. Base pairs are stabilized by hydrogen bonds.
D. Base pairing occurs at the interior of the double helix.
If a DNA strand contains the sequence 5ʹ-ATTCCGGATCGA-3ʹ, which of the following is the sequence of the complementary strand of DNA?
During denaturation of DNA, which of the following happens?
A. Hydrogen bonds between complementary bases break.
B. Phosphodiester bonds break within the sugar-phosphate backbone.
C. Hydrogen bonds within the sugar-phosphate backbone break.
D. Phosphodiester bonds between complementary bases break.
Phosphate Group Function
In Cellular Energy
One of the main functions of a phosphate group within cells is as an energy storage molecule. When a phosphate group is added to a molecule of adenosine, it becomes adenosine monophosphate, or AMP. This molecule is used in a number of biochemical reactions, and is heavily involved in both storing energy and as a second messenger in cellular signaling.
When you add another phosphate group, you get adenosine diphosphate (ADP). This molecule has an additional phosphate group bound to the first, and stores energy in this bond. This ADP molecule can accept another phosphate group, and become adenosine triphosphate. Commonly called ATP, this molecule can transfer the third phosphate group to a number of enzymes, activating them or imbuing energy to some process. You can see the recycling of phosphate groups between ADP and ATP in the diagram below.
Phosphate groups are one of the most important cellular components. Unfortunately for organisms, it is primarily based on a source of phosphorous atoms. This is the reason phosphorous is commonly a limiting nutrient. It is commonly a component of fertilizer for agricultural crops, which allows both the plants and the microorganisms in the soil to thrive.
A phosphate group is also a key component of life itself. A constituent of deoxyribonucleic acid is a number of individual phosphates. DNA is composed of individual units, called nucleotides. Each free nucleotide has two additional phosphate groups, which will be used in the reaction binding it to the DNA chain. The process can be seen in the following diagram.
Each nucleotide contains a nucleotide base (A,T,C, or G), a sugar (deoxyribose), and a phosphate group. The chain of DNA is formed by bonds between the phosphate group of one molecule to the sugar molecule of the next. These series of phosphodiester bonds become the sugar-phosphate backbone of the molecule. This is also true of RNA, but the sugar is different (ribose).
Another major function of a phosphate group within biological systems is as part of the cellular messenger, cyclic adenosine monophosphate. Also known as cyclic AMP, or simply cAMP, this molecule is used in a number of signal transduction pathways. Signal transduction is the process of transmitting a chemical signal through the cellular membrane. It involves a number of proteins, and often a phosphorous group or two.
Typically, a signal transduction pathway starts by a chemical arriving at an integral membrane protein. These proteins cross the cellular membrane. When the protein is activated by the chemical, it changes shape slightly, activating another enzyme inside of the cell membrane. This enzyme, adenylate cyclase, uses the energy from two phosphate groups of an ATP molecule to produce a cAMP. These signal molecule then affect a number of other proteins, channels, and enzymes, leading to an overall cellular reaction. This use of a phosphate group (or many) is seen in many cellular communication channels.
Other Uses of a Phosphate Group
A phosphate group is also a component of the lipid bilayer which creates cellular membranes. Each phospholipid molecule within the bilayer has a phosphate group at the head of the molecule. The phosphate group is hydrophilic, attracting the head of the molecule towards water. The hydrophobic tails are collected together, forming a semi-permeable membrane which separates the contents of the cell from the outside.
A free phosphate group within the cytoplasm can also act as a buffer, attaching to strong acids or bases, and decreasing their effect on environment as a whole. This helps cells maintain an regular and consistent pH, and allows cellular processes to evolve.
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DNA provides living organisms with guidelines&mdashgenetic information in chromosomal DNA&mdashthat help determine the nature of an organism's biology, how it will look and function, based on information passed down from former generations through reproduction. The slow, steady changes found in DNA over time, known as mutations, which can be destructive, neutral, or beneficial to an organism, are at the core of the theory of evolution.
Genes are found in small segments of long DNA strands humans have around 19,000 genes. The detailed instructions found in genes&mdashdetermined by how nucleobases in DNA are ordered&mdashare responsible for both the big and small differences between different living organisms and even among similar living organisms. The genetic information in DNA is what makes plants look like plants, dogs look like dogs, and humans look like humans it is also what prevents different species from producing offspring (their DNA will not match up to form new, healthy life). Genetic DNA is what causes some people to have curly, black hair and others to have straight, blond hair, and what makes identical twins look so similar. (See also Genotype vs Phenotype.)
RNA has several different functions that, though all interconnected, vary slightly depending on the type. There are three main types of RNA:
- Messenger RNA (mRNA)transcribes genetic information from the DNA found in a cell's nucleus, and then carries this information to the cell's cytoplasm and ribosome.
- Transfer RNA (tRNA) is found in a cell's cytoplasm and is closely related to mRNA as its helper. tRNA literally transfers amino acids, the core components of proteins, to the mRNA in a ribosome.
- Ribosomal RNA (rRNA) is found in a cell's cytoplasm. In the ribosome, it takes mRNA and tRNA and translates the information they provide. From this information, it "learns" whether it should create, or synthesize, a polypeptide or protein.
DNA's genes are expressed, or manifested, through the proteins that its nucleotides produce with the help of RNA. Traits (phenotypes) come from which proteins are made and which are switched on or off. The information found in DNA determines which traits are to be created, activated, or deactivated, while the various forms of RNA do the work.
One hypothesis suggests that RNA existed before DNA and that DNA was a mutation of RNA. The video below discusses this hypothesis in greater depth.
Nuclear Structure and Function Research Group
1. How is it possible to discover the functions of the 'non-coding' sequences in and around a gene? To what extent have these techniques yielded satisfactory answers?
2. Repetitive DNA sequences are a major component of mammalian genomes. Describe the different classes of such sequences, and outline what - if any - biological function they may serve.
3. Write an essay on the recognition of information in nucleic acids.
4. What forces maintain the structure of a DNA duplex?
5. Illustrate how differences between the structure of DNA and RNA are reflected in the ways that proteins interact with them.
6. Genes have been defined in many different ways over the years. Describe as many of these ways as you can. What definition is appropriate today?
7. What is DNA supercoiling? How is it generated? What are its biological roles?
8. Discuss redundancy in the genome, and the roles that it plays.
9. Estimates for gene numbers suggest that mammals have four times more genes than flies, and ten times more than yeast. Discuss.
10. What is the role of the nuclear membrane?
11. What are the three primary lineages of the living world, and how do they differ?
12. What design principles are used in the construction of large biological structures like virus particles, the cytoskeleton, and chromosomes.
1. How is the structure of the nuclear pore related to its function?
2. Discuss how proteins are imported into the nucleus.
3. Describe what we know about the synthesis and processing of ribosomal RNA.
4. Would you describe the nucleolus as a ribosome factory, when we know so little about the assembly of ribosomal RNA into a ribosome?
5. Describe the hierarchies of organization of DNA from the double helix to the chromosome. What problems does the organization pose for transcription and replication?
6. How is the structure of DNA in the isolated 'nucleoid' related to that found in vivo?
7. What is the evidence that clusters of chromatin loops are organized into 'clouds' around transcription 'factories'?
8. The cytoplasm contains a well-characterized skeleton. Discuss the evidence for and against the existence of an analogous skeleton within the nucleus.
9. Write an essay on compartmentalization in the nucleus.
10. Why has it been so difficult to determine the structure of a eukaryotic chromosome, whether in interphase or mitosis?
11. Most eukaryotic chromosomes have similar shapes, even though they may contain very different amounts of DNA. How adequately do current models for the organization of the DNA fiber within a chromosome account for its general shape?
12. Discuss current models for the structure of chromatin and chromosomes. How far do they account for the various functions of DNA?
13. What are polytene chromosomes, and how are they formed?
14. Why do mitotic chromosomes have the shape they do?
15. Discuss telomeres in terms of their discovery, location, universality, duplication, and relationship with ageing and cancer.
1. Discuss the evidence for and against the idea that active DNA polymerases are organized into factories.
2. What problems does the double-helical structure of DNA pose for the process of replication?
3. Describe the roles of the different proteins involved in replicating a DNA duplex.
4. How does the process of replication on one side of a replication fork differ from that on the other?
5. DNA polymerases make mistakes. Describe the mechanisms that ensure that parental and daughter duplexes have the same DNA sequences.
6. Describe how the origins of replication in pro- and eu-karyotes can be defined.
7. Compare and contrast the origins of replication found in simple organisms with those of mammalian cells.
8. 'There is no such thing as a specific origin of DNA replication in eukaryotes'. Discuss.
9. Discuss the role played by transcription during replication.
10. Discuss the problems associated with replicating the ends of a chromosome. How are these problems solved?
1. Describe the topological problems associated with transcribing a double-helical template. How are these problems solved?
2. Outline the molecular events that lead to the synthesis of a primary transcript by RNA polymerase II, and describe how evidence for the process was obtained.
3. Discuss the evidence for and against the idea that active RNA polymerases are organized into factories.
4. Describe the properties of the three eukaryotic RNA polymerases and their templates.
5. Comparison of the promoter sequences of a family of mammalian genes reveals that all share a sequence of eight nucleotides. Outline how you would test experimentally the possible role of this octamer sequence in regulating the expression of these genes.
6. Outline the modifications that occur to ribosomal RNA as it matures. How were these modifications discovered?
7. The initiation of transcription by eukaryotic RNA polymerases requires the assembly of a large complex. Outline the order of events that result in initiation, and indicate the type of molecular interactions that are involved.
8. RNA polymerases make mistakes. Describe the mechanisms that ensure that messages contain the correct coding information.
9. To what extent can a transcriptional activity found in vivo be reproduced in vitro?
10. Discuss the role played by the C-terminal domain of RNA polymerase II in the production of a transcript.
11. Describe how a transcript made by RNA polymerase II is modified.
12. How are primary transcripts processed and what roles do such modifications play?
13. Describe the role played by RNA:RNA interactions in the removal of introns from the primary transcript of eukaryotic genes transcribed by RNA polymerase II.
1. Describe the lesions that are commonly found in DNA. What are the consequences if they go unrepaired?
2. Discuss the advantages and disadvantages of the different approaches that have been used to detect the ways in which damaged templates are normally repaired.
3. Illustrate how the study of human disease has helped us to understand the different pathways involved in repairing damage in DNA.
4. Compare and contrast the major pathways involved in repairing damage in human DNA.
5. What are the consequences of a failure to repair damaged templates?
6. Genomes seem to contain more genes involved in repairing DNA than in replicating it. Why?
7. Outline the evidence that some repair of damage in DNA is coupled to transcription.
6: REGULATION OF GENE EXPRESSION
1. The expression of bacterial genes is controlled by the action of diffusible repressors and activators. To what extent is the expression of mammalian genes controlled similarly?
2. How true is the statement that all cells in a mammal contain the same genetic information?
3. Outline the various levels at which the expression of genes is controlled? What methods would you use to identify which control mechanisms were operating in a particular case?
4. The differentiated state is generally stable and can be inherited from one somatic cell to another. What mechanisms might account for this stability and how might you distinguish experimentally between them?
5. Describe the experimental approaches that have been used to analyze how gene expression is regulated at the level of the nucleosome (and/or) chromatin loop?
6. How do covalent modifications of histones and DNA affect gene expression?
7. How far has a detailed knowledge of the nucleotide sequence in and around genes helped to explain their tissue-specific expression?
8. Discuss the relative importance of cis- and trans-acting factors in the control of transcription.
9. Discuss the advantages and disadvantages of the various approaches being used to obtain an understanding of tissue-specific gene expression?
10. Are locus control regions any different from transcriptional enhancers?
11. Describe the experimental approaches used to define enhancers and locus control regions, and explain how the functions of the two sequences differ.
12. What are the major factors underlying the inactivity of heterochromatin?
13. 'Methylation of DNA results from, but does not cause, differentiation.' Discuss.
14. Assess the significance of DNA methylation as a mechanism for suppressing gene expression.
15. How close are we to a complete molecular definition of the inactivity of heterochromatin?
16. Outline the various mechanisms that are involved in creating (and/or maintaining) the differentiated state.
17. 'The techniques of structural biology have told us little about the regulation of gene expression that we did not already know'. Discuss.
18. What does the birth of the first parthenogenetic mouse tell us about imprinting and mammalian development?
19. Cellular protein levels can be controlled by regulating the rate of translation. Give some examples of the mechanisms involved, and discuss the experimental approaches used to confirm that control is exerted at the level of translation.
20. To what extent does the position of a gene in the genome determine gene expression? Outline the experimental approaches that have been used to answer this question.
21. An enduring idea in biology sees genomes as looped, with the ties that maintain loops as barriers between different functional domains. What is the evidence for looping, and how might those barriers work?
22. A histone 'code' is thought to regulate gene expression. Describe the experimental approaches that have been used to establish how this code might operate.
1. Discuss the role that microtubules play in chromosome segregation.
2. The spindle contains millions of moving parts. How are these movements controlled?
3. Centromeres exhibit a bewildering structural variation. What are their main functions?
4. The cell cycle is regulated by the reversible phosphorylation of proteins. Discuss.
5. How is the synthesis of DNA controlled in eukaryotes?
6. Review the evidence supporting current models for the initiation of DNA replication in eukaryotic cells.
7. Compare the checkpoints in the cell cycles of yeast and man.
8. Assess the evidence that the mechanisms for controlling passage through the cell cycle are conserved in eukaryotes.
9. Review the mechanisms that ensure orderly progression through the cell cycle.
10 .Discuss the evidence that genetic defects are responsible for malignancy.
11. Describe the advantages and disadvantages of the various approaches being used to identify genes involved in cancer.
12. Discuss the view that malignancy results from an imbalance in the activity of oncogenes and anti-oncogenes.
13. Cancer is a multi-step process. Discuss.
14. How have studies of the nematode, Caenorhabditis elegans, contributed to our understanding of apoptosis? How does the process in the worm differ from that in higher vertebrates?
15. How is the apoptotic machinery controlled?
16. 'Cancer chemotherapy owes nothing to molecular biology.' Discuss.
17. How has the study of developmental biology impinged upon our understanding of cancer?
8: MEIOSIS AND RECOMBINATION
1. Compare and contrast the processes of mitosis and meiosis.
2. Discuss the roles that the synaptonemal complex and the chiasma play during meiosis.
3. Describe the mechanisms involved in the exchange of genetic information from one chromosome to another.
4. Describe the phenomenon of gene conversion in yeast.
5. How effectively do current models account for the properties of meiotic and mitotic recombination?
6. The breaking and joining of DNA are widespread in both prokaryotes and eukaryotes. What do we know of the various mechanisms that are used in these processes?
How Does DNA Perform Its Function?
The nucleotides A, T, C, and G act as the four letters of the genetic alphabet. Everyone (except identical twins) has a unique set of DNA called their genome. This is why everyone is unique&mdasheach person has a slightly different set of instructions leading to a slightly different person. Maybe one person has a T at a certain spot in their DNA and so has red hair and the person with a G is blonde.
A cell reads the instructions in the DNA with something called an RNA polymerase. This RNA polymerase separates the two strands of the DNA helix and copies the DNA of one strand into a molecule called RNA.
RNA is very similar to DNA except that instead of thymine (T), it has uracil (U). So when RNA and DNA pair up, G pairs with C, and U pairs with A (the T of DNA still pairs with the A of RNA). While some of the instructions stop at the RNA stage, most go on to an additional step.
For this step, the letters of DNA are grouped into three-letter words, which are then recognized as full sentences, called genes. This process can be illustrated in the following example:
Letters (nucleotides): A, C, G, T . . .
A sentence (gene): &ldquoCAT ACT TAG . . . &rdquo
All of the possible four-letter combinations give a total of 64 three-letter words, commonly called the genetic code. This code is read and translated into different compounds, called RNA and proteins, which do important jobs in your body. These proteins perform jobs like carrying oxygen to your cells or making the pigment that gives your eye color.
The termination has two types, the first is Rho-independent and the second is Rho-dependent (Rho factor is a protein). Let&rsquos look at them one by one.
- Rho-independent transcription termination &ndash In this process, the polymerase reaches a termination sequence of guanines and cytosines. This is followed by a sequence of repeating adenines. Then, a loop is formed in the guanine-cytosine region, and as guanine forms three hydrogen bonds with cytosine, it takes longer for the RNA Polymerase to join these. All this puts a strain on the adenine region and causes the strand to break off, releasing the polymerase!
Rho-independent termination. (Photo Credit: Oalnafo1 / Wikimedia Commons)
- Rho-dependent transcription termination &ndash The Rho protein moves along the RNA sequence until it reaches the termination sequence. Once it reaches the termination sequence, the Rho protein destabilizes the interaction between template and RNA sequence.
Rho-dependent transcriptional termination. (Photo Credit : Oalnafo1 / Wikimedia Commons)