question archive Structure of DNA and RNA
Structure of DNA and RNA
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Structure of DNA and RNA . Plasmids structure and function . Methods of identifying mutants and mutagens . How DNA is repaired . Transformation, transduction, conjugation . Distinguish between inducible and repressible operons . Structure of operons . Table 7.2 . Okazaki fragments . Virus, Viroids, Prions
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- DNA and RNA are different from their structure, functions, and stabilities. DNA has four nitrogen bases adenine, thymine, cytosine, and guanine and for RNA instead of thymine, it has uracil. Also, DNA is double-stranded and RNA is single-stranded which is why RNA can leave the nucleus and DNA can't. Another thing is that DNA is missing an oxygen.
- A plasmid is a small, circular, double-stranded DNA molecule that is distinct from a cell's chromosomal DNA. Plasmids naturally exist in bacterial cells, and they also occur in some eukaryotes. Often, the genes carried in plasmids provide bacteria with genetic advantages, such as antibiotic resistance. Plasmids have many different functions. They may contain genes that enhance the survival of an organism, either by killing other organisms or by defending the host cell by producing toxins. Some plasmids facilitate the process of replication in bacteria.
- With the development of new technologies for more accurate understanding of the genome and potential gene therapies, the detection of mutations has an increasingly central role in various areas of genetic diagnosis including preimplantation genetic diagnosis (PGD), prenatal diagnosis (PND), presymptomatic testing, confirmational diagnosis and forensic/identity testing. Two groups of tests, molecular and cytogenetic, are used in genetic syndromes. In general, single base pair mutations are identified by direct sequencing, DNA hybridization and/or restriction enzyme digestion methods. However, there are two approaches for genetic diagnosis; indirect approach depends on the results from a genetic linkage analysis using DNA markers such as STR(short tandem repeat) or VNTR (variable number tandem repeat) markers flanking or within the gene. The direct approach for diagnosis essentially depends on the detection of the genetic variations responsible for the disease. Ames test is one of the best examples for testing mutagenicity, while some new methods are still at the stage of validation. Mutagenicity methods have different levels of sensitivity and specificity, which contributes to false positive and false negative results.
- Most damage to DNA is repaired by removal of the damaged bases followed by resynthesis of the excised region. Some lesions in DNA, however, can be repaired by direct reversal of the damage, which may be a more efficient way of dealing with specific types of DNA damage that occur frequently.
- Conjugation- Conjugation is the transfer of DNA directly from one cell to another through cell-cell contact. The DNA transferred by conjugation often involve plasmids. Plasmids are circular pieces of DNA that can replicate in the bacterial cell, independently of the chromosome. The conjugative transfer of plasmids is carried out by cell surface structures that act like syringes, injecting the plasmid into neighbouring cells. Transformation- Unlike humans, bacteria are capable of taking up DNA directly from their enviroment and incorporating it into their genomes.This process is known as natural transformation. This DNA usually comes from dead bacteria lysing (splitting open) and releasing their genetic contents into the surrounding area. Transduction- Transduction is the transfer of DNA from one cell to another by a virus. These viruses are known as bacteriophage and they specifically infect bacteria. Bacteriophage don't have the machinery to replicate their own genomes or express their own genes, so instead, they hijjack the bacterial machinery to do so. Host cells will continue to express phage proteins and replicate the phage genome forming new virus particles. This process continues until the cell is so full of phage particles that it splits open (lyses), releasing phage into the surrounding area. This is known as the lytic cycle. Some phage can switch between this life cycle and a state of lysogeny, where they combine their genome with the bacterial chromosome, and remain silent for many generations. When lysogenic phage remove (excise) their genomes from the host chromosome, they occasionally take small sequences of bacterial DNA with them. Phage genome containing bacterial DNA is then packaged into phage coat proteins to form a complete, recombinant virus particle. When these phage lyse the bacterial cell and re-infect a new host, they take bacterial DNA with them.
- operons are inducible, meaning that they can be turned on by the presence of a particular small molecule. Others are repressible, meaning that they are on by default but can be turned off by a small molecule
- Operons are regions of DNA that contain clusters of related genes. They are made up of a promoter region, an operator, and multiple related genes. The operator can be located either within the promoter or between the promoter and the genes. RNA polymerase initiates transcription by binding to the promoter region. The location of the operator is important as its regulation either allows or prevents transcription of the genes into mRNA.
- Okazaki fragments are short, newly synthesized DNA fragments that are formed on the lagging template strand during DNA replication. They are complementary to the lagging template strand, together forming short double-stranded DNA sections.
- A prion is a misfolded rogue form of a normal protein (PrPc) found in the cell. This rogue prion protein (PrPsc), which may be caused by a genetic mutation or occur spontaneously, can be infectious, stimulating other endogenous normal proteins to become misfolded, forming plaques. Today, prions are known to cause various forms of transmissible spongiform encephalopathy (TSE) in human and animals. Viroids consist only of a short strand of circular RNA capable of self-replication Unlike viruses, viroids do not have a protein coat to protect their genetic information. Virus is a small entity consist of either DNA or RNA as genetic material. Genetic material is enclosed by protein coat called capsid. Virus may or may not surrounded by outer lipid membrane called envelope. Virus infect all types of organisms.
