1. What are viruses?

2018 ◽  
pp. 2-16
Author(s):  
Dorothy H. Crawford
Keyword(s):  
Electron Microscope ◽  
Mutation Rate ◽  
Molecular Clock ◽  
Antiviral Drugs ◽  
High Mutation Rate ◽  
Virus Identification ◽  
Obligate Parasites ◽  
Protein Coat ◽  
A Cell

‘What are viruses?’ introduces viruses and their structure. Martinus Beijerinck, in 1898, was the first to coin the term ‘virus’, and invention of the electron microscope in the late 1930s greatly enhanced virus identification. Viruses are not cells, but obligate parasites that must infect a cell and use its organelles in order to reproduce. They carry either DNA or RNA, and have a protein coat called a capsid. The whole structure is called a virion. Viruses have a high mutation rate, which helps them to survive and boost their resistance to antiviral drugs. The molecular clock technique to track a virus’s history is also explained.

scholarly journals A larger target leads to faster evolution

eLife ◽  
2020 ◽  
Vol 9 ◽  
Author(s):  
Bing Yang ◽  
Scott A Rifkin
Keyword(s):  
Cell Fate ◽  
Mutation Rate ◽  
High Mutation Rate ◽  
Cell Fate Decision ◽  
Number Of Genes ◽  
A Cell ◽  
Fate Decision

The speed at which a cell fate decision in nematode worms evolves is due to the number of genes that control the decision, rather than to a high mutation rate.

scholarly journals Generation of Rodent Malaria Parasites with a High Mutation Rate by Destructing Proofreading Activity of DNA Polymerase δ

DNA Research ◽  
2014 ◽  
Vol 21 (4) ◽  
pp. 439-446 ◽  
Author(s):  
Hajime Honma ◽  
Makoto Hirai ◽  
Shota Nakamura ◽  
Hassan Hakimi ◽  
Shin-ichiro Kawazu ◽  
...  
Keyword(s):  
Dna Polymerase ◽  
Mutation Rate ◽  
High Mutation Rate ◽  
Malaria Parasites ◽  
Rodent Malaria ◽  
Dna Polymerase Δ

Electron microscopy of the replicative events of A25 bacteriophages in group A streptococci

1972 ◽  
Vol 18 (1) ◽  
pp. 93-96 ◽  
Author(s):  
S. E. Read ◽  
R. W. Reed
Keyword(s):  
Electron Microscopy ◽  
Cell Wall ◽  
Electron Microscope ◽  
Nucleic Acid ◽  
Group A Streptococcus ◽  
Group A Streptococci ◽  
Virulent Phage ◽  
Acid Pool ◽  
Group A ◽  
A Cell

The replicative events of a virulent phage (A25) infection of a group A Streptococcus (T253) were studied using the electron microscope. The first intracellular evidence of phage replication in a cell occurred 30 min after infection with arrest of cell division and increase in the nucleic acid pool. Phage heads were evident in the nucleic acid pool of the cells 45 min after infection. Release of phages occurred by splitting of the cell wall along discrete lines. This appeared to be at sites of active wall synthesis, i.e., near the region of septum formation. Many phage components were released but relatively few complete phages indicating a relatively inefficient replicative system.

An Ultrastructural and Radioautographic Study of Early Oogenesis in the Toad Xenopus Laevis

1973 ◽  
Vol 12 (1) ◽  
pp. 71-93
Author(s):  
LESLEY WATSON COGGINS
Keyword(s):  
Electron Microscope ◽  
Xenopus Laevis ◽  
Thymidine Incorporation ◽  
Ultrastructural Level ◽  
Extrachromosomal Dna ◽  
Late Pachytene ◽  
Synaptonemal Complexes ◽  
Round Nucleus ◽  
A Cell ◽  
Major Period

Early oogenesis in the toad Xenopus laevis has been investigated at the ultrastructural level, with particular reference to the formation of extrachromosomal DNA. Thymidine incorporation was localized by electron microscope radioautography. In oogonia, the nucleus is irregular in outline and may contain several nucleoli. Oocytes, from premeiotic interphase to late pachytene, are found in cell nests which are estimated to consist of about 16 cells each. Adjacent oocytes within a nest are connected by intercellular bridges and develop synchronously. Each premeiotic interphase-leptotene oocyte has a round nucleus which contains one or two centrally located, spherical nucleoli. Electron-microscope radioautography showed that all nuclei in a cell nest incorporate thymidine synchronously during premeiotic S-phase. In zygotene oocytes, axial cores and synaptonemal complexes are observed in the nucleus and abut against the inner nuclear membrane in the region nearest the centre of the cell nest. The nucleolus is still more-or-less round in outline, but is asymmetrically positioned in the nucleus. It lies near the nuclear envelope on the side of the nucleus furthest away from the attachment of the chromosome ends, that is, nearest the outside of the cell nest. Each nucleolus is surrounded by a fibrillar ‘halo’ of nucleolus-associated chromatin into which a low level of thymidine incorporation occurs during zygotene. This is thought to represent the start of the major period of amplification of the ribosomal DNA. Pachytene is characterized by the presence of synaptonemal complexes in the nucleus. The nucleolus becomes very irregular in outline. The fibrillar area around it, which represents the extrachromosomal DNA, increases in size and thymidine is incorporated over the whole of this region. In late pachytene, many small fibrogranular bodies, the multiple nucleoli, are formed in it. The members of a cell nest become separated from one another at this time and begin to develop asynchronously. In diplotene, synaptonemal complexes are no longer observed in the nucleus. The most prominent structures in the nucleus are now the multiple nucleoli, which increase greatly in number in early diplotene. A large increase in cytoplasmic volume occurs and the oocyte grows in size.

scholarly journals Rhopalocnemis phalloides has one of the most reduced and mutated plastid genomes known

PeerJ ◽  
2019 ◽  
Vol 7 ◽  
pp. e7500 ◽  
Author(s):  
Mikhail I. Schelkunov ◽  
Maxim S. Nuraliev ◽  
Maria D. Logacheva
Keyword(s):  
Plant Species ◽  
Mutation Rate ◽  
Plastid Genome ◽  
Gene Content ◽  
High Mutation Rate ◽  
Base Pairs ◽  
Plastid Genomes ◽  
The Family

Although most plant species are photosynthetic, several hundred species have lost the ability to photosynthesize and instead obtain nutrients via various types of heterotrophic feeding. Their plastid genomes markedly differ from the plastid genomes of photosynthetic plants. In this work, we describe the sequenced plastid genome of the heterotrophic plant Rhopalocnemis phalloides, which belongs to the family Balanophoraceae and feeds by parasitizing other plants. The genome is highly reduced (18,622 base pairs vs. approximately 150 kbp in autotrophic plants) and possesses an extraordinarily high AT content, 86.8%, which is inferior only to AT contents of plastid genomes of Balanophora, a genus from the same family. The gene content of this genome is quite typical of heterotrophic plants, with all of the genes related to photosynthesis having been lost. The remaining genes are notably distorted by a high mutation rate and the aforementioned AT content. The high AT content has led to sequence convergence between some of the remaining genes and their homologs from AT-rich plastid genomes of protists. Overall, the plastid genome of R. phalloides is one of the most unusual plastid genomes known.

204. Isolation of CAG repeat containing genes from human placenta and decidua

2005 ◽  
Vol 17 (9) ◽  
pp. 77
Author(s):  
K. A. Freed ◽  
S. P. Brennecke ◽  
E. K. Moses
Keyword(s):  
Mutation Rate ◽  
Dna Sequences ◽  
Human Placenta ◽  
Trinucleotide Repeat ◽  
Normal Population ◽  
Repeat Expansion ◽  
Cag Repeat ◽  
High Mutation Rate ◽  
Strong Negative Selection ◽  
Expansion Mutation

Pre-eclampsia is a serious disorder of pregnancy that manifests clinically in the mother as new-onset hypertension and proteinuria. Although the precise cause remains unknown, the placenta and the decidua play a fundamental role. The worldwide incidence of pre-eclampsia is 2–5% and such a high incidence, in the face of strong negative selection, suggests that the gene(s) involved have a selective advantage and/or a high mutation rate. One class of genetic diseases that involve a high mutation rate are the trinucleotide repeat expansion diseases. In these diseases repeated trinucleotide DNA sequences within specific genes multiply or expand up to 1000-fold. The result of this gene expansion/mutation is altered gene function that confers genetic susceptibility. Thus, the overall objective of this study was to determine whether there is an association between a trinucleotide (CAG) repeat expansion and pre-eclampsia. The specific aim of this study was to isolate CAG repeat containing genes from human placenta and decidua. An adaptation of the mRNA differential display technique and traditional cDNA library screening was used. In total, 72 placental and 51 decidual sequences were analyzed using the BLAST nucleotide comparison program. Five cDNAs were analyzed further. The unique sequences surrounding the CAG repeat regions of these five genes will be used to generate primers to ascertain if any of these repeat DNA sequences vary in number in the normal population. If polymorphic genes are identified, the primers will be used on pre-eclamptic pedigrees to determine if pre-eclampsia is associated with a repeat expansion mutation.

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