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Pre-print https://www.biorxiv.org/content/10.1101/2020.10.01.322164v4 

Address of the bookmark: https://metagraph.ethz.ch/

Bdelloid rotifers are small animals well known for their ability to survive in harsh conditions. They can endure extreme dehydration by entering a state of dormancy and then revive when water becomes available again. Scientists have always been interested in the degree to which these animals resist radiation, since radiation can severely damage DNA by causing both of its strands to break. Interestingly, Adineta vaga's ability to survive radiation may be connected to the way in which it tolerates drying out, as dehydration can also damage DNA. The rotifer probably evolved strong DNA protection and repair mechanisms primarily as a response to its severe environment. Its resistance to radiation is merely a byproduct of adapting to the cycles of drying and rehydration.

In order to find out more about this ability, the researchers subjected A. vaga to proton radiation and then examined what effect it had on its chromosomes. The radiation caused serious damage, resulting in hundreds of breaks in the DNA. For the vast majority of organisms, such extensive chromosome damage would be disastrous; it's similar to going through an instruction manual in a shredder and ending up with a large number of separate fragments. The scientists believed that the rotifers would repair their chromosomes before producing offspring. However, what they discovered was unexpected: some of the broken pieces of chromosome did survive and were passed on to the next generations, where the repair process continued.

The discovery took on even greater interest when the scientists examined the offspring of the irradiated rotifers. Rather than observing the same DNA damage in all the generations, they found different sized sections missing and evidence that some of the genetic regions which had been lost were being slowly restored. The researchers proposed that the rotifers use a repair mechanism known as Break-Induced Homologous Extension Repair, or BIHER. In simple terms, a broken piece of DNA can locate a matching sequence on the other chromosome and use it as a guide to rebuild itself. If the damage is beyond what can be repaired in one generation, the repair process can continue in the next.

What makes the rotifer's method of repairing DNA so unusual is that, normally, we consider DNA repair to take place immediately after damage has occurred, with the cell rapidly repairing the damaged DNA. In A. vaga, however, the repair process can be a lot longer. Certain pieces of chromosome survive the initial damage, are inherited, and then serve as the basis for further repairs in later generations. It's as if one generation begins mending a broken book, the next one continues the work, and another eventually completes the job until all the missing parts have been restored.

One reason why this might work is that bdelloid rotifers have holocentric chromosomes. Rather than having a single point, as most chromosomes do, which controls movement during cell division, the ability to move is distributed along the entire length of their chromosomes. As a result, fragments of chromosomes are sometimes able to retain the parts that are necessary for them to move and divide, and this may account for how broken pieces of chromosome can last long enough to be passed on and then repaired.

This finding could have implications that go beyond those concerning rotifers. Scientists are interested in understanding how genomes cope with severe chromosome damage, for example in the case of chromothripsis, where chromosomes break and are then reassembled in new configurations. Investigating A. vaga could help us to learn how cells manage large genetic issues. Moreover, its high resistance to radiation might also prove useful in research into extreme environments, such as the radiation hazards encountered on long space missions. However, these are ideas for future investigation; at present the rotifer's DNA repair mechanism cannot be used to protect people from radiation.

What is perhaps the most interesting aspect of this discovery is that the rotifer's remarkable ability could have developed for a very simple reason. Since an animal living in moss constantly undergoes periods of drying out and then getting wet again, the same mechanisms that protect its DNA when it is dehydrated might also enable it to survive radiation which would damage most other animals. Therefore, the rotifer probably did not evolve solely in order to resist radiation; instead, it became extremely good at coping with the difficult conditions it encounters, and radiation resistance was just a byproduct.

The case of Adineta vaga demonstrates that survival does not always mean avoiding damage; on some occasions it involves being able to recover from damage which appears to be impossible to repair. This small animal can survive having its chromosomes broken into many pieces, retain some of those pieces, pass them on to its offspring, and continue to repair them over successive generations. Although we generally regard DNA as something fragile that must always remain intact, the rotifer indicates an alternative possibility: perhaps a genome does not have to be perfect in order to survive, since sometimes the pieces last long enough for life to reassemble them, bit by bit, generation after generation.

Ream more at https://www.science.org/doi/10.1126/sciadv.aee0891

In the mid 1960's scientists discovered that many genomes contain stretches of highly repetitive DNA sequences ( see Reassociation Kinetics Experiments, and C-Value Paradox ). These sequences were later characterized and placed into five categories:

Simple Repeats - Duplications of simple sets of DNA bases (typically 1-5bp) such as A, CA, CGG etc.
Tandem Repeats - Typically found at the centromeres and telomeres of chromosomes these are duplications of more complex 100-200 base sequences.
Segmental Duplications - Large blocks of 10-300 kilobases which are that have been copied to another region of the genome.
Interspersed Repeats
Processed Pseudogenes, Retrotranscripts, SINES - Non-functional copies of RNA genes which have been reintegrated into the genome with the assitance of a reverse transcriptase.
DNA Transposons
Retrovirus Retrotransposons
Non-Retrovirus Retrotransposons ( LINES )

Currently up to 50% of the human genome is repetitive in nature and as improvements are made in detection methods this number is expected to increase.

On the other hand; In genetics, the term palindrome refers to a sequence of nucleotides along a DNA (deoxyribonucleic acid) or RNA (ribonucleic acid) strand that contains the same series of nitrogenous bases regardless from which direction the strand is analyzed. Akin to a language palindrome—wherein a word or phrase is spelled the same left-to-right as right-to-left (e.g., the word RADAR or the phrase "able was I ere I saw elba")—with genetic palindromes it does not matter whether the nucleic acid strand is read starting from the 3' (three prime) end or the 5' (five prime) end of the strand.

Recent research on palindromes centers on understanding palindrome formation during gene amplification. Other studies have attempted to relate palindrome formation to molecular mechanisms involved in double stranded breaks and in the formation of inverted repeats. Assisted by high speed computers, other groups of scientists link palindrome formation to the conservation of genetic information.

Related to the direction of transcription by RNA polymerase, DNA strands have upstream and downstream terminus defined by differing chemical groups at each end. The ends of each strand of DNA or RNA are termed the 5' (phosphate bound to the 5' position carbon) and 3' (phosphate bound to the 3' carbon) ends to indicate a polarity within the molecule. Using the letters A, T, C, G, to represent the nitrogenous bases adenine, thymine, cytosine, and guanine found in DNA, and the letters A, U, C, G to represent the nitrogenous bases adenine, uracil, cytosine, guanine found in RNA (Note that uracil in RNA replaces the thymine found in DNA), geneticists usually represent DNA by a series of base codes (e.g., 5' AATCGGATTGCA 3'). The base codes are usually arranged from the 5' end to the 3' end.

Because of specific base pairing in DNA (i.e., adenine (A) always bonds with (thymine (T) and cytosine (C) always bonds with guanine (G)) the complimentary stand to the sequence 5' AATCGGATTGCA 3' would be 3' TTAGCCTAACGT 5'.

With palindromes the sequences on the complimentary strands read the same in either direction. For example, a sequence of 5' GAATTC3' on one strand would be complimented by a 3' CTTAAG 5' strand. In either case, when either strand is read from the 5' prime end the sequence is GAATTC. Another example of a palindrome would be the sequence 5' CGAAGC 3' that, when reversed, still reads CGAAGC.

Palindromes are important sequences within nucleic acids. Often they are the site of binding for specific enzymes (e.g., restriction endobucleases) designed to cut the DNA strands at specific locations (i.e., at palindromes).

Palindromes may arise from brakeage and chromosomal inversions that form inverted repeats that compliment each other. When a palindrome results from an inversion, it is often referred to as an inverted repeat. For example, the sequence 5' CGAAGC 3', if inverted (reversed 180°), still reads CGAAGC.

The European Molecular Biology Open Software Suite (EMBOSS) includes some basic tools for finding tandem repeats and inverted repeats (see B.6.22. Applications in group Nucleic:repeats). There are many on-line services providing the EMBOSS tools, for example:

• Wageningen Bioinformatics Webportal EMBOSS explorer
• Mobyle@Pasteur
• Soaplab2 Web Services at Vital-IT

For more sophisticated repeat finding you will want to look at tools using Repbase for example:

• CENSOR
  • CENSOR@GIRI
  • CENSOR@EMBL-EBI
• RepeatMasker
• MUMmer (scan_for_match)
• Emboss Palindrome

Other nucleotide repeat finding methods found by a couple of web searches:

• Tandem Repeats Finder
• RECON
• RepeatRunner
• REPuter
• Imperfect Microsatellite Extractor (IMEx)
• Spectral Repeat Finder (SRF)
• REPFIND
• CRISPRfinder
• GrailEXP
• CONREPP
• find_palindromes
• Palindrome
• EMBOSS Palindrome
• Palindrome Search

Address of the bookmark: https://gvolante.riken.jp/

• Step 1. Run mugsy to get multiple sequence alignment results.
• Step 2 & 3. Extraction of the Coordinates of Core Blocks, Construction of Synteny Blocks and Generating Signed Permutations.
• Step 4. Generate pairwise genome rearrangement scenarios and find repeats at the breakpoints of each rearrangement events.

https://github.com/DanwangJessica/GRSR

Address of the bookmark: https://github.com/DanwangJessica/GRSR

The Program first constructs all exact match pairs by a suffix-array based algorithm and extends them to long highly similar pairs. Then Smith-Waterman algorithm is used to adjust the ends of LTR pair candidates to get alignment boundaries. These boundaries are subject to re-adjustment using supporting information of TG..CA box and TSRs and reliable LTRs are selected. Next, LTR_FINDER tries to identify PBS, PPT and RT inside LTR pairs by build-in aligning and counting modules. RT identification includes a dynamic programming to process frame shift. For other protein domains, LTR_FINDER calls ps_scan (from PROSITE, http://www.expasy.org/prosite/) to locate cores of important enzymes if they occur.

Address of the bookmark: https://github.com/xzhub/LTR_Finder

Align sequences and output the alignment in MSF format:

kalign -i BB11001.tfa -f msf  -o out.msf

Align sequences and output the alignment in clustal format:

kalign -i BB11001.tfa -f clu -o out.clu

Re-align sequences in an existing alignment:

kalign -i BB11001.msf  -o out.afa

Reformat existing alignment:

kalign -i BB11001.msf -r afa -o out.afa

Address of the bookmark: https://github.com/TimoLassmann/kalign

http://www.bioinf.jku.at/software/kebabs/

http://bioconductor.org/packages/release/bioc/vignettes/kebabs/inst/doc/kebabs.pdf

Address of the bookmark: http://www.bioinf.jku.at/software/kebabs/