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Nombre de programmes trouvés : 762
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le (5m48s)

2.6. Algorithms + data structures = programs

By writing the Lookup GeneticCode Function, we completed our translation algorithm. So we may ask the question about the algorithm, does it terminate? Andthe answer is yes, obviously. Is it pertinent, that is, doesit return the expected answer? The answer is yes, if you giveas an input a sequence of DNA, you will get as an output asequence of amino acids unless, of course, one of the tripletsis not one of the 64 expected triplets and then you will get, ofcourse, a nonsense protein sequence. Is it efficient? Well, for measuring the efficiency of an algorithm, you can ask the question, how manybasic operations you have to execute. In ...
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le (6m58s)

2.7. The algorithm design trade-off

We saw how to increase the efficiencyof our algorithm through the introduction of a data structure. Now let's see if we can do even better. We had a table of index and weexplain how the use of these small arrays allowed us to increase the efficiency that is to reduce the number of comparison to be executed when looking up a triplet in the genetic code. Now what I propose is an alternative to this data structure, it's to compute the indexes. OK. So we have this algorithm which uses here a function. You are now familiar with thisnotion of function, the idea is to fragment the complexity ofan algorithm ...
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le (8m22s)

2.8. DNA sequencing

During the last session, I explained several times how it was important to increase the efficiency of sequences processing algorithm because sequences arevery long and there are large volumes of sequences, so it's now the time to ask: but where these sequences come from? This is the process of sequencing. DNA sequencing is a physical operation through which a DNA molecule is read, that is every nucleotide along the strand of the molecule is read and then a text is producedas a succession of the nucleotides as letters. So from the DNA molecule tothe text through what is a sequencer. Sequencers are smaller and smaller and smaller and they ...
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le (5m54s)

2.3. The genetic code

Genes code for proteins. What is the correspondence betweenthe genes, DNA sequences, and the structure of proteins? The correspondence isthe genetic code. Proteins have indeedsequences of amino acids. There are 20 amino acidsin the living world. They can be named by a single letter,3 letters or their full name. It means that a protein can berepresented by a sequence of letters in a 20 letter alphabet. Let's come back again on thiscorrespondence between gene and protein. Genes are regions of DNA. These regions are first transcribedinto RNA and then RNA into proteins. And proteins’ sequences of aminoacids fold into 3D structures. Like here, some helixes. Translation is the process whichgoes from RNA to ...
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le (4m55s)

2.9. Whole genome sequencing

Sequencing is anexponential technology. The progresses in this technologyallow now to a sequence whole genome, complete genome. What does it mean? Well let'stake two examples: some twenty years ago, to sequence the bacillus subtilis bacteria genome took something like ten years,thirty five laboratories and several millions of euros. It was partly a European project,now some hundreds of dollars and it can be done within a day. The human genome project, famoushuman genome project, more than ten years, three billiondollars, 19-91 dollars OK.Tomorrow certainly less than 1000 dollars per genome, it means that we can now sequence humangenomes, not one but many many human genomes for the sake ofcomparison, diagnosis and so ...
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le (5m38s)

2.10. How to find genes?

Getting the sequence of the genome is only the beginning, as I explained, once you have the sequence what you want to do is to locate the gene, to predict the function of the gene and maybe study the interaction between genes and proteins. Let's concentrate on the prediction of genes on a genome. How can we find genes using,of course, algorithms? That's what we call genome annotation, the prediction of gene location and the prediction ofthe function of the genes, of the protein coded by the genes. A typical bacterial genome like the E. coli genome is four by five megabases and is the support of 4,500 genes. A ...
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le (5m42s)

3.1. All genes end on a stop codon

Last week we studied genes and proteins and so how genes, portions of DNA, are translated into proteins.We also saw the very fast evolutionof the sequencing technology which allows for producing large genomic texts, it is now possible to sequence a whole genome. But it is just thebeginning of the story. The challenge to come is to analysethe texts of these genomes and find genes, so this week wewill see how we can design avery first algorithm for predicting genes on a bacterial genome. We will first remember the conditionfor finding genes, we will design and propose an algorithmfor that and we will see that a part of ...
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le (5m14s)

3.2. A simple algorithm for gene prediction

Based on the principle we statedin the last session, we will now write in pseudo code a firstalgorithm for locating genes on a bacterial genome. Remember first how this algorithm should work, we first need to find two consecutive stop triplets in the same phase, same phase meansthe number of letters between these two stop triplets might bea multiple of three so that this sequence here can be divided into triplets. This is called an open reading frame. Once we have an open reading framewe look for the start triplet which is situated leftmost onthe open reading frame and we declare, we make the hypothesis that thisis a coding ...
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le (4m46s)

3.3. Searching for start and stop codons

We have written an algorithm for finding genes. But you remember that we arestill to write the two functions for finding the next stop codonand the next start codon. Let's see how we can do that. We are looking for triplets. We use the term triplets as long as wehave no proof that they are codons. You can have triplets outside genes. Within genes, they are called codons. In general, we arelooking for triplets. If you have a sequence like thisone and you are looking for occurrences of this triplet, whatyou have to do is: position your triplet at the beginning of the sequence. Compare the first letter. If it is not ...
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le (6m23s)

3.4. Predicting all the genes in a sequence

We have written an algorithm whichis able to locate potential genes on a sequence but only on one phase because we are looking triplets after triplets. Now remember that the genes maybe located on different phases and on the two strands. It means that to retrieve all the genes on a genome we have to look on six different sequences, three phases on each strand. Let's looknow how we can deal with this kind of search. First we have to modify a little bit our algorithm so that instead of starting at position One, I want to introduce a variable, a parameter which could be One or Two ...
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