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No human genome has ever been completely sequenced

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Re: No human genome has ever been completely sequenced

#61
post #20

Earlier quoted context omitted.

Not only do different individuals have different genomes, each cell from the same person will have different genomes. It is more like an "average" human genome, one that no actual cell has ever contained.

"each cell from the same person will have different genomes" Did you mean each cell will have different gene expression ? I thought all the cells have the same nucleotide sequences barring maybe occasional mutations (like in cancer)

Mutation rates are high enough that we should expect at least a few every division. There are supposedly n~6e9 base pairs[1] and mutation rate is said to be p~1e-8 per bp per division[2].

Assuming each mutation is independent, etc, etc we get a back of the napkin via the binomial distribution with mean = n*p ~ 60 mutations per division.

[1] https://en.wikipedia.org/wiki/Human_genome

[2] https://en.wikipedia.org/wiki/Mutation_rate

Re: No human genome has ever been completely sequenced

#62
post #37

> * A gene called ARHGAP11B, which was created by one such duplication, causes the cortex to develop the myriad folds that support complex thought; SRGAP2C, also a duplication, triggers brain development.* A question I've never seen addressed: is one justification for junk DNA to create space for beneficial mutations? Obviously some mutations are actively harmful, but the reason most mutations are destructive is that…

>A question I've never seen addressed: is one justification for junk DNA to create space for beneficial mutations? Yes, gene/genome duplications can help create raw material for adaptation, or be adaptive by their own right. >That's not a risk with non-coding (or irrelevantly-coding) DNA It's not risk free. It's just less risky. There are all sorts of elements within the "intergenic" region of genomes which can impac…

Thanks!

And yes, I'll be dropping "junk DNA" from my vocabulary immediately. I knew some non-coding regions had been found to have uses, but this thread taught me just how extensively relevant it is!

Re: No human genome has ever been completely sequenced

#63
post #20

Earlier quoted context omitted.

Not only do different individuals have different genomes, each cell from the same person will have different genomes. It is more like an "average" human genome, one that no actual cell has ever contained.

"each cell from the same person will have different genomes" Did you mean each cell will have different gene expression ? I thought all the cells have the same nucleotide sequences barring maybe occasional mutations (like in cancer)

There are somatic mutations (vs. germ line, which would be inherited).

Somatic mutations are just errors in genome duplication in somatic cell lines - i.e. not germ line. These accumulate over time and, yes, can be oncogenic. Many others simply have no consequence at all.

For many organisms, the germ line is 'sequestered' fairly early in development, in part to limit the number of replications needed, and thus errors that accumulate.

Re: No human genome has ever been completely sequenced

#64
post #55
post #43

Earlier quoted context omitted.

You should be able to scroll it.

Thanks, you’re right. I wasn’t aware of that. The experience is rather crappy though. Cumbersome to scroll on a smartphone screen and I’d love to see to entire statement not parts of only at a time.

> dna = ''.join(random.choices('atgc', weights=[30.9, 29.4, 19.9, 19.8], k=3_234_830_000))

On a tablet, I highlight and pull down. It grabs it all and I can then read it. It is a very awful workaround that impacts the site's usability.

Re: No human genome has ever been completely sequenced

#65
A question and a story for any intrepid biologists and geneticists. I'm seeing in other comments that this is fundamentally a problem with how genome sequencing works today.

When I was a movie theatre projectionist, I had a similar problem. Movies have to be assembled onto platters, from a handful of reels which contain 10-20 minute lengths of film. There's about a mile of film per hour, and to assemble it quickly you have a motorized platter and a table with its own motor with dials to control the speed of one or the other. How it should work is you take a center ring - a circle with a gap to wind film through and metal spokes that sit in the platter - and you spin the film on. You put the reels one by one onto this, stopping with each one to cut the footer of one and the header of the next to splice them together with tape.

Well, that's how it's supposed to work. Sometimes, the film tightens up, when moving the film from one platter to another. The center ring spokes might not fit, or might create additional stress.

Well, I went full speed ahead anyway, and when a tiny bit of slack resulted in a sudden jerk on the platter, the center ring popped off, and about an hour of film flew over my hand and into the wall behind me. What happened next is almost impossible to describe. A circular disk of film and metal hitting a solid wall briefly became a vertical column as the tension-less film was forced to go any direction except forward, and the result then splayed itself out on the floor in front of me in a tangled mess a mile long.

Perhaps like an early sequencing machine, my first attempt at recovering this was to look at a piece of the film and determine what part of the movie it was from, and to begin throwing away the parts I thought were from trailers.

They weren't. It was the wrong movie.

I despaired, the film was a mile long and tangled into unbelievable knots. I had to cut it to untangle it. But how?

And it hit me, I knew where a handful of colored markers were, and I began randomly pulling segments of film out from the mess, placing masking tape across a frame, and making 3 lines in 3 different colors. I made dozens of these little loops, and that was how I put it together. The process: cutting, labeling, and placing the segments into new reels made my shift from that night run into the next day's matinee.

This process was painstaking, and that's where my question leads: can we label the ends when we cut up the DNA, like I did with my film? Are we stuck watching chromosomes splash against the wall and shatter into indecipherable pieces?

Re: No human genome has ever been completely sequenced

#66
Haven't paid attention to this area in a while but back in the day I've wondered about the garbage-in garbage-out problem in genome databases. A lot of subsequent sequence assemblies were made on the basis of approximate matching to certain results in the database that have no quality information, and conclusions made on the basis of further approximate matching across datasets. Has anyone seriously worked out how reliable some of these conclusions are? At least physicists spend a lot of time worrying about p-values.

Re: No human genome has ever been completely sequenced

#67

A question and a story for any intrepid biologists and geneticists. I'm seeing in other comments that this is fundamentally a problem with how genome sequencing works today. When I was a movie theatre projectionist, I had a similar problem. Movies have to be assembled onto platters, from a handful of reels which contain 10-20 minute lengths of film. There's about a mile of film per hour, and to assemble it quickly yo…

This is a nice analogy to genome assembly and there are some technologies that try to label DNA before sequencing to aid reconstruction. The one that comes to mind when reading your post is the 10X Genomics system, which acts as a preprocessor before Illumina sequencing. The idea is that you can introduce nucleotide tags into the DNA molecule(s) before sequencing, then use the tags to figure out what short reads came from the same DNA fragment later on. There is a nice video on 10X's website explaining how this works and how it helps:

https://www.10xgenomics.com/technology/

Re: No human genome has ever been completely sequenced

#68

As usual, the journalist spends many paragraphs painting a picture of human conflict before actually getting on to the interesting claim. > The reason for these gaps is that DNA sequencing machines don’t read genomes like humans read books, from the first word to the last. Instead, they first randomly chop up copies of the 23 pairs of chromosomes, which total some 3 billion “letters,” so the machines aren’t overwhelm…

My biologist friend isn't patient enough to ELI5, but AFAIU don't genes realign through the natural course of replication? That is, there is no specific, canonical sequence at large scales, neither between two individuals nor even two cells, even if the complete set of base pairs[1] is otherwise identical (i.e. no mutations).

I realize that the magnitude of this natural shuffling, even during meiosis, is nowhere near like what happens when sequencing DNA[2], but (assuming I'm correct) it's helpful to realize that this problem of deciphering the "correct" sequence reflects something intrinsic to the operation of the machinery; that this applies even when the relevant macro sequences are all genes that directly code for phenotypes as traditionally understood; and that gene sequencing works precisely because many (most?) genes are relatively tolerant to being spliced in at random locations in the strand. (Which is different than saying all locations--or even any two locations--produce identical results.)

[1] I refrain from saying "complete set of genes" because I imagine for some genes location at both the micro- and macro-scale is everything. AFAIU the term gene is sometimes better understood as more a description of the result of possibly non-local sequences which maintain reproductive affinity than an identification of a specifically adjoining sequence of base pairs.

[2] Or is it?

Re: No human genome has ever been completely sequenced

#69
post #68

As usual, the journalist spends many paragraphs painting a picture of human conflict before actually getting on to the interesting claim. > The reason for these gaps is that DNA sequencing machines don’t read genomes like humans read books, from the first word to the last. Instead, they first randomly chop up copies of the 23 pairs of chromosomes, which total some 3 billion “letters,” so the machines aren’t overwhelm…

My biologist friend isn't patient enough to ELI5, but AFAIU don't genes realign through the natural course of replication? That is, there is no specific, canonical sequence at large scales, neither between two individuals nor even two cells, even if the complete set of base pairs[1] is otherwise identical (i.e. no mutations). I realize that the magnitude of this natural shuffling, even during meiosis, is nowhere near…

there is no specific, canonical sequence at large scales, neither between two individuals nor even two cells

This is correct, and is a reason why genome informatics has been moving away from representing the genome as a single linear "reference" sequence of 3 billion nucleotides, and towards a graph-based model: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5411762/.

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