pz

What do you mean, that you can't "format" my reply?

You replied to the comment that "every cell with a nucleus has a complete set of genes" with "Gametes however are haploid". That's a non sequitur. Haploid cells still have a complete set of genes, they simply have a single copy of each instead of two.

Does that help?

zwi

pz

I tried hard to work the quote function but my clipboard kept rejecting your comments

Perhaps a very bright clipboard

Precisely not so Youre half right

They have a half a complete set

In mammals a complete set is two pair Thats a DNA full house

It is subtle but not cruel Think it over

Let me try it this way. The DNA in one of your spermatozoa comes from both of your parents That solitary sperm cell has DNA that is distinct from all the other cells of your body

If you are female, just consider one of your ova

But the DNA of the gamete is definitely not indentical with the rest of the body

The DNA of one of your liver cells is identical to the DNA of one of your brain cells

If you cant work it out let me know

Zwi

Doubting Didymus

Uh-oh.

pz

You are quibbling over the meaning of "complete set", and are using it in a nonstandard way. Diploid cells have two sets of genes; that's implicit in the name. A gamete has all of the genes that a somatic cell has, but it has only one copy of each rather than two.

You are exhibiting a common metazoan bias, that gametes are in some way an incomplete cell. You might want to think more about plants which have alternation of generations and a haploid-diploid life cycle. The haploid form can be just as complex and highly developed as the diploid form; it possesses a complete set of genes.

KC

Male honeybees are haploid. Are they incomplete?

KC

Non-praying Mantis

Actually, I don't think that this is true. Here is my reasoning:

1) When DNA replicates, errors are made. Most are corrected, but some errors will remain. I have seen some estimates of the numbers of these errors, but they remain around 1 error per 1 gigabase (1 billion (American billion) bases) of DNA. That's just for point mutations.

2) The human genome has approximately 3 gigabases of DNA, so on average, there will be about 3 errors per cell replication.

3) Most of these errors will crop up in the "junk" DNA of the cell, which is about 95% to 99% of the DNA, but occasionally one will crop up in coding DNA. Most of those probably occur in DNA that is packed away and not expressed in the active cell.

4) As an organism develops, cellular divisions are used to make new cells. I'm not sure of the number of cell divisions between zygote and finished infant, but it is probably on the order of 100 cell divisions (educated guess). Also, as the organism grows, more cell divisions are made.

5) Liver cells divide much faster than brain cells. Brain cells do not divide when they are fully developed (except for stem cells). Liver cells probably divide about every 2 days or so (the fastest cell divisions in the body occur in skin and digestive tract cells, which is about 1 cell division per day).

6) The liver you had when you were three years old is not the same liver you have now. The original cells are all dead, completely replaced by new cells from more cellular divisions. All of these cellular divisions add up more mutations in the DNA.

7) Errors in coding DNA usually lead to mutant cells which do not do their jobs, and are therefore killed off by the immune system. Cancer is a good example of this: most cancers are killed off by the immune system, but those that get through are the ones that we notice.

This means that your liver cells DO have different DNA than your brain cells, however they share so much DNA, that the differences are probably minimal.

However, what this probably means for cloning is that clones probably don't resemble their donors as much as we expect that they should, especially if an intestinal cell was used for the procedure. Clones are not exact copies in any respect!

However, there are occasional errors to the coding DNA which produce proteins that are slightly better adapted to their jobs. Might this mean that intraorganismal evolution goes on???

I wonder if anyone else has had this idea?

NPM

zwi

Nice post from the non praying mantis

There is a very efficient system for the correction of copying errors. Part of this is related to what is one of the very hot areas of genetics today, and that is small RNA One of the functions of these very common 25 to 60 base snippets of RNA is thought to be creeping up and down the DNA strand and excising copying errors

Another is part of the functions of heterochromatin This was formerly thought to be just structural support for the chromosome, but now seems to have a great deal to do with gene expression, at least for a few generations This is the whole sub-subject of epigenetics. Apparfently the addition of methyl grouip or a phenyl group at critical parts of the chromosome will alter the expression of the genes

If your father is a stallion and your Mom is a donkey you are a mule. But if your mom is a mare and your daddy a jackass you are a hinny. A hinny and a mule are clearly different beasts, yet they seem to have the same genome

All of this is 21st century genetics, and I dont guarantee its detailed accuracy, but clearly it will have a great deal to do with the success or failure of cloning efforts

Another point to what is already an obscenely long post There are certain genes that are the same in unicellular eukaryotes as in humans That means that they have been faithfully copied for over a billion years

I have great respect for the fidelity of the copying system

Zwi

pz

Yes. There are examples of intentional sloppy copying in the immune system, for instance, in a process called hypermutation. The variable regions of the Ig molecule acquire greater diversity than you inherit in that way. We also have a selection process going on in the amplification of lymphocytes that bind active antigens.

In most tissues, though, you won't get selection for better performance. A gastric epithelial cell that produces a more efficient enzyme is not going to have any clonal advantage over its less effective neighbors. Maybe the individual with the lucky cell will have better digestion, but there's no mechanism in place to 'reward' that particular cell. And, of course, as a somatic mutation, it's not going to get passed on to any progeny.

There are, however, certain kinds of somatic mutations in which cells acquire properties that allow them to proliferate at a greater rate than their neighbors. We call these cells cancers. They aren't a good thing.

zwi

Another point arising from the non praying mantis

That leads to this question

How old is you ear?

As you said, all the cells have been replaced, so your ear is much younger than you Similarly all your DNA, with a few trivial exceptions, has been replaced

In fact the only thing that is more than a few years old in your body is the organization of your DNA, that is the genes

A copy of the DNA that is in your liver cell is effectively equivalent to the copy that is in your heart cells

But an exact copy of half your genes was present in your Mom, and of another half in your son, not the same as in your Mom, OK?

Follow this line of thought out and you will see that your genes are practically immortal

And that dear friends is the truth about immortality

Zwi

pz

This isn't quite true, but I know what you mean. They use haplodiploid sex determination, so the males have an unpaired X chromosome.

The only animal species I know of in which the males can be called truly haploid is Myrmecia pilosula, an ant.