When does the first "random mutation" of DNA (have to) occur?

Question

When reading about how evolution happens, "random mutations" are often spoken of.

As far as I understand, "random mutation" refers to the unsuccessful self-replication of DNA (the new version of which is then passed on correctly). But when does this change happen? During fertilization? Shortly before (in the sperm or egg itself) or shortly thereafter? And what is the last possible time for that to happen so that it gets passed onto every cell in the body and eventually brings forth a new physical trait?

I hope the questions make sense, I don't have a lot of background in biology.

Edit for completeness: When is the earliest possible moment for a mutation to take place? (A mutation that will ultimately make it to the offspring and eventually cause a new trait.) Is it in the ovaries/testicles or even before that?

Answer 1

Random mutations happen everywhere in the body all the time. Most are harmless, some cause problems, some cause cancer. Some are beneficial. If a beneficial mutation occurs in the germ cell line, it will be passed on to the offspring. Depending on how beneficial it is, it can spread like wildfire (the mutation allowing lactase to continue to be expressed after infancy is a good example of this, as is sickle cell trait); if it is harmful, the carrier may die before birth, in infancy, etc. If it causes a later onset disease (e.g. early Parkinson's)¹, the carrier has time to mate, reproduce and pass the disease on to their offspring. More and more diseases are being found to be genetic in nature now that genetic studies are so much easier to perform.

Again, for a mutation to be passed on, it must occur anywhere in the germ cell line of a parent, right up to the last stage before being released as an ova or a sperm cell, during fertilization, or during the very early stages of embryonic development in cells genetically programmed to become germ cells. If it occurs after fertilization, it will not affect that person, but its offspring. Depending on how early the mutation occurs, it may affect other parts of the reproductive system as well.

If a mutation happens in an embryo in a non-germ cell line, it may affect that person mildly, profoundly, not at all, or the embryo may die if it is a deleterious mutation in a critical organ system, say the heart.

But you need not put random mutations in scare quotes. I'm pretty sure you don't trust me, but they happen all the time, in every tissue, in you yourself as well as in everyone you know and everyone you don't know. And, by the grace of God, many of them can be repaired, and we don't ever have any ill effects from them. Alas, not all are repaired, and some lead to cancer.

_{Hereditary Early-Onset Parkinson's Disease Caused by Mutations in PINK1}

Answer 2

You need to understand that whenever a cell duplicate there is a DNA replication process, which means even when you are growing up there is DNA replication process, whatever or wherever you are there is DNA replication because cells in your body don't stop duplicating, which means every cell duplication hold a risk potential of causing a DNA mutation but that potential's probability is as low as one in a billion cell duplication and even if there is any mistake in the DNA-replication process the DNA-Polymerize-enzyme have repairing abilities which correct the mistakes, well sometimes even DNA-Polymerize-enzyme don't repair these mistakes which cause a disorder in some gene's task, these disorders might be helpful and make the creature grow faster or reproduce faster, or it might be harmful and cause diseases or some kind of three hand human or any kind of disorder and that depends on which gene that has been affected by the mistake in the DNA-replication, that means if the gene was in charge of the cell's duplication process this might make the cell duplicate forever and cause cancer, on the other hand, most of the times the mutation doesn't happen normaly because DNA-Polymerize-enzyme failed to do it's job but it happens because of some chemicals or even being exposed a lot of time to the sun rays. Bear in mind that fertilization or anytime before or after it have anything to do with DNA mutation as far as I know, anyway I might be mistaken, I have answered you by the information I got in the school.