In the 2nd episode of the new Cosmos series, the host Neil deGrasse Tyson shows how the white-furred bear could have evolved (reasonable scientific speculation, of course).

If you haven't seen that episode, here's the link. Great show, by the way.

So, it shows the bears eggs, and then goes on to show how there can happen an error in the DNA copying, that leads to the brown pigment production malfunction. Here's an excerpt from the subtitles text:

- great bears roamed the frozen wastes of Ireland.
- This might look like an ordinary bear,
- but something extraordinary is happening inside her.
- Something that will give rise to a new species.
- In order to see it, we'll need to descend down to a much smaller scale, to     the cellular level, so that we can explore the bear's reproductive system.
- Those are some of her eggs.
- To see what's going on in one of them,
- we'll have to get even smaller.
- We'll have to shrink down to the molecular level.
- When a living cell divides in two,
- each one takes away with it a complete copy of the DNA.
- A specialized protein proofreads to make sure
- that only the right letters are accepted
- so that the DNA is accurately copied.
- But nobody's perfect.
- Occasionally, a proofreading error slips through,
- making a small, random change in the genetic instructions.
- A mutation has occurred in the bear's egg cell.
- A random event as tiny as this one can have consequences on a far grander scale.
- That mutation altered the gene that controls fur color.
- It will affect the production of dark pigment in the fur
- of the bear's offspring.

From my (not professional but kind of passionate amateur) knowledge of biology, I I was under impression that all of the mammalian eggs are present at birth. But the story depicted in the show involves the (erroneous) DNA copying. I see dissonance there, and I'd like to solve it.

  • $\begingroup$ The episode isn't available outside the US, so can you provide some more details? When does the DNA copying happen? When the eggs are produced? $\endgroup$
    – jarlemag
    Commented Apr 7, 2014 at 8:59
  • $\begingroup$ I edited the port to include the text. Perhaps this is the point where I fail to grasp what's happening: "A mutation has occurred in the bear's egg cell" - at which point a mutation could in fact happen in an egg cell? To my understanding, only a fertilized egg cell divides. $\endgroup$
    – Passiday
    Commented Apr 7, 2014 at 12:27

1 Answer 1


The writers of the show may have been somewhat imprecise either by accident or intentionally to avoid excessive details.

There are several different stages of egg cells, with distinct names for each and for the process that leads from one to the next. The whole process of egg cell creation is called [Oogenesis].1 To quote Wikipedia:

Oogenesis starts with the process of developing oogonia, which occurs via the transformation of primordial follicles into primary oocytes, a process called oocytogenesis.[4] Oocytogenesis is complete either before or shortly after birth.

To our current knowledge all primary Oocytes are present at birth. The next stage in the maturing process is called ootidogenesis, which produces an ootid.

The succeeding phase of ootidogenesis occurs when the primary oocyte develops into an ootid. This is achieved by the process of meiosis. In fact, a primary oocyte is, by its biological definition, a cell whose primary function is to divide by the process of meiosis.[8]

However, although this process begins at prenatal age, it stops at prophase I. In late fetal life, all oocytes, still primary oocytes, have halted at this stage of development, called the dictyate. After menarche, these cells then continue to develop, although only a few do so every menstrual cycle.

Meiosis I of ootidogenesis begins during embryonic development, but halts in the diplotene stage of prophase I until puberty. The mouse oocyte in the dictyate (prolonged diplotene) stage actively repairs DNA damage, whereas DNA repair is not detectable in the pre-dictyate (leptotene, zygotene and pachytene) stages of meiosis

DNA replication is completed before Meiosis occurs. So oocytes do not replicate DNA after birth, but they do repair DNA to fix mutations that may occour after birth but before conception. If DNA repair fails, then mutations may persist. So the show would appear to be wrong about egg cells copying DNA in an adult bear, but right in that mutations might occur during its lifetime affecting its offspring.

  • $\begingroup$ Wow, that's elaborate, thank you. On what authority I could decide that I can mark your answer as "correct"? I am not competent to decide :) Why would the show writers choose this particular stage for addressing the mutation introduction? Is that the prevailing hypothesis of how the persistent, evolution-driving mutations occur? $\endgroup$
    – Passiday
    Commented Apr 7, 2014 at 18:00

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