All of these statements about female gametogenesis are correct, EXCEPT:
A. Just like in the case of the spermatogonia, the oogonia population is renewed throughout adult life.
B. Primary oocyte is a diploid cell.
C. Primary oocyte remains in prophase I until ovulation.
D. Secondary oocyte comes out of metaphase II only if fertilization occurs.
Answer: A.
SHORT-AND-SWEET:
Female gametogenesis beings during the fetal life, with ovarian germ cells dividing mitotically to produce oogonia. However, while the female fetus is still in utero, the germ cells will produce a finite number of oogonia and then stop the production (unlike spermatogonia production, which continues throughout the adult life -- answer A). Diploid oogonia will then undergo meiosis, and arrest in prophase I as diploid primary oocyte. Ovulation will trigger primary oocyte to continue through meiosis, but it will arrest again in metaphase II as the haploid secondary oocyte, to continue development into haploid mature ovum only if fertilization occurs.
THE WHOLE STORY:
Before reviewing female gametogenesis (oogenesis), let's make sure we have solid understanding of the more fundamental matters.
First, let's clarify the cellular DNA content.
- With the exception of gametes, all other healthy human cells have 23 pairs of chromosomes -- each pair consisting of one Mom chromosome and one Dad chromosome, giving a grand total of 46 chromosomes per cell. These cells are considered DIPLOID (2n).
- When the DNA replicates, each of the parent's chromosomes will consist of two identical sister chromatids. That means that the cell which has undergone DNA replication will still have 46 different chromosomes, but each chromosome will consist of two sister chromatids (92 chromatids total per cell). However, this cell is still considered DIPLOID (2n), because it has 46 chromosomes total.
- HAPLOID (n) cells, such as gametes, will contain only one copy of each chromosome (essentially, 23 chromatids).
Moving on to discuss M&M -- mitosis and meiosis. These are the two types of cellular division. Both are preceded by one round of DNA replication.
- Mitosis consists of one round of cell division following DNA replication, and it produces two daughter cells which are identical to their mother cell (all diploid, with 23 pairs of chromosomes). Most of the cells in the human body which can replicate do so by mitosis.
- Meiosis, which occurs only in reproductive cells, consists of two sequential rounds of cell division (meiosis I and meiosis II) following DNA replication. The final result of meiosis are four haploid gametes. In male, these are four sperm cells, whereas female produces one ovum and three polar bodies. During meiosis I there will be an opportunity for exchange of Mom's and Dad's genetic material, which ensures genetic diversity.
Now that we understand the basics, let's look at what happens during oogenesis.
Female gametogenesis begins during the fetal period. The ovarian germ cells divide mitotically, producing multiple identical diploid oogonia. Once they have produced several million oogonia, they stop dividing -- and ALL BEFORE BIRTH! This is one of the key differences from the male gametogenesis, where the germ cells continue producing spermatogonia throughout adult life. This helps us pick out answer choice A as the correct answer. (If you were on the actual test you would go on to the next question...... How nice. Wish every other question was this easy!)
But let's review the rest of the oogenesis. In order to produce its progeny, oogonium cell will enter meiosis -- meiosis I, to be more specific. However, it will not get far. It will arrest in the first phase of meiosis I, prophase I, as the PRIMARY OOCYTE.
How "much" DNA does the primary oocyte have? Well, we know that the DNA was replicated prior to meiosis, so the cell will have total of 46 chromosomes (92 sister chromatids). Such cell is still considered diploid. There is no division of genetic material during prophase I, which means that primary oocyte will also be diploid (eliminate answer choice B).
The lazy primary oocyte will remain in prophase I for years, until puberty and the first ovulation (eliminate answer choice C). Just before ovulation the "selected" primary follicle and its primary oocyte will complete meiosis I, producing one SECONDARY OOCYTE (and one polar body to be discarded), which heads into meiosis II.
What about secondary oocyte's DNA content? What transpires during the rest of meiosis I? The DNA is first exchanged between the homologous Mom and Dad chromosomes, and then during anaphase I, the homologous chromosomes (each consisting of two chromatids) are separated, yielding two haploid daughter cells, each with 23 chromosomes (either of Mom or Dad origin, each having two chromatids).
The secondary oocyte likes to chill out too, and arrests not long after its formation -- in metaphase II (good bye, answer choice D), and it stays there until the ovulated egg is fertilized by sperm. While the sperm is trying to break in, the haploid secondary oocyte hurries to finish meiosis II, producing haploid ootid (and another polar body), which has one copy of each chromosome.
The sperm and ootid nuclei will not fuse until the second polar body is released, and the ootid matures producing a mature ovum. After their nuclei fuse, they will produce a diploid fertilized egg, or a zygote.
Basically, you can think of oogenesis as one big arrested development (like the TV show!), with lazy primary and secondary oocytes napping whenever they can. But how to remember when each will take a nap? Easy -- number 1 and number 2!
Number 1: The PRIMARY oocyte will arrest in the FIRST phase (prophase) of meiosis I.
Number 2: SECONDARY oocyte will arrest in the SECOND phase (metaphase) of meiosis II.
Be selected or die is the other motto of oogenesis! Only the primary oocyte which gets "selected" for ovulation in a particular month continues its development. All of the other ones prepping for that month's ovulation die. Similarly, the ovulated oocyte arrested in metaphase II needs to be fertilized in order to continue its development. If there is no fertilization, it will die a sad and lonely death.
BIG PICTURE:
1. Diploid cells have 23 pairs of chromosomes, each pair consisting of one Mom and one Dad chromosome. Haploid cells have 23 chromosomes, each either being of Mom origin, Dad origin, or mixed Mam/Dad origin (due to recombination which occurs during meiosis).
2. DNA replication precedes both mitosis and meiosis. Mitosis consists of one round of cell division, which produces two daughter cells, genetically identical to their mother cell (all 2n). Meiosis consists of two rounds of cell division, producing four haploid cells.
3. Oogenesis is one big arrested development show! Diploid oogonium produces diploid primary oocyte, which arrests in prophase I. Ovulation pushes it forward, but the oocyte arrests again in metaphase II as the secondary oocyte. Finally, if it gets fertilized it will continue its development to mature ovum.
~The MCAT POD Team~
