AssessmentsMitosis and meiosis
USMLE® Step 1 style questions USMLE
A 36 year old G0P0 comes to the fertility clinic because she has been having trouble getting pregnant. She is a high school biology teacher and just taught her students about mitosis and meiosis. She asks you to explain how eggs are formed and you start out by explaining the maturation process from primordial germ cell to primary oocyte. Primary oocytes are arrested in which phase of cell division?
Content Reviewers:Rishi Desai, MD, MPH
Pretty much everything alive - from a humble bacteria, to a baby blue whale, has two main goals - to grow and to reproduce.
Our cells are no different. They grow and they reproduce.
But the reproduction part can be done in two distinct ways - mitosis, or meiosis.
Mitosis gives rise to daughter cells that are almost identical to the parent cell in terms of their genetic information.
Whereas in meiosis the daughter cells or gametes, sperm in males and eggs in females, get only half of the genetic information of the parent cell.
These gametes can then pair with completely different gametes to form a cell that’s quite different from the parent cell.
This newly formed cell can then do mitosis to form an entirely new organism.
Mitosis is one part of the cell cycle.
Most of the cell cycle is taken by interphase - which is usual cellular day-to-day activities, including growth, protein synthesis, making new organelles, and so on.
Interphase has three phases: G1, S and G2.
The first phase is called G1.
During G1, the cell gets bigger in preparation for cell division.
At this point the 46 chromosomes look like spaghetti and are called chromatin fibers.
Each chromatin fiber is made of a single copy of the genetic material - called a chromatid.
The second phase is the S phase.
During S phase, each chromatid is copied and pasted so there are still 46 chromosomes, but each chromosomes now has two sister chromatids.
The two chromatids are joined together in a region called the centromere - adding up to 92 chromatids total.
The third phase is G2.
During G2, the cell does some more growing before finally entering mitosis.
Some cells, like neurons, enter what’s called the G0 phase - where they basically continue to live but don’t divide.
But most cells do enter mitosis after G2, and mitosis can be divided into four distinct phases: prophase, metaphase, anaphase, and telophase - and you can remember them as PMAT - a mat that you pee on.
At the beginning of prophase, the chromatin fibers condense, meaning that they start to untangle to form individual chromosomes.
Each chromosome starts mitosis with two sister chromatids.
During prophase, there are two organelles called centrosomes that are located opposite to one another just outside the nucleus.
So, if the nucleus is a “globe”, one centrosome is at the North Pole, while the other is at the South Pole.
Each centrosome has two centrioles, which are protein structures placed at right angles to each other, so they sort of look like a “+” sign.
Now, each centriole sends off spindle fibers made of microtubule proteins that stretch from the centriole out towards the centromeres on all of the chromosomes.
These spindle fibers help to gently position the chromosome towards the exact middle between the two centrosomes.
The next step in mitosis is metaphase, which has an early phase called prometaphase, and a late phase which is referred to as the actual “metaphase”.
During the prometaphase, the nuclear membrane and the nucleolus disintegrate.
Then during metaphase, the chromosomes neatly align on the midline of the cell - along what’s called the metaphase plate.
Throughout metaphase, spindle fibers continue to attach to a specific point on the centromere of each chromosome, called the kinetochore.
When a microtubule from each centrosome has connected to the kinetochore of each chromosome, anaphase begins.
During anaphase, the centrosomes start pulling on the spindle fibers to pull the sister chromatids apart - like a brief, but powerful game of tug of war.
Since both centrosomes pull with equal force in opposite directions, the sister chromatids get separated, and pulled to opposite poles of the cell.
Finally, during telophase, a new nuclear envelope forms around each centrosome and the chromosomes - which, at this point, are made up of a single chromatid each.
So, for a brief moment, the cell has two nuclei.
Now - all throughout mitosis, the cell membrane gradually pinches in tighter and tighter, like an invisible belt, until it actually separates the cell into two daughter cells.