Mitosis and meiosis

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.