Genetics Basic Concepts

Mitosis - 0:26 s


Meiosis, Walter and Eliza Hall Institute of Medical Research (Australia) - 1:09 min 

Cellular reproduction
(mitosis and meiosis)

Our cells reproduce so that we may grow and regenerate. For example, cells are constantly duplicating so that our nails and hair keep growing. 

The process by which a cell splits into two genetically identical copies is called mitosis. To achieve this, the cell must first make a second copy of all the DNA in its nucleus. DNA is then condensed into rod-like structures known as chromosomes. Both copies of the chromosomes stick together in the middle, which is why they look like an X under the microscope. Once the nuclear membrane breaks down, the chromosomes line up in a neat row at the centre of the cell. The chromosome pairs then split and move apart toward opposite poles of the cell before it divides into two genetically identical daughter cells. 

When sex cells, the sperm and the egg, are involved, cell division is taken to a whole other level! The process is called meiosis. Regular cells have 23 pairs of chromosomes (for a total of 46 chromosomes), and each pair contains one chromosome from our father and another chromosome from our mother. But a sex cell can only hold half of the genetic material, since it must unite with the other sex cell to produce a new individual. DNA in sex cells must undergo another round of division: with 23 chromosomes on one side and 23 chromosomes on the other. Meiosis produces four genetically different cells containing half of the genetic material.

Meiosis is similar to mitosis, but it has an extra round of cellular division. During meiosis, the mother cell copies its DNA molecules and condenses them into rods (chromosomes). Both copies of the same rod are bound together at the middle to form an X. We have 23 pairs of rods, for a total of 46 chromosomes. The chromosomes then pair up. Once they have found their partner, they line up at the centre of the cell for the first round of division. The two resulting cells contain 23 chromosomes. A second division occurs. During this round, the chromosomes are divided into 23 rods in each of the 4 cells.  

During the process, chromosomes can exchange pieces of DNA. This is known as recombination. This shuffling of genetic material between the maternal and paternal chromosomes during meiosis leads to greater genetic diversity. It’s also what makes each of us unique!