Where do cells come from?

The human body is made of over 100 trillion cells. There are over 200 different types of cells, each with their own special function.

Cells give our bodies their shape, allow us to move, provide us with energy, let us have children, and much more.

What do you remember about cells?

There are two categories of cells: prokaryotic cells and eukaryotic cells.

The biggest difference between these two groups is that prokaryotic cells (bacteria) DO NOT have a nucleus. They are small and simple.

In addition to having DNA inside a nucleus, eukaryotic cells (plant and animal cells) are also large, complex, and have membrane-bound organelles.

Regardless of whether a cell is prokaryotic or eukaryotic, all cells have the ability to make more cells. This is known as cellular reproduction.

Cells can reproduce?

In short, yes. Think back to cell theory: all cells come from pre-existing cells.

Cellular reproduction might not happen like how you're imagining it.

Prokaryotes divide to create two new cells in a process called binary fission.

Eukaryotes also divide, but in a different process called mitosis. This type of cellular reproduction produces genetically identical body cells (all cells EXCEPT for sperm and egg). After a sperm and egg cell come together, the single cell starts going through mitosis in order to create more cells to build-a-baby.

Why do cells need to reproduce?

In this unit, we will take an in-depth look at mitosis, the process of how eukaryotic cells reproduce.

Remember: most eukaryotes (plants and animals) are multicellular organisms. Mitosis is how multicellular eukaryotes grow larger in size, repair themselves, and replace old cells.

What is the Cell Cycle?

Living organisms are constantly making new cells for growth, repair, and to replace old dead cells.

In the previous section, you read that eukaryotes make new cells through mitosis. This is the final step in cell division in the cell cycle.

The Cell Cycle is broken up into 2 main phases.

The first phase is called Interphase. It is made up of 3 parts: G₁, S, and G₂. This is where the cell spends most of its time doing normal cellular processes (maintaining homeostasis, endo/exocytosis, cellular respiration, photosynthesis, etc.)

The second phase is called M phase. It is made up of 4 parts: Prophase, Metaphase, Anaphase, Telophase. This is where the DNA within the cell is moved into two nuclei (plural form of "nucleus").

The "split second" where the Cell Cycle ends is called Cytokinesis. This is when the cell finally splits into two identical daughter cells.

The Longest Part of the Cell Cycle - INTERPHASE

Our cycle begins with a single parent cell - the cell that first enters the cycle and goes through the process of cell division (cellular reproduction). This cell starts with interphase, the longest phase of the cell cycle.

INTERPHASE: G_¹, S, and G₂

As soon as the parent cell enters the Cell Cycle, it starts G₁. This is where the cell grows in size and organelles replicate (double).

Next, the cell goes through Synthesis (S) Phase. This is where the DNA is doubled / replicated / copied.

The last part of Interphase is G₂. This is where the cell grows for the second time and has a final checkpoint before starting cell division. During G₂, the cell has to make sure all of the organelles have been copied, all of the DNA has been copied correctly, and that the cell has grown large enough to produce two normal-sized daughter cells.

The Shortest Part of the Cell Cycle - M PHASE

Once the cell has passed the final checkpoint at G₂, the cell will make final preparations to divide.

During M Phase, DNA within the cell is moved into two nuclei. This ensures that each daughter cell has a full and complete set of instructions (DNA).

Once DNA has been separated equally, the cell finally splits, and two identical daughter cells form. They are the same size, shape, and contain the exact same information as the original parent cell. They are like clones of each other!