Cell Reproduction

Keywords

Cells, mitosis, meiosis, division, multiplication, cytokinesis, prophase, split nucleus, doubling chromosomes, cell cycle, differentiation, stem cells.

Introduction

Cell reproduction. You would think it was simple. It can be, but there are different ways of doing it. Let's take a look further at what happens.

Mitosis

When an organism grows, cells have to divide or multiply. This is biology 101, as all organisms that want to grow or recreate, have to do this. It also occurs when an organism becomes damaged in some way. For humans, this would be when we cut ourselves, or get damaged in other ways.

Mitosis is a specific cell cycle. It creates a cell in a five-stage circle:

  1. Cell growth: the first stage is that the cell grows. It's that simple. It has to grow as what is about to happen is amazing.
  2. DNA synthesis: what this means, is that the cell has to create copies of the DNA inside the cell and each chromosome. So, if you have a cell that is beginning to perform mitosis, it will copy each chromosome with the DNA inside it.
  3. Nucleus splitting: this happens. The nucleus that is contained within the cell, duplicates itself and splits in half.
  4. Chromosome lineup: while the nucleus splits in two, the chromosomes line up across the centre of the cell, with each pair ready to duplicate.
  5. Chromosome splits: so, once they are ready to, each set of chromosomes split in half, and moves to the sides of the cell. They are now a duplicate of each other, and will form the new cells.
  6. Mitosis occcurs: actual mitosis happens. The cell membrane and cytoplasm creates two new cells from one, and splits into two new cells. This last part is called Cytokinesis. It is the only part of the phases that has any significant movement when watched under a microscope.

Interesting fact: a cell takes just 24 hours to form and then divide to form new cells. This is through either process of mitosis or meiosis.

What Does This Mean?

Well, when a cell divides by mitosis, it becomes two daughter cells of the original parent. They are each identical to the original parent cell.

Cell Differentiation

When animals and plants are created, they start off as a single-celled organism. Mitosis has to occur to enable division of cells to become the multi-cellular organisms we are today. But, another point worth mentioning, is that we have to differentiate some of the cells for certain functions.

What I mean by this, is that some cells will differentiate (change) into cells used for respiration. Some cells will change into cells that are used for reproduction. Some cells will change for use in circulation and immune system use. And so on.

All these cells are called specialised cells, and they are created to work for a specific function.

Meiosis

The other type of cell reproduction is called Meiosis. This is the type of cell division that creates gametes. A gamete is a sex cell, and works in the reproduction organs of an organism.

How the Process Happens

The process is as follows:

  1. Starting with one parent cell, the process of meiosis is defined by:
  2. Identical copies: each chromosome makes an identical copy of itself.
  3. Pairing up: similar chromsomes pair up. This is still within the same cell.
  4. DNA swap: sections of DNA get swapped over.
  5. Chromosome divide: new pairs of chromosomes divide. Specifically, they are still in pairs at this point.
  6. Chromosome divide #2: now, the pairs of chromosomes divide further to create single chromosomes in each cell.

Meiosis works to create four cells, each with a single set of chromosomes within. So, each cell holds 23 chromosomes, instead of 46, which is what our cells carry. This process is known as reduction division, as our gamete cells (sperm, egg) are haploid, rather than diploid.

Haploid & Diploid

I need to explain what haploid is. A gamete (sex cell - sperm and egg) is a haploid cell. This means it hold one set (23) of chromosomes. If I use a sperm cell for the example here, it carries those 23 chromosomes until it pairs up with an egg to reproduce - make an embryo.

A diploid cell is the most common cell type in our body, as it carries all 46 pairs of chromosomes we carry in our DNA. This is obviously different to a haploid cell.

Back to Meiosis...

Now that's explained, you may understand that reduction division is the process where a diploid cell becomes a haploid cell through division and the process of meiosis.