Cell reproduction

Introduction

Previous chapters looked at a cell's genetic material and main principles of inheritance. When cells reproduce, the genetic material is distributed between the newly-produced cells, called daughter cells. This topic looks at different types of cell reproduction.

Mitosis

Somatic cells and gametes reproduce differently. Somatic cells divide by mitosis. Mitosis is the process by which the parent cell divides into two identical daughter cells. These daughter cells are genetically identical to their parent cell. Mitosis is the type of cell division that leads to the growth and development of an organism. All cells, except gametes, divide by mitosis.

The process of mitosis can be divided into the following stages. See image 1.

1. Interphase

Before actual cell division occurs, the cell grows in size. This growth stage can take a few hours or a few months. DNA molecules replicate but are still in the form of chromatin. The cell's nucleus is still intact.

2. Prophase

DNA molecules are more condensed and coiled. The nucleus is no longer a membrane-bound organelle. A spindle apparatus starts forming, which is also called a mitotic spindle. It is called that because all the microtubules attached to centrosomes at opposite poles of the cell, make it look like a spindle. The mitotic spindle helps chromosomes move in opposite directions. It is formed by the centrosome - the non-membrane bound organelle of the animal cell. Spindle micro-tubules 'drag' chromosomes apart to the newly-formed daughter cells. See image 2.

3. Metaphase

Spindle microtubules attach themselves to the chromosome centromeres. Chromosomes get aligned along the cellular 'equator'.

4. Anaphase

The micro-tubules of the spindle shorten and separate and drag sister chromatids towards the opposite poles of the cell.

5. Telophase

At this stage, chromosomes reach the opposite poles of the parent cell and the nuclear envelope reforms. Chromosomes uncoil and turn into chromatin again. The spindle fibres disintegrate. The parent cell splits into two daughter cells in the process called cytokinesis. Each daughter cell is diploid and contains the same type of chromosomes as the parent cell.

The process of mitosis can happen in any part of the body.

Meiosis

A cell division, in which eggs and sperm are produced, is called meiosis. Meiosis takes place in all sexually reproducing organisms. In the process of meiosis, the number of chromosomes is reduced by half, so all gametes are haploid cells (1n). Sometimes meiosis is also called reductive cell division. During fertilisation gametes fuse, forming a diploid zygote: 1n+1n=2n. Meiosis consists of two, consecutive nuclear divisions with only one round of DNA replication. At the end of meiosis, four non-identical haploid daughter cells are produced.

Meiosis consists of two stages: meiosis I and meiosis II. See image 3.

Meiosis I

1. Interphase 1

As in mitosis, at this stage the cell grows in size getting ready for division. Chromosomes are duplicated, but are still in the cell's nucleus in the form of chromatin.

2. Prophase 1

DNA molecules are more condensed and visible. The key, new events (compared to mitosis,) are the formation of tetrads and a crossover. Each tetrad is composed of four chromatids. Non-sister chromatids of the homologous chromosomes exchange pieces of DNA in the process of crossing over. Later, the envelope of the nucleus breaks down and tetrads migrate to the opposite cell poles.

3. Metaphase 1

Tetrads line up along the cell's equator with their centromeres facing the opposite cell poles.

4. Anaphase 1

Chromosomes move to the opposite cell poles with the help of centrosome micro-tubules. The homologous chromosomes move to the opposite cell poles, yet sister chromatids remain together.

5. Telophase 1

Meiotic spindles continue to move the homologous chromosomes to the poles. Once movement is complete, each pole has a haploid number of chromosomes. The chromosomes are double stranded. Two genetically non-identical daughter cells are formed in the process of cytokinesis.

The genetic material does not replicate again.

Meiosis II

There is no interphase stage in meiosis II. It starts with prophase.

1. Prophase 2

The nuclear envelope breaks and the spindle apparatus forms. Chromosomes begin to migrate towards the cell's equator.

2. Metaphase 2

Chromosomes line up at the cell's equator with their centromeres facing opposite poles of the cell.

3. Anaphase 2

Sister chromatids separate and move toward opposite cell poles.

4. Telophase 2

The cellular nucleus is formed and daughter cells are formed after cytokinesis is complete. At the end of meiosis II, there are four daughter cells, each with half the number of chromosomes of the original parent cell. Each daughter cell gets one of the four chromatids from the tetrads.

The process of meiosis can happen only in organs which produce sex cells. It is possible to see different stages of cell division through the microscope because condensed chromosomes stain well with biological dyes.