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Gametogenesis: Definition & Concept

Instructor: Shannon Compton

Shannon teaches Microbiology and has a Master's and a PhD in Biomedical Science. She also researches cancer and neurodegenerative diseases.

Birds do it, bees do it, even educated fleas do it. Let's do it, let's make some gametes. This lesson discusses how sperm and egg are produced and why these cell types are different from any other cell in your body.

Gametogenesis Defined

Gametogenesis, by definition, is the development of mature haploid gametes from either haploid or diploid precursor cells. The precursor cells undergo cell division in order to become gametes. This may sound like a very technical definition, but by the end of this lesson, you'll understand it.

Organisms can be either diploid or haploid. Those that are diploid, like you and me, have two copies of their DNA per cell. Those that are haploid have one copy of their DNA per cell. As mentioned in the definition, gametes are all haploid. So if you are already a haploid cell, you undergo regular cell division (mitosis). However, if you are diploid, you have to make haploid gametes. That is, you have to create cells with only one copy of DNA each. This is also done by a special type of cell division called meiosis.

During the process of mitotic cell division, a cell makes a complete copy of its DNA. Then, when the cell divides, the DNA is split between the two daughter cells. Thus, each daughter cell gets a complete, exact copy of the parent cell's genetic information (DNA). This type of cell division is a one-step process.

Meiotic cell division is a two-step process. Meiosis begins with a diploid cell that has two copies of DNA. One comes from the father and one from the mother. The cell divides twice producing four haploid cells. The first division is called Meiosis I. It involves replication of chromosomes but allows 'gene shuffling' between the maternal and paternal chromosomes. The second division is called Meiosis II. It results in haploid cells.

What is gene shuffling? Imagine you have a red blanket from your dad and a light blue one from your mom. Mitotic cell division keeps the blankets (or chromosomes) separate. So, all cells have red blankets and light blue blankets. Meiosis allows the cells to make a patchwork quilt out of mom and dad's blankets. This creates genetic diversity which is an advantage to sexual reproduction. The general steps for meiosis are described in the first figure.

Meiosis Overview
Image of Meiosis Overview

In Animals

Most animals reproduce sexually. Both males and females undergo gametogenesis through meiosis. Gametogenesis happens in specialized organs called gonads. Gametogenesis in males is called spermatogenesis. Female gametogenesis is called oogenesis.

The second figure shows the similarities and differences between maturation of the ovum and the spermatids. Both the oocytes and spermatocytes are called gametocytes. Ova (eggs) and spermatids (or sperm) are mature, haploid gametes. The main difference between them is that in females, polar bodies typically die without dividing. In males, all the immature gametes develop into mature spermatids. Polar bodies do not form in males.

Gametogenesis
Image of Gametogenesis

In Flowering Plants

Angiosperms (flowering plants) also develop ova and sperm. The sex organ for angiosperms is the flower. A flower can be male (producing sperm), female (producing eggs), or both (produces sperm and eggs). Whether the flower is male, female, or both depends on the species of plant. Male gametes develop inside the pollen grains. Female gametes develop inside the ovule.

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