Topics in Depth
The Theme of Gametes in Animal Reproduction
As we delve deeper into the world of sexual reproduction we must discuss the star players: the gametes. Gametes are the haploid sperm and egg cells which must combine during fertilization to kick-off the formation of a new animal. The production of gametes is the first step of sexual reproduction. And we're off.
An important part of gamete production is meiosis. This is the process which turns diploid cells into haploid cells. Although sperm and egg production differ in many ways, meiosis is a critical to both processes. The basic process of meiosis is described a little bit in another Shmoop guide, but we will give you a quick rundown again right here because it is an important part of gamete formation.
Meiosis begins with a regular diploid cell. Let's call him Larry. Remember, diploid means two full "sets" of chromosomes.
Think of a "set" of chromosomes like a set of 4 cards: Jack, Queen, King, and Ace (each card represents a different number chromosome). Larry is diploid and has one "set" from his mother (red hearts) and he has one "set" from his father (black clubs). Sometimes the number of "sets" is represented by a #n. So, a diploid cell like this one would be 2n because there are two versions of each card (Jack of hearts and Jack of clubs).
1) Copying chromosomes: Larry copies his chromosomes so that he now has two Jacks of hearts, two Jacks of clubs, etc. These duplicates remain attached. The cell is still considered 2n because there are still only two versions of each number (hearts and clubs) even though there are 4 total Jacks.
2) First Division: The two Jacks of hearts and the two Jacks of clubs line up near each other and swap DNA in a process called recombination (discussed below). Then, the hearts go one way and the clubs go the other and the cell splits in two. Each cell will have either two Jacks of hearts or two Jacks of clubs. Since each cell now has only one version of a Jack, it is haploid (1n).
Don't forget that the Queens, Kings, and Aces are also doing all the same steps as the Jacks. It is important to mention that although the Jacks of hearts might go to the left, the Queens of hearts might go to the right. So, the two cells formed at this point might each have a combination of pairs of hearts and pairs of clubs.
3) Second Division: In the last step, the two Jacks of hearts finally split and the two Jacks of clubs finally split (as well as the same suit pairs of Queens and Kings). This means there are now four 1n cells, each one with either a heart OR a club version of the Jack, Queen, King, and Ace.
Easy as 1, 2, 3. One diploid cell becomes 4 haploid cells.
Gamete Diversity and Recombination
We have already learned that sexual reproduction leads to diversity because offspring have a combination genome from two parents. Meiosis doubles the diversity because each parent produces a variety of gametes. After all, if every gamete from a parent were the same, then all siblings would be identical twins. That would be confusing with a capital C. You can find more detail about some of these topics in the Shmoop genetics guide, but we will discuss them here specifically in regards to producing diverse gametes.
Law of Independent Segregation
This means that each gamete will receive only one of Larry's two alleles, either the maternal hearts allele OR the paternal clubs allele.
Law of Independent Assortment
This means that each chromosome is sorted and packaged independently from the others. A maternal Jack of hearts has an equal chance of being packaged with a maternal Queen of hearts or a paternal Queen of clubs.
The first split in meiosis separates the maternal (hearts) and paternal (clubs) versions of each chromosome. To do this, the 2 versions line up next to each other during the sorting. While they are next to each other they swap bits of DNA in a process called recombination.
This means that the 2 versions are no longer straight mother versions and father versions. They are now more like mofather and famother hybrids. This is important because it helps genes to get switched around so that all offspring are completely unique.
Let's think about how this works. We will pretend that chromosome #3 has a gene for eye color and nose size. The chromosome Larry got from his mother has the alleles for green eyes and a giant nose. The chromosome from his father has the alleles for brown eyes and a tiny nose. Physically, Larry actually has brown eyes and a giant nose because those alleles are dominant, but that is another story.
If there was no recombination during meiosis then the brown eyes and giant nose alleles would be permanently located on separate versions of his chromosomes. If they are never on the same chromosome then they will never be in the same gamete (since each gamete only gets one copy of each chromosome). Larry's children would never inherit his brown eyes and giant nose.
Luckily, recombination means that the brown eyes and the blue eyes could swap and create a chromosome with both brown eyes and a giant nose. Recombination is what makes it possible for Larry's offspring to have a mix of Larry's genes, not just a mix of Larry's chromosomes.
The gamete cells produced by males are called spermatozoa, or sperm for short (we tried to call them "erm" instead, but it never caught on). The process by which sperm are produced is called spermatogenesis. The headquarters for sperm production are the testicles or testes.
Behold, sperm, the male gamete (10 μm is about 10 times skinnier than human hair).
Most animals have two testicles, although some jawless fish only have one. Without a jaw, two testicles just would not look right. Spermatogenesis occurs in an area of the testes called the seminiferous tubules. If you were to unwind a human's seminiferous tubules, they would be 20 feet long.
The cells that want to be gametes when they grow up are called germ cells. The germ cells for males are called spermatogonia and they are located in the seminiferous tubules. These germ cells can undergo either mitosis to create more spermatogonia or they can undergo meiosis and development.
Towards the end of development, they grow a small tail or flagellum for swimming. They also develop a sac on their heads called an acrosome which contains special enzymes to eat through the egg's wall. Tasty. At the appropriate moment, the sperm are ejaculated from the body. They are immersed in a combination of helpful liquids which provide nutrition and structural support to help them on their quest to fertilize an egg. It is like the Fellowship of the Sperm.
Most animals do not begin spermatogenesis until puberty and then sperm production is a continuous event throughout the course of a male's life. The testes might get bored, but they have no choice. Sperm production is controlled by hormones. Special testosterone producing Leydig cells in the testes are stimulated by hormones signals sent from the brain and pituitary gland such as LH, FSH, and GnRH.
The length of time to complete spermatogenesis differs between animals, but in mammals it is between 30-78 days. In humans, it takes approximately 74 days from start to finish. This means that spermatogonia which start meiosis today might be fertilizing an egg 74 days from now.
The female gamete is called an oocyte, or egg. The process of egg production is called oogenesis. Oogenesis occurs in the ovaries, which are the female equivalent of the testes. Ooooh boy, that's a lot of o's.
Oogenesis varies greatly between animals. Some animals produce a single egg at a time while others produce millions of eggs at a time. We will describe oogenesis as it occurs in most mammals and then talk about the difference in other animals.
When a female mammal is born, her ovaries are filled with a finite number (this means a set-in-stone limited number) of small sacs called follicles. Each follicle contains a single primary oocyte. These primary oocytes are diploid cells that began meiosis before birth. Since then, they have been waiting ever-so-patiently in the beginning stage of meiosis.
The process of oogenesis.
These primary oocytes do not continue through meiosis until puberty when certain hormones tell them to wake up and start dividing. A primary oocyte then undergoes the first split of meiosis.
However, it does not split equally. Instead, one large cell and one teensy tiny cell form. The tiny cell is called a polar body, which basically serves as a chromosome dump. It allows the egg to get rid of the excess genetic material without splitting the nutrient rich cytoplasm which it needs for development. The polar body usually deteriorates over time. The remaining large cell is called a secondary oocyte.
Secondary oocytes are released from the ovary during ovulation with a dream of being fertilized. If they do meet a handsome sperm, then they quickly undergo the second meiotic division. It is an uneven split again, and another polar body forms. This means there are now 2 polar bodies (or 3 if the first polar body underwent the second split of meiosis as well). It is practically the North Pole.
This process can be slightly different in other animals:
- Some animals produce more than one egg at a time (like dogs and cats), and some only produce one at a time (like elephants).
- Animals that produce many eggs at one time (like fish and frogs) are not born with a finite number of eggs. They can replicate their germ cells just like males can. Eggs are on the menu every day.
- Most birds only have one functional ovary.
- Some insects undergo a different type of meiosis where each germ cell forms 16 connected cells. Only one of these cells actually develops into the oocyte and the others are supportive nurse cells.
- The hormones involved in regulating oogenesis vary from animal to animal.
Differences in Sperm and Egg Production
|Gamete size||Small, fast, mobile, expendable||Large, nutritious, less mobile|
Sperm are typically produced by males and eggs are traditionally produced by females, but that is not always the case. There are animals that can produce both types of gametes. These animals are called hermaphrodites.
Some hermaphrodites can always produce both gametes, and they play both the male and the female roles at the same time. Snails, slugs, and earthworms fit into this category. Self-fertilization can occur in some species, although mating with a partner is still common (probably for genetic mixing). This can lead to either one or both partners becoming fertilized.
Other types of hermaphrodites are born either male or female, but they can switch sexes later in life. Many species of fish, including clownfish, can do this. The change often occurs to maintain a social dynamic which is dependent upon a set proportion of males and females.
Hermaphroditic clownfish. If Nemo’s father had waited awhile, he could have been a female.
Gametes are a delicacy in many cultures. Humans and other animals eat a lot of eggs: turtle eggs, fish eggs, chicken eggs, etc. Since chickens lay their ovulated eggs whether they are fertilized or not, most of the chicken eggs eaten by humans are unfertilized.
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