What is reproduction?

SEXUAL REPRODUCTION

The process in which the male and female gametes fuse together to form a new individual is called sexual reproduction.  

The male reproductive organs are the testes, sperm ducts, and the penis. The sperms are produced by the testes. The sperms are very small, and they have a head, a middle piece, and a tail.

The female reproductive organs are the ovaries, oviducts, and the uterus. The eggs (ova) are produced by the ovaries. The development of the baby takes place in the uterus.

            SPERM                                            EGG (OVA)

The semen contains millions of sperms. A single sperm fuses with the ova during fertilization. The nuclei of the egg and the sperm fuse together to form a single nucleus. So, a zygote is formed. 

There are two types of fertilization: 

• Internal fertilization: It takes place inside the body of the female, like humans, cows, dogs, etc. This method is more frecuent in terrestrial animals, but some aquatic animals also adopt this method, like sharks and rays. 

There are three ways of internal fertilization: 

- Oviparity: The fertilized eggs are laid outside, and they receive food from the yolk.

- Ovoviviparity: The fertilized eggs are retained in the female's body where they receive food from the yolk. The eggs are laid right before they are hatched.

- Viviparity: The youngs are born directly from the mother and they feed from her. This can be seen in mammals.

A mammal is an animal that breathes air, has a backbone, and grows hair at some point during its life. In addition, all female mammals have glands that can produce milk. Mammals are among the most intelligent of all living creatures. Mammals include a wide variety of animals, from cats to humans to whales.  

Mammals are the only animals that produce milk to nourish their young. The female has special glands called mammary glands. After childbirth, the mother's glands produce milk. The mother feeds the young with this milk until the young are old enough to get food for themselves. 

• External Fertilization

It is the fertilization that takes place outside the female. For example: frogs, fish. Most fertilization takes place during the process of spawning. 

ASEXUAL REPRODUCTION

We can see this type of reproduction in lower organisms and unicellular microbes. It is the process in which a new individual is formed by the involvement of a single parent without gametes. The individuals produced are similar in genetic and form. The cells divide by mitotic division and no fertilization takes place. The division occurs very rapidly.

There are five types of asexual reproduction:

• Binary Fission: It is seen in amoeba and euglena. The parent cell undergoes mitosis and increases in size. The nucleus also divides. Two identical daughter cells are obtained, each containing a nucleus. 

• Budding: In this, the offspring grows out of the body of the parent. It remains attached to the parent until it matures. After maturation, it detached itself from the parent and lives as an individual organism. This form of reproduction is most common in Hydras.

• Fragmentation: In some organisms, when the body of an organism breaks into several pieces each piece grows into an individual offspring. In few animals such as sea star, a broken arm grows into a complete organism.

• Regeneration: It is a modified form of fragmentation and occurs mostly in Echinoderms. When a part of an organism, like an arm, detaches from the parent body, it grows into a completely new individual. 

• Parthenogenesis: This is a form of asexual reproduction where the egg develops without fertilization. This process occurs in bees, wasps, ants, etc. Ants, wasps, and bees produce haploid males. Parthenogenesis can occur in a few vertebrates such as hammerhead sharks or Komodo dragons.

References:

Figure 50: The Weird Science Teacher. (2019). Anchor Charts: Sexual vs. Asexual Reproduction [figure]. Retrieved from https://www.teacherspayteachers.com/Product/ANCHOR-CHARTS-SEXUAL-VS-ASEXUAL-REPRODUCTION-1-CHART-2-FILES-4233833 

Figure 51: Khan Academy. (n.d.). Egg meets sperm [figure]. Retrieved from: https://www.khanacademy.org/test-prep/mcat/cells/embryology/a/egg-meets-sperm

Figure 52: Khan Academy. (n.d.). Egg meets sperm [figure]. Retrieved from: https://www.khanacademy.org/test-prep/mcat/cells/embryology/a/egg-meets-sperm

Figure 53: Broomfield, K. (2019). The spawn of spring [figure]. Retrieved from https://www.thecourier.co.uk/fp/lifestyle/outdoors/nature-watch/832730/the-spawn-of-springxxx/

Figure 54: University of Gothenburg. (2015). Coscinasterias tenuispina starfish [figure]. Retrieved from https://phys.org/news/2015-06-starfish-clone-longer.html