Why, scientifically, can't human eggs be fertilized by animal sperm? (this is for school)?

Question

Alright- this is extra credit for me and I can't seem to find a clear answer. I read already that there are differences in the number of genes, but is that truly the only reason? There have to be enzymes or blocks of some kind- species identifiers? If you know, please help me out. Thank You

Answer 1

There's two main mechanisms that prevent this from happening:

Prezygotic barrier. This prevents the gametes of different species ever coming into contact with each other. Often this is a behavioural instinct, after all, would you want to mate with an animal other than a human? However sometimes its not physically possible, for instance no matter how hard you tried, you couldn't mate with a fish, or a badger, just because the dimensions are wrong.

Postzygotic barriers are methods that prevent fertilsation even if the gametes do get close to each other. In the case of humans, the head of a sperm is covered with an enzyme that helps it break through the wall of an ovum. The sperm of other animals do not have this, so a would-be sperm trying to fertilise an egg just couldn't get in. Even if it could, it wouldn't necessarily have the right number of chromosones. If there is a significant defeceit in one number [just one missing doesn't always make a difference, Down's Syndrome babies are formed when there is a chromosone missing, and horses and donkeys can mate to form mules but a horse has a different number of chromosones to a donkey (this is why mules are sterile)]. If there happens to be the same number of chromosones, the DNA sequence itself must still be approximately the same to express the same commands and synthesise the correct protiens in order for the zygote to develop into a blastocyst and then a fetus and finally a baby.

Even if this happens, we move on to hybrid breakdown. Either the hybrid offspring is extremely weak and unlikely to survive naturally [as in crosses between tigers and lions, the hybrid offspring have some difficulties suriving, giantism and tumours are common in these hybrids] or the hybrid itself is sterile [in the case of mules].

If you manage to get past this, well then you are the same species. A two animals are of the same species if the opposite sexes can successfully mate and produce fertile offspring.

Answer 2

Well, the short answer is no. Both animals and plants have evolved extensive mechanisms which prevent this from happening.

First, sperm have to find their way to an egg. In mammals, when the egg is released from the ovary it is surrounded by a loose group of follicle cells. These cells release a chemical signal that the sperm swim toward. If the signal is not correct, the sperm won t find the egg.

While the sperm are swimming up the female s reproductive tract, ions that her body secretes are absorbed into the sperm cells, causing internal changes that are referred to as capacitation. In humans, capacitation takes 5 to 6 hours and must be completed before the sperm can accomplish its next job, which is to penetrate the zona pellucida, or egg coat. The zona pellucida has proteins that are similar to the proteins used by the immune system, and these proteins make cross-species fertilization nearly impossible.

Once the sperm has successfully penetrated the zona pellucida, the acrosome reaction allows the sperm to fuse with the membrane of the egg so that the male s DNA can enter the egg cell. The acrosome reaction requires specific proteins that again prevent hybridization between species.

In order for cell division to occur and create an embryo, the paternal and maternal DNA must line up - another block to hybridization, as DNA varies from species to species enough for this to be unlikely to work.

An interesting side note is that fertility clinics sometimes use enzymes to strip the zona pellucida away from hamster eggs, and then use the stripped eggs to test human sperm for penetrating power. Although healthy sperm can penetrate the hamster eggs in this case, the DNA alignment fails and the fertilized eggs die without undergoing mitosis.

Answer 3

Simply because the human genome is arranged differently from another species genome. When the DNA in the chromosomes line up during fertilisation the process would stop there because each strand of code would not correspond to each other. Thus a spontaneous abortion would occur. If the process did continue, there would be a waste of resources growing the being as it would die fairly quickly. It's a bit like sticking a playstation 2 disc into an x box. The x box wouldn't be able to read the code, so it can't run the program.

Answer 4

Animal Fertilization

Answer 5

Tom's answer is good. There are multiple barriers

Sperm generally can not bind and/or penetrate into the egg of another species. There are some exceptions so that is why there are more barriers, such as the ones that Tom described. The binding of the sperm to egg requires a sperm specific protein on its outer membrane to bind to a membrane receptor protein found on the egg. If these two proteins can't bind, which is often the case between different species, the sperm can't penetrate the egg and eventually induce cell division.

Answer 6

Mechanically it is not possible in most instances. That is known as a prezygotic barrier. The haploid gametes would be unable to produce a viable diploid organism since the numbers would not match. Even if two species are able to procreate, their offspring would not be fertile as a result of the mismatched gametes, for example, a horse and a donkey produce a mule which is infertile. The animal sperm would not be able to survive in the human envirornment and it would not be able to penetrate the human egg. Oh yea AP bio :D
hope that helped

Answer 7

each species have a different number of chromosomes in their cells. Humans have 46 in a normal cell, 23 in sex cells (cause sperm and ovum combine to make 46). That's what makes us human. Mice for example, have 34 in a normal cell, 17 in a sex cell meaning if a mouse were to combine with a human the resulting cell would only have 40 chromosomes, so it wouldn't work.

Answer 8

The genetic code needs to be able to link up on both sides of the joining...therefore it needs to be of the same make-up. For instance if I had a genenic code ABC...I would need to have another ABC code to join...A to A, B to B and C to C. I would not be able to join with a code of say BCA...because it is not in the same order. There is a lot more to it of course, including total number of chromosomes, chemical make-up of the cells, the environment the division is trying to occur in ect. The womb will kill anything that it sees as foreign to its self. People have been able to modify a human's chromosome to bond with a chimps in an experiment in Japan, because they are so close.

Answer 9

Genes are very smart, think of a lock and key, not any key can open any lock, same thing with chromosomes, they can recognize when the human genes wont fit with that of an animals so it wouldnt fertilize.m

Answer 10

Species' gametes are largely identified by their chromosome numbers. If gametes don't have the same chromosome numbers they aren't compatible. End of story.