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12.10Problems

 

 

Explain the mechanism that guarantees the species-specificity observed during mammalian fertilization.

 

Describe the biological significance of meiosis.

 

 

Outline the advantages and disadvantages of both asexual and sexual reproduction in terms of species survival and propagation.

Determine whether the following statements are correct and, if incorrect, provide the reasons.

A)

The mechanism of producing haploid cells from diploid cells is called meiosis.

B)

In meiosis, only one copy of each chromosome type is allocated to a germ cell.

C)

Variations that occur during meiosis are not passed on to the next generation

D)

Since a germinated pollen has two spermatids, two embryos are produced by fertilization.

E)

Multicellular organisms do not perform asexual reproduction under natural conditions.

 

 

Explain the mechanism that guarantees the species-specificity observed during mammalian fertilization.

The zona pellucida surrounding an ovum contains the glycoprotein, Z3, which species-specifically binds to Z3 receptors on the plasma membrane of a sperm. This mechanism prevents interspecies fertilization. The species-specific binding of a sperm to the zona pellucida triggers the acrosome reaction. Hydrolase stored in the acrosomal vesicle of a sperm is then released to the zona pellucida and degrades it, thus allowing the sperm to penetrate. A sperm cannot enter an ovum without this species-specific binding to the zona pellucida.

 

Describe the biological significance of meiosis.

The purpose of meiosis is to provide gametes - the origins of next-generation progeny - with diverse gene combinations by mixing paternal and maternal genes. In meiosis, crossover occurs between paired homologous chromosomes, resulting in gene recombination. During chromosome segregation, chromosomes are also distributed to gametes in many combinations, thus producing gametes with diverse gene sets.

 

 

Outline the advantages and disadvantages of both asexual and sexual reproduction in terms of species survival and propagation.

The number of progenies can quickly be increased through asexual reproduction, although their genetic information is essentially the same. The probability of producing individuals that can adapt to environmental changes is low, so the number of individuals that can deal with such changes is enhanced by increasing the number of progenies. In sexual reproduction, the process of creating progeny is complex and takes time. However, since individuals are produced through the fusion of paternal and maternal gametes, gene combinations are diverse, and virtually all individuals have different genetic information. Despite the small number of progenies created, the probability of them withstanding environmental changes and surviving is higher.

Determine whether the following statements are correct and, if incorrect, provide the reasons.

A)

The mechanism of producing haploid cells from diploid cells is called meiosis.

Correct.

B)

In meiosis, only one copy of each chromosome type is allocated to a germ cell.

Correct.

C)

Variations that occur during meiosis are not passed on to the next generation

Incorrect. Variations that occur during meiosis can be passed on to the next generation, unless they make gametes inviable.

D)

Since a germinated pollen has two spermatids, two embryos are produced by fertilization.

Incorrect. Of the two spermatids, one fuses with the ovum and the other fuses with the central cell (a process known as double fertilization). As a result, one embryo is produced.

E)

Multicellular organisms do not perform asexual reproduction under natural conditions.

Incorrect, as demonstrated by the phenomenon of parthenogenesis. Examples include vegetative reproduction in plants (e.g., potatoes and bulbs) and water fleas under favorable reproduction conditions (water fleas perform sexual reproduction when they lay dispause eggs).

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