Reproduction

The ability of cells to divide allows them to reproduce and pass along their genetic information to offspring, which is essential for life. There are two kinds of reproduction: asexual and sexual. Students experiment by analyzing the growth rates of dissected planarians to evaluate the advantages and disadvantages of asexual reproduction.

• One essential purpose of cell division is reproduction—the ability of a mature organism to have offspring.
• When organisms reproduce, their DNA gets passed along to their offspring, which is why organisms have different strategies for increasing their chances of reproduction.
• Sexual reproduction is one of two forms of reproduction (the other being asexual reproduction), and it results in offspring that are genetically different from their parents.
• Asexual reproduction is different from sexual reproduction because it requires just one parent. The offspring of an asexually reproducing organism possess nearly all the same genes as their parent.

• Why do cells reproduce?
• Why does sexual reproduction result in genetically different offspring?
• How are gametes different from other cells?
• How does meiosis allow for genetic information from two parents?
• Why is conjugation another form of sexual reproduction?
• How closely do the offspring of organisms that reproduce asexually resemble their parents?
• How is fragmentation a form of asexual reproduction?
• Why can asexual reproduction be advantageous?
• What are some disadvantages to asexual reproduction?
• How might sexual reproduction ensure greater survival of offspring?

Misconception: Organisms grow because cells get bigger.

Fact: Organisms grow because cells divide. 

Misconception: Cell division results in a decrease in the number of chromosomes.

Fact: In mitosis, cells divide to create exact replicas of themselves, with the same number and kind of chromosome. In meiosis, the daughter cells have half the number of chromosomes as the parent cell.

Asexual reproduction : reproduction that requires only one parent (e.g., binary fission, budding, and fragmentation)

Fertilization : the fusion of two gametes (cells formed from meiotic division), one from a male and the other from a female, to produce a new organism

Fragmentation : the development of a new individual from parts of a parent that broke off and regenerated into a complete organism

Meiosis : a form of cell division that results in four daughter cells, each with half the chromosome number of the parent cell

Reproduction : the ability of a mature organism to have offspring

Sexual reproduction : the creation of a new individual from combined genetic information of two parents of different sexes

The Peacock Spider

One day, a scientist named Jürgen Otto went for a walk in the woods when he noticed an unusual tiny jumping spider. Otto is a scientist who studies insects, so he was particularly interested in the nimbleness of the spider, called a peacock spider.

Otto became intrigued by the spider and researched it. He learned that not a lot was known about the spider, so he began to study it. After a couple of years, he documented something no one had seen before. The male peacock spider performs an elaborate dance that includes hopping, waving its legs, and vibrating its abdomen. The spider also raises a colorful fan-like flap during the dance.

Scientists believe that the male spider’s dance is so elaborate, and its fan so colorful, for a reason: it is trying to attract a female. An elaborate dance and a vibrant colorful fan are adaptations that indicate to the potential female mate that the male has good genes. This is important to the female who wants to produce offspring with the same good genes.

A driving force behind all life is the urge to reproduce and pass along genes to future offspring. Reproduction is the ability of a mature organism to have offspring. When organisms reproduce, their DNA gets passed along to their offspring. This is why children resemble their parents in different ways.

There are two types of reproduction: sexual reproduction and asexual reproduction. The peacock spider uses sexual reproduction, which combines the genetic information of two parents of different sexes to create a new individual or individuals.

Sexual Reproduction

Multi-celled organisms such as the peacock spider use a sexual reproduction strategy called fertilization. Fertilization is the fusion of two gametes, one from a male and the other from a female, to produce a new organism. Gametes are cells that only contain half of an organism’s chromosomes. When two gametes combine, they create a new individual that has a complete set of chromosomes.

Gametes are created by another form of cell division that takes place in eukaryotic cells called meiosis, during which the cell divides twice (once in meiosis I and once in meiosis II). The result is four daughter cells, each with half the chromosome number of the parent cell.

IMeiosis only happens in cells that are preparing for reproduction. Just as in mitosis, the chromosomes must duplicate before meiosis can take place. The same four phases of mitosis—prophase, metaphase, anaphase, and telophase—occur during both meiosis I and meiosis II. However, there are a few key differences at various steps in the process.

One significant difference in meiosis is that the non-sister chromatids exchange genetic information. This is called crossing-over, and it occurs at random places along the chromosomes. This is an important difference from mitosis because it results in chromosomes that have different genetic information from the parent chromosomes. Another key difference is that the nucleus divides twice, which produces four daughter cells.

Human males produce gametes called sperm. Human females produce gametes called eggs. When a sperm and egg fuse together, they form an embryo. The embryo grows into a complete human being. In flowering plants, pollen is the male gamete and the ovule is the female gamete. Fertilization is common for multi-celled organisms. The most common sexual reproduction strategy for single-celled organisms is conjugation. Parameciums “conjugate” by forming a bridge between two cells. A donor cell transfers its genetic information over the bridge, which then becomes a part of the receiving cell’s DNA.

There are some advantages to sexual reproduction. For example, because each offspring has a different set of traits, certain individuals have a greater advantage over threats than others.

This isn’t true with asexual reproduction. Asexual reproduction requires only one parent. Because of this, the offspring of an asexually reproducing organism possess nearly all the same genes as their parent. Some differences happen as a result of genetic mutations during the DNA replication.

Asexual Reproduction

The most common strategy for prokaryote reproduction is splitting in half. This is called fission, and it is similar to but not the same as mitosis because prokaryotes do not have a nucleus. The single-celled parent divides into two or more daughter cells of equal size. The daughter cells grow in size and then also divide. Some eukaryotic organisms such as paramecium also use fission to split in half.

Yeast are single-celled fungi that reproduce asexually by budding. A bud appears on the parent cell and receives nutrients from the parent until it is fully mature. The bud then breaks off and becomes a unique individual with the same genes as its parent.

Fragmentation is another form of asexual reproduction where a new individual develops from a part of a parent that broke off and regenerated into a complete organism. Some sea stars and flatworms can reproduce this way.

Asexual reproduction is common in organisms with very short life cycles and with species that need dense populations to overcome predation and other threats. The problem with asexual reproduction is that if a parent organism cannot survive in a changing environment, neither will any of its offspring. The exception is when one of the offspring has a mutation that allows it to better survive. These mutations often lead to new types of organisms.

Asexual or Sexual?

There are some organisms that are able to reproduce both asexually and sexually, including sea stars, sponges, mushrooms, and parameciums.

Sea stars can reproduce through fertilization, with male sea stars releasing sperm and females releasing eggs into the environment. When a sperm joins with an egg, it forms a free-floating embryo that will eventually grow into an adult that has genetic material from both the male and the female. However, sometimes part of a sea star’s arm and central disk become detached from the rest of the body. In certain cases, this detached portion can grow into another organism that is genetically identical to the original sea star.

The ability to reproduce both asexually and sexually offers organisms the benefits of both kinds of reproduction. For example, being able to reproduce asexually eliminates the need for finding a mate and ensures that all of an organism’s genes get passed along to offspring, which is beneficial for organisms that are already well adapted to their environment. As a result, asexual reproduction is much more efficient than sexual reproduction, which requires time and energy. However, sexual reproduction ensures genetic variation, which increases the likelihood that some individuals will be able to survive challenging environments.

For example, the paramecium is a unicellular eukaryote that alternates between asexual and sexual reproduction. It mostly reproduces asexually by fission. The organism divides, splitting in two by pinching off in the middle. In ideal conditions, a paramecium can reproduce asexually two or three times a day. However, if the environment becomes stressful, such as when there is overcrowding or scarce food, paramecium will begin to reproduce sexually by conjugation—two paramecium will join together and exchange genetic information.

In this lesson, students use a microscope to observe prepared slides of paramecium undergoing phenomena asexual and sexual reproduction, and then carry out an experiment to analyze the regeneration rate of fragmented planarians. Students gather data from their experiment as evidence to figure out and explain the advantages and disadvantages of asexual reproduction compared to sexual reproduction.