- Identify three methods of asexual reproduction.
- Give an overview of sexual reproduction.
- Explain how meiosis produces haploid gametes.
- State advantages of asexual and sexual reproduction.
- asexual reproduction
- homologous chromosomes
- sexual reproduction
Reproduction is how organisms produce offspring. The ability to reproduce is a characteristic of all living things. In some species, all the offspring are genetically identical to the parent. In other species, each offspring is genetically unique. Look at the kittens in Figure below. They are brothers and sisters, but they are all different from each other. Why does this happen in some species but not others? It’s because there are two types of reproduction. Reproduction can be sexual or asexual.
These kittens have the same parents, but each kitten is unique.
Asexual reproduction is simpler than sexual reproduction. It involves just one parent. The offspring are genetically identical to each other and to the parent. All prokaryotes and some eukaryotes reproduce this way. There are several different methods of asexual reproduction. They include binary fission, fragmentation, and budding.
Binary fission occurs when a parent cell simply splits into two daughter cells. This method is described in detail in the lesson "Cell Division." Bacteria reproduce this way. You can see a bacterial cell reproducing by binary fission in Figure below.
Binary fission in a bacterium
Fragmentation occurs when a piece breaks off from a parent organism. Then the piece develops into a new organism. Sea stars, like the one in Figure below, can reproduce this way. In fact, a new sea star can form from a single “arm.”
A sea star can reproduce by asexually by fragmentation. It can also reproduce sexually.
Budding occurs when a parent cell forms a bubble-like bud. The bud stays attached to the parent while it grows and develops. It breaks away from the parent only after it is fully formed. Yeasts can reproduce this way. You can see two yeast cells budding in Figure below.
Budding in yeast cells
Sexual reproduction is more complicated. It involves two parents. Special cells called gametes are produced by the parents. A gamete produced by a female parent is generally called an egg. A gamete produced by a male parent is usually called a sperm. An offspring forms when two gametes unite. The union of the two gametes is called fertilization. You can see a human sperm and egg uniting in Figure below. The initial cell that forms when two gametes unite is called a zygote.
Fertilization: human sperm and egg
In species with sexual reproduction, each cell of the body has two copies of each chromosome. For example, human beings have 23 different chromosomes. Each body cell contains two of each chromosome, for a total of 46 chromosomes. You can see the 23 pairs of human chromosomes in Figure below. The number of different types of chromosomes is called the haploid number. In humans, the haploid number is 23. The number of chromosomes in normal body cells is called the diploid number. The diploid number is twice the haploid number. In humans, the diploid number is two times 23, or 46.
Humans have 23 pairs of chromosomes in each body cell
The two members of a given pair of chromosomes are called homologous chromosomes. We get one of each homologous pair, or 23 chromosomes, from our father. We get the other one of each pair, or 23 chromosomes, from our mother. A gamete must have the haploid number of chromosomes. That way, when two gametes unite, the zygote will have the diploid number. How are haploid cells produced? The answer is meiosis.
Meiosis is a special type of cell division. It produces haploid daughter cells. It occurs when an organism makes gametes. Meiosis is basically mitosis times two. The original diploid cell divides twice. The first time is called meiosis I. The second time is called meiosis II. However, the DNA replicates only once. It replicates before meiosis I but not before meiosis II. This results in four haploid daughter cells.
Meiosis I and meiosis II occurs in the same four phases as mitosis. The phases are prophase, metaphase, anaphase, and telophase. However, meiosis I has an important difference. In meiosis I, homologous chromosomes pair up and then separate. As a result, each daughter cell has only one chromosome from each homologous pair.
Figure below is a simple model of meiosis. It shows both meiosis I and II. You can read more about the stages below. You can also learn more about them by watching this video:
Meiosis occurs in two stages: meiosis I and meiosis II
After DNA replicates during interphase, the nucleus of the cell undergoes the four phases of meiosis I:
- Prophase I: Chromosomes form, and the nuclear membrane breaks down. Centrioles move to opposite poles of the cell. Spindle fibers form between the centrioles. Here’s what’s special about meiosis: Homologous chromosomes pair up! You can see this in Figure below.
- Metaphase I: Spindle fibers attach to the centromeres of the paired homologous chromosomes. The paired chromosomes line up at the center of the cell.
- Anaphase I: Spindle fibers shorten, pulling apart the chromosome pairs. The chromosomes are pulled toward opposite poles of the cell. One of each pair goes to one pole. The other of each pair goes to the opposite pole.
- Telophase I: The chromosomes uncoil, and the spindle fibers break down. New nuclear membranes form.
Meiosis I is followed by cytokinesis. That's when the cytoplasm of the cell divides. Two haploid daughter cells result. Both of these cells go on to meiosis II.
Meiosis II is just like mitosis.
- Prophase II: Chromosomes form. The nuclear membrane breaks down. Centrioles move to opposite poles. Spindle fibers form.
- Metaphase II: Spindle fibers attach to the centromeres of sister chromatids. Sister chromatids line up at the center of the cell.
- Anaphase II: Spindle fibers shorten. They pull the sister chromatids to opposite poles.
- Telophase II: The chromosomes uncoil. The spindle fibers break down. New nuclear membranes form.
Meiosis II is also followed by cytokinesis. This time, four haploid daughter cells result. That’s because both daughter cells from meiosis I have gone through meiosis II. The four daughter cells must continue to develop before they become gametes. For example, in males, the cells must develop tails, among other changes, in order to become sperm.
Advantages of Sexual and Asexual Reproduction
Both types of reproduction have certain advantages.
Advantage of Asexual Reproduction
Asexual reproduction can happen very quickly. It doesn’t require two parents to meet and mate. Under ideal conditions, 100 bacteria can divide to produce millions of bacteria in just a few hours! Most bacteria don’t live under ideal conditions. Even so, rapid reproduction may allow asexual organisms to be very successful. They may crowd out other species that reproduce more slowly.
Advantage of Sexual Reproduction
Sexual reproduction is typically slower. However, it also has an advantage. Sexual reproduction results in offspring that are all genetically different. This can be a big plus for a species. The variation may help it adapt to changes in the environment.
How does genetic variation arise during sexual reproduction? It happens in three ways: crossing over, independent assortment, and the random union of gametes.
- Crossing over occurs during meiosis I. It happens when homologous chromosomes pair up during prophase I. The paired chromosomes exchange bits of DNA. This recombines their genetic material. You can see where crossing over has occurred in Figures 5.15 and 5.16.
- Independent assortment occurs when chromosomes go to opposite poles of the cell in anaphase I. Which chromosomes end up together at each pole is a matter of chance. You can see this in Figures 5.15 and 5.16 as well.
- In sexual reproduction, two gametes unite to produce an offspring. Which two gametes is a matter of chance. The union of gametes occurs randomly.
Due to these sources of variation, each human couple has the potential to produce more than 64 trillion unique offspring. No wonder we are all different!
- Asexual reproduction involves just one parent. It produces offspring that are genetically identical to the parent. Methods of asexual reproduction include binary fission, fragmentation, and budding.
- Sexual reproduction involves two parents. It produces offspring that are all genetically unique. It requires the production of haploid gametes. The union of gametes is called fertilization. It results in a diploid zygote.
- Haploid gametes are produced through meiosis. This is a special type of cell division. The cell divides twice, called meiosis I and meiosis II. However, the DNA replicates just once. Homologous chromosomes separate. The outcome is four haploid cells.
- Asexual reproduction has the advantage of occurring quickly. Sexual reproduction has the advantage of creating genetic variation. This can help a species adapt to environmental change. The genetic variation arises due to crossing over, independent assortment, and the random union of gametes.
Lesson Review Questions
- What are three methods of asexual reproduction? For each method, give an example of an organism that can reproduce that way.
- Briefly describe sexual reproduction.
- Define haploid and diploid numbers. Which cells are haploid and which are diploid?
- If you don’t have an identical twin, how likely is it that a brother or sister would be just like you?
- A single-celled organism belongs to the Eukarya Domain. Apply lesson concepts to describe how the organism divides.”
- Some organisms can reproduce sexually or asexually. Under what conditions might each type of reproduction be an advantage?
Points to Consider
All of our cells contain DNA. Meiosis ensures that each gamete receives a copy of each chromosome.
- Why do cells need DNA?
- What specific role does DNA play?