Asexual and Sexual Reproduction

Reproduction and Meiosis

Use this to help you find and remember the most important parts of the reading material.

Lesson Objectives

• Compare and contrast asexual and sexual reproduction.
• Give an overview of sexual reproduction, and outline the phases of meiosis.
• Explain why sexual reproduction leads to variation in offspring.
• Define life cycle, and identify different types of sexual life cycles.

Introduction

Cell division is how organisms grow and repair themselves. It is also how they produce offspring. Many single-celled organisms reproduce by binary fission. The parent cell simply divides to form two daughter cells that are identical to the parent. In many other organisms, two parents are involved, and the offspring are not identical to the parents. In fact, each offspring is unique. Look at the family in Figure below. The children resemble their parents, but they are not identical to them. Instead, each has a unique combination of characteristics inherited from both parents. In this lesson, you will learn how this happens.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Vocabulary

allele

An alternative form of a gene.

asexual reproduction

A form of reproduction in which a new individual is created by only one parent.

binary fission

An asexual form of reproduction where a cell splits into two daughter cells.

crossing-over

Exchange of DNA segments between homologous chromosomes; occurs during prophase I of meiosis.

cross-pollination

Sexual reproduction in plants where sperm from the pollen of one flower is received by the ovary of another flower.

diploid

When a cell has two sets of chromosomes.

gametes

Cells involved in sexual reproduction; typically egg and sperm cells.

gonads

Organ that produces gametes, such as the ovaries and testes.

haploid

When a cell has only one set of chromosomes, typical of a gamete.

internal fertilization

Reproduction occurs through the internal deposit of gametes.

external fertilization

Reproduction where the eggs are fertilized outside the body.

meiosis

Nuclear division that results in haploid gametes.

ovaries

Female gonads in animals that produce eggs.

parthenogenesis

Reproduction where an unfertilized egg develops into a new individual.

sexual reproduction

Reproduction where gametes from two parents combine to make an individual with an unique set of genes.

sister chromatids

Two genetically identical chromosome segments that form after DNA replication.

testes

Male gonads in animals that produce sperm.

zygote

Single cell that is formed after the fertilization of an egg; the first cell of a new organism.

Reproduction: Asexual vs. Sexual

Production of new individuals along a leaf margin of the air plant, Kalanchoe pinnata. The small plant in front is about 1 cm tall. The concept of "individual" is obviously stretched by this asexual reproductive process.

Reproduction is the process by which organisms give rise to offspring. It is one of the defining characteristics of living things. There are two basic types of reproduction: asexual reproduction and sexual reproduction.

Asexual Reproduction

Asexual reproduction involves a single parent. It results in offspring that 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 splits into two identical daughter cells of the same size. This process was described in detail in Lesson 12.1.
• Fragmentation occurs when a parent organism breaks into fragments, or pieces, and each fragment develops into a new organism. Starfish, like the one in Figure below, reproduce this way. A new starfish can develop from a single ray, or arm.
• Budding occurs when a parent cell forms a bubble-like bud. The bud stays attached to the parent cell while it grows and develops. When the bud is fully developed, it breaks away from the parent cell and forms a new organism. Budding in yeast is shown in Figure below.

Asexual reproduction can be very rapid. This is an advantage for many organisms. It allows them to crowd out other organisms that reproduce more slowly. Bacteria, for example, may divide several times per hour. Under ideal conditions, 100 bacteria can divide to produce millions of bacterial cells in just a few hours! However, most bacteria do not live under ideal conditions. If they did, the entire surface of the planet would soon be covered with them. Instead, their reproduction is kept in check by limited resources, predators, and their own wastes. This is true of most other organisms as well.

Sexual Reproduction

Sexual reproduction involves two parents. As you can see from Figure below, in sexual reproduction, parents produce reproductive cells—called gametes—that unite to form an offspring. Gametes are haploid cells. This means they contain only half the number of chromosomes found in other cells of the organism. Gametes are produced by a type of cell division called meiosis, which is described in detail below. The process in which two gametes unite is called fertilization. The fertilized cell that results is referred to as a zygote. A zygote is diploid cell, which means that it has twice the number of chromosomes as a gamete.

Mitosis, Meiosis and Sexual Reproduction is discussed in the following video:

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Meiosis and Gametogenesis

The process that produces haploid gametes is meiosis. Meiosis is a type of cell division in which the number of chromosomes is reduced by half. It occurs only in certain special cells of the organisms. During meiosis, homologous chromosomes separate, and haploid cells form that have only one chromosome from each pair. Two cell divisions occur during meiosis, and a total of four haploid cells are produced. The two cell divisions are called meiosis I and meiosis II. The overall process of meiosis is summarized in Figure below. It is also described in detail below.

Overview of Meiosis. During meiosis, homologous chromosomes separate and go to different daughter cells. This diagram shows just the nuclei of the cells.

Phases of Meiosis

Meiosis I begins after DNA replicates during interphase. In both meiosis I and meiosis II, cells go through the same four phases as mitosis. However, there are important differences between meiosis I and mitosis. The flowchart in the figure below shows what happens in both meiosis I and II. You can follow the changes in the flowchart as you read about them below.

The phases of meiosis are discussed in this video:

Meiosis I

1. Prophase I: The nuclear envelope begins to break down, and the chromosomes condense. Centrioles start moving to opposite poles of the cell, and a starts begins to grow. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of mitosis and meiosis II, homologous chromosomes do not form pairs in this way.
2. Metaphase I: Spindle fibers attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.
3. Anaphase I: Spindle fibers shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.
4. Telophase I and Cytokinesis: The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.

Meiosis II

1. Prophase II: The nuclear envelope breaks down and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.
2. Metaphase II: Spindle fibers line up the sister chromatids of each chromosome along the equator of the cell.
3. Anaphase II: Sister chromatids separate and move to opposite poles.
4. Telophase II and Cytokinesis: The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.

Mitosis, Meiosis and Sexual Reproduction is discussed in the following video:

Gametogenesis

At the end of meiosis, four haploid cells have been produced, but the cells are not yet gametes. The cells need to develop before they become mature gametes capable of fertilization. The development of haploid cells into gametes is called gametogenesis. Gametogenesis may differ between males and females. Male gametes are called sperm. Female gametes are called eggs. In human males, for example, the process that produces mature sperm cells is called spermatogenesis. During this process, sperm cells grow a tail and gain the ability to “swim,” like the human sperm cell shown in Figure below. In human females, the process that produces mature eggs is called oogenesis. Just one egg is produced from the four haploid cells that result from meiosis. The single egg is a very large cell, as you can see from the human egg in Figure below.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Sexual Reproduction and Genetic Variation

Sexual reproduction results in offspring that are genetically unique. They differ from both parents and also from each other. This occurs for a number of reasons.

• When homologous chromosomes pair up during meiosis I, crossing-over can occur. Crossing-over is the exchange of genetic material between homologous chromosomes. It results in new combinations of genes on each chromosome.
• When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This called is called independent assortment. It results in gametes that have unique combinations of chromosomes.
• In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is likely to be a matter of chance. It is obviously another source of genetic variation in offspring.

All of these mechanisms working together result in an amazing amount of potential variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Sexual Reproduction and Life Cycles

Sexual reproduction occurs in a cycle. Diploid parents produce haploid gametes that unite and develop into diploid adults, which repeat the cycle. This series of life stages and events that a sexually reproducing organism goes through is called its life cycle. Sexually reproducing organisms can have different types of life cycles. Three are represented in Figure below and described following sections.

Haploid Life Cycle

The haploid life cycle is the simplest life cycle. It is found in many single-celled organisms. Organisms with a haploid life cycle spend the majority of their lives as haploid gametes. When the haploid gametes fuse, they form a diploid zygote. It quickly undergoes meiosis to produce more haploid gametes that repeat the life cycle.

Diploid Life Cycle

Organisms with a diploid life cycle spend the majority of their lives as diploid adults. When they are ready to reproduce, they undergo meiosis and produce haploid gametes. Gametes then unite in fertilization and form a diploid zygote. The zygote develops into a diploid adult that repeats the life cycle. Can you think of an organism with a diploid life cycle? (Hint: What type of life cycle do humans have?)

Alternation of Generations

Organisms that have a life cycle with alternating generations switch back and forth between diploid and haploid stages. Organisms with this type of life cycle include plants, algae, and some protists. These life cycles may be quite complicated. You can read about them in later chapters.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/

Lesson Summary

• Asexual reproduction involves one parent and produces offspring that are genetically identical to each other and to the parent. Sexual reproduction involves two parents and produces offspring that are genetically unique.
• During sexual reproduction, two haploid gametes join in the process of fertilization to produce a diploid zygote. Meiosis is the type of cell division that produces gametes. It involves two cell divisions and produces four haploid cells.
• Sexual reproduction has the potential to produce tremendous genetic variation in offspring. This variation is due to independent assortment and crossing-over during meiosis and random union of gametes during fertilization.
• A life cycle is the sequence of stages an organisms goes through from one generation to the next. Organisms that reproduce sexually can have different types of life cycles, such as haploid or diploid life cycles.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/