5.3 Reproduction and Meiosis

Learning Objectives

Name the types of asexual reproduction.
Explain the advantage of sexual reproduction.
List the stages of meiosis and explain what happens in each stage.

Introduction

Can some animals have one parent instead of two?

Not all animals have two parents. When necessary, some animals can be produced from just one parent. Some reptiles, such as this Komodo dragon, can have only one parent. The process of creating offspring from just one individual is called asexual reproduction.

Guided Practice

What is Reproduction?

What does reproduction mean? Can an organism be considered alive if it cannot make the next generation? Since individuals cannot live forever, they must reproduce for the species to survive. Reproduction is the ability to make the next generation.

Two methods of reproduction are:

Asexual reproduction, or the process of forming a new individual from a single parent.
Sexual reproduction, or the process of forming a new individual from two parents.

There are advantages and disadvantages to each method, but the result is always the same: a new life begins.

Asexual Reproduction

For humans to reproduce, DNA must be passed from the mother and father to the child. Humans cannot reproduce with just one parent, but it is possible in other organisms, like bacteria, and some insects and some fish. These organisms can reproduce asexually, meaning that the offspring (children) have a single parent and share the exact same genetic material as the parent. This is very different from humans.

The advantage of asexual reproduction is that it can be very quick and does not require the meeting of a male and female organism. The disadvantage of asexual reproduction is that organisms cannot mix beneficial traits from both parents. An organism that is born through asexual reproduction only has the DNA from the one parent, and it is the exact copy of that parent. This can cause problems for the individual. For example, if the parent organism has a gene that causes cancer, the offspring will also have the gene that causes cancer. Organisms produced sexually may or may not inherit the cancerous gene because there are two parents mixing up their genes.

Types of organisms that reproduce asexually include:

Prokaryotic organisms, like bacteria. Bacteria reproduce through binary fission, where they grow and divide in half (Figure below). First, their chromosome replicates (bacteria only have one chromosome) and the cell enlarges. After cell division, the two new cells each have one identical chromosome (mitosis is not necessary because bacteria do not have nuclei). Then, new membranes form to separate the two cells. This simple process allows bacteria to reproduce very rapidly.
Flatworms divide in two, then each half regenerates into a new flatworm identical to the original.
Different types of insects, fish, and lizards. These organisms can reproduce asexually through a process called parthenogenesis (Figure below). Parthenogenesis happens when an unfertilized egg cell grows into a new organism. The resulting organism has half the amount of genetic material of the parent. Parthenogenesis is common in honeybees. In a hive, the sexually produced eggs become workers, while the asexually produced eggs become drones.

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Bacteria reproduce by binary fission. Shown is one bacterium reproducing and becoming two bacteria.

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This Komodo dragon was born by parthenogenesis.

Sexual Reproduction

During sexual reproduction, two parents are involved, with the male producing sperm and the female producing eggs. When a sperm and egg meet, a zygote, the first cell of a new organism, is formed (Figure below). The zygote will divide and grow into the embryo.

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During sexual reproduction, a sperm fertilizes an egg.

Let's explore how animals, plants, and fungi reproduce sexually:

The reproductive organs of animals produce eggs or sperm. The male reproductive organ is the testes, which produce the sperm, and the female reproductive organs are the ovaries, which produce the eggs. Sperm and egg, the two sex cells, are known as gametes, and can combine two different ways:

Fish and other aquatic animals release their gametes in the water, which is called external fertilization. These gametes will combine by chance. (Figure below).
Animals that live on land reproduce by internal fertilization. Typically males have a penis that deposits sperm into the vagina of the female. Birds do not have penises, but they do have a chamber called the cloaca that they place close to another bird’s cloaca to deposit sperm.

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This fish guards her eggs, which will be fertilized externally.

Plants can also reproduce sexually, but their reproductive organs are different from animals. Plants that have flowers have their reproductive parts in the flower. The sperm is contained in the pollen, while the egg is contained in the ovary, deep within the flower. The sperm can reach the egg two different ways:

In self-pollination, the egg is fertilized by the pollen of the same flower.
In cross-pollination, sperm from the pollen of one flower fertilizes the egg of another flower. Like other types of sexual reproduction, cross-pollination allows new combinations of traits. Cross-pollination occurs when pollen is carried by the wind to another flower. It can also occur when animal pollinators, like honeybees, or butterflies (Figure below) carry the pollen from flower to flower.

Fungi can also reproduce sexually, but instead of female and male sexes, they have (+) and (-) strains. When the filaments of a (+) and (-) fungi meet, the zygote is formed. Just like in plants and animals, each zygote receives DNA from two parent strains.

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Butterflies receive nectar when they deposit pollen into flowers, resulting in cross-pollination.

Meiosis and Gametes

Meiosis is a process of cell division that produces sex cells, or gametes. Gametes are reproductive cells, such as sperm and egg. As gametes are produced, the number of chromosomes must be reduced by half. Why? The zygote must contain information from the mother and from the father, so the gametes must contain half of the chromosomes found in normal body cells.

In humans, our cells have 23 pairs of chromosomes, or 46 total. For each of the 23 chromosome pairs, you received one chromosome from your father and one chromosome from your mother. The chromosomes are separated when gametes are formed. Therefore, gametes have only 23 chromosomes, not 23 pairs.

Before meiosis begins, DNA replication occurs, so each chromosome contains two sister chromatids that are identical to the original chromosome. Then the cell must divide twice to resulting in 23 chromosomes in each cell.

The simplified process is below.

The normal number of chromosomes doubles. Using humans as an example, the nucleus would go from 46 chromosomes to 92 chromosomes.
First Cell Division: occurs as the cell divides into two. Each cell contains 46 chromosomes; but because gametes contain 23 chromosomes, the cells must divide again.
Second Cell Division: occurs as the two cells divide into four. This division results in cells with 23 chromosomes each. These cells are gametes, or sex cells: the sperm for males and the eggs for females.

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An overview of meiosis.

Alleles are alternate forms of genes found on chromosomes. Since the separation of chromosomes into gametes is random, it results in different combinations of chromosomes (and alleles) in each gamete. With 23 pairs of chromosomes, there is a possibility of over 8 million different combinations of chromosomes in a gamete.

Haploid vs. Diploid

A cell with two sets of chromosomes is diploid, referred to as 2n, where n is the number of sets of chromosomes. Most of the cells in a human body are diploid. A cell with one set of chromosomes, such as a gamete, is haploid, referred to as n. Sex cells are haploid. When a haploid sperm (n) and a haploid egg (n) combine, a diploid zygote will be formed (2n). In short, when a diploid zygote is formed, half of the DNA comes from each parent.

	Mitosis	Meiosis
Purpose:	To produce new cells	To produce gametes
Number of cells produced:	2	4
Rounds of Cell Division:	1	2
Haploid or Diploid:	Diploid	Haploid
Daughter cells identical to parent cells?	Yes	No
Daughter cells identical to each other?	Yes	No