Download Chapter 4 • Lesson 23

Chapter 4 • Lesson 23
Meiosis and Genetic Variation
Objective: 3,2,1
Key Words
• sexual reproduction • gamete • somatic cell • mitosis • variations • haploid number
• fertilization • diploid number • homologous chromosomes • meiosis • crossing-over
• independent assortment • asexual reproduction
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Getting the Idea
About 7 billion people live on Earth. With the exception of identical twins, no two of those
people are genetically alike. Earth's human population is tremendously diverse. For
example, there are probably thousands of shades of skin color among the world's
population. Many other traits also vary widely. You may have inherited your mother's blood
type and your father's hair color. Such diversity, although perhaps more subtle, exists
among all organisms that reproduce sexually.
Sexual Reproduction and Variation
Humans and most other multicellular organisms reproduce sexually. Recall that in sexual
reproduction, cells from two parents join to form a new individual. Offspring produced
sexually are genetically different from either parent because sexual reproduction combines
two genetically unique reproductive cells, called gametes or sex cells.
In multicellular organisms, such as humans, the somatic cells, or body cells, divide by
mitosis. Recall from Lesson 5 that mitosis is a type of cell division in which the cell nucleus
divides in two. In this process, the DMA of the original cell is divided equally between two
daughter cells. Each daughter cell has the same number and kinds of chromosomes as the
parent cell.
If two somatic cells joined together, the resulting cell would have twice as many
chromosomes as it should. This does not happen because gametes are not produced by
mitosis. Instead, they are produced by a kind of cell division called meiosis. You will learn
how this process occurs later in the lesson. Meiosis results in variations—differences in
traits among the members of a species or population.
Chromosomes
Recall that chromosomes are structures that contain the cell's genetic material. Each
human gamete has 23 chromosomes. The gametes of other organisms may have more or
fewer chromosomes. For example, each gamete of a cat contains 19 chromosomes, a
rabbit 22, a crayfish 100, and a tomato 12. The number of chromosomes in a gamete is
called the haploid number. Haploid refers to "half." The chromosome number in gametes
is haploid because sex cells contain half as many chromosomes as somatic cells.
Sexual reproduction requires the joining of two gametes, a sperm from the male parent and
an egg from the female parent. The process in which a sperm and an egg combine is called
fertilization. The zygote, or fertilized egg that results from this process, develops into a new
multicellular organism.
During fertilization in humans, two gametes combine to form a cell with 46 chromosomes.
The resulting offspring has 46 chromosomes in every somatic cell, or body cell. The
chromosomes are arranged in 23 pairs.
The number of chromosomes in each somatic cell is the diploid number. A zygote receives
one set of chromosomes from each gamete. The chromosomes from the two parents
combine to form pairs of homologous chromosomes. Homologous chromosomes are
paired chromosomes that have matching genes but may have different alleles. Each
gamete is different, so each organism inherits a different combination of alleles. The varied
combinations of alleles give different offspring different traits.
Meiosis and Gamete Formation
Sperm and egg cells form by meiosis. Meiosis is a process of cell division that reduces the
number of chromosomes by half. During meiosis, each sex cell divides twice, in stages
called meiosis I and meiosis II. As the diagram below illustrates, meiosis begins with a
single diploid cell and produces four genetically different haploid cells.
During meiosis I (the first division in the diagram above), the homologous chromosomes of
the parent cell pair up. While the homologous chromosomes are paired up, crossing-over
can occur. Crossing-over is a process in which segments of homologous chromosomes
break off and are exchanged. When the cell divides to produce two daughter cells, each
daughter cell receives one chromosome from each homologous pair. In this way, crossingover increases the number of possible genetic combinations in the offspring.
Recall Mendel's law of independent assortment from the last lesson. This law states that
different pairs of genes separate independently of one another when gametes form. After
crossing-over, pairs of homologous chromosomes line up along the center of the cell in a
random fashion called independent assortment. Like crossing-over, independent
assortment leads to genetic variation.
When the cell divides, each daughter cell receives a mix of chromosomes that differs from
that of the original cell. The exact mix depends on how the chromosomes lined up before
the cell divided.
Look again at the diagram of meiosis. The stages of meiosis II (the second division in the
diagram) are nearly identical to those of mitosis except that meiosis II begins with haploid
cells instead of a diploid cell. The two haploid daughter cells formed by meiosis I divide to
form four haploid cells in meiosis II. Each haploid cell has a unique set of chromosomes.
The rearrangement of genes during sexual reproduction gives organisms combinations of
genes that differ from those of their parents. Each human gamete has 23 chromosomes,
containing a total of more than 20,000 genes. These genes are shuffled during meiosis I
through crossing-over and independent assortment. The number of possible gene
combinations is many times greater than the number of humans who have ever lived. This
huge variety of possible combinations accounts for the diversity of traits that result from
sexual reproduction.
Comparing Methods of Reproduction
Recall that asexual reproduction is the production of offspring by a single parent. Some
eukaryotes reproduce asexually by mitosis. Yeasts and freshwater animals called hydras
reproduce in this way. Their offspring develop from buds on the parent's body. By contrast,
meiosis is used to make gametes, specialized cells used only for sexual reproduction.
The steps of meiosis are similar to those of mitosis, but there are important differences.
Cells that undergo mitosis divide only once, to form two genetically identical diploid cells. By
contrast, cells divide twice during meiosis, in stages called meiosis I and meiosis II, to
produce four genetically different haploid cells.
Asexual reproduction and sexual reproduction both have advantages and disadvantages.
Asexual reproduction enables an organism to produce many offspring quickly. However,
because the offspring are genetically identical, a factor such as a toxin in the environment
that harms one offspring can harm all of them.
Sexual reproduction produces relatively few offspring. However, because the offspring are
genetically diverse, they may be able to survive in more varied conditions than organisms
produced by asexual reproduction. For example, suppose a disease strikes a particular type
of crop plant. If the plants are genetically different, a few may have genes that enable them
to resist the disease. Although many individuals will die, some resistant plants will survive
and reproduce. Those plants can then pass on the genes for disease resistance to their
offspring.