Sexual Reproduction Advanced
Douglas Wilkin, Ph.D.
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Printed: November 10, 2016
AUTHOR
Douglas Wilkin, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Sexual Reproduction - Advanced
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Sexual Reproduction Advanced
• Describe sexual reproduction; explain the genetic relationship between parent and offspring.
Sexual vs. asexual: what’s the main difference?
Truly, it’s just that sexual reproduction is the combining of sex cells from two parents, each providing half the
amount of genetic material.
Sexual Reproduction
Why do you look similar to your parents, but not identical? First, it is because you have two parents. Second,
it is because of sexual reproduction. Whereas asexual reproduction produces genetically identical clones, sexual
reproduction produces genetically diverse individuals. Sexual reproduction is the creation of a new organism by
combining the genetic material of two organisms. As both parents contribute half of the new organism’s genetic
material, the offspring will have traits of both parents, but will not be exactly like either parent.
Organisms that reproduce sexually by joining gametes, a process known as fertilization, must have a mechanism
to produce haploid gametes. This mechanism is meiosis, a type of cell division that halves the number of chromosomes. Meiosis occurs only in gamete producing cells within the gonads. During meiosis the pairs of chromosomes
separate and segregate randomly to produce gametes with one chromosome from each pair. Meiosis involves two
nuclear and cell divisions without an interphase in between, starting with one diploid cell and generating four haploid
cells (Figure 1.1). Each division, named meiosis I and meiosis II, has four stages: prophase, metaphase, anaphase,
and telophase. These stages are similar to those of mitosis, but there are distinct and important differences.
Prior to meiosis, the cell’s DNA is replicated, generating chromosomes with two sister chromatids. A human cell
prior to meiosis will have 46 chromosomes, 22 pairs of homologous autosomes, and 1 pair of sex chromosomes.
Homologous chromosomes, or homologues, are similar in size, shape, and genetic content; they contain the same
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genes, though they may have different alleles of those genes. The genes/alleles are at the same loci on homologous
chromosomes. You inherit one chromosome of each pair of homologues from your mother and the other one
from your father. Sexual reproduction is the primary method of reproduction for the vast majority of multicellular
organisms, including almost all animals and plants. Fertilization joins two haploid gametes into a diploid zygote,
the first cell of a new organism. The zygote enters G1 of the first cell cycle, and the organism begins to grow and
develop through mitosis and cell division.
FIGURE 1.1
During meiosis the number of chromosomes is reduced from a diploid number
(2n) to a haploid number (n). During fertilization, haploid gametes come together
to form a diploid zygote and the original
number of chromosomes (2n) is restored.
Summary
• Sexual reproduction involves haploid gametes and produces a diploid zygote through fertilization.
• Fertilization joins two haploid gametes into a diploid zygote.
Review
1.
2.
3.
4.
Define sexual reproduction.
List the main differences between asexual and sexual reproduction.
Define meiosis.
How many chromosomes does a diploid human cell have? How many chromosomes does a haploid human
cell have?
References
1. Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. CK-12 Foundation . CC BY-NC 3.0
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