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Unit 1:
Ch. 1
Ch. 2
Ch. 3
Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
Ch. 10
Ch. 11
Ch. 12
Ch. 13
Ch. 14
Ch. 15
Interlude C
Unit 4:
Ch. 16
Ch. 17
Ch. 18
Ch. 19
Interlude D
Unit 5:
Ch. 20
Ch. 21
Ch. 22
Ch. 23
Ch. 24
Ch. 25
Ch. 26
Ch. 27
Ch. 28
Ch. 29
Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
Ch. 33
Ch. 34
Ch. 35
Ch. 36
Ch. 37
Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Chapter 29. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. What steps are involved in human reproduction and development?
  2. What is the difference between sexual and asexual reproduction?
  3. What is the importance of mating in sexual reproduction?
  4. Early in development, how does an amphibian embryo differ from a mammalian embryo?
  5. What are the three cell layers that develop in the early embryo?
  6. What occurs developmentally in each of the three trimesters of human pregnancy?
  7. How do genes control the fate and development of the cells in the body?
  8. Is the fetus’ environment important for development?  If so, why?
  9. How do changes in development influence the evolution of new species?
  10. What development occurs after the birth of an animal?

A Guide to the Reading

When exploring the content in Chapter 29 for the first time, the following concepts typically give students the most difficulty. For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas.

Animals Have Different Modes for Reproduction

The two basic ways in which animals reproduce are sexual reproduction and asexual reproduction.  Asexual reproduction occurs when a new individual is produced by only one parent.  This can occur in many ways, including by budding, in which the new animal buds off from the parent or by fragmentation.   The offspring produced by asexual reproduction do not have any genetic variation.  Asexual reproduction is beneficial in areas of low population, few mates, and stable environmental conditions.   Sexual reproduction involves the production of a haploid egg and a haploid sperm produced by meiosis.   Spermatogenesis is the process by which human males produce haploid sperm.   Oogenesis involves the production of a mature haploid egg by the human female.  The egg and sperm fuse during fertilization to produce a diploid zygote that contains genetic material from both parents.  This produces genetic variability within the population.  Animals that sexually reproduce can be male, female, or both.  Hermaphrodites are individuals of a species that contain both testes for production of sperm and ovaries for production of eggs.  Species with animals that are either male or female have evolved means to ensure that sperm successfully fertilizes the egg.  This involves finding a suitable mate and ensuring that the sperm and egg come into contact with each other under ideal environmental conditions.  Aquatic males will typically release sperm directly in the water around a female who will then release her eggs.  Terrestrial animals must deal with the potential for evaporation and many times sperm is released directly into the female’s reproductive tract as occurs when humans copulate.

For more information on this concept, be sure to focus on:

  • Section 29.2, Sexual reproduction requires meiosis
  • In Section 29.2, Sexually reproducing animals may be male, female, or both
  • In Section 29.3, Successful fertilization requires strategies to bring sperm and egg closer together
  • In Section 29.3, Gametes are usually released into a favorable environment
  • Figure 29.2, The Production of Male and Female Gametes
  • Figure 29.4, Asexual Reproduction Doesn’t Require Mates or Gametes
  • Figure 29.7, Sperm Transfer in Animals

Development From Fertilization to Birth

The first step in the development of any sexually reproducing animal is the fertilization of the egg by the sperm.  This occurs when the acrosome at the tip of the sperm releases enzymes that digest the outer covering of the egg, and the nucleus from both the egg and sperm fuse.  Upon fertilization, the cells of the zygote begin to divide and become an embryo.  The developing human is considered an embryo for the first 8 weeks of development, after which time it is considered a fetus.  This initial division of the zygote involves subdivision of the cells with little growth in size to form a hollow sphere known as a blastula in most vertebrates.  Mammalian early development differs from most other vertebrates and the blastula is called a blastocyst.  The mammalian blastocyst is composed of two cell types:  the outer trophoblast that will become the placenta and the inner cell mass from which the embryo will develop.   The cells of the trophoblast are involved in secreting enzymes that signal the blastocyst to implant in the endometrium lining the walls of the uterus and to form the placenta.  As development continues, the embryo forms three distinct cell layers.  The inner layer of cells is the endoderm that will become the gut, lungs, liver, and glands of the endocrine system.  The middle layer of cells is the mesoderm that will develop into the muscles, heart, kidneys, reproductive system, and skeletal system.     The outer layer of cells is the ectoderm that will develop into the nervous system and skin.  These cell types move into their correct positions during gastrulation and form the gastrula.  As the embryo develops, the fate of the cells of each layer becomes increasingly limited.  Reproductive structures arise from undifferentiated groups of cells in the yolk sac.  Human development in the uterus is divided into three trimesters, each lasting 3 months.  During the first trimester, the embryo is very susceptible to disruption and can result in miscarriages.  During the second and third trimester, the development of the fetus involves further development of the different organ systems.

For more information on this concept, be sure to focus on:

  • Section 29.2, Sexual and Asexual Reproduction in Animals
  • Section 29.4, Development From Zygote to Birth
  • Figure 29.3, Fertilization Occurs When a Single Sperm Enters the Egg
  • Figure 29.8, Early Development in Vertebrates
  • Figure 29.11,The Three Cell Layers Have Developmentally Distinct Fates

Control of Development Dictates the Form, Function, and Evolution of Animals

During development, the fate of an embryo’s cells changes dramatically.  The fate of a given cell depends on both the presence of genes in the DNA and the transcription of the gene.  During development, differential transcription of genes produces different proteins involved in the control of cellular development.  Additionally, by cutting mRNA in different locations, a developing animal can produce different gene products.  Morphogens are regulatory proteins that allow gene expression to be controlled at specific locations within the developing embryo.  Morphogens are typically released by one cell and bind to receptors on nearby cells, producing the localized effect.  Morphogens can also help cells determine what other cells are in their surroundings.  Environmental control of development also occurs in animals as can be seen in turtles and alligators, where the temperature at which the embryo develops governs the sex of the adult.  Changes in development have played an invaluable role in the evolution of life.  Changes in the timing and activation of single genes can have profound effects on the organisms.  An example of this is apoptosis or programmed cell death where cells are genetically programmed to die at specific times in development.  Differential gene expression leading to cell determination may be controlled at the level of transcription (from DNA to mRNA) or the level of translation (from mRNA to amino acid chain). Gene expression of a cell is influenced by neighboring cells through hormones, morphogens, and control genes.

For more information on this concept, be sure to focus on:

  • Section 29.5, How Development is Controlled
  • Section 29.6, The Effect of Development on Evolution
  • Figure 29.14, Morphogens Often Control Development
  • Figure 29.15, The Identity of Surrounding Cells Influences Development
  • Figure 29.16, Why Chickens Don’t Have Duck Feet

Tying it all together

Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:

Sexual and Asexual Reproduction in Animals

  • Chapter 9 – Section 9.4, Meiosis: Halving the Chromosome Number

Human Reproduction and Development: A Brief Overview

  • Chapter 24 – Section 24.3, Regulating Medium and Long-Term Events: Human Reproduction

How Development is Controlled

  • Chapter 13 – Section 13.3, Transcription: Information Flow from DNA to RNA
  • Chapter 13 – Section 13.5,  Translation: Information Flow from mRNA to Protein
  • Chapter 14 – Section 14.3, Patterns of Gene Expression
  • Chapter 14 – Section 14.4,  How Cells Control Gene Expression

Development after Birth

  • Chapter 15 – Science Toolkit, Human Cloning

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