Chapter 1. Genetics and Early Prenatal Development

Synopsis

Human Development Video Activity
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You must read each slide, and complete any questions on the slide, in sequence.
amino acids
Organic compounds that combine to make up proteins. Proteins are critical to human function and are involved in digestion, growth, repair of the body, and other bodily operations. Some amino acids are acquired through diet, and others are manufactured by the body.
chromosome
A structure that contains protein and a single DNA molecule. Every human cell nucleus, except for reproductive gamete cell nuclei, contains 46 chromosomes, 23 of which came from the mother and 23 of which came from the father.
deoxyribonucleic acid (DNA)
A large, complex molecule found in the cell nucleus that acts as a blueprint for an organism. It contains the genetic information that determines the characteristics of an organism and dictates how an organism develops, survives, and reproduces.
DNA
A large, complex molecule found in the cell nucleus that acts as a blueprint for an organism. It contains the genetic information that determines the characteristics of an organism and dictates how an organism develops, survives, and reproduces.
gene
A short segment of a DNA molecule. Genes are the detailed components that make up the DNA blueprint for an organism.
ovum
The female reproductive cell, or female gamete.
protein synthesis
The process in which individual amino acids are connected to each other in a specific order so as to create a new protein.
sperm
The male reproductive cell, or gamete.
zygote
A fertilized egg cell that is formed when the female gamete (ovum) and a male gamete (sperm) unite.
A color image showing the double-helix of DNA

Genetics and Early Prenatal Development

Author

A color image showing the double-helix of DNA

S. Stavros Valenti, Hofstra University

Synopsis

With the help of an animation, this activity explores the genetic aspects of prenatal development from the creation of the zygote through implantation.

REFERENCES

Edelman, G. M. (1992). Bright air, brilliant fire: On the Matter of the Mind. New York: Basic Books.

The Zygote

A color drawing of sperm approaching an ovum

Each human person begins as a single cell, the zygote. This single cell is formed from the fusion of two cells:

• A sperm − the male reproductive cell, which is produced in one or two testicles;

• An ovum − the female reproductive cell, vastly larger than the sperm, which is produced in one of two ovaries.

The amazing result of this fusion is a tiny entity that could fit through the eye of a needle. In spite of its size, the zygote contains a detailed blueprint for the growth of the full human being. In this activity, we will take a closer look at the earliest biological mechanisms that make human development possible.

Instructions for Building a Human Body

A color image the 23 pairs of chromosomes
23 pairs of chromosomes

A zygote contains 46 chromosomes, 23 from the father and 23 from the mother. Imagine these 46 chromosomes as separate volumes of an instruction manual called How to Build a Human Body. This set of 46 volumes is arranged in 23 pairs and contains all the necessary instructions for making the future baby similar to, yet different from, all the rest of us.

Each chromosome consists of a long double-stranded molecule of deoxyribonucleic acid (DNA). These DNA molecules contain thousands of genes, short segments of DNA that are arranged like beads on a string and are the basic units of heredity. To continue with our instruction manual analogy, you can think of these separate genes as pages in each of the 46 volumes.

Click the image to view a further zoom of the DNA structure.

Question 1.1

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You should explain that the genes are composed of the chemical DNA. Thousands of genes reside on each chromosome. The male parent and the female parent each contribute 23 chromosomes yielding 23 pairs total. The 23 pairs of chromosomes are present inside of the zygote.

Division of the Zygote and Its DNA

As the illustration on the right shows, the process of building a body follows a very orderly sequence, at least in the first few days. The zygote begins by duplicating each chromosome’s DNA. With enough chromosomes to form two identical sets, the zygote separates and splits down the middle to form two identical cells. In a similar way but at different times, these “daughter” cells divide over and over again. The zygote and every new cell of the body contain the developing individual’s unique set of 23 pairs of chromosomes. Therefore, a complete set of building instructions, carried on coiled strands of DNA, are replicated every time the cell divides, so each new cell always has a copy of the genetic code.

With these early cell divisions, each new daughter cell is half the size of the original cell, so the entire ball of cells is no larger than the original zygote. Soon, this new organism will take nourishment from the mother’s uterus and begin to grow into a tiny embryo.

Question 1.2

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Your response should be 46 volumes, or 23 pairs of volumes. Each new daughter cell will have the same, complete set of instructions as did the parent cell has.

Implantation and Beyond

By about the twelfth day of pregnancy, the ball of cells has embedded itself in the mother’s uterine wall, and the cells now can acquire nutrients from her bloodstream. At this point, the new human being’s DNA instructs each cell to grow by building proteins out of amino acids, a process called protein synthesis. The proteins will function both as the “bricks and mortar” and as the chemical “messengers” of the body. Depending on the kinds of proteins grown inside these miniature chemical factories, each cell multiplies and gives rise to a specific type of tissue, such as brain tissue, heart tissue, skin tissue, and so on. As these tissues grow, a human body begins to take shape and the developing organs begin to function.

So, in order to build a body, the DNA must accomplish two tasks:

• to replicate every time the cell divides and

• to synthesize proteins as needed for growth and repair of cells, tissues, and organs.

Summary

A zygote separating and splitting down the middle to form two identical cells
A zygote separating and splitting down the middle to form two identical cells

Let’s review the main points that were discussed in this activity.

• The zygote and every new cell of the body contain the individual’s unique set of 23 pairs of chromosomes, which function as the complete instructions for building and maintaining the body.

• These instructions are carried on coiled strands of DNA and are replicated every time that the cell divides. Therefore, each new cell contains a copy of the individual’s genetic blueprint.

• DNA and the cell’s chemical factory allow each cell to grow, multiply, and give rise to various tissues and organs as the human body develops.

It is amazing to think that the entire process of DNA replication and protein synthesis is going on in every cell throughout the body for the individual’s entire life.

Assessment: Check Your Understanding

A color image of sperm approaching an ovum

Question 1.3

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Good job! You know that each cell of the body contains 23 pairs of chromosomes.
Sorry. This statement is false. Each cell of the body contains 23 pairs of chromosomes.

Assessment: Check Your Understanding

A color image showing the double-helix structure of DNA

Question 1.4

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Good job! A chromosome is made of DNA, a long double-stranded molecule.
Sorry. This statement is false. A chromosome is made of DNA, a long double-stranded molecule.

Assessment: Check Your Understanding

A color image the 23 pairs of chromosomes

Question 1.5

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Good job! You know that chromosomes contain thousands, not millions, of genes.
Sorry. This statement is false. Chromosomes contain thousands, not millions, of genes.

Assessment: Check Your Understanding

A color image showing the double-helix of DNA

Question 1.6

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Good job! Genes instruct cells to make specific proteins.
Sorry. This statement is correct. Genes instruct cells to make specific proteins.

Congratulations! You have completed this activity.Total Score: x out of x points (x%) You have received a provisional score for your essay answers, which have been submitted to your instructor.