The Reproductive System, Third Edition

Randolph Krohmer

Description

Organisms reproduce to ensure the continued survival of their respective species. For humans, our ability to produce offspring and contribute to genetic variability in the world is made possible by our body's reproductive system. In The Reproductive System, Third Edition, learn how the development of the reproductive systems in both males and females depends on the delicate and coordinated balance of genetic makeup, hormones, and the nervous system. Also examined are the reproductive systems of males and females, and how the body develops from conception through puberty and into maturity. Packed with full-color photographs and illustrations, this absorbing book provides students with sufficient background information through references, websites, and a bibliography.

Sujets

• Life Sciences
• Medical
• Sciences et techniques
• Biology
• Medicine
• Anatomy
• Health
• Human
• Reproduction
• Science

Informations

• Publié par: Infobase Publishing
• Date de parution: 01 octobre 2021
• Nombre de lectures: 1
• EAN13: 9781646937233
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

The Reproductive System, Third Edition
Copyright © 2021 by Infobase
All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval systems, without permission in writing from the publisher. For more information, contact:
Chelsea House An imprint of Infobase 132 West 31st Street New York NY 10001
ISBN 978-1-64693-723-3
You can find Chelsea House on the World Wide Web at http://www.infobase.com
Contents Cover Copyright Chapters Reproduction: A Characteristic of Life Early Embryonic Development Development of the Reproductive System Developmental Differences in Brain and Behavior Adolescent Puberty Puberty in the Male Puberty in the Female Concerns and Complications Support Materials Glossary Bibliography Further Resources About the Author Index
Chapters
Reproduction: A Characteristic of Life

The fact that this book is not a living organism should not be much of a surprise to anyone over the age of five. But how do we know that it is an inanimate (not living) object? The scientific community has developed a list of characteristics that can be used to determine if an object is truly alive. One of those characteristics is its ability to reproduce, thus ensuring the continued existence of the organism's species. Although this text has been reproduced many times for many different readers, it has no self-regulating mechanism for reproducing. However, all living organisms, from a single-celled amoeba to a 72 trillion-celled human, have an innate drive—not simply a desire—to reproduce.
There are two kinds of reproduction: asexual and sexual. Many biochemical events must occur before an organism can reproduce either way. Asexual reproduction, the simplest form of reproduction, literally means "without sex." In organisms that reproduce asexually, reproduction occurs without partners coming together. Asexual, single-celled organisms grow to a certain stage or size and then divide into two identical organisms. This division, which is a complex process, is called mitosis. It requires the organized division of the genetic material (see figure below). This division must be coordinated with the division of the cytoplasm, a process called cytokinesis. The end result of asexual reproduction is two identical daughter cells.

Mitosis and cytokinesis result in the equal division of the nucleus and cytoplasm, respectively, producing two identical daughter cells. During interphase, the cell grows and the genetic material in the nucleus is duplicated. The cell then enters prophase in which the nuclear envelope breaks down, and the paired centrioles migrate to opposite sides of the cell while sending out fibers to form the mitotic spindle. During metaphase, the chromosomes line up in the middle of the cell and fibers from both centrioles attach to each pair of chromosomes. Prometaphase is the stage during which the nuclear membrane begins to disintegrate. During anaphase, the daughter chromosomes are pulled by the spindle fibers to opposite sides of the cell. By late anaphase, as the daughter chromosomes near their destination, a cleavage furrow begins to form in the cell membrane indicating the beginning of cytokinesis. In the final stage, telophase, the cell membrane continues to constrict and eventually divides into daughter cells. As this is occurring, the nucleus is reestablished, and the daughter cells are once again in prophase.
Multicellular organisms that reproduce asexually have developed several unique reproductive strategies. For example, the jellyfish reproduces by budding, a process in which a new individual begins to grow, or bud, from the original organism and is eventually released as a small, free-swimming organism. Starfish have a similar method of reproduction. More than 100 years ago, men working the oyster beds wanted to eradicate starfish because starfish eat oysters. When workers brought up a starfish with their catch, they would cut it into pieces and throw it back into the water thinking they had put an end to that starfish. Little did the workers know that an entire starfish could be regenerated from each piece. Obviously, this put the oyster farmers at an even greater disadvantage as they caused an increase in the starfish population rather than eliminating it. Asexual plants, such as strawberries, propagate new individuals by sending out shoots that will develop into new plants. This reproductive process is also how new plants can be generated from "cuttings" of existing plants. All of these reproductive methods produce offspring that are clones (genetically identical) to the parent organism.
The benefits of asexual reproduction include the fact that an organism can reproduce independently. That is, no individual is dependent on another to reproduce. Organisms that reproduce by asexual means are capable of creating a large population in a relatively short time. Because the organisms are genetically identical, they will all be equally successful in the same constant environment. The genetic similarities, however, confer some disadvantages to asexual organisms. For example, if a population of clones is perfectly suited for an environment that has a pH of 7.0 and a temperature range of 77°F-86°F (25°C-30°C), what happens if the environment changes? If the temperature increases and the pH of the environment becomes more acidic, the population has no genetic variability and, therefore, no way to compensate for changes in this new environment. The entire population will likely disappear under these altered conditions because it could not tolerate or live in the new environment.
Although sexual reproduction is much more complex than asexual reproduction, it offers the benefit of genetic variability. While sexual reproduction may waste some nutrients on males who cannot add to the population number directly, these males offer a different set of chromosomes with different characteristics, thus generating genetic variability and allowing sexually reproducing species to evolve and occupy essentially every corner of the Earth. As described previously, a cell copies its genetic blueprint during mitosis before it divides into two identical daughter cells. Unlike mitosis, sexual reproduction is accomplished by the fusion of an egg nucleus with a sperm nucleus, or fertilization. However, the resulting cell, a fertilized egg (or zygote) must not contain more genetic material than is present in the somatic (nonsex) cells of that species. This is critical because too many chromosomes may result in a "genetic burden" that can result in problems by providing too much genetic information. One universal example is trisomy 21.  Trisomy 21, or Down Syndrome, results when a fertilized egg has three copies of chromosome 21 instead of the normal two copies. Down Syndrome usually causes varying degrees of intellectual and physical disability and associated medical issues. 1  In sexually reproducing animals, all of the cells in the body, except the sex cells, are called somatic cells and contain two copies of every chromosome, one from the father and one from the mother. A cell with two sets of chromosomes is described as diploid (2 n ). The sex cells-eggs and sperm-differ from somatic cells in that they are haploid ( n ), which means that they have only one set of chromosomes.
Somatic cells contain all of the genetic information that makes you who and what you are. In humans, all of the genetic information is contained on 23 pairs (46 total) of chromosomes housed within the nucleus. Chromosomes are the blueprint that makes each individual unique. They are composed of millions upon millions of DNA (deoxyribonucleic acid) molecules that in turn code for (or direct) the development of each and every characteristic of an individual, such as hair, skin, and eye color. In somatic cells, each pair of chromosomes represents equally the genetic information from the two parents. Sex cells develop by meiosis (see figures below), a process that requires the developing sperm or egg to undergo two nuclear divisions during which the genetic material is reshuffled and reduced by half.

This illustration shows a cellular view of the stages of meiosis. Meiosis is the process by which the number of chromosomes in developing gametes (egg or sperm) is reduced by half (to the haploid number). In meiosis I, chromosome pairs are drawn to opposite poles of the cell, establishing genetic variation. This is a major difference from mitosis.
Source: Infobase Learning.
Because the meiotic process is very efficient at mixing up the genetic material and each individual has an equal complement from both mother and father, no two individuals (except for identical twins) have exactly the same genetic profile. Although you and your siblings (brothers and/or sisters) may have characteristics in common, such as the color of hair and eyes, hundreds and maybe even thousands of other characteristics within your genetic profile make you a unique individual. Since each individual is unique, each person can now be identified by his or her specific genetic profile. This profile is most commonly used in law enforcement to convict and sometimes exonerate suspects in a crime. During the last decade or so, using your genetic profile to trace your ancestry has become quite popular. Genetic ancestry testing, or genetic genealogy, is a way to explore family history (genealogy) beyond what can be learned from relatives or historical records. Genetic variations (or similarities) can provide clues as to where a person's ancestors might have come from and reveal familial relationships. 2  With current technology, most kits available have on average an accuracy rate of more than 99%.


This illustration shows the chromosomal changes during meiosis. The chromosomes duplicate themselves once and then undergo two cell divi