Section 5 – Reproduction and Heredity
Reproduction and development are necessary for the continuation of any species. Asexual reproduction is a
method of reproduction with all the genetic information coming from one parent.
Some Methods of Asexual Reproduction
1. binary fission -- involves
an equal division of both the
organism cytoplasm and
nucleus to form two identical
-- the diagram of the protist at
the right is example of this
2. budding -- involves one
parent dividing its nucleus
(genetic material) equally, but
-- the diagram of a yeast at
the right is an example of this
3. sporulation (spore
formation) -- is reproduction
involving specialized single
cells coming from one parent
-- the diagram of mold spores
being formed at the right is an
example of this
Asexual reproduction is sometimes called cloning. Cloning is the production of identical genetic copies. All
forms of asexual reproduction are variations of the cell division process of mitosis. Mitosis is associated with
asexual reproduction, as well as growth and repair in sexually reproducing organisms.
Mitosis is the method used for cell division and reproduction in cells not involved in sexual reproduction.
This process starts with one replication (copying of the chromosome material) and one division of the
chromosome material. This results in the chromosome numbers in the two cells produced being the same as in
the parent cell. This process is represented in the graphic which follows.
An Overview of the Process of Mitosis
2 Key Results of Mitosis
1. The same chromosome number is retained from generation to generation.
2. Each daughter cell receives an exact copy of the chromosomes of the parent cell. (clones)
Sexual Reproduction Process
The process of sexual reproduction involves two parents. Both parents normally contribute one gamete or
sex cell to the process. This process assures that the genetic information given to the offspring will be obtained
equally from each parent. The female gamete is called the egg or the ovum and the male gamete is called a
sperm. These gametes are formed in specialized reproductive structures called gonads. The sperm is much
smaller than the egg, but is capable of moving on its own power using a whip-like tail called a flagellum.
Sperm and Egg
The sperm and egg unite in a process called fertilization. This process forms a single celled structure called a
zygote which contains the complete genetic information to develop into a complete new organism having
characteristics of both parents.
Process of Fertilization
This zygote will then divide by mitosis and form the specialized cells, tissues, and organs of the organism.
This development of specialized structures from the zygote is called differentiation.
The process of meiosis produces gametes or sex cells. While some parts of this cell division process are
similar to the asexual cell division process of mitosis, there are several key differences. Meiosis produces
gametes, while mitosis produces other cell types. The process of meiosis halves the chromosome number from
the original parent cell in the four cells it forms. It does this by having two cell divisions forming four cells,
where mitosis has only one cell division forming two cells. Both processes start out with one doubling or
replication of the chromosome material. The graphic below will help to visually illustrate some of the key
events of meiosis.
Process of Meiosis
Another important way that meiosis differs from mitosis is the exchange of chromosome pieces which occurs in
the first division of this process. This exchange of chromosome pieces is called crossing over. Crossing over
assures that the cells produced as a result of meiosis will be different from and exhibit variations from the
parent cell that produced them. This process is chiefly responsible for the variations seen in members of the
same species of sexually reproducing organisms. These variations are the driving force for the process of
The process of crossing over and how it produces variation when these chromosomes are
recombined in the process of fertilization is illustrated in the graphic below.
Crossing Over and Genetic Recombination
Comparative Reproduction and Development
Different organisms possess different adaptations for reproduction and development. Organisms which spend
their lives or a large proportion of their lives in the water tend to lay their eggs in great numbers (thousands) in
the water and wait for the male of the species to release sperm near them to fertilize them. The fertilization
which occurs in the water in this case outside the body of the organism is called external fertilization. These
young organisms then develop outside the mother in the water once this has occurred, which is called external
development. A disadvantage of this process is that the eggs and developing young have little or no parental
protection. Many fish and amphibians like frogs undergo fertilization and development in this manner.
Reptiles and birds engage use the process of internal fertilization to fertilize their eggs. In this situation, the
male of the species inserts his sperm inside the female, who then lays her fertilized eggs outside her body. The
process of development is then external. Reptiles and especially birds tend to lay fewer eggs and provide much
more parental protection for their developing young. Organisms (with some exceptions) which use the process
of internal fertilization tend to spend much of their lives on land. Mammals like humans have both their
fertilization and initial stages of development occur within the female organism. This is referred to as internal
fertilization and internal development. These organisms tend to release very few eggs, but those eggs and
the developing organism are very well protected by one or both parents.
Human Reproductive System
The structure and function of the human male reproductive system, is very similar to that of many other
mammals. The male system is designed to make sperm or male gametes and is adapted to provide for the
delivery of these gametes to the female to allow for fertilization.
Male Reproductive System
Male Reproductive System Structures
1. testes -- produces sperm and the hormone testosterone
2. scrotum -- pouch enclosing the testes keeping the sperm at an
optimum temperature for development
3. vas deferens -- tube carrying sperm away from the testes
4. prostate gland -- the largest of several glands which add
lubricating and other fluids to the sperm
-- this combination of sperm and fluids is called semen
5. urethra -- tube through the penis carrying sperm to the outside
of the body
6. penis -- adaptation for internal fertilization of the female
The structure and function of the human female reproductive system, is very similar to that of many other
mammals. It is designed to produce female gametes or eggs, allow for internal fertilization, support the
internal development of the embryo and fetus, and provide nutrition through milk for the newborn.
Female Reproductive System
Female Reproductive System Structures
1. ovary -- (females have two of these) -- produce female
gametes or eggs and the hormone estrogen
2. oviduct (fallopian tube) -- carries the egg away from the
-- internal fertilization normally occurs here
3. uterus -- implantation and development of the embryo and
fetus before birth occurs here
4. vagina or birth canal -- entry point for sperm from the male
and exit tube for the baby when it is born
Human reproduction and development are influenced by factors such as gene expression, hormones, and the
environment. The reproductive cycle in both males and females is regulated by several different hormones.
Some of these hormones include:
testosterone -- produced by the testes in the male and stimulates the development of male secondary sex
characteristics (like facial hair and deeper voice).
estrogen -- produced by ovaries in the female and stimulates the development of female secondary
sex characteristics (wider hips and mammary glands) as well as starting the thickening of
the uterus lining in preparation for a possible pregnancy after the egg is released by the
female each month.
progesterone -- produced by yellow tissue called corpus luteum in the empty ovarian follicle (place in
ovary producing and releasing the egg) -- this hormone maintains the thickness of the
uterus lining in case fertilization occurs and development of a fetus occurs.
In human females of reproductive age, these hormones interact in a cyclic pattern called the menstrual cycle.
This pattern of events repeats itself on average every 28 days unless a pregnancy or other disruption occurs. A
graphic representation and written description of the stages of the human menstrual cycle is provided below.
Human Menstrual Cycle
Note the influence of the hormone progesterone in beginning the
thickening of the uterus lining and the role of the hormone
estrogen in maintaining the thickness of that lining. Ovulation
or release of the egg occurs at the midpoint of this cycle, while the
uterine lining thins and is shed (menstruation) when the level of
estrogen begins to decline to a large extent.
Menstrual Cycle Stages
1. follicle stage (10-14 days average duration)
production of ova/eggs occurs in tiny cavities in the ovary called follicles
enlarging follicle produces estrogen which causes the uterus to get ready for embryo implantation
(uterus thickens its lining)
2. ovulation (1 day)
follicle enlarges and ruptures ovary wall
egg is released to the oviduct (usually only 1 is released at a time)
3. corpus luteum stage (10 -14 days average duration)
yellow tissue fills the follicle after ovulation called the corpus luteum
"yellow body" secretes progesterone which maintains the thickness of the uterus in case a pregnancy
4. menstruation (3-5 days average duration)
periodic shedding of the thickened lining of the uterus which occurs if fertilization does not occur
Initial Development and Differentiation
The processes of gamete production, fertilization, and development follow an orderly sequence of events.
Zygotes contain all the information necessary for growth, development, and eventual reproduction of the
The zygote, which is a fertilized egg consisting of one cell, will begin to divide rapidly by mitosis forming the
early developing human embryo. Fertilization and the initial stages of this mitotic cell division occur in the
oviduct. The early embryo is migrates down the fallopian tube and completes most of its development in the
wall of the uterus.
Fertilization and Initial Development of the
The placenta is a combination of maternal and fetal tissue which allows for the exchange of materials with the
fetus and mother. Needed materials such as food and oxygen diffuse through the placenta to the fetus, while
wastes from the fetus diffuse to the mother. The umbilical cord is a fetal structure containing blood vessels
which allows materials to be carried between the fetus and placenta in both directions. The amniotic fluid
surrounds the fetus and helps to provide a shock absorber to protect the fetus against mechanical injury in the
event the mother is shaken or injured in some manner.
Fetal Development in the Uterus
The embryo will eventually develop into a three cell layered structure. This structure is called a gastrula and
will eventually differentiate to form the specialized cells. Differentiation means that the cells will develop
specific jobs and develop into specific tissues in the maturing organism. An example of this is that the outer
cell layer of the developing gastrula will develop into the skin and nervous system of a mature human
organisms. Most multicellular animals undergo a similar pattern
of development and differentiation.
Development is a highly regulated process After this small cluster of cells called the gastrula forms in humans,
tissues begin to form. In humans, the embryonic development of essential organs occurs in early stages of pregnancy. During
the first three months of human development, organs begin to form. The human embryo is usually referred to
as a fetus when human like features become visible in its structure. All organs and body features are
developed by the end of the sixth month. During the last three months of pregnancy, organs and features
develop well enough to function after birth.
Human Development at 6 weeks Human Development at 4 months
The embryo (or fetus) may encounter risks from faults in its genes and from its mother's exposure to
environmental factors such as inadequate diet, use of alcohol, tobacco, drugs, other toxins, or infections.
While the patterns of development discussed previously hold true for humans, these developmental patterns
vary between different plants and different animals.
Aging is a complex series of developmental changes which occur with the passage of time. This
process is influenced by both heredity and the environment. This process eventually leads to the
death of the organism.
Reproduction and development are subject to environmental
impact. Human development, birth, and aging should be viewed as
a predictable pattern of events.
Reproductive technology has medical, agricultural, and
ecological applications. In many instances, these technologies
have progressed at a faster rate than the ethical considerations
resulting from these technologies. Some of these techniques
include birth control methods used to block the process of
fertilization. Many technologies now exist to enhance the process
of fertilization and development in humans and other organisms.
Hormone therapy can cause increased egg production. Surgery can open blocked fallopian tubes in females and
the vas deferens in males. In vitro fertilization (test-tube babies) is a widely used technique to aid infertile
couples, allowing them to have children where this otherwise would not be possible.
The in vitro Fertilization Process