Asexual and Sexual Reproduction
I. Reproduction
A. reproduction = the process by which living things produce other living things of the same kind (the
same species).
1. The new organisms that result from reproduction are called offspring.
B. By reproducing, organisms are accomplishing 2 major goals:
1. they are helping that species to continue to exist.
2. they are passing on their own genes (DNA) to the next generation (for an individual organism, this
is its most important goal).
C. There are 2 different types of reproduction—asexual reproduction and sexual reproduction.
II. Asexual Reproduction
A. asexual reproduction = the process by which offspring are produced from one parent.
1. Since only one parent is involved, the DNA passed on to the offspring will be identical to the DNA
of the parent (this process basically involves mitosis).
B. There are 3 main types of asexual reproduction:
1. fission = the division of an organism into two equal parts (this occurs in single-celled organisms).
a. The organism goes through mitosis to make an exact copy of its DNA and then splits into two
new organisms.
b. Examples: bacteria and some protists reproduction by fission.
2. budding = the process of growing a new organism from the body of the parent; once large enough
to perform the life functions, the “bud” breaks away from the parent to live on its own.
a. Examples: hydra and yeast reproduce by budding.
3. regeneration = the ability of an organism to regrow lost or damaged body parts; also, the ability to
grow a new organism from a piece of the parent.
a. Examples: a salamander or gecko lizard can regrow a new tail if it loses the original; a sea star
can regrow a new ray if one is lost; each piece of a cut up natural sponge will grow into a new
adult sponge; and you can grow a new plant from a “cutting” of certain plants.
C. Advantages and Disadvantages of Asexual Reproduction
1. Advantages
a. No time or energy is needed to find a mate.
b. It is “convenient”—it can be done at any time and is very useful in the repair and/or replacement
of body parts that have been damaged or lost.
2. Disadvantages
a. The identical DNA from the parent is being passed on. If there is an error or negative mutation in
the parent’s DNA, this negative trait will be passed on to the offspring. This could weaken the
species.
b. Since there is no “mixing” of DNA, there is no chance that any weakness in the DNA will be
“corrected” (made stronger by a dominant trait).
III. Sexual Reproduction
A. sexual reproduction = the process by which offspring are produced by the combining of sex cells from
2 parents.
B. There are 2 types of cells in most higher-level organisms: body cells and sex cells
1. Body cells make up the vast major of the organism. The tissues and organs in an organism are
composed of body cells.
a. There are many more body cells than sex cells.
2. Sex cells (also called gametes) are produced in the reproductive organs of the parents for the
specific purpose of reproduction and passing on their traits (DNA) to the next generation.
a. The process of producing sex cells in the reproductive organs is called meiosis.
b. egg = the sex cell produced by the female parent or the female part of the organism.
c. sperm = the sex cell produced by the male parent or the male part of the organism.
C. Diploid vs. Haploid
1. Remember that each cell of an organism has a certain number of chromosomes and that these
chromosomes exist in pairs (ex. humans have 46 chromosomes in 23 pairs).
2. diploid = a cell that has a complete set of chromosomes (two of every kind of chromosome—a pair
of each).
a. Normal body cells are diploid—they have a complete set of chromosomes.
1. The diploid number for human cells is 46.
3. haploid = a cell that contains only half of the chromosomes (only one chromosome from the pair)
a. Sex cells are haploid—they need to have only half the chromosomes so that when they combine
with another sex cell there will be one complete set of chromosomes for the new organism’s cells.
1. The haploid number for humans is 23.
D. Fertilization
1. fertilization = the joining of the egg and sperm cells.
a. Fertilization assures that the new cell will have a complete set of chromosomes (diploid).
2. The new fertilized egg with a complete set of chromosomes is called a zygote.
3. The zygote will undergo mitosis to become 2 cells. Both of these cells will go through mitosis to
become 4 cells. The 4 cells go through mitosis to become 8 cells, and so on to develop into a new
organism.
E. The Process of Meiosis
1. Meiosis occurs in 2 divisions of the nucleus—called meiosis I and meiosis II.
a. This process is very similar to mitosis and uses the same names for the steps in each division.
2. In meiosis I, the chromosomes (which have made a copy of themselves) split to produce 2 daughter
cells that have a complete set of chromosomes each—they are diploid at this point.
a. The chromosomes DO NOT make a copy before the 2 daughter cells move into meiosis II.
3. In meiosis II, the 2 new daughter cells immediately begin to go through the stages to divide again.
a. Since each cell started with a complete set of chromosomes, this second division will result in
4 sex cells with half the number of chromosomes—they are now haploid.
4. The end result of meiosis is the creation of 4 sex cells that have half the number of chromosomes as
a body cell.
F. Advantages and Disadvantages of Sexual Reproduction
1. Advantages
a. Even if one of the sex cells has a negative mutation or a weak trait in its DNA, the other sex cell
is likely to be “normal”. The organism will usually show the stronger trait of the parent that did
not have a problem with its DNA. This process strengthens that organism and the species as a
whole.
b. “Mixing” of different DNA in the offspring creates differences in the way organisms look, act,
and respond to different situations. It provides for diversity in the species.
2. Disadvantages
a. Finding a mate often requires a lot of time and energy and may even rely on chance (for example,
pollen in the wind may or may not land on the female part of the “right” flower—a flower of the
same species as the flower that produced the pollen.)
1. In the animal kingdom, males often fight for the right to mate with the female. This requires
large amounts of energy and may result in injury and/or death. Also, the loser has expended
all this energy and still does not get to pass on his genes.
2. In some animal species, the male dances or builds something for the female—this also requires
energy.
3. In animals like black widow spiders and praying mantises, the male mates with the female
and then his body is eaten by the female to provide her with enough energy to produce the
offspring.
b. When a weak trait in the DNA is hidden by the stronger trait in the DNA of a mate, the
weakness is still there—the offspring is said to “carry” the weakness in its DNA. If it then mates
with another organism with that same weakness, the weakness may “show up” in their offspring.