Reproduction
Directions: Read, highlight, and complete the chart at the end.
We’ve been tossing around the words sexual and asexual reproduction for some time now, and I could tell from your
stories that it’s still a little confusing. So let’s take a step back and review.
From our 6 characteristics of living things, we know that all living things reproduce. Reproduction is making an
offspring, whether it is a new cell or a new baby. For us though, we are talking on the cellular level. The problem I can
see your having is that there are different ways for reproduction to happen, sexually and asexually. So let’s start with
asexual reproduction.
In the end regardless of which process a cell uses, chromosomes are copied and new cells are made.
Asexual
Asexual reproduction happens without a partner so no male-female interactions (1 parent), but chromosomes are still
copied. In most cases of cell reproduction, the nucleus (the control center organelle) divides into two parts and then
moves to opposite sides of the cell. This is what we have been calling mitosis (prophase, metaphase, anaphase,
telephase).
When a cell splits in half (cytokinesis),
two cells are formed. Even though one
of the cells was the original cell, they
still refer to this as creating two new
cells. These new cells are called
daughter cells.
Every living species (prokaryotic and
eukaryotic) has cells undergoing asexual reproduction including you. This process is responsible for the increase in
number of cells which happens when you grow and develop or even when your body repairs tissues when you get
hurt. The neat thing about the asexual process is that it happens quickly which makes it a great advantage.
In asexual reproduction all cells are exact copies of each other. They can even be described as clones. This is a
disadvantage for this type of reproduction because if something can kill one cell, it can probably kill them all.
Even though we are talking mainly about single cells reproducing, some species use asexual reproduction to make new
offspring. Occasionally, under extreme conditions, some animals can go through asexual reproduction and create new
offspring. This is very unusual, but it does happen. This only occurs with the females of a species. The resulting
offspring is an exact clone of the mother.
Identical twins are a form of asexual reproduction. The fertilizing of the egg is sexual reproduction (we’ll talk about
that in a second). Once the embryo forms it splits in half creating two identical embryos. This split is asexual
reproduction.
Mitosis is one of the forms of asexual reproduction, but the world isn’t always so neat since we have billions of
different organisms. Two alternatives to mitosis that are also forms of asexual reproduction are binary fission and
budding.
Binary Fission
For single celled organisms, asexual reproduction is the only way living things can create new offspring. Prokaryotic
organisms (no nucleus) such as bacteria use asexual reproduction to reproduce. Because they are missing a nucleus,
their asexual reproduction process is called binary fission which is the simplest way for cells to reproduce.
Binary fission is simple because it doesn’t require a nucleus to break apart, but it still requires the chromosomes to be
copied. The nucleus doesn’t break apart because they do not have one. Remember, bacteria are prokaryotic. So
although it is simple (and faster), it still is
reproduction just like mitosis but without the
nucleus. When complete, the cells created through
binary fission are exact duplicates in both size and
chromosomes.
Budding
Since some multi-cellular organisms such as yeast and sea sponges have only a few different
cell types, they are able to reproduce using a type of asexual reproduction called budding. In
this case, a mother cell creates a small daughter cell (the bud) that continues to grow. Once
the bud is created, the nucleus splits in two using mitosis.
The difference between budding and mitosis is that the mother cell creates the offspring
(the bud) before it starts reproducing a nucleus. Once the bud is created, the mother cell
completes mitosis. The second nucleus goes to the bud and the bud may detach from the
parent and become a new individual or remain attached. Once detached though, the small
bud is an exact smaller copy of the parent cell just like mitosis.
Budding is advantageous for animals such as sponges, which attach to rock and move very
little. Colonies can be maintained in the same place because the bud can stay attached, or
new colonies can be established when buds break away from their parents and are carried to
new locations.
Sexual
In sexual reproduction, there are partners (male and female) that create offspring (2 parents). In each partner there
are sex cells (sperm and egg in animals; pollen and ovules in plants). In this type of reproduction, the chromosomes of
the new cell are a blend of chromosomes from the male and female. In fact, they are an equal blend of chromosomes
from each parent so the offspring are different.
At the beginning of sexual reproduction, the male sex cell (sperm or pollen) combines with the female sex cell (egg or
ovule). In this case only one cell is made when these two cells combine. Soon after that cell is made, asexual
reproduction (mitosis) takes over.
Take us for example. We have 46 chromosomes. 23 come from your mom and 23 come from your dad. Because we
are a blend of chromosomes, we are not copies of our parents even though your parents say “you’re just like your dad
(or mom).”
This is a big difference between sexual reproduction and asexual reproduction. Asexual reproduction creates identical
cells while sexual reproduction creates new cells that are not exact copies. In this case, we consider this an advantage
for sexual reproduction since the offspring are different. A major disadvantage for this reproductive process is that it
is slow. Think about how long it takes for us to be born (9 months) where bacteria can reproduce in 30 minutes.
Asexual Reproduction
Number of parents
Are offspring identical or different?
Advantage
Disadvantage
Sexual Reproduction
http://www.biotopics.co.uk/genes1/asexual_and_sexual_reproduction.html