Asexual and Sexual
Reproduction
Fertilization of an egg cell by a sperm cell. In sexual
reproduction, haploid gametes fuse to produce a
diploid zygote.
Some species have two parents, whereas
others have just one.
Asexual reproduction produces an identical individual
Sexual reproduction produces a similar but unique individual
In sexual reproduction, a cell division process called meiosis
produces eggs and sperm that join during fertilization to
produce a new individual.
Asexual Reproduction and mitosis
Are there male and female bacteria?
How would you be able you tell?
• Bacteria have just one chromosome;
they do not have an X or Y
chromosome. They have a very
simplified form of reproduction.
All prokaryotic and many eukaryotic
organisms reproduce asexually.
• Cell divides after DNA is replicated.
• No gametes are formed,
• also occur by fragmentation (a piece of
the organism breaking off)
Asexual reproduction has advantages and
disadvantages.
Advantages of asexual reproduction
• Simplest and quickest method of reproduction
• Produces a clone (an organism that is genetically identical to its
parent)
• Requires only one parent
• Stronger populations in a more favorable environment
Disadvantages of asexual reproduction
• No genetic variation
Without genetic variation, populations can only evolve very slowly
because it depends on random mutations for evolution.
• Does not allow organisms to adapt to changing environment
Sexual Reproduction and Meiosis
Each gamete is haploid
• Each parent contributes ½ of the genetic
material for the offspring.
• Gametes fertilize to make a diploid
offspring.
Gametes form through the cell division
process called meiosis
Sexual reproduction has advantages and
disadvantages
Advantages of sexual reproduction
• Offspring are all different, organisms are able to adapt and evolve to a
changing environment
• Stronger populations in a less desirable environment
• Highly diverse, offspring are a genetic recombination of parents, there
is also opportunity for many mutations in meiosis
Disadvantages of sexual reproduction
• Two parents are required, process is longer the population cannot
grow as quickly
Similarities and/or
Differences
Both are methods of
reproduction
• Asexual one parent is needed
and offspring are identical
• Sexual two parents are
needed and offspring are
genetically unique
Mutation is a source of
variation in both
• Asexual offspring are
otherwise identical to parent
• Sexual offspring are a genetic
recombination of parents
Start Day 2
Meiosis and Reproduction
Do you have ALL your parents' chromosomes?
No, you only received half of your mother's
chromosomes and half of your father's
chromosomes.
If you inherited them all, you would have
twice the number of chromosomes that
you're supposed to have.
Humans have 23 pairs.
If you received all your parents'
chromosomes, you would have 46 pairs!
What monomer is DNA is made of?
1.
2.
3.
4.
Glucose
Amino acid
Nucleic acid
Phospholipids
HAPLOID VS. DIPLOID
• A cell with two sets of chromosomes is diploid, referred to as 2n,
where n is the number of sets of chromosomes.
• Most of the cells in a human body are diploid.
• A cell with one set of chromosomes, such as a gamete, is haploid,
referred to as n.
Gametes are haploid
When a haploid sperm (n) and a haploid egg (n) combine, it is called
fertilization, and a diploid zygote will be formed (2n).
In short, when a diploid zygote is formed, half of the DNA comes from
each parent.
Which process creates gametes?
1.
2.
3.
4.
Meiosis
Mitosis
Electron transfer chain
Gametification
During meiosis, four haploid cells are created
from one diploid parent cell.
MEIOSIS I
Meiosis I is where the genetic variation is introduced into the daughter
cells.
During meiosis I, the pairs of homologous chromosomes are separated
from each other.
1. Prophase I:
The homologous chromosomes line up together.
During this time, a process that only happens in meiosis can occur.
This process is called crossing-over, which is the exchange of DNA
between homologous chromosomes.
Crossing-over forms new combinations of alleles on the resulting
chromosome, in other words it creates genetic variation.
Without crossing-over, the offspring would always inherit all of the
alleles on one of the homologous chromosomes.
Also during prophase I, the spindle forms, the chromosomes condense
as they coil up tightly, and the nuclear envelope disappears.
2. Metaphase I:
• The homologous chromosomes line up
in their pairs in the middle of the cell.
• Chromosomes from the mother or from
the father can each attach to either side
of the spindle.
• Their attachment is random, so all of the
chromosomes from the mother or father
do not end up in the same gamete. This
is called random alignment.
3. Anaphase I:
The homologous chromosomes are separated as the spindle shortens,
and begin to move to opposite sides of the cell.
4. Telophase I:
• The spindle fibers dissolves, but a new nuclear
envelope does not need to form.
• No DNA replication occurs between meiosis I and
meiosis II because the chromosomes are already
duplicated.
After Meiosis 1 how many copies of DNA are
in the dividing cells?
1.
2.
3.
4.
1n
2n
3n
4n
During crossing-over, segments of DNA are exchanged
between non-sister chromatids. Notice how this can
result in an allele (A) on one homologous chromosome
being moved onto the other member of the pair.
MEIOSIS II
During meiosis II, the sister chromatids are separated and the gametes are
generated. The steps are outlined below:
1. Prophase II: The chromosomes condense.
2. Metaphase II: The chromosomes line up one on top of each other along the
middle of the cell. The spindle is attached to the centromere of each
chromosome.
3. Anaphase II: The sister chromatids separate as the spindle shortens and move
to opposite ends of the cell.
4. Telophase II: A nuclear envelope forms around the chromosomes in all four
cells.
This is followed by cytokinesis.
An overview of meiosis.
Fertilization happens when two gametes join
to form a zygote
Fertilized egg cells contain all of the genes to make a new organism.
Fertilization is an additional way that sexual reproduction increases
genetic variation, because it results in the combination of genes from
two separate individuals.
Review Questions
1. What are the advantages and disadvantages of asexual
reproduction?
2. What are the advantages and disadvantages of sexual
reproduction?
3. How many chromosomes does a diploid human cell have? How
many chromosomes does a haploid human gamete cell have?
4. What happens during metaphase I of meiosis? How does this
compare to the metaphase of mitosis?
5. What is the product of meiosis?
6. Describe crossing-over.