1. Meiosis

2. Advantages of Asexual Reproduction
Eliminates the need to find a mate
May reproduce at a faster rate
May produce larger number of offspring using less overall energy and resources
Optimum in stable unchanging environments
Uses mitosis
Emphasize that the following are all mitotic divisions.
Types of asexual reproduction:
Budding - an offspring grows out of the body of the parent. An example is the hydra.
Gemmules- In this form of asexual reproduction, a parent releases a specialized mass of cells that can develop into offspring. An example is the sponge.
Fragmentation- In this type of reproduction, the body of the parent breaks into distinct pieces, each of which can produce an offspring. Planarians are examples.
Regeneration- In regeneration, if a piece of a parent is detached, it can grow and develop into a completely new individual. This includes starfish.
Parthenogenesis- This type of reproduction involves the development of an egg that has not been fertilized into an individual. This includes the whipped lizard.

3. Advantages of Sexual Reproduction
Varied number phenotypic offspring.
Better response to varying environment.
Sexual reproduction involves the
union of two haploid cells from different parents to produce a diploid zygote.
The two photos are of Sissy Spacek and her family and Lloyd Bridges ( ) and his family. Notice the similarities and the differences between the siblings and the parents. Sexual reproduction shuffles the genes to produce new combinations. The purpose of sexual reproduction is to produce offspring that are genetically different from the parents and from other siblings. It is the force that drives evolution along with varying environments.
During the process of sexual reproduction, two sets of chromosomes unite (fertilization) to produce a diploid cell (2N). Then at some point in time, the diploid cell(s) must undergo a special type of cell division (meiosis) and produce haploid cells to continue this process.
Meiosis is a special type of cell reproduction found in organisms that reproduce sexually.

4. Diploid Life Cycle
Haploid (n) - A cell or an organism with just one complete set of chromosomes.
Diploid (2n) - A cell or an organism with just two complete sets of chromosomes. Usually one set comes from one parent and the other set comes from a second parent as a result of fertilization.
Emphasize that there are three different types of life cycles that involve sexual reproduction:
Diploid (2n)-The organism is diploid until it is time to reproduce, then it undergoes meiosis to produce haploid gametes. Then fertilization of the gametes takes to restore the diploid condition. This is typical of animals.
The advantage of being diploid is that deleterious genes can be masked if they are recessive.
The disadvantage is that with more chromosomes to manage, there is an increased likelihood that something will go awry!

5. A Diploid Cell
This is an illustration of a diploid cell. Emphasize that there are two complete sets of chromosomes 2n=6. The blue set originated from the father or is the paternal set of chromosomes, and the red set originated from the mother or is the maternal set. This cell has gone through the S phase of the cell cycle, so the chromosomes are double stranded.
Emphasize that a pair of homologous chromosomes will have the same genes in the same order. The chromosomes may have different forms or alleles. For example perhaps the homologous chromosomes each have the eye color at the same locus, but one chromosome may have the brown allele at that locus, while the other chromosome may have the blue allele.
The purpose of meiosis is:
Take a diploid cell and produce four haploid cells
Produce haploid cells that are genetically different from one another.

6. Human Somatic Chromosomes
Humans have 23 pairs of chromosomes in their somatic cells.
These chromosomes have been stained and certain banding patterns appear.
This is a picture of a human cell’s with 23 pairs of homologous chromosomes. Karyotypes are made from photos such as the one shown. This photo of the chromosomes would be cut apart, then paired according with respect to length, the location of the centromere and the banding pattern. Currently computer programs play a role in pairing up the chromosomes.
The two chromosomes in each pair are called homologous chromosomes, or homologs.
Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters.

7. Human Karyotype
A karyotype is an ordered display of the pairs of chromosomes from a given cell arranged from longest to shortest.
Autosomes are non-sex chromosomes.
Homologous chromosomes are two chromosomes with the same genes in the same order (i.e. at the same loci). A karyotype matches homologous chromosomes and puts them in order from longest to shortest.
This slide presents a human karyotype with 23 pairs of homologous chromosomes. This karyotype belongs to a human male since the sex chromosomes in males are not entirely homologous, though a few small regions of the X and Y are homologous. Autosomes are non-sex chromosomes.
Emphasize that if this were a karyotype of a human gamete, it would contain only 23 chromosomes.
Notice that the sex chromosomes in this karyotype are not homologous. What does that signify?

8. (Caution! More Details Coming…)
Meiosis Overview
(Caution! More Details Coming…)
This is a special type of cell division where ONE diploid cell produces FOUR haploid cells that are genetically different from one another.
There are TWO cell division events, and they are called meiosis I and II.
Meiosis I produces 2 haploid cells with double stranded chromosomes (chromosomes have 2 chromatids). The cells are genetically different.
Meiosis II produces 4 haploid cells that separate the hybrid chromatids.

9. Meiosis I: Cut Chromosome Number in Half
A diploid cell with paired double stranded chromosomes will divide, producing two haploid cells also containing double stranded chromosomes. These two cells will be genetically different from one another.
Emphasize that the original cell has four chromosomes that are double stranded. At the end of meiosis I, two daughter cells are formed each having only TWO chromosomes and that these chromosomes are still double stranded. 1 cell (4 double-stranded chromosomes)
This cell division results in two genetically different haploid cells that contain double stranded chromosomes.
1 cell (4 double-stranded chromosomes)  2 cells (2 double-stranded chromosomes)

10. Meiosis II: Produce 4 Haploid Gametes
It is just like mitosis in that double stranded chromosomes are separated to produce single stranded chromosomes in the resulting daughter cells. The DNA, however has already been replicated Its purpose is to separate double stranded chromosomes into single stranded chromosomes.
Results in four haploid cells by separating the 2 cells with double-stranded chromosomes into 4 cells with 2 single-stranded chromosomes.
2 cells (2 double-stranded, haploid chromosomes)  4 cells (2 single-stranded, haploid chromosomes)

11. Recap the “bones” of the process and check for understanding.

12. What’s the Scoop?
In mitosis, diploid cells replicate chromosomes during S phase and segregate sister chromatids during M phase, so that diploid daughter cells are produced.
In meiosis, two chromosome-segregation phases, meiosis I and meiosis II, follow a single round of DNA replication during pre-meiotic S phase. During meiosis I, homologous chromosomes (shown in red and blue) are segregated to opposite poles. Sister chromatids segregate to opposite poles during meiosis II, which results in the formation of non-identical haploid gametes. Please note that the lengths of the cell-cycle stages are not drawn to scale.
Image:

13. Gametogenesis in Humans
Spermatogenesis: The four equal sized cells produced in meiosis will differentiate and become sperm cells.
Oogenesis: During cytokinesis, there is unequal division of the cytoplasm with usually one large cell and three smaller cells.
Cytokinesis in the formation of human female gametes is different than males. Males result in four cells of equal size to become sperm cells, but in females, meiosis results in one large egg and three polar bodies. What’s the advantage?
Many plant species have a similar unequal division of the cytoplasm to produce a megaspore involved in the female gametophyte generation.
Fungal cells will undergo meiosis after fertilization and will form haploid spores not gametes. The haploid spores will produce haploid hyphae.

14. Sexual Reproduction Provides Genetic Variability to Population
Crossing over
Random arrangement of chromosome when lining up on the metaphase plate
Random fertilization of gametes
Choice of mates
Ask students to explain how each of the items in the bulleted list contribute to genetic variability within the population being studied.
While sexual reproduction is more difficult, requires more energy and resources and is less successful than asexual reproduction, the advantage of providing more genetic variation to a species in a changing environment is more important in the long run.