1. DNA Part V
Meiosis

2. 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!

3. Haploid Life Cycle
2. Haploid (1n)-The organism, a fungus in this example, is haploid until it is time to reproduce, then cells (hyphae) fuse (fertilization). Meiosis takes place almost immediately to restore the haploid condition. Most fungi that reproduce sexually have the haploid life cycle. The advantage is that there is less DNA to manage. The disadvantage is that all genes are expressed.
Disadvantage of being haploid is that all the genes are expressed. “What you see is what you get.”
Advantage of being haploid is that it is easier to reproduce with fewer chromosomes.

4. 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.

5. 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.

6. (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.

7. 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)

8. 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)

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

10. Interphase G1, S, G2 of the cell cycle ( DNA is replicated in S)
Centrosomes with a pair of centrioles (not in higher plant cells) replicate.
Centrosomes are microtubules organizing centers
We’ll pick up after the DNA has been replicated...

11. Prophase/Prometaphase I
Prophase I
Chromosomes are visible as long separated filaments
Homologous chromosomes pair and align as long well-separated filaments.
Recombination nodules attach and crossing over occurs. .
Prophase I- lasts longer and is more complicated than prophase of mitosis. Chromosomes become visible as long, well separated filaments.
Homologous chromosomes pair and align gene for gene. During this time, recombination nodules made of proteins move down the synapsed chromosomes. Each nodule will clip the homologs at precisely the same place and the resulting fragments are spliced to one another forming. This is called crossing over. This results in hybrid chromatids that are no longer identical.
Prometaphase the nuclear membrane disappears, and kinetochore microtubules attach to the kinetochore.

12. Synapsing
Next, the homologous pairs form a protein structure between them called the synaptonemal complex. This keeps the homologs as a tetrad. Places where crossing over occurs are called chiasma. In mitosis, kinetochore microtubules attach to both sides of the centromere, but in meiosis I the kinetochore microtubules on attach only to one side.
Homologous pairs form a protein structure between them called the synaptonemal complex. This keeps the homologs as a tetrad.
Places where crossing over occurs is called a chiasma.
In mitosis, kinetochore microtubules attach to both sides of the centromere, but in meiosis I they on attach to one side.

13. Forming the Tetrad
The protein involved in holding the two homologs together is called ZIP 1 and forms the synaptonemal complex.
A large recombination nodule clips the DNA on both sides of the chromatids and recombines them to form hybrid sister chromatids.
When synapsing occurs, the chromosomes do not lie side by side but rather on top of each other with a protein called the synaptonemal complex holding them together.

14. Effects of Crossing Over on Genetic Variation
Compare the results of the chromosomes in which no crossing occurs over versus the result of crossing over.

15. Attaching Kinetochore Microtubules
During Meiosis I, kinetochore microtubules attach to only to one side.

16. Metaphase I Metaphase I
The homologous chromosomes move as unit to the metaphase plate. Each homolog has different kinetochore microtubules attached to the centromeres from opposite poles. Which side of the metaphase plate the chromosomes line up on is a matter of chance.
The homologous chromosomes move as unit to the metaphase plate. They are still considered tetrads. Each homolog has different kinetochore microtubules attached to the centromeres from opposite poles. The chromosomes line up on the plate in tetrads (two by two instead of single file like mitosis). Which side of the metaphase plate the chromosomes line up on is a matter of chance. This allows different combinations of chromosomal arrangements.

17. The Effect of Adding an Additional Chromosome Pair on the Metaphase Plate
The number of possible gametes made is 2n where n is equal to number of haploid chromosomes. Humans have the possibility of making 223 different types of gametes which is why no two siblings are never genetically identical unless they are identical twins.
In our example, we increased the number of homologous pairs from 2 in previous slides to 3 in this slide, so n is now equal to 3, thus 23 = 8 possible gamete combinations (neglecting any crossing over). Emphasize that humans can make 223 different gametes, not including the variations that result from crossing over. As a result, any two human mates can produce 223 x 223 different offspring!
SO, each person represents 1 out of 70,368,744,177,664 different offspring their parents could have produced! And crossing over makes that number much higher!
This concept and its importance in Mendelian genetics is often overlooked by students.
If we increase the number of chromosomes from two to three, how many different gametes can be formed?

18. Anaphase I Centromeres do not separate.
Homologous chromosomes do separate and move apart to opposite poles.
Spindle fibers from opposite poles interact to force poles apart.

19. Nondisjunction
Nondisjunction occurs when problems with the meiotic spindle cause errors in daughter cells.
This may occur if tetrad chromosomes do not separate properly during meiosis I.
Alternatively, sister chromatids may fail to separate during meiosis II.
Trisomy (2n + 1) 21 is also known as Down’s syndrome. It is characterized by the following:
Lowered IQ
Rounded mouth with protruding tongue
Almond shaped eyes
Heart defects
Monosyomy (2n-1) results in Turner’s syndrome where a female has only one copy of the X chromosome (X0). It is characterized by:
Small stature
Sexual immaturity
Aneuploidy is often fatal in humans. By comparison, plants can handle aneuploidy much better than animals. There are examples of plants having experienced total nondisjunction of chromosomes resulting in new polyploid plants. This concept was examined in evolution and speciation.
Graphic-

20. Aneuploidy
If the gamete is fertilized, the resulting cells will not have the proper number of chromosomes and is called aneuploidy.
Graphic-

21. Telophase I
The chromosomes are still double stranded containing two hybrid chromatids.
The double-stranded chromosomes have migrated to the poles.
The number of chromosomes is the haploid number of chromosomes.
Telophase I- The chromosomes have migrated to the poles. The number of chromosomes is the haploid number of chromosomes BUT the chromosomes are still double stranded containing two hybrid chromatids. Cytokinesis occurs to form two haploid cells. New nuclear membrane begins to form. Chromosomes become longer and thinner and as they uncoil. Cells finishing telophase I results in two haploid cells that have double stranded chromosomes.

22. Meiosis II
Interkinesis- This is the time between meiosis I and meiosis II. No DNA replication occurs during this time.
Meiosis II-
It is just like mitosis but the DNA has already been replicated. Its purpose is to separate double stranded chromosomes into single stranded chromosomes.
It is like mitosis but the DNA has already been replicated. It separates double stranded chromosomes. into single stranded chromosomes.

23. Interkinesis This is the time between meiosis I and meiosis II.
Haploid daughter cells; no further DNA replication occurs during this time.

24. Prophase/Prometaphase II
Centrosomes replicate and begin to move apart.
Chromosomes appear as long thin threads Nucleolus becomes less distinct. (Middle)
Asters begin to form. Twin chromatids become visible.
Kinetochore microtubules attach to the kinetochores. Spindle forms. Membrane begins to disappear.

25. Prometaphase I vs. Prometaphase II
Compare how the kinetochore microtubules attach during the process of prometaphase I with prometaphase II. Note that most books are a little behind and do not include prometaphase in the process of meiosis.
On the AP Biology exam students will not be responsible for naming and/or distinguishing between the various phases, but they do need to be able to explain, and represent with diagrams, the events of meiosis.

26. Metaphase II Membrane has disappeared.
Double stranded chromosomes have moved single file to the metaphase plate. Microtubules attach to both sides of the kinetochore.
The spindle fibers over lap one another. They will interact with one another to push the poles apart.

27. Anaphase II
Centromeres have separated and move the single stranded chromosomes toward opposite poles.
Overlapping spindle microtubules from opposite poles interact and to pull the poles apart.
Cytokinesis may begin.

28. Telophase II
Nuclear membrane begins to form. Chromosomes begin to uncoil, become thinner.
Nucleolus becomes less distinct. Cytokinesis is nearly complete.

29. 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.

30. Mitosis Versus Meiosis
Students should be able to compare and contrast mitosis with meiosis.

31. Mitosis Versus Meiosis

32. 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.