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Learning Objectives Associated with 2020_Winter_Bis2a_Facciotti_Lecture_26
- Compare and contrast the sequence of events that need to occur during mitosis versus meiosis and why they are necessary: include the roles of microtubules, motor proteins, centrosomes, and the level of DNA condensation.
- Compare and contrast the behaviors of sister chromatids, chromosomes, and homologous chromosomes in mitosis versus meiosis.
- Create and discuss a picture that illustrates the importance of crossing over and chromatid exchange during meiosis I and explain what happens if these crossover events do not occur.
- Define and be able to use the terms heterozygous, homozygous, mutant, wild type, dominant, recessive, allele, gene, loci, and chromosomes correctly.
- Define haploid and polyploid and
describesome costs and benefits of polyploidy.
- Describe how allelic segregation and independent assortment result in the inheritance of characteristics through the process of meiosis and sexual reproduction.
Sexual reproduction was an early evolutionary innovation after the appearance of eukaryotic cells. On the surface, offspring that are genetically identical to the parent may appear to be more
However, multicellular organisms that
depend on asexual reproduction are rare.
So why is sexual reproduction so common?
This is one of the important questions in biology and has been the focus of much research from the latter half of the twentieth century until now. A likely explanation is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of those offspring. The only source of genetic variation in asexual organisms is mutation. In sexually reproducing organisms, mutations
between generations when parents combine their unique genomes, and
into different combinations by the process of meiosis.
The Red Queen Hypothesis
Each tiny advantage gained by favorable variation gives a species an edge over close competitors, predators, parasites, or even prey. The only method that will allow a
Sexual reproduction requires fertilization, the union of two cells from two individual organisms. If those two cells each contain one set of chromosomes, then the resulting cell contains two sets of chromosomes. Haploid cells contain one set of chromosomes, diploid cells contain two sets of chromosomes. The number of sets of chromosomes in a cell
its ploidy level. If the reproductive cycle is to continue, then the diploid cell must somehow reduce its number of chromosome sets before fertilization can occur again, or there will be a continual doubling in the number of chromosome sets in every generation. So,
fertilization, sexual reproduction includes a nuclear division that reduces the number of chromosome sets.
The nuclear division that forms haploid cells, which
to mitosis. In mitosis, both the parent and the daughter nuclei are at the same ploidy level—diploid for most plants and animals. Meiosis
many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid. To achieve this reduction in chromosome number, meiosis
one round of chromosome duplication and two rounds of nuclear division. Because the events that occur during each of the division stages are analogous
. However, because there are two rounds of division,
with a “I” or a “II.” Thus, meiosis I
the first round of meiotic division and
prophase I, prometaphase I, and so on. Meiosis II, in which the second round of meiotic division takes place, includes prophase II, prometaphase II, and so on.
Early in prophase I, before the chromosomes can
As the nuclear envelope
Early in prophase I, homologous chromosomes come together to form a synapse.
The crossover events are the first source of genetic variation in the nuclei produced by meiosis. A single crossover event between homologous non-sister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. Now, when that sister chromatid
Crossover occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes.
Possible NB Discussion Point
What are the major differences between Prophase I of Meiosis and Prophase of Mitosis? Why are these distinctions so significant?
The key event in prometaphase I
This randomness is the physical basis for the creation of the second form of genetic variation in offspring. Consider that the homologous chromosomes of a sexually reproducing organism
This event—the random (or independent) assortment of homologous chromosomes at the metaphase plate—is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations
To summarize the genetic consequences of meiosis I, the maternal and paternal genes
Random, independent assortment during
In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound
Telophase I and Cytokinesis
In telophase, the separated chromosomes arrive at opposite poles. The
Two haploid cells are the
In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so
If the chromosomes
The nuclear envelopes
The sister chromatids are maximally condensed and aligned at the equator of the cell.
The sister chromatids
The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase
Telophase II and Cytokinesis
The chromosomes arrive at opposite poles and begin to
An animal cell with a diploid number of four (2n = 4) proceeds through the stages of meiosis to form four haploid daughter cells.
Possible NB Discussion Point
Have you ever enjoyed the convenience of a seedless fruit? If you’ve eaten the modern day banana, then you have consumed a triploid fruit. While the wild fruit is diploid and can sexually reproduce, seedless bananas arise from mutations, planned hybridizations, and can propagate asexually. Explain why triploid organisms are incapable of successfully undergoing meiosis. Can you think of any benefits to being triploid instead of diploid?
Comparing Mitosis and Meiosis
Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but also exhibit distinct differences that lead to very different outcomes. Mitosis is a single nuclear division that results in two nuclei that
The main differences between mitosis and meiosis occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs become associated with each other,
When the chiasmata resolve and the tetrad
Meiosis II is much more analogous to a mitotic division.
Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell.
The Mystery of the Evolution of Meiosis
Some characteristics of organisms are so widespread and fundamental that it is sometimes difficult to remember that they evolved like other simpler traits. Meiosis is such an extraordinarily complex series of cellular events that biologists have had trouble hypothesizing and testing how it may have evolved. Although meiosis
Meiosis and mitosis share obvious cellular
There are other approaches to understanding the evolution of meiosis in progress. Different
Link to Learning
Click through the steps of this interactive animation to compare the meiotic process of cell division to that of mitosis: How Cells Divide.
- Leigh Van Valen, “A new evolutionary law,” Evolutionary Theory 1 (1973): 1–30.
- Adam S. Wilkins and Robin Holliday, “The Evolution of Meiosis from Mitosis,” Genetics 181 (2009): 3–12.
- Marilee A. Ramesh,
Shehre-Banoo Malik and John M. Logsdon, Jr, “A Phylogenetic Inventory of Meiotic Genes: Evidence for Sex in Giardia and an Early Eukaryotic Origin of Meiosis,” Current Biology 15 (2005):185–91.