Unit 8: Cellular Reproduction: Meiosis
Meiosis is a specialized type of cellular reproduction that only occurs in the ovaries and testes and results in an egg or sperm, respectively. Sexual reproduction is responsible for the amazing amount of diversity within a species. When sperm fertilizes an egg, the resulting offspring contain genes from the father and the mother. In essence, you contain genes from ALL of your ancestors, at least in a small part.
Completing this unit should take you approximately 4 hours.
Upon successful completion of this unit, you will be able to:
- identify the different types of daughter cells produced by cell division;
- diagram and label the phases of meiosis given a number of chromosomes or chromosome pairs;
- compare and contrast mitosis and meiosis; and
- explain the role of meiosis.
8.1: Cell Division, Meiosis, and Sexual Reproduction
Meiosis is a type of cell division that leads to the production of non-identical daughter cells. These sex cells contain half the genetic information and are combined for sexual reproduction. Sexual reproduction is critical for the diversity of living things.
The process of meiosis is unique to animals that reproduce sexually. The life cycle of any sexual species features fertilization, which is the fusion of unicellular gametes (one male gamete and one female gamete) to produce a unicellular zygote. The unicellular zygote produced by fertilization carries the chromosomes from both gametes.
Consequently, the ploidy (the number of sets of chromosomes in a cell) of the zygote is double the ploidy of the gametes. If fertilization were the only process occurring in each generation, then the ploidy would double each generation (tetraploid, then octoploid, etc.), and the zygote would not be able to contain the DNA. To prevent the ploidy from doubling each generation, a separate process is needed to cut the ploidy in half.
Read this introduction to sexual reproduction. Organisms give half of their chromosomes to their offspring. Pay attention to the definition of haploid and diploid cells. As you read, think about the ways sexual reproduction benefits a species by creating diversity.
Another important purpose of meiosis is to increase the genetic variability of the gametes produced. The increase in genetic variability comes in the forms of crossing over (during prophase of meiosis I) and independent assortment (during metaphase of meiosis I). As you study the phases of meiosis, appreciate that crossing over and independent assortment produce new genetic combinations, and separation of homologs reduces the ploidy.
Read this section to learn about the events that occur during meiosis.
Watch these videos to review the steps of mitosis and meiosis. Notice the additional division in meiosis that leads to genetic variability. How would you describe stages of meiosis and how cells are produced? Can you explain why meiosis is needed for sexual reproduction? What cells are involved in fertilization? How does fertilization occur?
After you have watched the videos, you should be able to distinguish between somatic cell and gamete, distinguish between haploid and diploid cells, list the phases of meiosis I and meiosis II, describe the events of each phase, and illustrate the phases of meiosis.
Mitosis and meiosis are two alternative processes that can be involved in eukaryotic cell division.
Here is a summary of the key similarities:
- Mitosis and meiosis are both processes that divide the nucleus of a cell.
- Both processes occur in phases including prophase, prometaphase, metaphase, anaphase, and telophase.
- Meiosis II is essentially identical to mitosis, but meiosis II occurs in each of the two cells previously produced in meiosis I.
Here is a summary of the key differences:- Mitosis produces two cells that are genetically identical to the parent cell; meiosis produces four cells that are genetically distinct from each other and from the parent cell.
- Mitosis is used to produce duplicate cells for the purpose of growth of a multicellular organism or replacement of lost cells; meiosis is used to produce haploid cells out of a diploid cell for the purpose of sexual reproduction.
- Mitosis involves only one round of division; meiosis involves two rounds of division (meiosis I and II).
- In mitosis, chromosomes act individually, and homologous chromosomes do not synapse; in meiosis, homologous chromosomes synapse, and each homologous pair acts throughout meiosis I as a unit.
- Mitosis does not feature crossing over; meiosis I features crossing over.
- In metaphase of mitosis, individual, replicated chromosomes line up midway between poles (without the pairing of homologs); in metaphase of meiosis I, homologous pairs of replicated chromosomes line up midway between poles.
- In anaphase of mitosis, sister chromatids separate; in anaphase of meiosis I, homologs separate.
- Mitosis maintains the ploidy; meiosis cuts the ploidy in half.
Watch this video for a summary of the differences between mitosis and meiosis.
Watch this video for a guided tour of meiosis as seen on a microscopy slide of lily flower buds. The speaker presents examples of cells in four stages of meiosis: late prophase I, telophase I, anaphase II, and telophase II.
Read this text, which discusses how nondisjunction leads to disorders in chromosome number and how errors in chromosome structure occur through inversions and translocations.
8.2: Chromosomes, Chromatids, and Chromatin
Chromosomes are unique collections of genetic material that make up each of us as individuals. They are made up of chromatin which is composed of histone proteins and DNA. When DNA is copied, chromatids are formed to be passed on to cells upon cell division.
Watch this video for an overview of chromosomes.
Watch this video, which discusses the structure of chromosomes and their composition. At this point, you should be able to differentiate between the concepts of chromosomal structure, chromatin, and chromatids. You should also be able to define the role of the centromere.
Unit 8 Assessment
Take this assessment to see how well you understood this unit.
- This assessment does not count towards your grade. It is just for practice!
- You will see the correct answers when you submit your answers. Use this to help you study for the final exam!
- You can take this assessment as many times as you want, whenever you want.