Objectives

By the end of this lesson you should be able to:

  1. Define basic terminologies in genetics
  2. Differentiate between the three life cycles
  3. Describe the steps in meiosis
  4. Explain three key mechanisms accounting for genetic variation

In the last lesson we covered mitosis. Remember in mitosis, somatic (body) cells divide to produce exact replicas. In this lesson we will spend most of our time focusing on meiosis. In this type of cell division, sex cells divide to produce new sex cells that are not identical to each other. We will get into that later. Let us first review some basic terminologies.

Terminologies

Transmission of traits from one generation to the next is called inheritance or heredity (from the Latin heir). Genetics is the study of heredity and variation. In sexual reproduction, offspring are not exact replicas of their parents but show variation. In asexual reproduction, the offerings are exact replicas of the parent. We call these replicas clones. Variation among clones generally occur due to mutation that happens after they are produced.

The unit of heredity is the gene. Reproductive cells, called gametes are responsible for carrying genes from one generation to the next. A gene’s specific location along the length of a chromosome is called a locus.

Humans have 23 pairs of homologous chromosomes – one from each parent. Homologous chromosomes are those that share structural features such as size, band patterns and centromere position. Homologous chromosomes appear alike under a microscope but may vary in their versions of genes. The version of gene at each locus is called an allele. We call our full set of chromosomes karyotype. Chromosome 1 to 22 is referred to as autosomal chromosomes and chromosome 23 is our sex chromosome – XY for males and XX for females.

Figure 1. Human karyotype. Image source: Wiki Commons.

The sex chromosome is an exception to the others since they are not homologous. The Y chromosome is shorter than the X and so it does not have all the genes that the X chromosome has.

The number of chromosomes on the full set of 23 chromosomes is represented as n. So, organisms with 2 chromosome sets are diploid (2n). Since humans have 2 sets of 23 chromosomes, we are diploid. Somatic sells have 46 chromosomes (2 sets of 23), but gametes (egg and sperm cells) have 23 chromosomes. So, they are haploid (n).

Gametes are the only cells in the body that are not produced by mitosis. They develop from specialized diploid cells in the ovaries and testes called oocytes and spermatocytes. When they divide, they produce egg and sperm cells respectively. The process of cell division in gametes is via meiosis. This will be the subject of our discussion next. But, just before we jump into meiosis, let’s refamiliarize ourselves with the 3 types of sexual life cycle. We use the term life cycle, to describe the generation to generation sequence of stages in the reproductive history of an organism.

  1. Diploid-dominant life cycle (e.g., animals)
    1. Spends most of its life as diploid adults
    1. Gametes (n + n)) fuse in fertilization to form a zygote (2n)
    1. Zygote undergoes mitosis to develop into a multicellular adult
    1. Adult produces gametes via meiosis which can then participate in fertilization to start the cycle again
  2. Haploid-dominant life cycle (e.g., protists, and fungi)
    1. Spends most of its life as a haploid cell
    1. Gametes (n + n) fuse in fertilization to form a zygote (2n)
    1. Zygote remains as a single cell and undergoes meiosis to develop into gametes
    1. New gametes are formed via mitosis which can then participate in fertilization to start the cycle again
  3. Alternation of generation (e.g., plants)
    1. Spends half of its life as diploid (sporophytes) and the other half as haploid (gametophytes)
    1. Zygote (seeds) undergoes mitosis to develop into a sporophyte
    1. Sporophyte produces spores via meiosis
    1. Spores develop via mitosis to produce gametophytes (flowers in angiosperms and cones in gymnosperms)
    1. Gametophytes produce gametes (sperms and eggs) which fertilize to produce zygote (the seed)
    1. Seeds grow and starts the cycle again  

Meiosis  

While mitosis maintains the total number of chromosomes. Meiosis reduce them to half (2n to n). Meiosis is divided into two divisions, i.e., meiosis I and meiosis II.

Meiosis I

  1. Prophase I
  2. Metaphase I
  3. Anaphase I
  4. Telophase I and cytokinesis

Meiosis II

  1. Prophase II
  2. Metaphase II
  3. Anaphase II
  4. Telophase II and cytokinesis

Mnemonic: PMAT x 2

In meiosis I, the number of chromosomes is halved, while in meiosis II, the sister chromatids are split. 

Figure 2. Summary of what happens in meiosis I and meiosis II

Steps in Meiosis

Let’s take a look now in some more details on what happens at each stage of meiosis. We have already talked about interphase in our discussion of mitosis. Interphase happens just before meiosis. Remember that in interphase, the cell grows, and the DNA is duplicated.

Figure 3. Meiosis I

Figure 4. Meiosis II

Prophase I

  • Homologous chromosomes pair up and form synapses. This is a physical connection of the pairs with the help of a protein called synaptonemal complex
  • Nuclear envelop disintegrates
  • Spindle fiber forms
  • Homologous chromosomes exchange DNA in an event called cross-over which takes place at a point known as the chiasmata. After this, the homologous pairs are no longer identical. Note that the synapse always occurs but exchange of DNA may or may not happen at the synapse. The synapse at which the exchange happens is the chiasmata

Metaphase I

  • Homologous chromosome pairs line up at the metaphase plate
  • Microtubules from opposite poles attach to kinetochores of the chromosomes

Anaphase I

  • Chromosomes are pulled along the microtubules to opposite ends of the cell

Telophase I and Cytokinesis

  • Microtubules disintegrate
  • Cleavage furrow forms
  • Cell divides (cytokinesis)

Prophase II

  • Centriole and microtubules   

Metaphase II

  • Chromosomes line up at the metaphase plate. (Note that chromatids are no longer identical)

Anaphase II

  • Proteins holding chromatids together break down
  • Chromatids move along spindle fiber towards opposite poles of the cell

Telophase and Cytokinesis

  • Nuclear envelop forms
  • Chromosome decondenses
  • Cleavage furrow forms and cell divides producing 4 daughter cells that are genetically different from each other

Genetic Variation

Genetic variation occurs within any given population of species. This feature is important in ensuring species survival. Three ways in which genetic variation is made possible are:

  1. Independent assortment
  2. Crossing over
  3. Random fertilization

Independent Assortment

Independent assortment is the random assignment of homologous pairs at the metaphase plate in metaphase I. How the chromosomes align will determine which cell they end up when the homologs are separated in later steps of meiosis. Take a look at the metaphase I diagram again. Why is the red homolog facing the top or the bottom as the case might be? Don’t sweat it. As far as we know, it’s just random. The number of possible combinations of chromosome alignment in metaphase I, given that we have 23 pairs of chromosomes, is 223 (8.4 million).

Crossing Over

In meiosis, an average of 1-3 cross-over events occur per chromosome pair, depending on the size of the chromosome and centromere position. As you can see in the figure below, one chromosome is maternal and the other paternal. Thus, meiosis produces new combinations of maternal and paternal chromosomes.

Figure 4. Cross-over event

Random Fertilization

Every sperm that makes its way to the egg has a different combination of genes due to independent assortment and cross over. Which sperm gets to the egg first is totally random. Fertilization produces a zygote of about 70 trillion (223 X 223) diploid combinations. It’s needless to say that each one of us is truly unique. 

Textbook: Textbook: Reece, J. B., & Campbell, N. A. (2011). Campbell biology. Boston: Benjamin Cummings / Pearson.

Courtney Simons
Administrator
Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University. He also holds Masters degrees in both Environmental Science and Instructional Design from Wright State University.
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