Meiosis and Sexual Life Cycles - 1 - 1 - STEM Digital · PDF fileMeiosis and Sexual Life Cycles ... generation by the process of meiosis and sexual reproduction. Although asexual reproduction,

Meiosis and Sexual Life Cycles - 1

We have just finished looking at the process of mitosis, which ensures that eachcell of an organism has the same DNA as the original fertilized egg or zygote(absent mutations).

Transmitting chromosomes and genetic information from generation to generationis equally important. A critical role of heredity is to maintain and obtain variationamong members of a species. These variations are the result of the specificgenes we inherit from our parents. We did not always know that genes werelocated on chromosomes. We didn't even know how genetic information wastransmitted from parent to offspring. The mechanism for transmitting geneticinformation was first proposed by Gregor Mendel in the mid-1800's. It was prettymuch forgotten until the early 1900's when Mendel's papers were "discovered"about the time other researchers were drawing the same conclusions based onsimilar research. Soon after, Walter Sutton showed that Mendel's principles ofinheritance applied to chromosomes and that chromosomes are the units ofheredity. We shall discuss Mendel's principles and inheritance patterns soon, butfirst we'll look at how chromosomes are transmitted from generation togeneration by the process of meiosis and sexual reproduction.

Although asexual reproduction, which uses mitosis to make new individuals(genetically the same as the parent) is common in protists, plants, fungi and someanimals, most organisms produce offspring by a process of sexual reproduction, inwhich a gamete from one parent joins a gamete from the other parent to form azygote (or fertilized egg). This process results in offspring that have acombination of parental chromosomes.

However, each generation of a species retains the same chromosome number asthe preceding generations. Meiosis is the process that ensures that each newgeneration has the same chromosome number as the preceding generation.

Meiosis is a process that reduces chromosome number by half and occurs at justone stage in an organism's life cycle (to form gametes in animals, or to start thegamete producing stage in plants, or for some organisms to restore theappropriate chromosome number for the assimilative stage of its life history).Sexual reproduction then restores the "typical" number of chromosomes forthe next generation. In this context, we need to look not just at the process ofmeiosis, but also take a look at the sexual life cycles of organisms.

In addition to providing a mechanism to reduce chromosome number for sexualreproduction, meiosis has a second, most important function for living organisms:maintaining genetic variation. Each time meiosis occurs, followed by, at somepoint, sexual reproduction, the new individual is genetically different from eitherparent. Because meiosis is involved with genetic variation and is needed for sexualreproduction, we will discuss this important genetic function of meiosis as well asits chromosome reduction function in this section.

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So how does meiosis work?To answer the question of how meiosis works, we need to first revisit thechromosome. If we look at the chromosomes of most eukaryotic organismscarefully, it can be seen that for each individual chromosome, a secondchromosome can be found that physically matches it in length and shape. This isbest seen with the karyotype. Closer inspection of the DNA shows that thematching chromosomes have very similar, but not identical DNA. These matchingchromosomes, with their similar DNA, form the basis of the variation we see in thegenetic traits of living organisms, as well as being a way that we can reducechromosome number during meiosis and still have the appropriate geneticinformation in each cell. The matching chromosome pairs are called homologouschromosome pairs, or homologues (homologs).

Cells that contain pairs of homologous chromosomes are called diploid (2n).When a cell has chromosome pairs, we refer to the diploid number ofchromosomes, again meaning that each chromosome has a match, or homologue inthat cell. For humans, the diploid number of chromosomes is 46. Again, these 46chromosomes are comprised of 23 homologous pairs of chromosomes.

Following meiosis, the product cells will not have pairs of chromosomes; there willbe one of each pair in each cell formed, so the chromosome number will have beenreduced by half, and is said to be haploid (n).

By the way, "ploid" as a general term also means a "set", so we can also say that adiploid cell has two sets of chromosomes, or two of each kind of chromosome. Ahaploid cell has one set of chromosomes, or one of each kind. (It's possible to havemore than 2 chromosomes of each kind. Polyploids are quite common inagriculture as a result of plant breeding. Polyploids are less common in animals.)

For many sexually reproducing organisms, one homologous pair of chromosomesdoes not precisely match in size in one gender, but does match in the other gender.This is the pair of sex-determining chromosomes, or the sex chromosomes. Theother pairs of chromosomes do match and are called autosomes. Humans have22 pairs of autosomes and 1 pair of sex chromosomes.

Meiosis and the Life Cycles of OrganismsJust as each cell has a cell cycle, organisms have a life cycle. For most, the lifecycle includes sexual reproduction. Meiosis is something that takes place atjust one point in any sexually reproducing organism's life cycle. Meiosisalways reduces the chromosome number (typically from diploid to haploid).

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In animals, meiosis generally occurs to form gametes: sperm or eggs. Egg andsperm are the only haploid cells of animals. In many other types of organisms,meiosis occurs at some point in the life cycle other than the direct formation ofgametes, and the products of meiosis may be spores, (as in plants) or the firstcells of the next generation (for most protists and most fungi). At some pointhowever, all organisms that sexually reproduce will make haploid gametes (spermand egg, or different genetic mating types).

Similarly, fertilization occurs at one point in an organism's life history.Fertilization occurs between two different haploid cells, called gametes, to formthe zygote, or fertilized egg. The zygote obtains half its chromosomes from thesperm and half from the egg (or half from one gamete and half from the second;yeasts, for example, have a and α gametes, not sperm and egg).

Fusion of gametes restores the diploid number, and in so doing, also restoreshomologous chromosomes (one of each kind being provided by the sperm and oneof each kind coming from the egg). Since each gamete has a unique combination ofchromosomes, each zygote will be unique, and genetic variation is both maintainedand obtained within the species.

Yet, as mentioned earlier, when meiosis occurs in the life cycle of organisms is notalways the same. Let's compare three typical life cycle patterns and look at thetiming of meiosis in the life cycle of each.

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Diploid Life Cycle• In animals, meiosis generally produces just haploid sex cells, or gametes, which

at fertilization start the next generation. The only haploid cells of the animalare egg or sperm, and the respective maturation processes are calledoogenesis and spermatogenesis.

• When gametes fuse, the zygote grows by mitosis producing the adult stage. Allcells will be diploid and all cells are produced by mitosis. The animal life cycle isa diploid life cycle.

Haploid Life Cycle• In many protists, and some fungi, haploid gametes fuse to form a zygote, but

the zygote immediately does meiosis forming single haploid cells.• In protists, which remain single cell organisms, the nucleus is then haploid.• At some time, a single cell may just decide to become a gamete and fuse with

another to make a zygote, or haploid cells may do mitosis to make moreindividuals asexually.

• Haploid life cycles can be more complex. Fungi, and some algae may makemulticellular haploid organisms from the single-celled meiotic product bymitosis. At some time, special areas of the haploid body will become gamete-making structures (often called gametangia), and haploid gametes are formedby mitosis.

Alternation of Generations• Most plants have both a multicellular haploid stage and a multicellular diploid

stage in their life histories. This is called the alternation of generations. (Sincewe are humans (and animals), we just haven't had the opportunity to becomeacquainted with these different forms of a plant's life.)

• In plants, the structure in which meiosis occurs is called a sporangium. Themulticellular diploid plants, or parts of plants, that produce sporangia are calledsporophytes (spore-making plant).

• Meiosis does not directly produce gametes, but produces haploid cells, calledspores that in turn, grow, by mitosis, into multicellular haploid structurescalled gametophytes (gamete-making plant). Gametophytes eventuallyproduce and contain gametes.

• Which stage (sporophyte or gametophyte) is predominant in the life of a plantvaries with different types of plants. Most "higher" plants have predominantsporophytes. A pollen grain of pine tree or flower, for example is the malegametophyte stage of those plants. Mosses, in contrast have predominantgametophyte stages. It is easiest to see sporangia and spores on ferns. Thefern plant we know (and love) is the diploid sporophyte generation. Thesporangia are located on the undersides of leaves. Once one knows what a ferngametophyte looks like, they can often be spotted growing in pots or on thesurfaces of pots in greenhouses.

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On to the Process of MeiosisRemember, the purposed of meiosis is to reduce the chromosome number by onehalf at one point in the life cycle of an organism so that following sexualreproduction (fertilization) the "typical" number of chromosomes will be maintainedfrom generation to generation. Meiosis and sexual reproduction are geneticevents, critical to maintaining (and obtaining) genetic variation among members ofa species. This variation is essential for most organisms so the species can beresponsive to changing environmental conditions through time. (More on this later.)

Homologous chromosome pairs are essential to how meiosis works. Inmeiosis the homologous chromosomes literally pair up prior to the reduction ofchromosome number. In meiosis, one of each type of chromosome (one of eachhomologous pair) is distributed to each meiotic product, so that the meioticproducts have half as many chromosomes as the "parent" cell. This is the crucialdifference between mitosis and meiosis, and explains why we can reducechromosome number and still have all of the genetic information needed to form anew organism after fertilization.

Some Notes:• "Sister" Chromatids are not pairs; they are the two identical parts of one

duplicated chromosome. You must make this distinction to understand howthe process of meiosis works!

• A pair of duplicated homologous chromosomes will have a total of 4chromatids, two chromatids for each homologue. It's just a fact that priorto any cell division, chromosomes duplicate, so meiosis starts with duplicatedchromosomes.

• After meiosis, the meiotic products have the haploid (half the parental)number of chromosomes, and no pairs of homologous chromosomes.Haploid also refers to the cell when there is just one of each kind ofchromosome, or the "n" number of chromosomes. (Diploid is the 2n number ofchromosomes.)

• Again, the diploid number of chromosomes will be restored when two gametes(egg and sperm) unite in sexual fertilization.

• In contrast, during mitosis, the DNA is precisely duplicated and each cellformed has exactly the same genes as the original. This means that anyoffspring that result from mitosis (as with asexual reproduction) aregenetically identical to the parent.

• The homologous pairs of chromosomes in diploid organisms do not interactduring mitosis; each chromosome is on its own.

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The Process of Meiosis – Details that Distinguish MeiosisJust to say it again, during meiosis, homologous chromosomes line up or literallypair with each other. Meiosis reduces the chromosome number by one-half in away that ensures that the gametes will get one of each pair of homologues. Theseparation of homologous chromosomes is critical to the process of meiosis. Theproducts of meiosis (such as the gametes in animals) have no pairs ofchromosomes and are haploid, since they have half the chromosomes of the parentcell. When fertilization occurs, the diploid number is restored, along with thematching chromosomes.

Prior to any cell division, chromosomes must undergo DNA duplication. To achievethe reduction in chromosome number and appropriate distribution ofchromosomes, meiosis requires two divisions, called Meiosis I and Meiosis II.At the completion of the second division, four cells will typically be produced. Thestages of meiosis resemble those of mitosis; the differences occur in thematching or pairing of the homologous chromosomes, which occurs during the firstdivision prophase.

The Stages of Meiosis (Or how we break up a continuous process into chunks)Meiosis is comprised of two divisions each with a prophase, metaphase, anaphaseand telophase. In addition, meiosis is preceded by pre-meiotic interphase in thegerm-line tissue and an interkinesis between the two divisions.

Pre-Meiotic InterphaseThe DNA of the cell* that will do meiosis duplicates. (DNA duplication must precedeany cell division.) The identical sister chromatids of each duplicated chromosomeare attached at their centromeres and have their kinetochores.

* Cells that do meiosis are restricted to specialized structures such as the sexorgans (ovary and the testis) of animals; or anther and ovule (which arespecialized sporangia) of "higher" plants; or sporangia of "lower" plants. Thesetissues are often referred to as the "germ-line tissues". These tissues arediploid, just as is the rest of the organism. Only the products of meiosis(gametes or gamete-producing structures) are haploid.

Note that some organisms have a haploid life cycle; most of their assimilativelife is spent with cells that are haploid. Meiosis immediately follows theformation of the zygote. The haploid cells produced are the first cells of thenext generation, which grow by mitosis to become adults.

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Meiosis IProphase IProphase I has been further broken down by some into four subphases:leptotene, zygotene, pachytene, and diplotene, plus one more: diakinesis.

Leptotene• Duplicated chromosomes condense

Zygotene• Homologous chromosomes pair up along their length at the start of prophase I

in a process called synapsis. This uses a protein lattice along thechromosomes to join the homologues together. The homologues literally join atseveral points (called chiasmata). All four chromatids of the homologous pairare aligned together, forming the synaptonemal complex.

Pachytene• The synaptonemal complex allows for a process of genetic importance to occur.

Portions of intertwined chromatids of the homologous chromosomes unwind andthe single-stranded DNA molecule sections base-pair to complementary strandsof a non-sister chromatid of the homologue. This results in an exchange of DNAportions between the homologous chromosomes with the structure of thesynaptic complex. This exchange is called crossing over. Crossing overoccurs between the non-sister chromatids, and is mediated by enzymes. If thespecific DNA of the homologues were different forms of genes, thenrecombination occurs. The sister chromatids now have some geneticvariation; they are no longer precisely identical.

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Diplotene• After crossing over takes place, the still-joined homologues pull apart,

decondense and active cell growth activities occur, including transcription ofgenes, gearing up for the divisions to follow. Homologous chromosomes arestill attached at the chiasmata, however.

Diakinesis• As homologous chromosomes ready for migration to metaphase I, transcription

activities ceases, and homologous chromosome pairs recondense.

In addition, all things that we normally think of taking place in a prophase alsooccur in prophase I of meiosis, including attaching spindle microtubules to eachchromosome of the attached homologous pairs. But, spindle microtubules do notattach to each kinetochore of each chromatid!

Metaphase I• Homologous pairs of chromosomes, still synapsed, are moved to the equator by

the spindle complex. The chiasmata ensure that homologous chromosome pairsalign as pairs.

• The alignment is random; some "maternal" chromosomes will orient facing onepole along the equator; others face the opposite pole.

• Spindle microtubules just attach to homologous chromosomes as they find themfrom the respective poles of the cell. They do not attach to both homologouschromosomes.

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Anaphase I• The homologous chromosomes are separated from each other and

pulled toward opposite poles by the microtubules.• Since spindle microtubules have attached to just one kinetochore of each

duplicated chromosome, duplicated chromosomes are pulled as a unit. Theduplicated chromosomes are not affected during Anaphase I. The sisterchromatids are still tightly bound to each other by their centromeres.

• The chromosome number is officially reduced at this time because eachnucleus that will form around the set of chromosomes at each pole will havehalf the number of chromosomes as the pre-meiotic cell. All of thechromosomes will still be duplicated. No sister chromatids have separated.

• No homologous chromosome pairs are present at the end of Anaphase I. Eachcluster of chromosomes at the respective poles of the cell will have one of eachtype of homologous chromosome. It is the pairing and separation ofhomologues that is the key to reducing chromosome number whilemaintaining all of the genetic information.

• Genetic variation is increased as a consequence of the random alignment ofhomologous chromosomes at metaphase I and separation of homologous pairs inanaphase I. This is called random assortment, and is important for geneticvariation.

Telophase I and Interkinesis• Typically two new nuclei are formed, each with one set of the homologous

chromosomes• Cytokinesis typically will form two cells. (Each chromosome is still duplicated;

this occurred in pre-meiotic interphase.) Essentially the cells are just preparingfor the second division. Some cells do not bother with cytokinesis here, or evenform new nuclear envelopes, which will just have to be degraded during meiosis IIanyway.

• Genetically speaking, the two cells formed are not identical – recombination andindependent assortment activities result in non-identical chromatids in the twocells.

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Meiosis IIProphase II• New spindle apparatus is formed in each of the two cells from telophase I• The still-duplicated chromosomes stretch out and then recondense• Spindle fibers attach to the kinetochores of each of the sister chromatids, one

from each pole.

Metaphase Ii• The duplicated chromosomes are aligned along the equator by the spindle

complex

Anaphase II• Centromeres of sister chromatids are detached from each other.• The now non-duplicated chromosomes are pulled to the poles of the cells.

Telophase II and Cytokinesis• Each new nucleus formed has half the number of the original chromosomes but

each nucleus has one of each type of homologous chromosome.• A total of four new cells will be produced.

Note: Meiosis II as a mechanism is just like mitosis; duplicated chromosomes aredistributed equally into new cells, each with the same number of chromosomes asthe original. The difference is that you are starting with two cells, and forming atotal of 4 new cells.

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Reviewing the genetic importance of meiosis and sexual reproductionWe have discussed that chromosomes occur as homologous pairs, which arephysically matched. The homologous pairs of chromosomes are also matchedgenetically; each homologous chromosome has a gene locus for a specific trait, sothat a diploid organism typically has two pieces of DNA for each geneticcharacteristic, one on each of its homologues.

During meiosis there is some shifting and recombining of alleles so that new genecombinations always occur in the gametes that are different from the parent.

In sexual reproduction, each parent typically has two "genes" (or morecorrectly, two alleles of a gene) for each characteristic, one on each of thehomologous chromosomes. Each parent passes one of these genes (but not both)and one of each of its homologous chromosomes to the offspring by meiosis andfertilization. The fertilized egg (zygote) will then have two genes for each trait,one from each parent. It's important to note that each individual will have apaternal set of chromosomes and a maternal set of chromosomes. Eachhomologous pair of chromosomes has one paternal and one maternal origin.

Since parents are not genetically identical, their gametes will have differentcombinations of genes. Each egg and each sperm (or each spore) is geneticallydifferent from the parent's DNA (having only half as much). The offspring(children) formed by sexual reproduction will have genetic variation, important forthe long-term response of species to their environment. Such variations amongoffspring lead to physical, behavioral and physiological differences. Thesedifferences may be more or less useful in the surroundings of that organism, andare subject to the agents of selection. This variation is an important basis forevolutionary change.

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Further genetic variation is achieved during meiosis when homologouschromosomes synapse during prophase I and recombination occurs so that sisterchromatids are no longer precisely identical. Each of the four meiotic productsformed will be a bit different as a result of this recombination.

Another source of genetic variation in meiosis is the alignment of homologouschromosomes at the equator during metaphase I. Some maternal chromosomeswill align towards one pole and some towards the other pole. Depending on thisalignment, chromosomes assort differently, relative to maternal/paternal originduring each meiotic event. This is known as Independent Assortment, and is"easily" demonstrated in inheritance tests. The number of possible independentassortments of the 23 pairs of human chromosomes is 223 or about 8 million.

Independent Assortment of Homologous Chromosomes in Meiosis

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Final Note: Not all organisms reproduce sexually. Many organisms have bothsexual and non-sexual means of propagation, or increasing the numbers ofindividuals. Although non-sexual reproduction increases numbers of individuals, allhave virtually the same genetic complement.

Development of eggs without fertilization occurs in both plants and animals.Haploid eggs of bees become males while fertilized eggs become females. Inplants, development of embryos without fertilization is called apomixis, in animals,parthenogenesis. Many invertebrates have parthenogenesis, as does the rarevertebrate.

Asexual reproduction, which uses mitosis, can be a good strategy in anenvironment that is constant if a species is well-suited to those conditions.Dandelions, for example, rarely reproduce sexually; their seeds develop withoutfertilization. They are highly successful in the suburban lawn. And suburban lawnowners spend a lot of time making sure that their lawn looks just the same yearafter year, carefully applying water and nutrients several times each growingseason. This is perfect for dandelions. Sexual reproduction might introducevariation that could result in a dandelion less fit for the suburban lawn. Withoutsexual reproduction, however, there can be little genetic variation, most specieswithout genetic variation cope poorly over time with changing environments. (Thisis not always the case with microorganisms that can mutate and reproducerapidly.)