Anaphase I: During anaphase I the spindle fibers shorten, and the homologous chromosome pairs are separated from each other. One chromosome from each pair moves toward one pole, with the other moving toward the other pole, resulting in a cell with 23 chromosomes at one pole and the other 23 at the other pole. The sister chromatids remain attached at the centromere. Because human cells have 23 pairs of chromosomes, this independent assortment of chromosomes produces 223, or 8,388,608 possible configurations. More on independent assortment of chromosomes will be presented in the chapter on Mendelian Genetics.
Telophase I: The spindle fiber disassembles and the nucleus reforms. This is quickly followed by cytokinesis and the formation of two haploid cells, each with a unique combination of chromosomes, some from the father and the rest from the mother. After cytokinesis, both cells immediately enter meiosis II; the DNA is not copied in between. Meiosis II is essentially the same as mitosis, separating the sister chromatids from each other.
Prophase II: Once again the nucleus breaks down, and the spindle begins to reform as the centrioles move to opposite sides of the cell.
Metaphase II: The spindle fibers align the 23 chromosomes, each made out of two sister chromatids, along the equator of the cell.
Anaphase II: The sister chromatids are separated and move to opposite poles of the cell. As the chromatids separate, each is known as a chromosome. Anaphase II results in a cell with 23 chromosomes at each end of the cell; each chromosome contains half as much genetic material as at the start of anaphase II.
Telophase II: The nucleus reforms and the spindle fibers break down. Each cell undergoes cytokinesis, producing four haploid cells, each with a unique combination of genes and chromosomes.
Figure 6.12
Meiosis is a process in which a diploid cell divides itself into four haploid cells. represents the number of chromosomes, represents a haploid cell, represents a diploid cell.
An excellent animation depicting meiosis can be viewed at
http://www.youtube.com/watch?v=D1_-mQS_FZ0&feature=related.
Meiosis and Genetic Variation
Sexual reproduction results in infinite possibilities of genetic variation. This occurs through a number of mechanisms, including crossing-over, the independent assortment of chromosomes during anaphase I, and random fertilization.
Crossing-over occurs during prophase I. Crossing-over is the exchange of genetic material between non-sister chromatids of homologous chromosomes. Recall during prophase I, homologous chromosomes line up in pairs, gene-for-gene down their entire length, forming a configuration with four chromatids, known as a tetrad. At this point, the chromatids are very close to each other and some material from two chromatids switch chromosomes, that is, the material breaks off and reattaches at the same position on the homologous chromosome (Figure below). This exchange of genetic material can happen many times within the same pair of homologous chromosomes, creating unique combinations of genes. This process is also known as recombination.
Figure 6.13
Crossing-over. A maternal strand of DNA is shown in red. Paternal strand of DNA is shown in blue. Crossing over produces two chromosomes that have not previously existed. The process of recombination involves the breakage and rejoining of parental chromosomes (M, F). This results in the generation of novel chromosomes (C1, C2) that share DNA from both parents.
As mentioned above, in humans there are over 8 million configurations in which the chromosomes can line up during metaphase I. It is the specific processes of meiosis, resulting in four unique haploid cells, that results in these many combinations. Figure below compares mitosis and meiosis. This independent assortment, in which the chromosome inherited from either the father or mother can sort into any gamete, produces the potential for tremendous genetic variation. Together with random fertilization, more possibilities for genetic variation exist between any two people than individuals alive today. Sexual reproduction is the random fertilization of a gamete from the female using a gamete from the male. In humans, over 8 million (223) chromosome combinations exist in the production of gametes in both the male and female. A sperm cell, with over 8 million chromosome combinations, fertilizes an egg cell, which also has over 8 million chromosome combinations. That is over 64 trillion unique combinations, not counting the unique combinations produced by crossing-over. In other words, each human couple could produce a child with over 64 trillion unique chromosome combinations.
Figure 6.14
Mitosis vs. Meiosis Comparison. Mitosis produces two diploid daughter cells, genetically identical to the parent cell. Meiosis produces four haploid daughter cells, each genetically unique. See for an animation comparing the two processes.
Gametogenesis
At the end of meiosis, haploid cells are produced. These cells need to further develop into mature gametes capable of fertilization, a process called gametogenesis (Figure below). Gametogenesis differs between the sexes. In the male, the production of mature sperm cells, or spermatogenesis, results in four haploid gametes, whereas, in the female, the production of a mature egg cell, oogenesis, results in just one mature gamete.
Figure 6.15
Analogies in the process of maturation of the ovum and the development of the spermatids. Four haploid spermatids form during meiosis from the primary spermatocyte, whereas only 1 mature ovum, or egg forms during meiosis from the primary oocyte. Three polar bodies may form during oogenesis. These polar bodies will not form mature gametes.
During spermatogenesis, primary spermatocytes go through the first cell division of meiosis to produce secondary spermatocytes. These are haploid cells. Secondary spermatocytes then quickly complete the meiotic division to become spermatids, which are also haploid cells. The four haploid cells produced from meiosis develop a flagellum tail and compact head piece to become mature sperm cells, capable of swimming and fertilizing an egg. The compact head, which has lost most of its cytoplasm, is key in the formation of a streamlined shape. The middle piece of the sperm, connecting the head to the tail, contains many mitochondria, providing energy to the cell. The sperm cell essentially contributes only DNA to the zygote.
On the other hand, the egg provides the other half of the DNA, but also organelles, building blocks for compounds such as proteins and nucleic acids, and other necessary materials. The egg, being much larger than a sperm cell, contains almost all of the cytoplasm a developing embryo will have during its first few days of life. Therefore, oogenesis is a much more complicated process than spermatogenesis.
Oogenesis begins before birth and is not completed until after fertilization. Oogenesis begins when an oogonia (singular, oogonium), which are the immature eggs that form in the ovaries before birth, with the diploid number of chromosomes undergoes mitosis to form primary oocytes, also with the diploid number. It proceeds as a primary oocyte undergoes the first cell division of meiosis to form secondary oocytes with the haploid number of chromosomes. A secondary oocyte undergoes the second meiotic cell division to form a haploid ovum if it is fertilized by a sperm. The one egg cell that results from meiosis contains most of the cytoplasm, nutrients, and organelles. This unequal distribution of materials produces one large cell, and one cell with little more than DNA. This other cell, known as a polar body, eventually breaks down. The larger cell undergoes meiosis II, once again producing a large cell and a polar body. The large cell develops into the mature gamete, called an ovum.
Sexual Life Cycles
Eukaryotes have three different versions of the sexual life cycle: a haploid life cycle, a diploid life cycle, and a life cycle known as the alternation of generations (Figure below). A life cycle is the span in the life of an organism from one generation to the next. All species that reproduce sexually follow a basic pattern, alternating between haploid and diploid chromosome numbers. The sexual life cycle depends on when meiosis occurs and the type of cell that undergoes meiosis.
Figure 6.16
Sexual Life Cycles.
Haploid Life C
ycles
The haploid life cycle is the simplest life cycle. Organisms with this life cycle, such as many protists and some fungi and algae, spend the majority of their life cycle as a haploid cell. In fact, the zygote is the only diploid cell. The zygote immediately undergoes meiosis, producing four haploid cells, which grow into haploid multicellular organisms. These organisms produce gametes by mitosis. The gametes fuse through a process called fusion to produce diploid zygotes which undergo meiosis, continuing the life cycle.
Diploid Life Cycles
Organisms that have a diploid life cycle spend the majority of their lives as diploid adults. All diploid adults inherit half of their DNA from each parent. When they are ready to reproduce, diploid reproductive cells undergo meiosis and produce haploid gametes. These gametes then fuse through fertilization and produce a diploid zygote, which immediately enters G1 of the cell cycle. Next, the zygote's DNA is replicated. Finally, the processes of mitosis and cytokinesis produce two genetically identical diploid cells. Through repeated rounds of growth and division, this organism becomes a diploid adult and the cycle continues.
Alternation of Generations
Plants, algae, and some protists have a life cycle that alternates between diploid and haploid phases, known as alternation of generations. In plants, the life cycle alternates between the diploid sporophyte and haploid gametophyte. Spore forming cells in the diploid sporophyte undergo meiosis to produce spores, a haploid reproductive cell. Spores can develop into an adult without fusing with another cell. The spores give rise to a multicellular haploid gametophyte, which produce gametes by mitosis. The gametes fuse, producing a diploid zygote, which grow into the diploid sporophyte.
Lesson Summary
Asexual reproduction produces a clone, an organism that is genetically identical to its parent.
Asexual reproduction includes fission, fragmentation and budding.
Sexual reproduction involves haploid gametes and produces a diploid zygote through fertilization.
Meiosis is a type of cell division that halves the number of chromosomes. There are eight stages of meiosis, divided into meiosis I and meiosis II. DNA is not replicated between meiosis I and meiosis II.
Crossing-over, the independent assortment of chromosomes during anaphase I, and random fertilization result in genetic variation.
Meiosis is a step during spermatogenesis and oogenesis. Spermatogenesis produces four haploid sperm cells, while oogenesis produces one mature ovum.
Eukaryotes have three different versions of the sexual life cycle: a haploid life cycle, a diploid life cycle, and a life cycle known as the alternation of generations. The sexual life cycle depends on when meiosis occurs and the type of cell that undergoes meiosis.
Review Questions
Define crossing-over in meiosis.
Describe how crossing-over, independent assortment, and random fertilization lead to genetic variation.
Compare and contrast mitosis and meiosis.
List the main differences between asexual and sexual reproduction.
How many chromosomes does a diploid human cell have? How many chromosomes does a haploid human cell have?
Name the three different sexual life cycles. What characterizes the differences between these life cycles?
Compare binary fission and asexual reproduction.
Further Reading / Supplemental Links
http://www.genome.gov
http://www.accessexcellence.org/RC/VL/GG/meiosis.html
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Meiosis.html
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookmeiosis.html
Vocabulary
alternation of generations
A life cycle that alternates between diploid and haploid phases.
asexual reproduction
Reproduction without gametes; the simplest and most primitive method of reproduction; produces a clone, an organism that is genetically identical to its parent.
budding
Asexual reproduction in which new individuals split off from existing ones; the bud may stay attached or break free from the parent.
crossing-over
The exchange of genetic material between non-sister chromatids of homologous chromosomes; also known as recombination.
diploid
A cell containing two sets of chromosomes; in human cells, two sets contains 46 chromosomes.
fertilization
The joining of gametes during reproduction.
fission
Asexual reproduction in which a parent separates into two or more individuals of about equal size.
fragmentation
Asexual reproduction in which the body breaks into several fragments, which later develop into complete adults.
gametes
An organism’s reproductive cells, such as sperm and egg cells.
gametogenesis
The further maturation of the haploid cells produced by meiosis into mature gametes capable of fertilization.
gametophyte
Produces gametes by mitosis; in alternation of generation life cycles.
haploid
A cell containing one set of chromosomes; in human gametes, one set is 23 chromosomes.
life cycle
The span in the life of an organism from one generation to the next.
meiosis
A type of cell division that halves the number of chromosomes.
oogenesis
The production of a mature egg cell; results in just one mature ovum, or egg cell.
polar body
Cell formed during oogenesis; contains little cytoplasm and eventually breaks down; does not form a gamete.
sexual reproduction
Reproduction involving the joining of haploid gametes, producing genetically diverse individuals.
spermatogenesis
The production of mature sperm cells; results in four haploid gametes.
spore
A haploid reproductive cell; found in plants, algae and some protists; can develop into an adult without fusing with another cell.
tetrad
A configuration with four chromatids; formed by the pairing of homologous chromosomes during prophase I of meiosis.
Points to Consider
The next unit, Genetics, discusses the branch of biology that studies heredity. What is heredity?
What role do you think meiosis plays in heredity?
Describe what would happen if gametes were formed by mitosis.
Human Genetics is an ever increasingly important field of medicine. Explain why this field of medicine is so important.
Chapter 7: Mendelian Genetics
Lesson 7.1: Mendel’s Investigations
Lesson Objectives
Identify how Mendel’s study of science and math was important to his success in research.
Distinguish between characteristics and traits.
Explain how Mendel was able to control pollination of the pea plants.
Identify the terms used to describe the three generations in Mendel’s studies.
State one reason for carrying out a monohybrid cross.
Identify the traits that appeared in Mendel’s F2 generation.
Identify the actions of dominant alleles and recessive alleles for a trait.
Outline the Law of Segregation.
Outline the Law of Independent Assortment.
Explain Mendel’s results in relation to genes and chromosomes.
Distinguish between genotype and phenotype.
Introduction
For thousands of years, humans have understood that characteristics such as eye color or flower color are passed from one generation to the next. The passing of characteristics from parent to offspring is called heredity. Humans have long been interested in understanding heredity. Many hereditary mechanisms were developed by scholars but were not properly tested or quantified. The scientific study of genetics did not begin until the late 19th cent
ury. In experiments with garden peas, Austrian monk Gregor Mendel described the patterns of inheritance.
Gregor Mendel: Teacher and Scientist
Gregor Johann Mendel was an Augustinian monk, a teacher, and a scientist (Figure below). He is often called the "father of modern genetics" for his study of the inheritance of traits in pea plants. Mendel showed that the inheritance of traits follows particular laws, which were later named after him. The significance of Mendel's work was not recognized until the turn of the 20th century. The rediscovery of his work led the foundation for the era of modern genetics, the branch of biology that focuses on heredity in organisms.
Figure 7.1
Gregor Johann Mendel The Father of Modern Genetics. 1822-1884.
Johann Mendel was born in 1822 and grew up on his parents’ farm in an area of Austria that is now in the Czech Republic. He overcame financial hardship and ill health to excel in school. In 1843 he entered the Augustinian Abbey in Brünn (now Brno, Czech Republic.) Upon entering monastic life, he took the name Gregor. While at the monastery, Mendel also attended lectures on the growing of fruit and agriculture at the Brünn Philosophical Institute. In 1849 he accepted a teaching job, but a year later he failed the state teaching examination. One of his examiners recommended that he be sent to university for further studies. In 1851 he was sent to the University of Vienna to study natural science and mathematics. Mendel’s time at Vienna was very important in his development as a scientist. His professors encouraged him to learn science through experimentation and to use mathematics to help explain observations of natural events. He returned to Brünn in 1854 as a natural history and physics teacher.
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