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Hints For Biology 101 Exam #3

Mitosis, Meiosis & Life Cycle Patterns

   Biology 100 Mitosis Laboratory
   Mitosis & Meiosis Compared
   Genetics of Seedless Watermelon
   Plant Hybridization & The Mule
   Nondisjunction in Spermatogenesis
   Life Cycle Patterns In Plants
   Botanical Record-Breakers
   Angiosperm Double Fertilization
   Go To Biology 100 Home Page
   Go To WAYNE'S WORD index
   WAYNE'S WORD Home Page

Questions 1 - 31: 

See Cell Division (Mitosis) in Eukaryotic Cells. This link explains animal & plant mitosis, the cell cycle, the structure of an unraveled chromosome doublet, and the difference between homologous chromosomes and materenal/paternal sets of chromosomes.

Questions 32 - 34: 

For the honeybee questions, see Parthenogenesis & Agamospermy.

Questions 35 - 42: 

For the question about gender verification in the Olympics, see Gender Determination in men and women.

Questions 43 - 47: 

For the question about spermatogenesis in human males, see Nondisjunction during Spermatogenesis

Questions 48 - 67: 

For the question about phases of cell division, see Mitosis in Eukarytic Cells. For the questions about chromosomes and chromatids in humans, see Homologous, Maternal & Paternal Chromosomes. For the questions about seedless watermelons and mules, see Hybridization in San Diego County.

Simple Analogy Between A Chromosome & Floppy Disk:

A simple comparison between a doubled eukaryotic chromosome (composed of two chromatids) and an ordinary floppy disk. Genes are stored as sections of nucleotides along the twisted, double-stranded DNA ladder which makes up each chromatid. In a sense, genes are like information files stored on the magnetic layer of a floppy disk. In this example, the entire chromosome could be compared with a floppy disk or CD.

Questions 68 - 77: 

For the questions about the meiosis diagram, refer to the Details of Meiosis. For questions about crossing over, see the Illustration of Crossing Over.

Questions 78 - 95: 

For the questions about haploid, diploid and triploid, refer to the Life Cycle Patterns In Plants. For questions about crossing over, see the Comparison of an Alga, Fungus, Moss, Fern & Flowering Plant. In the life cycle comparison, the only cells that are triploid are in the endosperm of a seed plant. Endosperm is formed by the fusion of a haploid sperm (n) with two haploid polar nuclei (n + n) resulting in triplolid endosperm cells (n + n + n = 3n).

Questions 96 - 109: 

For the questions about the moss, fern or both, refer to the Life Cycle Patterns and the life cycle Comparison.

Questions 110 - 115: 

For the questions about flowering seed plants, refer to the Flowering Plant Life Cycle (1) and Flowering Plant Life Cycle (2).

Questions 116 - 121: 

Note: Where I have used the term "chromosome complement," I am simply referring to the "complete" chromosome makeup of the nucleus of a cell in the flowering plant life cycle. The Merriam Webster Third New International Dictionary has many definitions for complement, but the one I am using is "the quantity or number required to fill a thing or make it complete." On the chromosome questions I want the "complete" chromosomes (represented by letters) for various cells and nuclei in the plant life cycle. For example, if ABC refers to the chromosomes of an egg and abc refers to the chromosomes of a sperm; then AaBbCc refers to the complete chromosomes of a fertilized egg or zygote (ABC + abc = AaBbCc). In a number of questions I have referred to the chromosome complement of a specific cell or nucleus, such as the zygote. In this case the chromosome complement of the zygote would be AaBbCc. The chromosome complement of the embryo would also be AaBbCc. Just think of the chromosome complement as the complete chromosome makeup of a cell where large and small case letters stand for the actual chromosomes. If there is one of each letter, then the cell is haploid (e.g. ABC or abc). If there are two of each letter, then the cell is diploid (e.g. AaBbCc). If there are three of each letter, then the cell is triploid (e.g. AAaBBbCCc). If there are four of each letter, then the cell is tetraploid (e.g. AAaaBBbbCCcc).

If the diploid pollen parent is aabbcc, then the haploid sperm would be abc. If the diploid seed parent is AABBCC, then the haploid egg would be ABC. In double fertilization, one abc sperm unites with one ABC egg to form a diploid AaBbCc zygote. Another haploid sperm abc unites with two haploid polar nuclei ABC + ABC to form a triploid AAaBBbCCc endosperm within the seed. Since the seed coat originates from the outer wall of the ovule (called the integument), which was part of the original maternal seed parent, it is chromosomally identical with the original diploid seed parent. The mature pollen grain contains a tube nucleus and generative nucleus, the latter of which divides into two sperm nuclei within the pollen tube. All of these pollen nuclei are chromosomally identical with each other and with the sperm nuclei. The following diagram summarizes double fertilization in this question: Remember that in your question the diploid pollen parent is aabbcc and the sperm is abc; the diploid seed parent is AABBCC and the egg is ABC.

Sperm #1 (abc) fuses with a haploid egg (ABC) resulting in a diploid zygote (AaBbCc) that grows into a diploid embryo (AaBbCc) within the seed. Sperm #2 (abc) fuses with the two haploid polar nuclei (ABC and ABC) within the endosperm mother cell resulting in a triploid endosperm cell (AAaBBbCCc) that develops into the nutritive endosperm tissue (AAaBBbCCc) surrounding the embryo. The following image shows a minute diploid coconut embryo embedded in the triploid, meaty endosperm within the seed of a coconut palm.

Close-up view through the inside of a coconut seed showing a small, cylindrical embryo (A) embedded in the fleshy meat or endosperm (B). The base of the embryo (pointing into the coconut) swells into an absorbing organ (cotyledon) that fills the entire cavity of the seed as it digests the endosperm. The wall of the endocarp (C) is a hard, woody layer that makes up the inner part of the fruit wall. The thick, fibrous husk (mesocarp) that surrounds the endocarp has been removed.

Sprouting fruit of a coconut Cocos nucifera. The hard inner layer (endocarp) contains the actual seed composed of a minute embryo and food storage tissue (endosperm). The base of the embryo (cotyledon) swells into an absorbing organ that fills the entire cavity of the seed as it digests the endosperm. The endocarp has three germination pores, one functional pore and two plugged pores. [In "blind coconuts" all three pores are plugged.] The three pores represent three carpels, typical of the palm family (Arecaceae). Just inside the functional germination pore is a minute embryo embedded in the endosperm tissue. During germination, a spongy mass develps from the base of the embryo and fills the seed cavity. This mass of tissue is called the "coconut apple" and is essentially the functional cotyledon of the seed. [The white color has been altered in order to clearly differentiate it from the endosperm.] It dissolves and absorbs the nutrient-rich endosperm tissue to supply the developing shoot with sugars and minerals. Eventualy, the developing palm becomes self sufficient, as its leaves produce sugars through photosynthesis and its roots absorb minerals from the soil. The coconut "apple" is rich in sugars and is a sweet delicacy in tropical countries. The endosperm is the coconut "meat" which is dried and sold as "copra." The coconut "water" is multinucleate liquid endosperm inside green coconuts that has not developed into solid tissue composed of cells. Before the liquid endosperm forms a solid "meat" it is jellylike and may be eaten with a spoon. This stage of the endosperm development is called "spoon meat." The "coconut milk" used in many Asian recipes is made by soaking grated coconut meat in water and squeezing out the oil-rich liquid. "Coir" fibers are derived from the fibrous mesocarp. The saturated fat called "coconut oil" is derived from the meaty endosperm.

A Note For Biology 101 Students:

In exalbuminous seeds, such as lima beans and walnuts, the endosperm has been completely absorbed by the embryo. The embryo of these seeds consists of two prominent halves called cotyledons. Attached between the cotyledons is a minute, primordial, leaf-bearing shoot called the plumule and an elongate primordial root called the radicle. See following photo:

The embryo of a lima bean seed showing the embryonic shoot or plumule (A), the embryonic root or radicle (B) and two cotyledons (C). The two fleshy halves called cotyledons are actually part of the embryo. The seed coat (D) has been partially removed from the cotyledons. Since the seed coat originates from the outer wall of the ovule (called the integument), which was part of the original maternal seed parent, it is chromosomally identical with the original diploid seed parent.

Chromosome Numbers In Polyploid Plants

I prefer the terms "gametophyte" and "sporophyte" when dicussing polyploid chromosome numbers. Gametophyte refers to the chromosone number of gametes and sporophyte refers to the chromosome number of cells in adult plants. In humans, haploid (n) refers to the chromosome number of gametes, while diploid (2n) refers to the chromosome number after fertilization; however, in polyploid plants the chromosome numbers are very different. For example, I have studied a rare hybrid Brodiaea in San Marcos with a sporophyte chromosome number of 36. This is a hexaploid hybrid because the haploid number in Brodiaea species is 6 (6n = 36). The hybrid was derived from a cross between B. terrestris ssp. kernensis with an octoploid (8n) sporophyte number of 48, and B. filifolia with a tetraploid (4n) sporophyte number of 24. The gametes of these two parents are tetraploid (4n) and diploid (2n), resulting in a hexaploid (6n) hybrid: 4n + 2n = 6n. In this case, referring to the gametes as haploid (n) and the hybrid offspring as diploid (2n), as we do in humans, would be incorrect.

Chromosome Numbers Of The Rare San Marcos Hybrid Beodiaea

Questions 122 - 125: 

If the diploid chromosome number of a flowering plant is 24, then the somatic cells of the plant each contain 24 chromosomes (12 maternal and 12 paternal). Each of the three haploid nuclei in the germinated pollen grain must contain 12 chromosomes, and each of the 8 nuclei within the embryo sac contain 12 chromosmes. A triploid endosperm cell inside a seed must contain 36 chromosomes (3 x 12 = 36).

Questions 126 - 129: 

The genotype for a polyploid plant cell is AAaaBBbbCCccDDdd. Since there are four haploid sets consisting of one of each letter, this cell must be tetraploid or 4n: ABCD + ABCD + abcd + abcd = AAaaBBbbCCccDDdd. The chromosome number refers to the total number of chromosomes in the cell, each of which is represented by one letter. If there are a total of 16 letters, how many chromosomes are in the cell? One set of chromosomes is represented by one of each letter (e.g. abcd). In the list of choices there are actually two possible haploid sets to choose from.

Questions 130 - 132: 

Some flowering plants have evolved a clevery strategy that favors cross pollination. The male and female sex organs on idividual plants mature at different times so that self pollination does not occur. Cross pollination between different individuals provides greater genetic variability and a more vigorous population. For example, in wolffia (the world's smallest flowering plant), the female stigma is receptive before the male anthers are mature. By the time the male anther is ready to release pollen, the female stigma is no longer receptive. This condition is termed protogyny (proto=1st + gyny= female). There is also a term for flowers in which the anthers release pollen before the female stigma is receptive. Although flowering plants have many advanced characteristics over ferns, they are both vascular plants with a water-conducting systems of tubular cells called xylem tissue. Without vascular tissue, the rise of water in plant stems would be limited to osmosis and imbibition, and land plants would be limited in size to that of low-growing mosses and liverworts. There is ample fossil evidence for the presence of vascular tissue during the age of dinosaurs dating back several hundred million years. During these prehistoric times, the earth contained lush forests of ancient tree ferns and conifers. These tall plants needed vascular tissue to conduct water from the soil to the leaves and tips of the stems.

Questions 133 - 137: 

For the answers to these questions, refer to the Flowering Plant Life Cycle. For the record-breaking plants, refer to the Wayne's Word article called Botanical Record-Breakers.

Questions 138 - 144: 

For the questions about flowering seed plants, refer to the Flowering Plant Life Cycle (1) and Flowering Plant Life Cycle (2). For the record-breaking plants, refer to the Wayne's Word article called Botanical Record-Breakers.

Questions 145 - 147: 

For the question about a plant chromosome consisting of a circular DNA molecule, refer to Symbiogenesis and the Origin of Chloroplasts. There are several causes for variegated leaves in plants, including chromosomal genes and viruses. Variegated leaves can also be caused by defects in chloroplast DNA.

'Pinachée,' a parthenocarpic variety of Ficus carica with alternating yellow and green striped syconia and stems. Variegated plants are often referred to as chimeras. Chimeras are organisms composed of two genetically different types of tissue. Chimeras may result from the fusion of cells or tissue. In variegated plants, a mutation in the chloroplast DNA often results in the loss of chloroplasts. Consequently, this mutant tissue has no green pigment and no photosynthesis. Colorless tissue in variegated plants may also be caused by viruses.

Questions 148 - 152: 

A remarkable organism with the greatest number of chromosomes per cell can be seen at the Pteridophte link. This plant can be seen in the endangered vernal pool habitat in San Diego County. For the record-breaking plants, refer to the Wayne's Word article called Botanical Record-Breakers. If a fertile tetraploid plant has a sporophyte chromosome number of 40, then the chromosome number of its sex cells (gametes) must be 20. If 3 gametes combine to form the endosperm (one sperm plus 2 polar nuclei), then the endosperm cells must contain 3 x 20 = 60 chromosomes.

Questions 153 - 156: 

The pollen tube contains 12 chromosomes and 3 haploid nuclei, so each nucleus will have 4 chromosomes. If each sperm nucleus has 4 chromosomes, then the egg must also have 4 chromosomes and the diploid number for the plant must be 8. If the 2n number is 8 then the endosperm must have 12 chromosomes because it is triploid (3n). Each haploid nucleus (such as a polar nucleus) inside the embryo sac must also have 4 chromosomes.

Question 157 - 160: 

The embryo sac contains 32 chromosomes and 8 haploid nuclei, so each nucleus must contain 4 chromosomes. Therefore the egg cell has 4 chromosomes, the synergid cell has 4 chromosomes, etc. The endosperm mother cell has two polar nuclei so it contains a total of 8 chromosomes. There are 7 cells in the embryo sac, one of which (the endosperm mother cell) contains 2 nuclei. The egg cell, 2 synergid cells and 3 antipodal cells each contain one nucleus with 4 chromosomes.

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