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11.1: Deuterostome Lab - Biology

11.1: Deuterostome Lab - Biology


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Learning Objectives

  • State the phyla of the organisms discussed in the lab activities
  • Use the characteristics of symmetry, coelom, embryo tissue layers, and patterns of development to differentiate between the different organisms
  • Describe the general characteristics of echinoderms
  • Identify and locate external and internal structures of a starfish
  • State the common characteristics of all chordates
  • Be able to identify the chordate characteristics on a model or picture
  • Identify and locate external and internal structures of a frog

A SlideShare element has been excluded from this version of the text. You can view it online here: pb.libretexts.org/blab/?p=92

Echinoderms

Procedure

Access the page “Reading: Echinoderms.”

Questions

  1. Skip viewing the slide of the different developmental phases of the sea stars
  2. Dissect the starfish following the directions on the website. Remember the oral end (with mouth) is actually on the underside of the starfish.
    1. On the oral side make sure you find the mouth.
    2. Also on the oral side in the center region of each leg look for the tube feet. Tube feet are used for locomotion powered by the water vascular system. How many rows of tube feet does your starfish have?
    3. Try to differentiate between the spines and the skin gills. The spines are longer are used for protection. The skin gills are smaller and used for gas exchange.
    4. Find the sieve plate/madreporite on the aboral side. This is the water entrance point for the water vascular system used for movement.
    5. The starfish has plates located underneath the skin for protection and support. What material comprises these plates?
    6. The starfish has a two part stomach, the upper pyloric stomach and the lower cardiac stomach. Can you differentiate between the two stomachs on your specimen?
    7. In the starfish arms you should find both digestive glands and gonads. The digestive glands are brown and typically on top of the off white gonads. Make sure you can identify both structures.
  3. The preserved echinoderm specimens will be on display, but may differ from the ones directly mentioned in the lab handout. Please make observations on the available specimens and fill in the chart below.
    Name of SpecimenPhysical Description

Chordates

Procedure

Access the page “Reading: Chordates.”

Questions

  1. There are two groups of invertebrate chordates, the cephalochordates and the urochordates. We don’t have any urochordate examples in the lab.
    1. View the lancelet slide. The lancelet is an example of a cephalochordate. It contains all four chordate characteristics. List the four chordate characteristics below.
    2. View the lancelet model. Make sure you can identify all four chordate characteristics on the model.

Dissection

Our vertebrate chordate example of today’s lab is the frog. Dissect a frog following the procedure below.

  1. External anatomy
    1. Place the frog in the dissection pan legs down.
    2. Identify the eyes, covered by a nicitating membrane, the external nares (nostrils), and the tympanum located behind each eye.
      1. What is the function of the tympanum?
    3. Examine the front and back limbs. How many phalanges are on the hindfeet? The forefeet? Which pair of limbs is the longest? How does this assist the frog in its movement?
  2. Mouth
    1. Turn the frog over and open the mouth as wide as you can. You can cut the hinges of the jaw if necessary. Identify the following structures:
      1. Two vomerine teeth located in the middle of the roof of the mouth
      2. Maxillary teeth (smaller) located on the sides of the upper jaw
      3. Tongue
      4. Pharynx (located behind the tongue)
      5. Esophagus, the opening leading to the stomach
      6. Glottis, slit where air passes through to enter the trachea, which leads to the lungs
      7. Eustacian tubes (2) openings that lead to the ears. They are located in the angle of the jaw.
  3. Body Dissection
    1. Place the frog belly side up in the dissecting tray. You can pin down the limbs if necessary.
    2. Lift up the skin with forceps midway between the hind legs of the frog. Use scissors to cut the skin along the midline of the frog starting between the hind legs and ending at the neck. Be careful not too cut too deeply.
    3. Cut the skin horizontally above the hind legs and below the front legs creating skin flaps.
    4. Pick up a skin flap with forceps and use a scalpel to separate the skin from the muscle below.
    5. Pin the skin flaps to the dissection tray.
    6. Repeat the same procedure to cut through the muscles. Create one long incision along the midline of the frog from between the hind legs to the neck. Be careful not to cut too deeply and damage the internal organs. When you reach the area just below the front legs of the frog, turn your scissors sideways to cut through the chest bones and avoid damaging the heart and lungs. Then make horizontal incisions above the rear legs and between the front legs. Use forceps and a scalpel to separate the muscle from the tissue below. Then pin the muscle to the dissection tray.
  4. Internal Organs
    1. The most prominent organ is the liver, dark brown in color, and taking up most of the abdominal cavity
    2. Identify the lungs, two small pouches on opposite sides of the frog midline. They may be partially hidden by the liver.
    3. Lift up the liver and underneath locate the gallbladder.
    4. Identify the heart covered by the protective pericardium. Frogs have a three chambered heart with two atria and one ventricle. Try to locate these different areas of the frog heart.
      1. How is it a disadvantage to have a 3 chambered heart?
    5. The stomach is a j-shaped organ located underneath the left lobe of the liver. It connects to the esophagus bringing food from the mouth and the small intestine used for nutrient absorption.
    6. The small intestine connect to the large intestine which carry any undigested material to the cloaca. Frogs have one opening to the outside environment and the cloaca receives materials from the intestine, the urinary system and the reproductive system.
    7. Find the pancreas, a yellow ribbon located between the stomach and the small intestine.
    8. Locate the spleen, shaped similarly to a pea and located near the stomach.
    9. You will be able to see the yellow, finger like, fat bodies, which the frog uses to store fat.
    10. The kidneys of the frog are long and narrow and located along the back body wall.
    11. Try to find the mesonephric ducts, thin white tubes that carry urine from the kidney to the cloaca.
    12. If your frog is female, the abdominal cavity will be filled with black and white eggs. The eggs are stored in the ovaries.
    13. If you have a male frog, locate the testes. The testes are shaped like a bean and located at the top of the kidneys. They are yellow/tan in color.
      1. Do you have a male or female frog?

Review Questions

Answer the review questions below. The phyla we viewed today were the echinodermata and the chordata.

  1. Which phyla observed today were deuterostomes?
  2. Which phyla exhibited cephalization?
  3. Which phyla were coelomates?
  4. What does the name “echinodermata” mean?
  5. What type of symmetry does the echinoderm larva display?
  6. Give an example of an echinoderm example other than a starfish.
  7. What unique system does the starfish use for movement?
  8. Give an example of a chordate that is not a vertebrate.
  9. State the four common characteristics shared by all chordates.
  10. Name the two types of teeth found in frogs.
  11. Frogs have small lungs that are inefficient. What other structure do frogs use for gas exchange?
  12. Frogs have one opening to the outside environment, the cloaca. What three areas transfer material outside through the cloaca?

Chordates

Procedure

Access the page “Reading: Chordates.”

Questions

  1. There are two groups of invertebrate chordates, the cephalochordates and the urochordates. We don’t have any urochordate examples in the lab.
    1. View the lancelet slide. The lancelet is an example of a cephalochordate. It contains all four chordate characteristics. List the four chordate characteristics below.
    2. View the lancelet model. Make sure you can identify all four chordate characteristics on the model.

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    11.1 The Process of Meiosis

    In this section, you will explore the following questions:

    • How do chromosomes behave during meiosis?
    • What cellular events occur during meiosis?
    • What are the similarities and differences between meiosis and mitosis?
    • How can the process of meiosis generate genetic variation?

    Connection for AP ® Courses

    As we explored the cell cycle and mitosis in a previous chapter, we learned that cells divide to grow, replace other cells, and reproduce asexually. Without mutation, or changes in the DNA, the daughter cells produced by mitosis receive a set of genetic instructions that is identical to that of the parent cell. Because changes in genes drive both the unity and diversity of life, organisms without genetic variation cannot evolve through natural selection. Evolution occurs only because organisms have developed ways to vary their genetic material. This occurs through mutations in DNA, recombination of genes during meiosis, and meiosis followed by fertilization in sexually reproducing organisms.

    Sexual reproduction requires that diploid (2n) organisms produce haploid (1n) cells through meiosis and that these haploid cells fuse to form new, diploid offspring. The union of these two haploid cells, one from each parent, is fertilization. Although the processes of meiosis and mitosis share similarities, their end products are different. Recall that eukaryotic DNA is contained in chromosomes, and that chromosomes occur in homologous pairs (homologues). At fertilization, the male parent contributes one member of each homologous pair to the offspring, and the female parent contributes the other. With the exception of the sex chromosomes, homologous chromosomes contain the same genes, but these genes can have different variations, called alleles. (For example, you might have inherited an allele for brown eyes from your father and an allele for blue eyes from your mother.) As in mitosis, homologous chromosomes are duplicated during the S-stage (synthesis) of interphase. However, unlike mitosis, in which there is just one nuclear division, meiosis has two complete rounds of nuclear division—meiosis I and meiosis II. These result in four nuclei and (usually) four daughter cells, each with half the number of chromosomes as the parent cell (1n). The first division, meiosis I, separates homologous chromosomes, and the second division, meiosis II, separates chromatids. (Remember: during meiosis, DNA replicates ONCE but divides TWICE, whereas in mitosis, DNA replicates ONCE but divides only ONCE.).

    Although mitosis and meiosis are similar in many ways, they have different outcomes. The main difference is in the type of cell produced: mitosis produces identical cells, allowing growth or repair of tissues meiosis generates reproductive cells, or gametes. Gametes, often called sex cells, unite with other sex cells to produce new, unique organisms.

    Genetic variation occurs during meiosis I, in which homologous chromosomes pair and exchange non-sister chromatid segments (crossover). Here the homologous chromosomes separate into different nuclei, causing a reduction in “ploidy.” During meiosis II—which is more similar to a mitotic division—the chromatids separate and segregate into four haploid sex cells. However, because of crossover, the resultant daughter cells do not contain identical genomes. As in mitosis, external factors and internal signals regulate the meiotic cell cycle. As we will explore in more detail in a later chapter, errors in meiosis can cause genetic disorders, such as Down syndrome.

    Information presented and the examples highlighted in the section support concepts and learning objectives outlined in Big Idea 3 of the AP ® Biology Curriculum Framework. The learning objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A learning objective merges required content with one or more of the seven science practices.

    Big Idea 3 Living systems store, retrieve, transmit and respond to information essential to life processes.
    Enduring Understanding 3.A Heritable information provides for continuity of life.
    Essential Knowledge 3.A.2 In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.
    Science Practice 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
    Learning Objective 3.9 The student is able to construct an explanation, using visual representations or narratives, as to how DNA in chromosomes is transmitted to the next generation via mitosis, or meiosis followed by fertilization.
    Essential Knowledge 3.A.2 In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.
    Science Practice 7.1 The student can connect phenomena and models across spatial and temporal scales.
    Learning Objective 3.10 The student is able to represent the connection between meiosis and increased genetic diversity necessary for evolution.

    The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
    [APLO 1.9][APLO 2.15][APLO 2.39][APLO 3.11][APLO 3.9]

    Teacher Support

    The process of meiosis can be confusing, especially if it is taught as just a series of steps. Initially, discuss the goal of the process. Explain that meiosis serves to produce reproductive cells with exactly half the number of chromosomes, and that once these haploid cells are fused during fertilization, a complete set of genetic instructions for a new individual is formed. Meiosis starts in a cell with chromosomes in pairs. Each chromosome has already been duplicated and the two sister strands are held together. Therefore, each pair consists of four chromatids. Because students have already learned about mitosis (the process whereby chromosomes are sorted and allocated to daughter cells), it might be helpful to teach meiosis as a special case of mitosis. The first division separates the pairs of chromosomes, reducing the number of duplicated chromosomes in the daughter cells by half. The second division separates the chromatids, creating daughter cells that each has one half of the total number of chromosomes of the original cell. An added benefit to an organism using meiosis is the increase in genetic variation that occurs during the process. Each individual born as a result of sexual reproduction truly has a unique assortment of genes.

    You read that fertilization is the union of two sex cells from two individual organisms. If these two cells each contain one set of chromosomes, the resulting fertilized cell contains two sets of chromosomes. Haploid cells contain one set of chromosomes. Cells containing two sets of chromosomes are called diploid. The number of sets of chromosomes in a cell is called its ploidy level. If the reproductive cycle is to continue, a diploid cell must reduce the number of its chromosome sets before fertilization can occur again. Otherwise, the number of chromosome sets would double, and continue to double in every generation. So, in addition to fertilization, sexual reproduction includes a nuclear division that reduces the number of chromosome sets.

    Most animals and plants are diploid, containing two sets of chromosomes. In an organism’s somatic cells , sometimes referred to as “body” cells (all cells of a multicellular organism except the reproductive cells), the nucleus contains two copies of each chromosome, called homologous chromosomes. Homologous chromosomes are matched pairs containing the same genes in identical locations along their length. Diploid organisms inherit one copy of each homologous chromosome from each parent all together, they are considered a full set of chromosomes. Haploid cells, containing a single copy of each homologous chromosome, are found only within an organism’’s reproductive structures, such as the ovaries and testes. Haploid cells can be either gametes or spores. Male gametes are sperm and female gametes are eggs. All animals and most plants produce gametes. Spores are haploid cells that can produce a haploid organism or can fuse with another spore to form a diploid cell. Some plants and all fungi produce spores.

    As you have learned, the nuclear division that forms haploid cells— meiosis —is closely related to mitosis. Mitosis is the part of a cell reproduction cycle that results in identical daughter nuclei that are also genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are at the same ploidy level—diploid for most plants and animals. Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Meiosis II , in which the second round of meiotic division takes place, includes prophase II, prometaphase II, and so on.

    Teacher Support

    Meiosis I has the same steps as mitosis, with the exception that the chromosome pairs, not the chromatids, are separated at anaphase I. Two other events occur during the first cell division to produce the genetic variation that results. In prophase I, when the pairs of chromosomes condense and tentatively join, parts of the arms and legs of the chromosomes can crossover, or exchange places, with corresponding parts on the other homologous chromosome. The resulting pair now has a configuration that was not present initially. The pairs line up in a double line during metaphase I, but the distribution of the pairs at the equator is random. Half of the original chromosomes came from one parent, half from the other. As the chromosomes line up and are pulled apart during anaphase I, each daughter cell will receive a chromosome mixture that was not present in the original germ cells. Figure 11.3 illustrates crossing over and Figure 11.4 illustrates the random distribution of pairs of chromosomes. Also use the Link to Learning: Meiosis: An Interactive Animation. Meiosis II finishes the process and closely resembles mitosis, except for the number of chromosomes present, as compared to somatic cells.

    Teacher Support

    Comparing meiosis and mitosis should be a review of the two processes, with a reinforcement of the similarities and differences.

    Meiosis I

    Meiosis is preceded by an interphase consisting of the G1, S, and G2 phases, which are nearly identical to the phases preceding mitosis. The G1 phase, which is also called the first gap phase, is the first phase of the interphase and is focused on cell growth. The S phase is the second phase of interphase, during which the DNA of the chromosomes is replicated. Finally, the G2 phase, also called the second gap phase, is the third and final phase of interphase in this phase, the cell undergoes the final preparations for meiosis.

    During DNA duplication in the S phase, each chromosome is replicated to produce two identical copies, called sister chromatids, that are held together at the centromere by cohesin proteins. Cohesin holds the chromatids together until anaphase II. The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate. This prepares the cell to enter prophase I, the first meiotic phase.

    Prophase I

    Early in prophase I, before the chromosomes can be seen clearly microscopically, the homologous chromosomes are attached at their tips to the nuclear envelope by proteins. As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. Recall that, in mitosis, homologous chromosomes do not pair together. In mitosis, homologous chromosomes line up end-to-end so that when they divide, each daughter cell receives a sister chromatid from both members of the homologous pair. The synaptonemal complex , a lattice of proteins between the homologous chromosomes, first forms at specific locations and then spreads to cover the entire length of the chromosomes. The tight pairing of the homologous chromosomes is called synapsis . In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other. The synaptonemal complex supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over. Crossing over can be observed visually after the exchange as chiasmata (singular = chiasma) (Figure 11.2).

    In species such as humans, even though the X and Y sex chromosomes are not homologous (most of their genes differ), they have a small region of homology that allows the X and Y chromosomes to pair up during prophase I. A partial synaptonemal complex develops only between the regions of homology.

    Located at intervals along the synaptonemal complex are large protein assemblies called recombination nodules . These assemblies mark the points of later chiasmata and mediate the multistep process of crossover —or genetic recombination—between the non-sister chromatids. Near the recombination nodule on each chromatid, the double-stranded DNA is cleaved, the cut ends are modified, and a new connection is made between the non-sister chromatids. As prophase I progresses, the synaptonemal complex begins to break down and the chromosomes begin to condense. When the synaptonemal complex is gone, the homologous chromosomes remain attached to each other at the centromere and at chiasmata. The chiasmata remain until anaphase I. The number of chiasmata varies according to the species and the length of the chromosome. There must be at least one chiasma per chromosome for proper separation of homologous chromosomes during meiosis I, but there may be as many as 25. Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata (Figure 11.3) and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.

    The crossover events are the first source of genetic variation in the nuclei produced by meiosis. A single crossover event between homologous non-sister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. Now, when that sister chromatid is moved into a gamete cell it will carry some DNA from one parent of the individual and some DNA from the other parent. The sister recombinant chromatid has a combination of maternal and paternal genes that did not exist before the crossover. Multiple crossovers in an arm of the chromosome have the same effect, exchanging segments of DNA to create recombinant chromosomes.

    Prometaphase I

    The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. Kinetochore proteins are multiprotein complexes that bind the centromeres of a chromosome to the microtubules of the mitotic spindle. Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes. The microtubules attach at each chromosomes' kinetochores. With each member of the homologous pair attached to opposite poles of the cell, in the next phase, the microtubules can pull the homologous pair apart. A spindle fiber that has attached to a kinetochore is called a kinetochore microtubule. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. The homologous chromosomes are still held together at chiasmata. In addition, the nuclear membrane has broken down entirely.

    Metaphase I

    During metaphase I, the homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. For example, if the two homologous members of chromosome 1 are labeled a and b, then the chromosomes could line up a-b, or b-a. This is important in determining the genes carried by a gamete, as each will only receive one of the two homologous chromosomes. Recall that homologous chromosomes are not identical. They contain slight differences in their genetic information, causing each gamete to have a unique genetic makeup.

    This randomness is the physical basis for the creation of the second form of genetic variation in offspring. Consider that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent. Using humans as an example, one set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. Every cell of the multicellular offspring has copies of the original two sets of homologous chromosomes. In prophase I of meiosis, the homologous chromosomes form the tetrads. In metaphase I, these pairs line up at the midway point between the two poles of the cell to form the metaphase plate. Because there is an equal chance that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.

    This event—the random (or independent) assortment of homologous chromosomes at the metaphase plate—is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate the possible number of alignments therefore equals 2n, where n is the number of chromosomes per set. Humans have 23 chromosome pairs, which results in over eight million (2 23 ) possible genetically-distinct gametes. This number does not include the variability that was previously created in the sister chromatids by crossover. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition (Figure 11.4).

    To summarize the genetic consequences of meiosis I, the maternal and paternal genes are recombined by crossover events that occur between each homologous pair during prophase I. In addition, the random assortment of tetrads on the metaphase plate produces a unique combination of maternal and paternal chromosomes that will make their way into the gametes.

    Anaphase I

    In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart (Figure 11.5).

    Telophase I and Cytokinesis

    In telophase, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase events may or may not occur, depending on the species. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. In other organisms, cytokinesis—the physical separation of the cytoplasmic components into two daughter cells—occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow (constriction of the actin ring that leads to cytoplasmic division). In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells.

    Two haploid cells are the end result of the first meiotic division. The cells are haploid because at each pole, there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though each homolog still consists of two sister chromatids. Recall that sister chromatids are merely duplicates of one of the two homologous chromosomes (except for changes that occurred during crossing over). In meiosis II, these two sister chromatids will separate, creating four haploid daughter cells.

    Link to Learning

    Review the process of meiosis, observing how chromosomes align and migrate, at Meiosis: An Interactive Animation.

    1. Errors can arise only during the recombination process, which may result in deletions, duplications or translocations causing such abnormalities.
    2. Errors occur when a pair of homologous chromosomes fails to separate during anaphase I or when sister chromatids fail to separate during anaphase II, producing daughter cells with unequal numbers of chromosomes.
    3. Errors occur only during anaphase I of meiosis as chromosomes separate prematurely, triggering aberrations that result in unequal numbers of chromosomes in daughter cells.
    4. Errors during meiosis introduce variations in the DNA sequence that cause changes throughout the phases of meiosis, the intensity of which depend specifically on the size of the variant.

    Meiosis II

    In some species, cells enter a brief interphase, or interkinesis , before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II in synchrony. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes. Therefore, each cell has half the number of sister chromatids to separate out as a diploid cell undergoing mitosis.

    Prophase II

    If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed.

    Prometaphase II

    The nuclear envelopes are completely broken down, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.

    Metaphase II

    The sister chromatids are maximally condensed and aligned at the equator of the cell.

    Anaphase II

    The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. Non-kinetochore microtubules elongate the cell.

    Telophase II and Cytokinesis

    The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four unique haploid cells. At this point, the newly formed nuclei are both haploid. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombining of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover. The entire process of meiosis is outlined in Figure 11.6.

    Comparing Meiosis and Mitosis

    Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but also exhibit distinct differences that lead to very different outcomes (Figure 11.7). Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new cells. The nuclei resulting from a mitotic division are genetically identical to the original nucleus. They have the same number of sets of chromosomes, one set in the case of haploid cells and two sets in the case of diploid cells. In most plants and all animal species, it is typically diploid cells that undergo mitosis to form new diploid cells. In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are usually partitioned into four new cells. The nuclei resulting from meiosis are not genetically identical and they contain one chromosome set only. This is half the number of chromosome sets in the original cell, which is diploid.

    The main differences between mitosis and meiosis occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs become associated with each other, are bound together with the synaptonemal complex, develop chiasmata and undergo crossover between sister chromatids, and line up along the metaphase plate in tetrads with kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad. All of these events occur only in meiosis I.

    When the chiasmata resolve and the tetrad is broken up with the homologs moving to one pole or another, the ploidy level—the number of sets of chromosomes in each future nucleus—has been reduced from two to one. For this reason, meiosis I is referred to as a reduction division . There is no such reduction in ploidy level during mitosis.

    Meiosis II is much more analogous to a mitotic division. In this case, the duplicated chromosomes (only one set of them) line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles. During anaphase II, as in mitotic anaphase, the kinetochores divide and one sister chromatid—now referred to as a chromosome—is pulled to one pole while the other sister chromatid is pulled to the other pole. If it were not for the fact that there had been crossover, the two products of each individual meiosis II division would be identical (like in mitosis). Instead, they are different because there has always been at least one crossover per chromosome. Meiosis II is not a reduction division because although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I.

    Evolution Connection

    The Mystery of the Evolution of Meiosis

    Some characteristics of organisms are so widespread and fundamental that it is sometimes difficult to remember that they evolved like other simpler traits. Meiosis is such an extraordinarily complex series of cellular events that biologists have had trouble hypothesizing and testing how it may have evolved. Although meiosis is inextricably entwined with sexual reproduction and its advantages and disadvantages, it is important to separate the questions of the evolution of meiosis and the evolution of sex, because early meiosis may have been advantageous for different reasons than it is now. Thinking outside the box and imagining what the early benefits from meiosis might have been is one approach to uncovering how it may have evolved.

    Meiosis and mitosis share obvious cellular processes and it makes sense that meiosis evolved from mitosis. The difficulty lies in the clear differences between meiosis I and mitosis. Adam Wilkins and Robin Holliday 1 summarized the unique events that needed to occur for the evolution of meiosis from mitosis. These steps are homologous chromosome pairing, crossover exchanges, sister chromatids remaining attached during anaphase, and suppression of DNA replication in interphase. They argue that the first step is the hardest and most important, and that understanding how it evolved would make the evolutionary process clearer. They suggest genetic experiments that might shed light on the evolution of synapsis.

    There are other approaches to understanding the evolution of meiosis in progress. Different forms of meiosis exist in single-celled protists. Some appear to be simpler or more “primitive” forms of meiosis. Comparing the meiotic divisions of different protists may shed light on the evolution of meiosis. Marilee Ramesh and colleagues 2 compared the genes involved in meiosis in protists to understand when and where meiosis might have evolved. Although research is still ongoing, recent scholarship into meiosis in protists suggests that some aspects of meiosis may have evolved later than others. This kind of genetic comparison can tell us what aspects of meiosis are the oldest and what cellular processes they may have borrowed from in earlier cells.


    A.6 Evolution

    Evolution is change in the heritable characteristics of biological populations over successive generations. Evolutionary processes give rise to biodiversity at every level of biological organization, including the levels of species, individual organisms, and molecules.

    Repeated formation of new species (speciation), change within species (anagenesis), and loss of species (extinction) throughout the evolutionary history of life on Earth are demonstrated by shared sets of morphological and biochemical traits, including shared DNA sequences. These shared traits are more similar among species that share a more recent common ancestor, and can be used to reconstruct a biological “tree of life” based on evolutionary relationships (phylogenetics), using both existing species and fossils. The fossil record includes a progression from early biogenic graphite, to microbial mat fossils, to fossilized multicellular organisms. Existing patterns of biodiversity have been shaped both by speciation and by extinction.

    Charles Darwin developed his theory of “natural selection” from 1838 onwards and was writing up his “big book” on the subject when Alfred Russel Wallace sent him a version of virtually the same theory in 1858. Their separate papers were presented together at an 1858 meeting of the Linnaean Society of London. At the end of 1859, Darwin’s book “On the Origin of Species” explained natural selection in detail and in a way, that led to an increasingly wide acceptance of Darwin’s concepts of evolution at the expense of alternative theories.

    According to Ernst Mayr, Darwin’s theory actually consists of a number of different theories that can be best understood when they are clearly distinguished from each other. Mayr distinguished five independent theories:

    1. The non-constancy of species (the basic theory of evolution)
    2. The descent of all organisms from common ancestors (branching evolution)
    3. The gradualness of evolution (no saltations, no discontinuities)
    4. The multiplication of species (the origin of diversity)
    5. Natural selection

    The first and second theories were widely accepted by biologists rather quickly following Darwin’s publication. The other three theories were not widely accepted until the arrival of the so-called modern synthesis in the 20th century (see below).

    Evolution by natural selection is a process demonstrated by the observation that more offspring are produced than can possibly survive, along with three facts about populations: 1) traits vary among individuals with respect to morphology, physiology, and behavior (phenotypic variation), 2) different traits confer different rates of survival and reproduction (differential fitness), and 3) traits can be passed from generation to generation (heritability of fitness). Thus, in successive generations members of a population are replaced by progeny of parents better adapted to survive and reproduce in the biophysical environment in which natural selection takes place.

    The four most widely recognized evolutionary processes are natural selection (including sexual selection), genetic drift, mutation and gene migration due to genetic admixture. Natural selection and genetic drift sort variation mutation and gene migration create variation.

    The mechanisms of reproductive heritability and the origin of new traits remained a mystery. Towards this end, Darwin developed his provisional theory of pangenesis. In 1865, Gregor Mendel reported that traits were inherited in a predictable manner through the independent assortment and segregation of elements (later known as genes). Mendel’s laws of inheritance eventually supplanted most of Darwin’s pangenesis theory. August Weismann made the important distinction between germ cells that give rise to gametes (such as sperm and egg cells) and the somatic cells of the body, demonstrating that heredity passes through the germ line only. Hugo de Vries connected Darwin’s pangenesis theory to Weismann’s germ/soma cell distinction and proposed that Darwin’s pangenes were concentrated in the cell nucleus and when expressed they could move into the cytoplasm to change the cells structure. de Vries was also one of the researchers who made Mendel’s work well-known, believing that Mendelian traits corresponded to the transfer of heritable variations along the germline. de Vries developed a mutation theory to explain how new variants originate. This led to a temporary rift between those who accepted Darwinian evolution and biometricians who allied with de Vries. In the 1930s, pioneers in the field of population genetics, such as Ronald Fisher, Sewall Wright and J. B. S. Haldane set the foundations of evolution onto a robust statistical philosophy. The false contradiction between Darwin’s theory, genetic mutations, and Mendelian inheritance was thus reconciled.

    In the 1920s and 1930s the so-called modern synthesis connected natural selection and population genetics, based on Mendelian inheritance, into a unified theory that applied generally to any branch of biology. The modern synthesis explained patterns observed across species in populations, through fossil transitions in paleontology, and complex cellular mechanisms in developmental biology. The publication of the structure of DNA by James Watson and Francis Crick in 1953 demonstrated a physical mechanism for inheritance. Molecular biology improved our understanding of the relationship between genotype and phenotype. Advancements were also made in phylogenetic systematics, mapping the transition of traits into a comparative and testable framework through the publication and use of evolutionary trees. In 1973, evolutionary biologist Theodosius Dobzhansky penned that “nothing in biology makes sense except in the light of evolution,” because it has brought to light the relations of what first seemed disjointed facts in natural history into a coherent explanatory body of knowledge that describes and predicts many observable facts about life on this planet.

    All life on Earth shares a common ancestor known as the last universal common ancestor (LUCA), which lived approximately 3.5–3.8 billion years ago.

    In terms of practical application, an understanding of evolution has been instrumental to developments in numerous scientific and industrial fields, including agriculture, human and veterinary medicine, and the life sciences in general. Discoveries in evolutionary biology have made a significant impact not just in the traditional branches of biology but also in other academic disciplines, including biological anthropology, and evolutionary psychology.


    Part 2: Mendel’s First Law: Law of Segregation

    The Law of Segregation states that alternative alleles of a trait segregate independently during meiosis.

    Using a technique known as Punnett Square analysis, we will see how Mendel analyzed his monohybrid crosses to come up with the Law of Segregation.

    Procedure

    Carefully follow each step to create a Punnett square analysis. You can use these same general procedures to analyze every Punnett Square you do!

    Problem: In pea plants, height is coded for by the “T” gene. The dominant allele (T) codes for the tall phenotype while the recessive allele (t) codes for the short phenotype. Make a cross between a true breeding tall pea plant and a true breeding short pea plant.

    1. What are the phenotypes of the parent plants? The parents are considered the P generation.
    2. Determine the genotypes of each parent plant.
    3. Imagine each parent goes through meiosis to produce gametes. List the genotype(s) of the possible gametes that each parent would produce.
    4. Create a Punnett square that displays the genotypes of the possible offspring. Also label the phenotypes of the possible offspring. These offspring are considered the F1 (first filial) generation.
    5. Now allow the F1 generation to self-pollinate. What are the possible gametes that each F1 parent can produce?
    6. Create a Punnett square that displays the genotypes of the possible offspring. Also label the phenotypes of the possible offspring. These offspring are considered the F2 (second filial) generation.

    Note: Always reduce the phenotypic and genotypic ratios to their lowest terms.

    1. What is the phenotypic ratio of the F1 generation?
    2. What is the genotypic ratio of the F1 generation?
    3. What is the phenotypic ratio of the F2 generation?
    4. What is the genotypic ratio of the F2 generation?

    Environmental and Health Challenges of Energy Use

    The environmental impacts of energy use on humans and the planet can happen anywhere during the life cycle of the energy source. The impacts begin with the extraction of the resource. They continue with the processing, purification or manufacture of the source its transportation to place of energy generation, and ends with the disposal of waste generated during use.

    Extraction of fossil fuels can be used as a case study because its use has significant impacts on the environment. As we mine deeper into mountains, farther out at sea, or farther into pristine habitats, we risk damaging fragile environments, and the results of accidents or natural disasters during extraction processes can be devastating. Fossils fuels are often located far from where they are utilized so they need to be transported by pipeline, tankers, rail or trucks. These all present the potential for accidents, leakage and spills. When transported by rail or truck energy must be expended and pollutants are generated. Processing of petroleum, gas and coal generates various types of emissions and wastes, as well as utilizes water resources. Production of energy at power plants results in air, water, and, often, waste emissions. Power plants are highly regulated in the Unites States by federal and state law under the Clean Air and Clean Water Acts, while nuclear power plants are regulated by the Nuclear Regulatory Commission.


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    Watch the video: Non vert deuterostomes Echinoderms u0026 basal-Chordates (July 2022).


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