Chromatid strips at the center with a paperclip so an x is formed. each paper clip represents a centromere
q1. what process did you model when you cut the strips in half?

q2. what is the function of the centromere?

prophase 1: remove the nuclear membrane. place the blue chromosomes side by side. two xx’s in a row. repeat for the green pairs. simulate crossing over by measuring and cutting a 2 cm tip for a light blue strip. do the same with a dark blue strip. tape the light blue tip to the dark blue strip and repeat for the dark tip onto a light blue strip. repeat for green.

dark blue light blue dark green light green

q3. what is the purpose of placing the light and dark strips of the same color side by side?

metaphase 1: place four 10 cm strings inside the cell, so that two strings extend from one side into the center of the cell and two strings extend from the opposite side into the center of the cell. the string represents the spindle fibers. attach a string to the centromere of each chromosome (chromatid pair) with tape. move the chromosomes to the center of the cell. note: make sure that the strings attached to similar colors come from opposite sides of the cell. tetrades are now at the equator.
anaphase 1: gather the loose ends of the strings on both sides of the cell, and pull the strings in opposite directions.
telophase 1: un-tape the string from each centromere. place a 40 cm piece of string around each group of chromatids, forming two nuclei. place a 1 m piece of string around each cell, forming two membranes.
q4. how many chromosomes are in each cell? describe what each part represents.

meiosis 2
prophase 2: remove the strings that represent the nuclear membrane and the cell membranes in both cells. attach a 10 cm piece of string to each chromatid.
q5. what must happen before the chromatids to separate?

metaphase 2: move the chromosomes to the center of each cell. make sure the strings attached to the two strips in each chromosome come from opposite sides of the cell.
anaphase 2: gather the strings on both sides of each cell, and pull the strings in opposite directions, separating the paper strips. note: only one strip in each pair will have a paper clip attached.
telophase 2: untape the strings. remove the strings and paper clips. each strip of paper now represents a chromosome. place a 40-cm. piece of string around each group of chromosomes, forming four nuclei. place a 1m string around each cell, forming four membranes.
q6: how many cells did you make? how many chromosomes are in each cell? are the cells haploid or diploid in number?

analysis:
q7. what is the diploid chromosome number of the original cell in your model? how many homologous pairs does this represent?

q8. if a cell with a diploid number of 6 chromosomes undergoes meiosis, what will the cell look like after telophase 1? draw the result below:

q9. give two reasons why meiosis is important in sexual reproduction.

q10. how might crossing over affect the rate of evolution?

q11. use your model to show what would happen if homologous chromosomes did not pair in prophase 1. predict the outcome.

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synovial joints are characterized by the presence of a joint cavity. the walls of this space are formed by the articular capsule, a fibrous connective tissue structure that is attached to each bone just outside the area of the bone’s articulating surface. the bones of the joint articulate with each other within the joint cavity.

friction between the bones at a synovial joint is prevented by the presence of the articular cartilage, a thin layer of hyaline cartilage that covers the entire articulating surface of each bone. however, unlike at a cartilaginous joint, the articular cartilages of each bone are not continuous with each other. instead, the articular cartilage acts like a teflon® coating over the bone surface, allowing the articulating bones to move smoothly against each other without damaging the underlying bone tissue. lining the inner surface of the articular capsule is a thin synovial membrane. the cells of this membrane secrete synovial fluid (synovia = “a thick fluid”), a thick, slimy fluid that provides lubrication to further reduce friction between the bones of the joint. this fluid also provides nourishment to the articular cartilage, which does not contain blood vessels. the ability of the bones to move smoothly against each other within the joint cavity, and the freedom of joint movement this provides, means that each synovial joint is functionally classified as a diarthrosis.

outside of their articulating surfaces, the bones are connected together by ligaments, which are strong bands of fibrous connective tissue. these strengthen and support the joint by anchoring the bones together and preventing their separation. ligaments allow for normal movements at a joint, but limit the range of these motions, thus preventing excessive or abnormal joint movements. ligaments are classified based on their relationship to the fibrous articular capsule. an extrinsic ligament is located outside of the articular capsule, an intrinsic ligament is fused to or incorporated into the wall of the articular capsule, and an intracapsular ligament is located inside of the articular capsule.

at many synovial joints, additional support is provided by the muscles and their tendons that act across the joint. a tendon is the dense connective tissue structure that attaches a muscle to bone. as forces acting on a joint increase, the body will automatically increase the overall strength of contraction of the muscles crossing that joint, thus allowing the muscle and its tendon to serve as a “dynamic ligament” to resist forces and support the joint. this type of indirect support by muscles is very important at the shoulder joint, for example, where the ligaments are relatively weak.

additional structures associated with synovial joints

a few synovial joints of the body have a fibrocartilage structure located between the articulating bones. this is called an articular disc, which is generally small and oval-shaped, or a meniscus, which is larger and c-shaped. these structures can serve several functions, depending on the specific joint. in some places, an articular disc may act to strongly unite the bones of the joint to each other. examples of this include the articular discs found at the sternoclavicular joint or between the distal ends of the radius and ulna bones. at other synovial joints, the disc can provide shock absorption and cushioning between the bones, which is the function of each meniscus within the knee joint. finally, an articular disc can serve to smooth the movements between the articulating bones, as seen at the temporomandibular joint. some synovial joints also have a fat pad, which can serve as a cushion between the bones.

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