How does sexual reproduction lead to genetic variation?
Why? Model 1 – Meiosis I
Cells reproduce through mitosis to make exact copies of the original cell. This is done for growth and repair. Sexually-reproducing organisms have a second form of cell division that produces reproductive cells with half the number of chromosomes. This process is called meiosis, and without it, humans, oak trees, beetles, and all other sexually-reproducing organisms would not be what they are today.
1. What organs do the cells that enter meiosis I come from?
2. Considering what you already know about mitosis in cells, what event must take place during interphase before a cell proceeds to division?
3. What two structures make up a single chromosome?
4. How many single chromosomes does the cell in prophase contain?
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5. At which stage in meiosis I do the homologous pairs come together?
6. Once the chromosomes have formed a pair, what are they now called?
7. At the end of meiosis I, two cells have been produced. How many single chromosomes are
there in each of these cells?
8. Cells with a full set of chromosomes are referred to as Diploid or 2n, whereas cells with half the chromosomes number are Haploid or n. At which stage(s) of meiosis I are the cells diploid and at which stage(s) are they haploid?
9. Considering the genetic makeup of the homologous pairs, will the cells at the end of telophase I be genetically identical to each other? Explain your answer fully, using homologous pairs and other terminology from the previous questions.
10. How do the cells at the end of meiosis I differ from the parent cells?
Alleles are alternative forms of the same gene. For example a gene A may contain the information for hair color. One allele “A” may result in blond hair, while the alternative allele “a” may result in black hair. Homologous chromosomes are chromosomes that contain the same genes although each chromosome in the homologous pair may have different alleles.
20. After fertilization what happens to the secondary oocyte?
21. During meiosis II the secondary oocyte divides unevenly, with one cell (the ovum) receiving half of the chromosomes and nearly all the cytoplasm and organelles, while the other cell, the polar body, is much smaller and eventually degenerates. With your group develop a hypothesis to explain why the secondary oocyte divides in this way.
22. What is the ploidy of the zygote produced by fertilization: haploid or diploid?
23. What would the ploidy of the zygote be if egg and sperm were produced by mitosis rather than meiosis? How would this affect the ploidy of each successive generation?
24. With your group write a statement to explain the origin of the chromosomes found in the zygote. Your statement must include the term homologous pair.
25. At which stage of meiosis are the chromosomes in diagram A?
26. What word describes the two chromosomes?
27. What do the letters A-D represent?
28. Are the letters exactly the same on each homologous chromosome? Be specific in your answer.
29. Using information from the previous models, what is the origin of each of the homologous chromosomes?
30. When the chromosomes come together as homologous pairs, the arms of the sister chromatids may cross over. What are these cross-over points called?
31. When the homologous pairs separate during anaphase I do the homologous chromosomes still contain the same type of genes? Explain your answer fully.
32. When the homologous pairs separate during anaphase I do the homologous chromosomes still contain the same type of alleles?
33. What phrase is used to describe the chromatids after separation of the homologous chromosomes?
34. In diagram C, describe the arrangement of the chromosomes during metaphase. Be specific in your answer as you describe the location of each homolog in relation to each other.
35. What is different about the arrangement of the chromosomes in diagram D?
36. What effect does the orientation of the homologous pairs have on the end products of meiosis I?
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37. With your group, calculate how many possible genetic combinations there are due to independent assortment.
38. Meiosis and sexual reproduction each lead to variation. With your group, explain how events that occur during meiosis, as well as the random fertilization of eggs and sperm, together lead to variation in the genetic make-up of every person.
When homologous chromosome pairs align on the spindle during metaphase I the orientation of one pair is independent of the orientation of any other pair. This is known as Independent Assortment. Humans have 46 chromosomes, arranged as 23 pairs. During metaphase I each pair lines up independently, which results in 223 possible combinations.
39. During anaphase II the sister chromatids separate and move towards the poles of the spindle. Occasionally NON-DISJUNCTION can occur, which means that the two chromatids stick together instead of separating. Using the cell outlines below, draw two cells produced from meiosis I each containing two chromosomes (four chromatids) as they would appear during anaphase II if non-disjunction of one of the chromatid pairs occurred. Show the resulting four cells at the end of telophase II. Use colored pencils or dashed lines to make your drawing clearer.
40. Do you think all four cells would eventually be able to be fertilized successfully?
41. If this was a human how many chromosomes could each of the resulting four embryos have, assuming that each cell is fertilized by another “normal” cell with the regular amount of chromosomes?
Learning Objectives: After completing the activity the students should be able to:
1. Explain the stages of meiosis and how haploid cells are produced for reproduction. 2. Describe how the events during meiosis and fertilization lead to genetic variation. 3. Show through simple diagrams how independent assortment leads to almost infinite
variation. 4. Explain how fertilization restores the diploid number and how meiosis maintains the diploid
number across generations.
Prerequisites: This activity should be preceded with the Mitosis activity as students will need to be familiar with all the events that occur at each stage of mitosis and interphase.
Assessment Questions: 1. If an organism contains 10 chromosomes in its body cells, how many chromosomes will
there be in the following cells? a. Spermatid -5 b. Cell prior to starting meiosis - 10 c. Ovum - 5 d. Sperm - 5
2. How do meiosis and fertilization lead to genetic variation? Meiosis reduces the chromosome
number by hal f . During this process a l l e l es are swapped and independent assortment shuf f l es chromosome combinat ions. Fert i l izat ion then causes further random recombinat ion o f the genes depending on which sperm fer t i l ized which egg .
3. With the aid of a simple diagram show how independent assortment leads to genetic variation. See Model 4(C & D) for appropriate diagrams .
Teacher Tips:
• It will be important to emphasize that no further DNA replication takes place between meiosis I and II but nuclear division occurs twice. Similarly the process of oogenesis is also abbreviated and the production of the three polar bodies and one ovum is not emphasized.
• There is much terminology in this activity, some of which you may need to check beforehand to see if it matches that which you currently use, specifically the term tetrad, which may also be interchanged with bivalent.
1. What organs do the cells that enter meiosis I come from? Sex organs(ovaries & testes)
2. Considering what you already know about mitosis in cells, what event must take place during interphase before a cell proceeds to division? DNA repl i cat ion .
3. What two structures make up a single chromosome? Each is made from a pair o f s i s t er chromatids .
4. How many single chromosomes does the cell in prophase contain? 4
5. At which stage in meiosis I do the homologous pairs come together? Prophase I
6. Once the chromosomes have formed a pair, what are they now called A tetrad .
7. At the end of meiosis I, two cells have been produced. How many single chromosomes are there in each of these cells? 2
8. Cells with a full set of chromosomes are referred to as Diploid or 2n, whereas cells with half
the chromosomes number are Haploid or n. At which stage(s) of meiosis I are the cells diploid and at which stage(s) are they haploid? Diploid at prophase , metaphase , anaphase . Haploid at the end o f t e lophase once the ce l l has spl i t .
9. Considering the genetic makeup of the homologous pairs, will the cells at the end of
telophase I be genetically identical to each other? Explain your answer fully, using homologous pairs and other terminology from the previous questions. No they wi l l not be genet i ca l ly ident i cal to each other because the homologous pairs separated and the al l e l es on each homologous pair are not necessar i ly ident i ca l .
10. How do the cells at the end of meiosis I differ from the parent cells? They are not the same
as the parent ce l l because they now have only hal f the genet i c mater ia l as the parent ce l l . They s t i l l have two copies o f each hal f o f a homologous pair .
11. Where did each of the cells come from that started meiosis II? From meios is I .
12. In meiosis I, during anaphase I, did homologous chromosomes separate or did sister chromatids separate? Homologous chromosomes .
13. In meiosis II, during anaphase II, did homologous chromosomes separate or did sister
chromatids separate? Sister chromatids .
14. At the end of the meiosis II there are four daughter cells. Are they haploid or diploid? Explain your answer in a complete sentence. The ce l l s are haploid, each containing one chromosome from each homologous pair (and s is ter chromatid se t ) .
15. What is the name given to the cells produced at the end of meiosis I in males? Secondary spermatocytes .
16. What is the name given to the cells produced at the end of meiosis I in females? Secondary
oocytes
17. At the end of meiosis II in males, what cells are produced? Spermatids .
18. What do these cells (from the previous question) eventually become? Mature sperm
19. During fertilization which two cells come together? Be specific in your answer. Mature sperm and secondary oocyte .
20. After fertilization what happens to the secondary oocyte? It proceeds to meios is II
21. During meiosis II the secondary oocyte divides unevenly, with one cell (the ovum) receiving
half of the chromosomes and nearly all the cytoplasm and organelles, while the other cell, the polar body, is much smaller and eventually degenerates. With your group develop a hypothesis to explain why the secondary oocyte divides in this way. The ovum wil l rece ive more organel l es , such as mitochondria for making energy , which wi l l provide the zygote with al l o f i t s cy toplasm and organel l es (note that the sperm contains only scant cy toplasm, a f ew mitochondria and haploid chromosomes) . But nuc lear div is ion is necessary to produce the correc t number o f chromosomes in the oocyte .
22. What is the ploidy of the zygote produced by fertilization: haploid or diploid? Diploid.
23. What would the ploidy of the zygote be if egg and sperm were produced by mitosis rather
than meiosis? How would this affect the ploidy of each successive generation? I f the egg and sperm were produced by mitos is , they would be diplo id, so the zygote would be te traplo id ( four se ts o f chromosomes) . I f this t e traplo id organism produced te traplo id egg and sperm, but next generat ion would be oc toplo id (e ight se ts o f chromosomes) , and this doubl ing would cont inue with each generat ion.
24. With your group write a statement to explain the origin of the chromosomes found in the
zygote. Your statement must include the term homologous pair. The homologous pairs o f chromosomes found in the zygote have come from each parent . One hal f o f each pair i s maternal and the other hal f i s paternal .
25. At which stage of meiosis are the chromosomes in diagram A? prophase I
26. What word describes the two chromosomes? Homologous
27. What do the letters A-D represent? The gene combinat ions on each arm of the s i s ter chromatid .
28. Are the letters exactly the same on each homologous chromosome? Be specific in your answer. They are the same le t t ers , but some are capi ta l l e t t ers and others are lower case .
29. Using information from the previous models, what is the origin of each of the homologous
chromosomes? One comes from the mother and one comes from the father , during f er t i l izat ion.
30. When the chromosomes come together as homologous pairs, the arms of the sister chromatids may cross over. What are these cross-over points called? Chiasma
31. When the homologous pairs separate during anaphase I do the homologous chromosomes still contain the same type of genes? Explain your answer fully. Yes they s t i l l have the same type o f genes (A-D)
32. When the homologous pairs separate during anaphase I do the homologous chromosomes
still contain the same type of alleles? No, the al l e l es may swap from one non s is t er chromatid to another within the homologous pair .
33. What phrase is used to describe the chromatids after separation of the homologous
chromosomes? Recombinant chromatids .
34. In diagram C, describe the arrangement of the chromosomes during metaphase. Be specific in your answer as you describe the location of each homolog in relation to each other. The larger homologous pair o f chromosomes have l ined up with the dashed chromosome on the l e f t and the so l id on the r ight . The smal ler homologous pair i s l ined up with the so l id chromosome on the l e f t and the dashed one on the r ight .
35. What is different about the arrangement of the chromosomes in diagram D? In diagram D the larger homologous pair has f l ipped, so the so l id one i s on the l e f t and the dashed one i s on the r ight .
36. What effect does the orientation of the homologous pairs have on the end products of
meiosis I? Depending on how the homologous pairs l ine up during metaphase I , i t wi l l determine how the homologs are spl i t during anaphase I and i t ul t imate ly determines the genet i c makeup of the daughter ce l l s .
37. With your group, calculate how many possible genetic combinations there are due to independent assortment. 223 = 8388608
38. Meiosis and sexual reproduction each lead to variation. With your group, explain how events that occur during meiosis, as well as the random fertilization of eggs and sperm, together lead to variation in the genetic make-up of every person. Independent assortment l eads to mult ip le di f f erent maternal & paternal gene combinat ions be ing produced in the ce l l s at the end o f meios is I . Chiasma formation between homologous pairs o f chromosomes l eads to blocks o f genes be ing swapped between non-s is t er chromatids during separat ion at anaphase I . This l eads to al l e l e combinat ions on the chromosomes be ing shuf f l ed , and further var ies the chromosomes that end up in egg and sperm af ter meios is II . Random fer t i l izat ion o f eggs and sperm means that when zygotes are formed the gene combinat ion wi l l be di f f erent each t ime, even i f they come from the same two parents .
39. During anaphase II the sister chromatids separate and move towards the poles of the spindle. Occasionally NON-DISJUNCTION can occur, which means that the two chromatids stick together instead of separating. Using the cell outlines below, draw two cells produced from meiosis I each containing two chromosomes (four chromatids) as they would appear during anaphase II if non-disjunction of one of the chromatid pairs occurred. Show the resulting four cells at the end of telophase II. Use colored pencils or dashed lines to make your drawing clearer. Accept any c l ear drawing showing non dis junct ion.
40. Do you think all four cells would eventually be able to be fertilized successfully? The ce l l s may fer t i l ize but the resul t ing embryos wi l l probably not survive for the ce l l s with a miss ing chromosome.
41. If this was a human how many chromosomes could each of the resulting four embryos have, assuming that each cell is fertilized by another “normal” cell with the regular amount of chromosomes? Two wi l l have 46 chromosomes; 1 wi l l have 47 and another wi l l have 45. [Indiv iduals with Trisomy 21 (Down Syndrome) have 47 chromosomes; indiv iduals with Turner ’s Syndrome (X0) have 45 chromosomes.]
