4. Why do gonosome
undergo meiosis?
Why do meiosis?
Meiosis is “REDUCTION
DIVISION”so Reduces
chromosome number, parent cell
is 2n while daughter cells are n.
5. Haploid gametes (n = 23)
A life cycle
n
Egg cell
n
Sperm cell
Meiosis
Ovary
Fertilization
Testis
Diploid
zygote
(2n = 46)
2n
Key
Multicellular diploid
adults (2n = 46)
Mitosis
Haploid stage (n)
Diploid stage (2n)
7. QUIZ TIME…
1. In streptomycin fungus n = 11. What is the diploid number
for this species?
2. If a horse egg has 98 chromosomes how many does a horse
egg cell have?
3. If n = 16 for goldfish how many chromosome in a fin cell?
4. If 2n = 108 for black spruce trees then what is the haploid
number?
2n = 14 for this species. Fill in
the chromosome numbers for
A
each cell.
B
C
What is process A,
B, and C?
9. Meiosis I (four phases)
• Cell division that reduces the chromosome
number by one-half.
• four phases:
phases
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
15. Metaphase I
• Shortest phase
• Tetrads align on the metaphase plate.
plate
• INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation
3. Formula: 2n
Example: 2n = 4
then n = 2
thus 22 = 4 combinations
32. Conditions caused by
non-disjunction
Downs syndrome = Trisomy 21 - 3 copies of
chromosomes 21 equalling a total of 47 chromosomes.
= 2n + 1(chance of occurring in oogenesis increases with
maternal age)
Turners syndrome = Monosomy X - has only one X
chromosomes totalling only 45 chromosomes in her
body cells. (monosomy X) = 2n-1
Klinefelter’s syndrome = XXY - male which has an
extra X chromosome = 2n + 1
33. Down’s syndrome – trisomy 21
A simple check for the
presence of a bone in the nose
could more accurately test
unborn babies for Down's
syndrome, scientists say.
Researchers say combining
the nose test with existing
screening methods
(amniocentesis, CVS) could
lead to a five-fold reduction in
the number of miscarriages
linked to an invasive
procedure used to confirm
Down's syndrome.
34. Klinefelter’s syndrome XXY
Normal boy who
develops some female
secondary sex
character-istics at
puberty.
-slightly lower IQ,
infertile, delayed
motor, speech,
maturation
-treated with
Editor's Notes
Student Misconceptions and Concerns
Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells.
Teaching Tips
1. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes.
2. In the shoe analogy, females have 23 pairs of matching shoes, while males have 22 matching pairs and 1 odd pair . . . maybe a sandal and a sneaker!
3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and growth.)
Figure 8.12A The human life cycle
Student Misconceptions and Concerns
Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells.
Teaching Tips
1. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes.
2. In the shoe analogy, females have 23 pairs of matching shoes, while males have 22 matching pairs and 1 odd pair . . . maybe a sandal and a sneaker!
3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and growth.)
Student Misconceptions and Concerns
Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells.
Teaching Tips
Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences.
Student Misconceptions and Concerns
Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells.
Teaching Tips
Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences.
Student Misconceptions and Concerns
Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells.
Teaching Tips
Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences.
Student Misconceptions and Concerns
Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells.
Teaching Tips
Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences.
Student Misconceptions and Concerns
Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells.
Teaching Tips
Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences.
Teaching Tips
1. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes (although this analogy is quite limited). A point to make is that the shoes (chromosomes) before crossing over are what you inherited . . . either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (think of shoes with switched shoelaces). Critiquing this limited analogy may also help students to think through the process of crossing over.
2. In the shoe analogy, after exchanging shoelaces, we have “recombinant shoes”!
3. Challenge students to consider the number of unique humans that can be formed by the processes of the independent orientation of chromosomes, random fertilization, and crossing over. Without crossing over, we already calculated over 70 trillion possibilities. But as the text notes in Module 8.17, there are typically one to three crossover events for each human chromosome, and these can occur at many different places along the length of the chromosome. The potential number of combinations far exceeds any number that humans can comprehend, representing the truly unique nature of each human being (an important point that delights many students!)
Teaching Tips
1. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes (although this analogy is quite limited). A point to make is that the shoes (chromosomes) before crossing over are what you inherited . . . either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (think of shoes with switched shoelaces). Critiquing this limited analogy may also help students to think through the process of crossing over.
2. In the shoe analogy, after exchanging shoelaces, we have “recombinant shoes”!
3. Challenge students to consider the number of unique humans that can be formed by the processes of the independent orientation of chromosomes, random fertilization, and crossing over. Without crossing over, we already calculated over 70 trillion possibilities. But as the text notes in Module 8.17, there are typically one to three crossover events for each human chromosome, and these can occur at many different places along the length of the chromosome. The potential number of combinations far exceeds any number that humans can comprehend, representing the truly unique nature of each human being (an important point that delights many students!)
