2.
Organisms receive half of
their DNA from each parent
Sperm and eggs cells
(haploid) combine to create a
zygote (diploid)
Used by most animals, fungi,
plants, and algae
3. 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)
4.
Humans body cells are
diploid (2N)
2N = 46
Gametes - sex cells
Males – sperm
Females – eggs
Gametes are HAPLOID
N = 23
Fertilization restores
the diploid state
(N + N = 2N)
Zygote - a fertilized
egg (2N)
9. 2N
• Prophase I – homologous
chromosomes form tetrads &
crossing over occurs
• Metaphase I – tetrads line up
along the middle
• Anaphase I – homologous
chromosomes separate
• Telophase I – two nuclei form
• Cytokinesis I – cells separate
N
N
10. • DNA replication DOES NOT occur
between Meiosis I & II
N
• Prophase II – spindle
forms, prepares for division
N
• Metaphase II – chromosomes
align along the center
• Anaphase II – sister
chromatids separate
• Telophase II – nuclei
reform; cytokinesis
N
N
N
N
Looks A LOT like
MITOSIS!
14.
Process to make haploid
gametes
Chromosome # is cut in half
2N
N
Occurs in ovaries & testes
TWO cell divisions
Increases genetic variation
22.
In meiosis, “crossing
over” occurs during
metaphase I.
Chromosomes form
tetrads
Homologous
chromosomes “swap”
parts of their DNA
leads to genetic variation
23.
Mitosis makes
cells for growth,
development,
repair, and
asexual
reproduction
Meiosis makes
cells for sexual
reproduction
26.
Campbell &Reece,(2010) Biology
Williamson, J. LS3AFET . Unit 3. University of
Johannesburg: Science Department, 2014.
Shilma, S. Meiosis. (2011).
http://www.worldstarhiphop.com/videos/vi
deo.php?v=wshhk0A758v5lEkmCKrk
Notas del editor
Figure 8.12A The human life cycle
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!)
Student Misconceptions and Concerns
Some students might conclude that sex chromosomes function only in determining the sex of the individual. As the authors note, sex chromosomes contain genes not involved in sex determination.
Teaching Tips
Students might recall some basic genetics, remembering that for many traits a person receives a separate “signal” from mom and dad. These separate signals for the same trait are carried on the same portion of homologous chromosomes, such as the freckle trait noted in Module 8.11.
Figure 8.13_4 The stages of meiosis: Meiosis II
Figure 8.12B How meiosis halves chromosome number
Figure 8.19A A karyotype showing trisomy 21, and an individual with Down syndrome
Figure 8.19A A karyotype showing trisomy 21, and an individual with Down syndrome
Student Misconceptions and Concerns
Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23.
Teaching Tips
1. The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at www.genomics.energy.gov.
2. Challenge students to create a simple sentence and then modify that sentence to represent (a) a deletion, (b) a duplication, and (c) an inversion as an analogy to these changes to a chromosome.
Student Misconceptions and Concerns
Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23.
Teaching Tips
1. The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at www.genomics.energy.gov.
2. Some syndromes related to human sexuality are not the result of abnormalities in sex chromosome number. Androgen insensitivity syndrome produces sterile males who possess mostly female sex characteristics. People with this condition are genetically male, but have bodies that fail to respond to male sex hormones. The National Institute of Health web site “Genetics Home Reference” can provide additional details about this and most genetic disorders at http://ghr.nlm.nih.gov/.