2. MEIOSIS
The form of cell division
by which gametes, with
half the number of
chromosomes also known
as haploid, are produced.
Diploid (2n)
(n)
→ haploid
Meiosis is sexual
reproduction.
3. MEIOSIS
cont..
Sex cells divide to produce
gametes (sperm or egg).
Gametes have half the # of
chromosomes.
Occurs only in gonads (testes or
ovaries).
Male: spermatogenesis
Female: oogenesis
Meiosis is similar to mitosis with
some chromosomal differences.
5. Interphase I
Similar to mitosis Interphase.
Chromosomes replicate (S phase).
Each duplicated chromosome consist of two
identical sister chromatids attached at their
centromeres.
centromeres
Centriole pairs also replicate, INSIDE THE
CENTROSOME
6. Interphase I
Nucleus and nucleolus visible.
chromatin
nuclear
membrane
cell membrane
nucleolus
7. Meiosis I
(four phases)
Cell division that reduces the
chromosome number by one-half.
a.
b.
c.
d.
Four phases:
phases
Prophase I
Metaphase I
Anaphase I
Telophase I
8. PROPHASE I
Longest and most complex phase (90%).
Chromosomes condense.
Synapsis occurs: homologous chromosomes come
together to form a BIVALENT TETRAD.
TETRAD
Tetrad= TWO chromosomes of THREE OR
Tetrad
FOUR chromatids (sister and nonsister chromatids).
IN THIS PHASE THE PROCESS CROSSING
OVER TAKES PLACE- EXCHANGE OF
GENETIC MATERIAL.
10. HOMOLOGOUS
CHROMOSOMES
PaIr Of chrOMOSOMES (MatErnal
and PatErnal) that arE SIMIlar In
ShaPE and SIzE.
hOMOlOgOuS PaIrS (tEtradS) carry
gEnES cOntrOllIng thE SaME
InhErItEd traItS.
Each lOcuS (POSItIOn Of a gEnE) IS In
thE SaME POSItIOn On hOMOlOguES.
huManS havE 23 PaIrS Of
hOMOlOgOuS chrOMOSOMES.
a. 22 PaIrS Of autOSOMES
b. 01 PaIr Of SEx chrOMOSOMES
12. Crossing
Over
????????????
Crossing over (variation) may occur between nonsister
chromatids at the chiasmata.
chiasmata
Crossing over: segments of nonsister chromatids break and
over
reattach to the other chromatid.
chromatid
Chiasmata (chiasma) are the sites of crossing over.
over
13. Crossing Over nonsister chromatids
chiasmata: site
of crossing over
vARIATION
Tetrad
variation
16. 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
26. Telophase II
Same as Telophase in Mitosis.
Mitosis
Nuclei form.
Cytokinesis occurs.
Remember: four haploid daughter
cells
produced.
gametes = sperm or egg
40. Trisomy 21 produces a characteristic set of
symptoms, which include:
•
•
•
•
•
mental retardation,
characteristic facial features,
short stature,
heart defects,
susceptibility to respiratory infections,
leukemia, and Alzheimer’s disease, and
• shortened life span.
The incidence increases with the age of the
mother.
41. B. Accidents during meiosis can alter
chromosome number
Nondisjunction is the failure of chromosomes or
chromatids to separate normally during meiosis.
This can happen during:
• meiosis I, if both members of a homologous
pair go to one pole or
• meiosis II if both sister chromatids go to one
pole.
Fertilization after nondisjunction yields zygotes
with altered numbers of chromosomes.
49. THESE REARRANGEMENTS MAY
INCLUDE:
• a Deletion, the loss of a chromosome
segment,
• a Duplication, the repeat of a
chromosome segment,
• an Inversion, the reversal of a
chromosome segment, or
• a Translocation, the attachment of a
segment to a non-homologous
chromosome that can be reciprocal.
51. PRESENTED and EDITED BY
KHANYISILE MASIKANE
ACKNOWLEDGEMENTS TO
Hasimah Azit
AND
Janice Williamson
Images taken from:
Campbell, N. Reece, J.2007.
BIOLOGY 8th edition. Pearson
Education.
Notes de l'éditeur
Figure 8.18_s4 Preparation of a karyotype from a blood sample (step 4)
Figure 8.18_s5 Preparation of a karyotype from a blood sample (step 5)
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. If you have several hundred students or more in your class, it is likely that at least one of your students has a sibling with Down syndrome. The authors note that, overall, about one in every 700 babies are born with Down syndrome.
3. The National Down Syndrome Society has a website at www.ndss.org. It is a wonderful resource.
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. If you have several hundred students or more in your class, it is likely that at least one of your students has a sibling with Down syndrome. The authors note that, overall, about one in every 700 babies are born with Down syndrome.
3. The National Down Syndrome Society has a website at www.ndss.org. It is a wonderful resource.
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. Students might be confused by the term nondisjunction. But simply put, it is an error in the sorting of chromosomes during mitosis or meiosis. Figure 8.20 illustrates two types of nondisjunction errors in meiosis.
Figure 8.20A_s3 Nondisjunction in meiosis I (step 3)
Figure 8.20B_s3 Nondisjunction in meiosis II (step 3)
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/.
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/.
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/.
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. In general, flowering plants are more likely to form new species through polyploidy than animals, because unlike most animals, many flowering plants can fertilize themselves.
3. The gray tree frog, which is found over most of the eastern half of the United States, from Florida and Texas to Ontario and Maine, consists of two species Hylachrysoscelis, which is diploid, and Hylaversicolor, which is tetraploid. The two species cannot be distinguished except by the number of chromosomes in their cells. The tetraploid species is thought to have been formed by an error in meiosis, similar to that frequently seen in plants.
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. 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. 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.