IB DP Biology Topic 3.3 Meiosis

1. Topic 3.3 Meiosis
IB Biology
R. Price
v. 1 2015

2. Allott 159

3. Allott 160

4. Human
Karyotype

5. Meiosis
Overview

6. #1: One diploid nucleus divides by meiosis to produce four haploid
nuclei
Meiosis I
• Start with diploid cell
• Reduction division -> ends with two
haploid cells
Meiosis II
• Start with haploid cells
• Start with chromosomes still
connected as two chromatids
• Chromatids separate -> ends with four
haploid cells
Allott 160

7. #2: The halving of the chromosome number allows a sexual life cycle
with fusion of gametes
• Fertilization is the union of sex
cells
• Fertilization doubles the number
of chromosomes
• Must halve the number of
chromosomes during life cycle
• Meiosis -> halving of
chromosomes

8. Allott 161

9. #3: DNA is replicated before meiosis so that all chromosomes consist of
two sister chomatids
• During interphase, right before
meiosis, DNA is duplicated
• Each chromosome consists of
two sister chromatids,
connected.
• Two sister chromatids are
genetically identical
Allott 162

10. #4: The early stages of meiosis involve pairing of homologous
chromosomes and crossing over followed by condensation
Meiosis I
• Bivalent = Pair of homologous
chromosomes
• Homologous chromosomes pair up
(synapsis)
• Crossing over -> Part of one
chromatid in each homologous
chromosome breaks and rejoins
with the other chromatid. Random
places.
• Chromatids with a new mix of
alleles result

11. #5: Orientation of pairs of homologous chromosomes prior to
separation is random
Early meiosis
• Microtubules growing from
poles
• Each chromosome attached to
only one pole
• Homologous chromosomes
attached to opposite poles
• Attachment due to which way
pair is facing: “orientation”
• Orientation is random
• Orientation does not affect
other bivalnts

12. #6: Separation of pairs of homologous chromosomes in the first division
of meiosis halves the chromosome number
• Disjunction: separation of
homologous chromosomes
during Meiosis I
• One chromosome from each
bivalent moves to one of the
poles and the other
chromosome to the other pole
• Separation of pairs halves the
chromosome number
• Meiosis I -> reduction division

13. Allott 163

14. Allott 164

15. Allott 164

16. Allott 164

17. Allott 165

18. Allott 165

19. #7: Crossing over an random orientation promotes genetic variation
Crossing over
• Prophase I
• Near infinite recombination of
alleles
Random orientation of bivalents
• Metaphase I
• Genetic variation among genes
on different chromosome types
Allott 166

20. #8: Fusion of gametes from different parents promotes genetic variation
• Fusion of gametes (sex cells)
produces a zygote
• Combination of alleles unlikely
ever to have existed before
• Fusion of gametes promotes
genetic variation in species
• Genetic variation is essential for
evolution
Allott 166

21. Allott 167

22. Background: Down Syndrome
(Trisomy 21)
Wikipedia contributors. "Down syndrome." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 26 May. 2015.
Web. 1 Jun. 2015.

23. Background: Klinefelter’s
Syndrome (XXY)
Primary symptoms:
• Sterility
• Weaker than average muscles
• Greater height
• Poor coordination
• Smaller genitals
Klinefelter syndrome is the most common chromosomal
disorder, and it occurs in 1:500 to 1:1000 live male births
Diagnosis:
• 10% during pre-natal testing
• 25% as an adult (generally for another issue)
• 65% never are diagnosed
Diagnosis is by karyotyping

24. Background: Turner’s Syndrome (X)
Wikipedia contributors. "Turner syndrome." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 19 May. 2015. Web. 1 Jun. 2015

25. Allott 167, 168

26. Sources
Content
Allott, Andrew, and David Mindorff. Biology: Course Companion. 2014
ed. Oxford: Oxford UP, 2014. Print. Oxford IB Diploma Programme.
Walpole, Brenda. Biology for the IB Diploma. 2nd ed. Cambridge:
Cambridge UP, 2014. Print.
Images
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