Topic 3.3 MeiosisIB Biology

R. Price

v. 1 2015

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Human Karyotype

Meiosis Overview

#1: One diploid nucleus divides by meiosis to produce four haploid nuclei

Meiosis I• Start with diploid cell• Reduction division -> ends with two

haploid cellsMeiosis II• Start with haploid cells• Start with chromosomes still

connected as two chromatids • Chromatids separate -> ends with four

haploid cells

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#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

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#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

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#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

#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

#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

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#7: Crossing over an random orientation promotes genetic variation

Crossing over• Prophase I• Near infinite recombination of

allelesRandom orientation of bivalents• Metaphase I• Genetic variation among genes

on different chromosome types

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#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

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Background: Down Syndrome (Trisomy 21)

Wikipedia contributors. "Down syndrome." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 26 May. 2015. Web. 1 Jun. 2015.

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 diagnosedDiagnosis is by karyotyping

Background: Turner’s Syndrome (X)

Wikipedia contributors. "Turner syndrome." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 19 May. 2015. Web. 1 Jun. 2015

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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.

ImagesUnless otherwise noted, images are obtained from Pixabay (www.pixabay.com) and used under the CC0 Public Domain license.