BIOLOGY 207 - Dr.McDermid
Lecture#14 Chromosome behavior in meiosis
Readings: Griffiths et al, 7th Edition: Ch. 3 pp 70-71, 76-85
Problems: Griffiths et al 7th Edition: Ch. 3 Tier 1: #2, 5, 8. Tier 2: #-3, 4, 7.
Concepts:
How do chromosomes behave in meiosis?
1. Meiosis occurs in diploid cells, called meiocytes, and produces four haploid nuclei.
2. The synapsis of structurally (and genetically) similar chromosomes (homologues) leads to
the orderly reduction in chromosome numbers in the first division of meiosis.
3. Random segregation of chromosomes and chromosome crossing-over are two features that
result in genetic diversity.
4. In eukaryotes the life- (or sexual-) cycle consists of haploid and diploid phases.
Meiosis
A cellular process
Meiocyte is a cell destined to enter meiosis
Meiocytes are diploid
The cell then enters meiosis
Meiosis I (MI) and Meiosis II (MII)
Meiosis I
Prophase I
1) Leptotene - The interphase nucleus begins to condense into chromosomes and are visible
as long thin threads
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2) Zygotene - chromosomes begin to pair up or synapse (Fig 3-27)
3) Pachytene - chromosomes are fully synapsed as a bivalent (bi = two chromosomes)
4) Diplotene - pairing becomes less tight, loosens and chromosomes separate such that the
chromatids become apparent
- can visualize chiasmata on chromosomes
5) Diakinesis - extension of Diplotene
- further chromosome contraction
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Metaphase I
- Nuclear membrane and nucleoli have disappeared and the chromosomes moved to the
equatorial plane by the spindle apparatus (filaments of microtubules) that attach to the
centromere of each chromosome
Anaphase I
- homologous chromosome centromeres begin to move towards opposite poles
Telophase I
- chromosomes reach the poles and may decondense
Result of MI = reduction division
Interphase - transient
Next: Meiosis II
- equational division occurs in each of the two MI products
Prophase II
- chromosomes condense and shorten
Metaphase II- chromosomes move to equatorial plane
Anaphase II - centromeres split and sister chromatids move to opposite poles
Note:chromatids now become chromosomes
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Telophase II - four product nuclei produced
The meiotic products are haploid
Homologous chromosomes segregate during Meiosis See Figure 3-2b, 3-15
Note:
- each chromosome pair orients independently in MI
- each chromatid orients independently in MII,
- independent assortment of chromosomes and genes on those chromosomes
- meiosis leads to diversity among the gametes and therefore of the progeny.
Crossing over
-Genetic Recombination
During prophase I there is a physical exchange which can occur between non-sister chromatids.
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- Follow the chromatids through MI
Life cycles
Prokaryotes
- reproduce by DNA replication --> cell division
Eukaryotes
In some organisms the conspicuous stage is diploid
In others the conspicuous stage is haploid
Both have alterating haploid/diploid stages.
Diploids
- Figure 3-14
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Haploids
- Figure 3-17
Alternating Haploid-diploid
- eg. ferns and mosses
- Figure 3-23
Message:
In all cases there is cycling between the stages:
haploid --> diploid --> haploid etc.
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Lecture notes: Copyright © 2002 Heather McDermid and the Department of Biological Sciences, University of Alberta
Images are Copyright©2000 by W.H. Freeman & Co. in Griffiths et al, Introduction to Genetic Analysis
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