Lecture 13; 2007
Biology 207; Section B2; Good
Lecture#13 - Chromosome behavior in meiosis
Readings: Griffiths et al (2004, 2000)
8th Edition: pp. 94-103
7th Edition: Ch. 3 pp 67-70; 76-90
Assigned Problems:
8th Ch. 3: 17, 35; 7th Ch. 3: 12, 16
Concepts:
How do chromosomes behave in meiosis??
1. The process of meiosis occurs in diploid cells, called meiocytes, and produces four
haploid nuclei (the meiotic products).
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.
Mitosis (FROM 1 – 2n CELL to 2 – 2n CELLS)
1) Prophase; Pairs of sister chromatids become visible. Chromosomes contract
and become shorter and more visible.
2) Metaphase. Sister chromatids move to and lie in equatorial plane of cell
3) Anaphase. Sister chromatids pulled to opposite ends of the cell
4) Telophase: Chromatids arrive at poles and pulling apart process is complete.
Nuclear membrane reforms and cell divides into two (2) daughter cells.
Meiosis - A cellular process - 1 2N cell --> 4 cells (with 1N nuclei)
- germline cells (not somatic)
- occurs in cells called meiocytes
Meiocyte is a cell destined to enter meiosis
- meiocyte will produce gametes - See Fig 3-24 (8th) 3-25(7th)
- in males -> sperm
- in females -> eggs
-are diploid - made from 2 chromosome sets
- 1 maternal + 1 paternal
--> normal S-phase - undergo chromosome replication
-->each chromosome has 2 sister chromatids
Then the cell enters meiosis - process has 2 cell divisions
Meiosis I (MI) and Meiosis II (MII)
both divisions follow the same basic steps (prophase, metaphase, anaphase, telophase)
the events in each are different and different from mitosis.
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Lecture 13; 2007
Biology 207; Section B2; Good
Meiosis I
Prophase I - complex and lengthy
- 5 separate cytological stages along a dynamic continuum
1) Leptotene - The interphase nucleus begins to condense into chromosomes and are
visible as long thin threads
2) Zygotene - chromosomes begin to pair up or synapse
- homologous chromosomes pair up
- chromosomes begin pairing at localized points
- then "zipper" up along the whole chromosome
- each homolog of a pair join together through some mechanism that is not fully
understood but involves an elaborate structure called the synaptonemal complex. The
synaptonemal complex consists of proteins, RNA and DNA.
3) Pachytene - chromosomes are fully synapsed as a bivalent (bi = two chromosomes)
- the 2 chromosomes have 4 chromatids
- chromomeres align along the chromosome
- crossing over takes place
- # of bivalents = # of chromosomes in one set = n
4) Diplotene - pairing becomes less tight, loosens and chromosomes separate such
that the chromatids become apparent
- can visualize chiasmata on chromosomes
- normally see one (or more) chiasmata chromosome pair
- chiasmata are thought to be needed for proper chromosome segregation during
meiosis
- chiasmata are the visible result of a cross-over event which had occurred earlier
(during zygotene or pachytene)
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Lecture 13; 2007
Biology 207; Section B2; Good
5) Diakinesis - extension of Diplotene
- further chromosome contraction (size reduction)
- form compact chromosomes which are easier to move than larger chromosomes
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
- chromosomes align such that homologous chromosomes face opposite poles
- both orientations are possible - for any given chromosome pair the orientation
happens at random
Anaphase I
- homologous chromosome centromeres begin to move towards opposite poles
Telophase I
- chromosomes reach the poles and may decondense
- not all organisms; many proceed directly to MII
Result of MI = reduction division
- homologous chromosome to each pole
- each resulting nucleus (cell) now has only one set of chromosomes (1n) - one
homolog from each pair of homologous chromosomes
- each chromosome still has two chromatids
Interphase - transient
- interkinesis - no DNA replication
Next: Meiosis II
- equational division occurs in each of the two MI products
Prophase II - haploid chromosome number
- chromosomes condense and shorten
Metaphase II- chromosomes move to equatorial plane
- each chromosome has 2 chromatids
Anaphase II - centromeres split and sister chromatids move to opposite poles
Note:chromatids now become chromosomes
Telophase II - four product nuclei produced - 4 X 1N products of meiosis from diploid
(2N) meiocyte
The meiotic products which are haploid, such as pollen, sperm, etc., go on and unite
with a complementary gamete (in the process of Syngamy) to form a zygote which is 2N
again (diploid).
Homologous chromosomes segregate during Meiosis
See Fig 3-24(8th) 3-26(7th) 2N (and 3-15 for two chromosomes)
This diagram follows a single chromosome pair in a diploid cell
through --> replication --> 2N --> MI --> 2 cells
Note:
-See animation on the WWW
- each chromosome pair orients independently in MI, the reductional division
- each chromatid orients independently in MII, the equational division
- get the independent assortment of chromosomes and of the genes on those
chromosomes
- meiosis leads to diversity among the gametes (not the same as the meiocyte) and
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Lecture 13; 2007
Biology 207; Section B2; Good
therefore of the progeny.
There is a second feature that leads to genetic diversity:
Crossing over - Genetic Recombination
During prophase I there is a physical exchange that can occur between non-sister
chromatids. This exchanges the alleles present on homologous chromosomes.
This process mixes up the combination of alleles further (in addition to independent
assortment).
Follow the chromatids through MI
- crossover results in different meiotic products being produced
- chromosomes are different in that they contain parts of both original parental
chromosomes.
- part maternal - part paternal
Note: We are discussing the nuclear genome, however in animals they also have a
mitochondrial genome (Mt) and in plants they have both the Mt genome and the
chloroplast genome. Both of these encode traits (a number of human disease are based
on mutations in the Mt genome and the Chl genome in plants also can have a number
of genomes included in it.
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