2/17/16
BIOLOGY 207 - Dr.Locke
Lecture#11 - Chromosomes and their behavior in meiosis
Required readings and problems:
Reading: Open Genetics, Chapter 2
Problems: Chapter 2
Optional
Griffiths (2008) 9th Ed. Readings: 32-57 (N.B. 47-49), 75-76, 103-105
Problems: 9th Ed. Ch. 3: 1-22 (not 18)
Campbell (2008) 8th Ed. Readings: Concept 14.1
Concepts:
How do chromosomes behave in meiosis??
1. A characteristic chromosome set (karyotype) is specific for a species.
2. The process of meiosis occurs in diploid cells, called meiocytes, and produces four
haploid nuclei (the meiotic products).
3. The synapsis of structurally (and genetically) similar chromosomes (homologues)
leads to the orderly reduction in chromosome numbers in the first division of
meiosis.
4. Random segregation of chromosomes and chromosome crossing-over are two
features that result in genetic diversity.
Biol207 Dr. Locke section
Lecture#11
Fall'11
page 1
2/17/16
Cytogenetics:
- cytology to study heredity (in eukaryotes)
 - use normal genome to identify the standard set of chromosomes in a species
 - use normal chromosome organization to identify abnormal

The normal set is based on several principles:
1) The chromosomes from a cell are usually observed at the metaphase stage of
mitosis
- this is a standard point in the cell cycle to see well differentiated, separate
chromosomes
2) Since all the cells of an organism have the same genetic material (same quality,
quantity and organization),
then the chromosomes of all body cells should be same.
-extrapolate from one cell --> all cells (not always true)
3) Because different individuals of a species have their DNA organized in a similar
manner the organization of their chromosomes will reflect this and be similar
within the species
-extrapolate: one individual --> all individuals in species
Biol207 Dr. Locke section
Lecture#11
Fall'11
page 2
2/17/16
Standard Metaphase chromosomes
Morphological differences among chromosomes make it possible to distinguish
chromosomes. Identify different chromosomes based on:
1) Size - overall length of the chromosome - varies among those in the set
2) Centromere position (primary constriction)
- location of the spindle attachment
telocentric - centromere is at the end, telomere
acrocentic - centromere is off centre
metacentric - centromere is in the middle
3) Secondary constrictions
- chromosome thickness varies along the length
- Nucleolus Organizer Regions (NOR) - contain the genes for rRNA
- one or a few sites per genome (not necessarily one on each chromosome)
- also other sites of 2° constrictions (not NOR)
4) Banding patterns - there are special staining procedures that reveal differences
in staining (or bands) along each chromosome.
-Each chromosome has a specific, reproducible, unique pattern that distinguishes
it from the others.
Biol207 Dr. Locke section
Lecture#11
Fall'11
page 3
2/17/16
Result of staining and cytology: a Karyotype
Each chromosome can be identified and assembled together to obtain a karyotype.
Karyotype - refers to a set of photographs organizing the metaphase chromosomes
based on number, size, and morphology for a cell, individual, or species
Idiogram - a diagrammatic representation of the karyotype
- standard diagram of the chromosomes
Karyotype of a Diploid organism
The karyotype would consist of "pairs of chromosomes"
- one from father - one from mother
- each "set" is a genome or "N" value's worth of chromosomes
Example:
eg. humans 46 chromosomes (23 pairs)
1N = 23 from mother - maternal in origin - N in the eggs
1N = 23 from father - paternal in origin - N in the sperm
2N = 46 chromosomes
Each species has its own chromosome set
- Each species' chromosome set can be defined based on number, size, and structure
- There is limited variation for normal and extreme differences are usually mutants
Example:
- human chromosome #15 are the same, however, close observation shows that slight
differences do exist among the chromosomes in the population
Biol207 Dr. Locke section
Lecture#11
Fall'11
page 4
2/17/16
Mitosis: 1 cell -> 2 cells – stability!
Cell cycle: G1, S, G2, Mitosis --> two daughter cells
- chromosome replication followed by mitosis maintains the chromosome complement
at each cell generation
Diploid cell Fig - replication and mitosis
Haploid cell Fig - process is exactly the same
Note:
- chromosome number and organization are maintained through each cell generation.
- daughter cells are genetically equivalent to the parent.
Meiosis - 1 cell --> 4 cells (nuclei) – diversity!
- germline cells (not somatic) called Meiocyte
- meiocyte will produce gametes- See Fig
- in males -> sperm - in females -> eggs
- 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
Biol207 Dr. Locke section
Lecture#11
Fall'11
page 5
2/17/16
Meiosis I
Prophase I - complex and lengthy
- 5 separate cytological stages along a dynamic continuum
1) Leptotene - The interphase nucleus begins to condense into visible long, thin
threads (chromosomes)
2) Zygotene - homologous chromosomes begin to pair up or synapse
- pair up at localized points, then "zipper" up along the whole chromosome
- each homolog of a pair join together through an elaborate structure called the
synaptonemal complex.
3) Pachytene - chromosomes are fully synapsed as a bivalent (bi = two
chromosomes) - the 2 chromosomes have 4 chromatids
****** crossing over takes place *******
- # of bivalents = # of chromosomes in one set = n
4) Diplotene - pairing loosens and chromosomes start to separate such that the
chromatids become visually apparent
- chiasmata appear and are the visible result of a crossover event that had occurred
earlier (during pachytene)
- normally see one (or more) chiasmata per chromosome pair
- chiasmata contribute to proper chromosome segregation during meiosis
5) Diakinesis – continuation/extension of Diplotene - further chromosome contraction
- form compact chromosomes which are easier to move to daughter cells than larger
chromosomes
Biol207 Dr. Locke section
Lecture#11
Fall'11
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2/17/16
Metaphase I
- Nuclear membrane and nucleoli disappear 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
- Note: 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 ture in all organisms, some 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
Biol207 Dr. Locke section
Lecture#11
Fall'11
page 7
2/17/16
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 one diploid (2N) meiocyte
The meiotic products which are haploid, such as pollen, sperm, etc.,
Gamete
Will go on and unite with a complementary gamete (in the process of Syngamy) to
form a zygote,
which is 2N again (diploid).
Biol207 Dr. Locke section
Lecture#11
Fall'11
page 8
2/17/16
Meiosis Notes:
- 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 alleles on those
chromosomes
- meiosis leads to diversity among the gametes (not the same as the meiocyte) and
therefore of the progeny.
There is a second feature, in addition to independent assortment, 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 ().
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
Biol207 Dr. Locke section
Lecture#11
Fall'11
page 9
2/17/16
Biol207 Dr. Locke section
Lecture#11
Fall'11
page 10