Download PROPHASE I

Chromosomes & Inheritance
MEIOSIS
-two successful divisions of a diploid
nucleus following only one DNA
replication cycle
- the original diploid cell contained a
haploid set of chromosomes from mom
and a haploid set from dad (except for
self-fertilizing organisms - plants)
- produces a haploid gamete
Chromosomes & Inheritance
INTERPHASE
- G1, S, G2
- chromosomes are duplicated
- produces a temporary 4N
- each “chromosome” consist of 2
sister chromatids attached at a
centromere
Chromosomes & Inheritance
MEIOSIS I
-chromosome number is reduced
-4 stages
- PROPHASE I
-similar to Prophase of Mitosis
except for crossing over of
homologous pairs of chromosomes
-produces greater
chromosomal
individuality of
offspring
Chromosomes & Inheritance
MEIOSIS I
- PROPHASE I
-Leptonema – chromosomes
begin to coil and become visible
-Telomere – end of
chromosomes that do not code for
proteins; protect and secure the
chromosome;
Chromosomes & Inheritance
MEIOSIS I
- PROPHASE I
-during Meiosis I these are
clustered on the nuclear envelope
to produce a bouquet of the
chromosomes
-Zygonema – chromosomes
continue to shorten (coil)
Chromosomes & Inheritance
MEIOSIS I
- PROPHASE I
-homologous pairs align and
undergo synapsis (formation of
the synaptonemal complex)
which aligns them base pair for
base pair
-made possible because of the
telomeres and bouquet
Chromosomes & Inheritance
MEIOSIS I
- PROPHASE I
-Pachynema – follows completion
of synapsis
-each synapsed set of
homologous chromosomes has 4
chromatids, this is called a tetrad
-in this tetrad is where
crossing-over now occurs
Chromosomes & Inheritance
MEIOSIS I
- PROPHASE I
-resulting genetic differences
between the homologs can
produce new gene combinations
-there is usually no loss or
gain of genetic material here, just
exchange – each homolog gives
and receives genes
Chromosomes & Inheritance
MEIOSIS I
- PROPHASE I
-recombinant chromosome chromosome that has a different
combination of genes than what it
began meiosis with
-genetic recombinantion process by which progeny receives
genotypes different from the
parents
Chromosomes & Inheritance
MEIOSIS I
- PROPHASE I
-Diplonema - synaptonemal
complex is disassembled and
homologous chromosomes move
apart
-chiasma – cross-shaped
structure resulting from the
overlapping of chromatids
Chromosomes & Inheritance
MEIOSIS I
- PROPHASE I
-at these locations the
homologous chromosomes are
very tightly connected
-in most organisms the remainder
of meiosis goes very quickly, except for
egg development in animals
-Humans: diplonema is
completed by the 7th month of fetal
development, then they wait, wait, wait
Chromosomes & Inheritance
MEIOSIS I
- PROPHASE I
-Humans: The onset of
puberty begins a cycle of one
oocyte per month completing
meiosis I and is ovulated.
-Humans: If fertilized, it
quickly completes meiosis II as it
passes down the fallopian tube and
then a functional zygote is
produced
Chromosomes & Inheritance
MEIOSIS I
- PROPHASE I
-Diakinesis – nucleolus and
nuclear envelope break down
-spindle is assembled
-chromosomes can be counted
fairly easily
* Crossing-over occurs between
homologous chromosomes, mainly
autosomes
Chromosomes & Inheritance
Chromosomes & Inheritance
MEIOSIS I
- METAPHASE I
-nuclear envelope is completely
broken down
-tetrads become aligned at the
equator of the cell
-spindle is completely formed
-microtubules are attached to the
homologs
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Chromosomes & Inheritance
MEIOSIS I
- ANAPHASE I
-chromosomes of the homologous
pair (dyads) disjoin and move to
opp. poles
-centromeres segregate randomly
-sister chromatids remain joined
- TELOPHASE I
-move to opp. poles is completed
-new nuclear envelope forms
-cytokinesis is happening
concurrently
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Chromosomes & Inheritance
MEIOSIS II
- PROPHASE II
-chromosomes condense
- METAPHASE II
-spindle is organized and attaches
to centromere (still connecting sister
chromatids)
-centromeres line up on the
equator of the cell
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Chromosomes & Inheritance
MEIOSIS II
- ANAPHASE II
-centromeres split
-chromatids are pulled to opp.
sides
-sister chromatids of each pair
move to opp. poles
- TELOPHASE II
-nuclear envelope forms around
each set of chromosomes
-cytokinesis takes place
-chromosomes become elongated
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Chromosomes & Inheritance
MEIOSIS II
- after TELOPHASE II
-chromosomes become elongated
and no longer easily visible
-end product is 4 haploid cells
(gametes)
-each gamete has one chromosome
(not exact copies) from each
homologous pair