Meiosis: reductive division
- What is meiosis?
- What are the stages?
- Independent assortment
- What can “go wrong”?
- Karyotyping
Refer to chapter 10 in text
- What is meiosis?
Gregor Mendel
- 19th century Austrian monk, (?)
looking at pea plants…
- Described patterns of trait inheritance
(e.g. segregated- not-blended),
but offered no mechanism.
- His work was not recognized until
half a century later,
in the wake of Darwin.
One version of each gene (one allele)
comes from each parent…
If all the parental genes were passed on
offspring would have twice as much DNA as parents.
We have two versions of each chromosome in our somatic cells (diploid).
Gametes have one version of each (haploid).
Meiosis is reduction division, forming haploid gametes.
- What are the stages?
Homologous chromosomes are the
two parental chromosome versions,
found in diploid organisms.
(This graphic shows a diploid number of 2:
one pair of homologous chromosomesIn humans 2n= 46, or 23 pairs.)
Interphase I, as in mitosis,
includes an S phase for the
replication of the DNA.
Sister chromatids are joined at centromeres.
meiosis I (first round),
results in two haploid cells,
with sister chromatids still attached.
meiosis II (second round)
results in four haploid cells, with
one half of the original amount of DNA.
*
NB →
names of phases, synapse, to make tetrads (aka bivalents)
chiasmata/crossover, kinetochore, metaphase plate, spindle,
centrosome…
* Some interesting details in three slides
rest of phase names,
cleavage, cytokinesis…
Prophase I
Homologous chromosomes together, forming tetrads.
Chiasmata
Metaphase I
Tetrads line up at center.
Anaphase I
Homologous pairs migrate apart.
Centromeres do not split.
Telophase I
Two cells, each with two copies of ½ info.
.
.
MEIOSIS I
.
RECAP
Interphase (like mitosis, ending with replicated DNA,
sister chromatids bound at centromere).
MEIOSIS II
Prophase II
Spindles form in each cell.
Metaphase II
Chromosomes line up, still joined to sisters.
Anaphase II
Pairs split at centromere, move apart.
.
Telophase II
New cells form.
Result: four haploid cells—gametes.
About that prophase I,
It can be divided into 5 sub-phases, with self explanatory names:
(b) Leptotene (thin threads): DNA condensing,
synaptonemal complex forming for…
(c) Zygotene (double threads): pairing/zipping together of homologous chromosomes
(d) Pachytene (thick threads): chiasmata form
(e) Diplotene (2 threads): SC degrades, homologs back off except at chiasmata
(e also) Dikinesis (move through): DNA packs tighter,
nucleus dissolves, spindles forming
Independent assortment
There is no control over
how the homologs align
in metaphase I …
In meiosis, the possible
outcomes equal 2n,
where n = the number of
chromosome pairs:
In you, 223, or
8,288,608 possible
combos of paternal and
maternal chromosomes!
(Mendel just happened to
pick traits for his experiments
that all sorted independently...)
What can “go wrong”?
Chiasmata, crossing over between non-sister chromatids
during tetrad formation in prophase I.
If the parts miss-align,
you get some of those chromosomal mutations.
If they line up,
it is an important contributor toward genetic variability.
Generally not
so tidy!
With this added in, the gamete genome possibilities
skyrocket well beyond 8 million: “ Effectively infinite”.
www.biologie.uni-hamburg.de/b-online/e11/4.htm
Gene Mapping
Without chiasmata,
genes on a given chromosome would
always be inherited together:
With chiasmata,
distant genes on the same
chromosome can recombine,
resulting in non-parental
mixes.
Very distant genes may
sort independently, as if on
separate chromosomes.
The closer the genes,
the less likely they are to sort
independently.
This is used to estimate gene locations.
(Note- it is not really this simple:
proximity to centromere has a role.
Male Drosophila chromosomes don’t cross over…!
What can “go wrong”? cont.
Nondisjunction occurs when chromosomes fail to separate in meiosis:
- tetrads do not split in meiosis I, OR
- sister chromatids fail to split in meiosis II
(primary nondisjunction)
(secondary nondisjunction)
If such a gamete is used,
result is aneuploidy of
the zygote.
Note: trisomy shown.
n-1 gamete results in
monosomy.
A karyogram is a tool used to screen for abnormalities in karyotype.
A sample can be taken from a fetus by
amniocentesis or chorionic villus sampling.
Mitosis in a somatic cell is arrested,
the chromosomes are stained, a micrograph is taken,
and using cut-and-paste chromosomes are ordered by
size, centromere location, and banding pattern.
In humans
1 through 22 are
autosomal chromosomes
The 23rd “pair” are the
sex chromosomes.
male = XY
female= XX
Female somatic cells
have one X disabled as
a Barr body.
(In cats this results in
calico and
tortoiseshell colors)
You are to actually work with karyotyping…
How is a karyotype done?
Why is a karyotype done?
What is meant by independent assortment?
Draw and label the stages of meiosis.
Compare meiosis and mitosis, thoroughly.
What is the purpose of meiosis?
What are chiasmata?
Significance? (“Good” and “bad”)
Describe how Patau syndrome happens (trisomy 13).
Gregor Mendel
telophase I
independent assortment
somatic cell
prophase II
chromosomal mutations
diploid
metaphase II
genetic variability
gamete
anaphase II
gene mapping
haploid
telophase II
nondisjunction
meiosis
synapse
aneuploidy
homologous chromosome
tetrad
trisomy
Interphase I
chiasma
zygote
S phase
crossover
monosomy
sister chromatid
kinetochore
karyotype
meiosis I
metaphase plate
amniocentesis
meiosis II
spindle
chorionic villus sampling
prophase I
centrosome
autosomal chromosome
metaphase I
cleavage
sex chromosome
cytokinesis
Barr body
karyogram
anaphase I