Download meiosis - lhsbiocaine

Cells IV
Meiosis
What is Meiosis?
 A special type of cell division used only for
the production of gametes
 Gametes are the reproductive cells of an
organism
 Typical mitosis cell division produces
genetically identical daughter cells
 Why wouldn’t this work for gametes?
Gamete Production
 Human somatic cells (those not used for
reproduction) have 46 chromosomes
 A chromatid has a complete set of an organisms DNA
 A chromosome has 2 copies of that set
 This is called a “diploid number” of chromosomes (2x)
 If gametes were made like somatic cells,
through mitosis, the sperm and the egg would
each have ______ chromosomes
 This means that upon conception, a zygote
would have ______ combined chromosomes
from its parents
46
46
?
Fertilized Egg
Gamete Production
 As a result of the problem of excess
chromosomes, gametes are produced in a
different way
 Meiosis
 Often called “reduction division”
 ________________________ must be reduced,
producing haploid cells
 Haploid = ½ x the number of necessary chromosomes
 _____________ must be divided
Chromosomes
Segments of DNA
found in our
chromosomes
contain genes
Since each of 2 parents donated
chromosomes to us, they are
said to come in pairs (23 pairs in
humans). These pairs are said
to be homologous pairs because
they each have genes for the
same traits. For example, a
homologous pair might have the
gene for eye color from the
mother on one chromosome and
a different gene for eye color
from the father on the other.
Each chromatid has a complete set of our genetic code, the
chromosome, then, has 2 copies of that complete code
The Phases
 Meiosis begins with the same type of
starting cell as mitosis (diploid cells)
 Meiosis undergoes the same phases as
mitosis (twice)
 Meiosis is split into 2 major parts:
 Meiosis I: reduction of chromosomes (by ½)
by separating homologous pairs
 Meiosis II: division of chromatids (just as in
mitosis)
The Phases: Meiosis I
Prophase I: formation of chromasomes, crossingover (explained later)
Metaphase I: homologous chromosomes line up
next to their homologues, spindle fibers begin to
attach to centromeres
Anaphase I: separation of homologous
chromosomes (reduces the # by ½)
Telephase I: parent cell begins to divide into 2
daughter cells (cleavage furrow in animal cells,
cell plate forms in plant cells)
2 daughter cells are made in Meiosis I
1 Chromosome from each homologous pair can
be found in each daughter cell
The Phases: Meiosis II
Prophase II: chromosomes remain intact
after first division on both new cells
Metaphase II: chromosomes now line up
single file as they do in mitosis; spindle
fibers attach to centromeres
Anaphase II: chromatids are divided with
one copy of the DNA going each way
Telophase II: each daughter cell from the
first division now begins to divide
The final product of meiosis is 4
daughter cells (all gametes, all haploid)
This would make 4 sperm or 1 egg/3
polar bodies (discarded)
Crossing Over
 In prophase I of meiosis, homologous
chromosomes allow genes to detach and
reattach (even on the homologue)
 This means that the chromosomes a
person receives from his/her parents are
not passed in their exact forms on to
his/her children, but rather some mixture
of the two
 Crossing over contributes to genetic
diversity
Crossing Over
Mitosis vs. Meiosis
 Generates somatic





daughter cells
Generates 2 cells
Generates genetically
identical copies
Undergoes 1 division
No crossing over
Homologous pairs
line up single file
 Generates gamete





daughter cells
Generates 4 cells
Generates genetically
unique copies
Undergoes 2 divisions
Crossing over in
prophase I
Homologous pairs
line up together in
metaphase I