Download Meiosis - Down the Rabbit Hole

Why do you share some but not
all characters of each parent?


Heredity
• Passing of genetic
traits from parent
to offspring
Chromosome theory
of heredity
• Chromosomes
carry genes
• Gene = unit of
heredity

Meiosis allows the creation of
unique individuals through sexual
reproduction.
 Somatic
Cells are “body” cells and
contain the normal number of
chromosomes . Examples would be …
skin cells, brain cells, etc. Diploid
 Gametes
are the “sex” cells and contain
only ½ the normal number of
chromosomes. Haploid Sperm cells and
ova are gametes.




Pair of chromosomes (maternal
and paternal) similar in shape and
size.
Each locus (position of a gene) is
in the same position on
homologues.
Humans have 23 pairs of
homologous chromosomes.
• 22 pairs of autosomes
• 1 pair of sex chromosomes
Homologous pairs (tetrads are
duplicated) carry genes
controlling the same inherited
traits.
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
• In Humans the
“Autosomes”
are sets 1 – 22
• (The Autosomes
code for most of
the offspring’s
traits)
In Humans the “Sex
Chromosomes” are
the 23rd set
XX chromosome - female
XY chromosome - male
Sex Chromosomes
the 23rd set
23
Y chromosome
X chromosome



Reproduction where the
genetic material combined is
called sexual reproduction
Two cells, a sperm and an
egg, unite to form a zygote,
the single cell from which the
organism develops
Meiosis is the process of
producing sperm and eggs
(gametes) – the number of
chromosomes are halved
 1)
Meiosis takes a cell with
two copies of every
chromosome (diploid) and
makes cells with a single
copy of every chromosome
(haploid).
 In
meiosis, one diploid cells
produces four haploid cells.




2) Meiosis scrambles the genes that each sex cell (egg
or sperm) receives.
This makes for a lot of genetic diversity.
This happens through independent assortment and
crossing-over.
Genetic diversity is important for the evolution of
populations and species.
Meiosis
Parent cell –
chromosome pair
Chromosomes
copied
1st division - pairs
split
2nd division –4
gamete cells with
½ the original
number of
chromosomes



Gametes have exactly one set of
chromosomes, this state is called
haploid (1n)
Regular cells have two sets of
chromosomes, this state is called
diploid (2n)
Why?
Meiosis – mouse testes
Parent cell
1st division
2nd division
4 gametes

Similar to mitosis
interphase.

Chromosomes
replicate (S phase).

Each duplicated
chromosome consist of
two identical sister
chromatids attached at
their centromeres.

Centriole pairs also
replicate.
The Key Difference Between Mitosis and
Meiosis is the Way Chromosomes Uniquely
Pair and Align in Meiosis
Mitosis
Meiosis
1st division
 Unlike
mitosis,
homologous chromosomes
line up next to each other
during prophase
 This
process is called
synapsing
 Lined
up homologues are
called tetrads

Longest and most complex spindle fiber
phase.

90% of the meiotic process
is spent in Prophase I

Chromosomes condense.

Synapsis occurs:
homologous
chromosomes come
together to form a tetrad.

Tetrad is two
chromosomes or four
chromatids (sister and
non-sister chromatids).
centrioles
homologues
Homologous chromosomes
sister chromatids
Tetrad
sister chromatids
Prophase 1– Crossing Over



Homologous
chromosomes
connect at areas
called chiasmata
Segments break
and reform at
similar locations.
This results in
new genetic
combinations
Non-sister chromatids
chiasmata: site of
crossing over
Tetrad
variation
Shortest phase
Tetrads align on the metaphase
plate.
INDEPENDENT
ASSORTMENT
OR
metaphase plate
metaphase plate
Independent Assortment
Independent assortment
produces 2n distinct gametes,
where n = the number of
unique chromosomes.
In humans, n = 23
223 = 6,000,0000.
That’s a lot of diversity by this
mechanism alone
Homologous chromosomes
separate and move towards
the poles.
Sister chromatids remain
attached at their
centromeres.
Each pole now has haploid set of
chromosomes.
Cytokinesis occurs and
two haploid daughter
cells are formed.
No interphase II
(or very short - no more DNA replication)
 same
as prophase in mitosis
 spindle reforms and chromosomes move
toward the metaphase plate
 same
as metaphase in mitosis
 sister chromatids lined up on the
metaphase plate
metaphase plate
metaphase plate
 same
as anaphase in mitosis
 sister chromatids are separated and pulled
toward opposite poles of the cell
Same as telophase in mitosis
Nuclei form, Cytokinesis occurs.
Four haploid daughter cells produced.
Example:
2n = 223
23 chromosomes in haploid
n = 23
= ~ 6 million possible combinations!


At least 6 million combinations from Mum and
another 6 million from Dad …
>36 trillion combinations for a diploid zygote!!!


In males is called spermatogenesis and
produces sperm.
In females is called oogenesis and
produces ova.
n=23
Primary
Spermatocyte
n=23
sperm
n=23
2n=46
Secondary
Spermatocyte
n=23
4 sperm cells are
produced from
each primary
spermatocyte
n=23
n=23
meiosis I
meiosis II



The male gamete is called a spermatozoa
Four spermatocytes are formed during meiosis
Men are busy - meiosis produces roughly 250,000
sperm a day.



The female gamete is
called an ovum
At birth each female
carries a lifetime supply of
developing oocytes, each
of which is in Prophase I.
A developing egg
(secondary oocyte) is
released each month from
puberty until menopause,
a total of 400-500 eggs.
Only one ovum is
produced during
meiosis
Oogenesis places
most of the
cytoplasm into
the large egg. The
other cells, the
polar bodies, do
not develop.
Any
Questions?