Download Meiosis

Homologous Chromosomes
• Homologous chromosomes contain the same genes,
•
one from each parent
Ex: Two copies of chromosome 3, one from mother and
one from father, form a homologous pair.
This paternal
chromosome 3
carries
instructions for
light hair
L
D
Homologous pair after
DNA replication:
This maternal
chromosome 3
carries
instructions for
dark hair
Both have the gene for
hair color, but the color
itself may be different!
L L
D D
Chromosome Number
All eukaryotic cells can be categorized as either…
Diploid
Haploid
• Contain homologous pairs
of chromosomes (di = two
sets)
• All body cells except sperm
and egg are diploid
• In humans, diploid
chromosome number is 46
(23 pairs)
• Can be written as 2n
• Contain only one of each
chromosome (hap = half as
much)
• Only sperm and egg are
haploid
• In humans, haploid
chromosome number is 23
(no pairs, only singles)
• Can be written as n
3
How many
chromosomes?
6
0
How many
homologous pairs?
3
Haploid
Haploid or
Diploid?
Diploid
n
2n or n?
2n
MEIOSIS OVERVIEW
Meiosis is the creation
of sex cells, or
gametes.
It occurs in the testes
of males and the
ovaries of females.
It results in eggs
(females) and sperm
(males).
Mitosis vs. Meiosis
Mitosis
• Produces identical copies of somatic
cells (body cells)
• Responsible for growth and healing
• Produces 2 diploid daughter cells
Meiosis
• Produces unique gametes – no two are
the same!
• Responsible for making egg and sperm
• Produces 4 haploid daughter cells
Phases of Meiosis
• Like mitosis, meiosis also follows the PMAT phases
• Unlike mitosis, meiosis goes through them twice (meiosis I
and meiosis II)
Mitosis
Meiosis
Prophase
Metaphase
Anaphase
Telophase
Prophase I
Metaphase I
Anaphase I
Telophase I & cytokinesis
Prophase II
Metaphase II
Anaphase II
Telophase II & cytokinesis
Meiosis I
• In meiosis I, a reduction division occurs: chromosome
number is halved from 2n to n (46 to 23)
• In metaphase I, homologous pairs line up at the equator
• In anaphase I, the pairs are separated
Meiosis I
Meiosis II
Animation
• Meiosis II looks identical to mitosis, except two cells are
becoming four
Meiosis II
Crossing Over
When homologous chromosomes pair
during meiosis I, crossing over may occur
• Chromosomes come into contact,
crossed sections are exchanged
• Produces brand new combinations of
genes
46 total in
each
46 total
92 total
23 total in
each
Chromosomes
replicate
Homologous
chromosomes
pair up
1st division –
homologous
chromosomes
separate
2nd division –
sister chromatids
separate
“FUN” FACTS
 Females are born with as many eggs as they need-
Around 1 million. About 40,000 will survive to puberty
and develop one at a time each month.
 Males can produce sperm until they die, nearly 100
million per day, on average. Sperm live a maximum of
five days.
 It takes only one sperm to fertilize one egg.