Meiosis and Gamete
Formation
Gametes
• Gametes =sex cells
• Gametes form from germline cells via meiosis
– Male gamete =sperm
– Female gamete =oocyte (egg)
• A male and a female gamete come together to
form a new individual
– Somatic (non-gamete) cells have 46 chromosomes
(diploid)
– Gametes have 23 chromosomes (haploid)
• ½ + ½ = 1 whole
Human Karyotype
Remember…
A chromosome
Replicated chromosome
(2 sister chromatids connected at centromere)
Figure 3.2
Homologous pair of chromosomes
Homologous pair of
replicated chromosomes
Meiosis
• Produces gametes (1n or haploid) from somatic
germline cells (2n or diploid)
from mother
from father
child
too
much!
meiosis reduces genetic content
Figure 3.1
Meiosis
Meiosis overview
• Produces gametes (1n or haploid) from somatic
germline cells (2n or diploid)
• Mixes up genetic information
– Diversity is important!
• Consists of 2 divisions
– Meiosis I
– Meiosis II
Meiosis I
(reduction
division)
Meiosis II
(equational
division)
Diploid
Figure 3.1
Haploid
Haploid
Meiosis I
•
•
•
•
•
Interphase (CHROMOSOMES REPLICATE)
Prophase I
Metaphase I
Anaphase I
Telophase I
Interphase
• Precedes Meiosis I
• Cell grows and organelles are made
• CHROMOSOMES ARE REPLICATED
Homologous pair of chromosomes
Homologous pair of
replicated chromosomes
Prophase I
• Pro =forward, forth
• Chromosomes condense
• Nuclear envelope breaks down
– Membrane surrounding the nucleus
• Centrioles move to opposite poles
• Spindle fibers grow
• Chromosomes synapse and crossing over occurs
Q4 Prophase
Crossing Over (Recombination)
Q7
• Replicated chromosomes pair up with their homologs
– i.e. chromosome from mom and chromosome from dad get
together
• Genetic information is exchanged
– Like shuffling a deck of cards and re-dealing
Homologous pair of
replicated chromosomes
Synapsis
New combinations of genetic info
Metaphase I
Q4 Metaphase
• Meta =after, with
• Homologous chromosome pairs line up at the midline
– Line up is random so that INDEPENDENT
ASSORTMENT takes place
• Spindle attaches to chromosomes at the centromere
– The centromere is where the sister
chromatids are attached
Independent Assortment
• Another way in which meiosis increases diversity
• Chromosomes line up at the midline in a random way
Q3
Anaphase I
• Ana =against
• Homologous pairs split
• Homologs are pulled to opposite poles by the
shortening of spindle fibers
Q4 Anaphase
Telophase I
• Telo =completion of a goal
• Spindle breaks down
• Nuclear envelope forms in each of the new cells
Q4 Telophase
Summary of Meiosis I
INTERPHASE
Prophase I
(early)
(diploid)
Prophase I
(late)
(diploid)
Metaphase I
(diploid)
Anaphase I
(diploid)
Telophase I
(diploid)
Meiosis II
•
•
•
•
•
Interphase –Chromosomes DO NOT replicate!
Prophase II
Metaphase II
Anaphase II
Telophase II
Prophase II
• Nuclear envelope breaks down
– Membrane surrounding the nucleus
• Centrioles move to opposite poles
• Spindle fibers grow
Q4 PII
Metaphase II
• Chromosomes line up at the midline
• Spindle attaches to chromosomes at the
centromere
– The centromere is where the sister
chromatids are attached
Q4 MII
Anaphase II
• Centromeres split
• Sister chromatids are pulled to opposite poles by
the shortening of spindle fibers
Q4 AII
Telophase II
• Spindle breaks down
• Nuclear envelope forms
in each of the new cells
• Chromosomes
decondense
Q4 TII
Summary of Meiosis II
Q2 Entire Overview
Prophase II
(haploid)
Metaphase II
(haploid)
Anaphase II
(haploid)
Telophase II
(haploid)
Four
nonidentical
haploid
daughter cells
Homologous
chromosomes in
germline cells
Synapsis
Square Dance
Homologous pair of
replicated chromosomes
New combinations of genetic info
Males and Females are Different!
Girls have a
“flower” and
boys have a
“noodle”
Male Reproductive System
• Gametes are made in the gonads
– Sperm are made in the testes
– Sperm are made and stored outside of the body
• 200-600 million sperm
are released with each
ejaculation!!!
• Trillions of sperm are
made in a lifetime
vas deferens
bladder
urethra
seminal
vesicle
prostate
bulbourethral
gland
epididymis
testis
Spermatogenesis
Entire process starts at puberty
Q1
Female Reproductive System
• Gametes are made in the gonads
– Oocytes are made in the ovaries
• A girl is born with just one million oocytes per
ovary
uterine tube
• Only 400 eggs will be
ovary
ovulated between
uterus
puberty and menopause
cervix
vagina
Oogenesis
Before birth
Arrested in Prophase I
After puberty (each month)
Arrested in Metaphase II
Upon fertilization
Polar Bodies
• Three polar bodies are
generated due to the
developing egg hogging
all of the cytoplasm
– Polar bodies usually
degenerate
– If fertilized by a sperm, a
blighted ovum develops
and results in miscarriage