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Topic 3
Meiosis (gamete formation)
Human chromosomes
• Human somatic cells
(any cell other than a
gamete) have:
• 22 pairs of homologous
chromosomes
• 1 pair of sex chromosomes
• 46 individual chromosomes
• Individuals inherit one
set from each parent
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Human chromosomes
•
The sex chromosomes are
called X and Y
•
Human females have a
homologous pair of X
chromosomes (XX)
•
Human males have one X and
one Y chromosome, which is
much smaller than an X
chromosome
•
The 22 pairs of chromosomes that
do not determine sex are called
…?
• Are these chromosomes
from a male or female?
• How do you know?
Sex chromosomes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
How we describe cellular chromosome number
Humans
Diploid (2n)
lll
ll
l
Chromosome # = 6
Cells with 2 complete sets of
chromosomes (1 set from mom, 1 set
from dad – pairs!…this means that you
have 2 versions of every gene.)
Human gametes (sex cells)
Haploid (n)
l
l
l
Chromosome # = 3
Cells with 1 complete set of
chromosomes (only one of each
gene)
Human gametes (n) are combined for reproduction.
sperm
n=23
n=23
egg
Fertilization
The fertilized
egg is now 2n
and is called a
zygote.
2n=46
zygote
5
Meiosis &
Sexual Reproduction
Double division
of meiosis
DNA replication
1st division of
meiosis separates
homologous pairs
Meiosis 1
Meiosis 2
2nd division of
meiosis separates
sister chromatids
Figure 13.7 The stages of meiotic cell division: Meiosis I
OBJ 15
8
Figure 13.7 The stages of meiotic cell division: Meiosis II
OBJ 15
9
Trading pieces of DNA creates
variation
Crossing over
during Prophase 1, sister
chromatids of homologous
chromosomes intertwine
homologous pairs swap
pieces of chromosome
• DNA breaks & re-attaches
tetrad
prophase 1
Variation from:
Independent assortment of chromosomes
gametes of offspring do not have same
combination of genes as gametes from parents
random assortment in humans produces
223 (8,388,608) different combinations in gametes
from Mom
from Dad
offspring
new gametes
made by offspring
Mitosis vs. Meiosis
Gametogenesis in humans
Oogenesis: formation of eggs
Spermatogenesis: formation of sperm
Occurs in the reproductive organs
Oogenesis
 During fetal development,
oocyte pauses after
Prophase I
 Meiosis 1 completed during
maturation
 Meiosis 2 pauses in
Metaphase II
 Meiosis 2 completed after
release from ovary
Sperm production
Variation from random fertilization
Sperm + Egg = ?
any 2 parents will produce a zygote with over 70
trillion (223 x 223) possible diploid combinations
Sexual reproduction creates variability
Sexual reproduction allows us to maintain both
genetic similarity & differences.
When meiosis goes wrong:
chromosomal disorders
Human chromosomes review
•
The sex chromosomes are
called X and Y
•
Human females have a
homologous pair of X
chromosomes (XX)
•
Human males have one X and
one Y chromosome, which is
much smaller than an X
chromosome
•
The 22 pairs of chromosomes that
do not determine sex are called
autosomes
Sex chromosomes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
ACTIVITY INTERLUDE
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Karyotype review
• A karyotype is an ordered
display of the pairs of
chromosomes from a cell
• Karyotypes can tell us the
sex of a fetus
• Karyotypes can also tell us
if there are chromosomal
abnormalities
• Autosomal disorders result
from extra autosomal
chromosomes
• Sex chromosome
disorders result from extra
sex chromosomes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Chromosomal disorders
 Result from errors during meiosis
Abnormal number of chromosomes (nondisjunction)
 Chromosomes fail to separate (either during meiosis 1 or
meiosis 2)
 Usually fatal if an autosome
 Usually nonfatal if a sex chromosome (if there is at least one
X)
Missing bits of chromosome (deletion)
Crossing over with nonhomologous chromosomes
(translocation)
ACTIVITY INTERLUDE
Patient A
Patient B
Patient C
What’s the diagnosis?
Patient A: Klinefelter’s syndrome
Extra sex chromosome (XXY)
Patient B: Patau syndrome (trisomy 13)
Extra autosomal chromosome 13
Patient C: Down’s syndrome (trisomy 21)
Extra autosomal chromosome 21
Variation from:
Crossing over: creates completely new
combinations of traits on each chromosome
Can create an infinite
variety in gametes
Sperm production
Epididymis
Testis
Coiled
seminiferous
tubules
germ cell
(diploid)
primary
spermatocyte
(diploid)
MEIOSIS I
secondary
spermatocytes
(haploid)
Vas deferens
spermatids
(haploid)
spermatozoa
Cross-section of
seminiferous tubule
MEIOSIS II
Oogenesis
primary follicles
germinal cell
(diploid)
fallopian tube
fertilization
primary
oocyte
(diploid)
MEIOSIS I
secondary
oocyte
(haploid)
first polar body
MEIOSIS II
second
polar body
developing
follicle
mature follicle with
secondary oocyte
ruptured follicle
(ovulation)
ovum
(haploid)
corpus luteum
The value of sexual reproduction
 Sexual reproduction introduces genetic variation
genetic recombination
independent assortment of chromosomes
crossing over
random fertilization
 Providing variation for natural selection
metaphase1