Mitosis (Review)
– Division of a somatic cell that results in 2 genetically
identical daughter cells
• Cell division begins in Interphase when the
chromosomes duplicate
• Nucleus divides by means of Mitosis (PMAT)
• Cytoplasm and organelles divide by Cytokinesis
Chromosomes
Parent cell duplicate
2 new daughter
cells identical to
parent cell
• Daughter cells are genetically identical to parent cell –
same kind and number of chromosomes
• Mitosis occurs in somatic or body cells
Ex: liver, heart, skin, stomach
• Every organism has its own unique number of
chromosomes. Humans have 46.
• Asexual reproduction for some species—ex: bacteria
Meiosis – process by which the number of chromosomes is
reduced to half the number in the daughter cells
A. Occurs in – sex organs
1. Males –
testes
2. Females –
ovaries
3. Sex organs of other organisms – plants, animals,
fungi
B. Produces – sex cells or
gametes
1. Males – sperm
a. process called –
spermatogenesis
b. meiosis produces - 4
sperm
2. Females – egg or ova
(singular = ovum)
a. process called –
oogenesis
b. meiosis produces - 1
egg
3. egg and sperm are
referred to as – gametes
C. When does meiosis occur?
1. Males – at puberty
2. Females – before birth all eggs are produced
At birth, development of eggs stops; at puberty
eggs mature
D. Meiosis is also referred to as – chromosome reduction
1. Chromosomes number is always reduced by ½
a. total number of chromosomes – diploid #, 2N
1. all somatic cells – (body cells) diploid
2. humans 2N number - 46
b. half the number of chromosomes – haploid #, N
1. all sex cells – haploid
2. humans 1N number - 23
2. This is necessary for any organism that reproduces sexually
a. with meiosis………..
N
Fertilized egg –
zygote
23
46
N
23
2N
Zygote
develops into
embryo and
finally adult
organism by
mitosis
b. without meiosis …………
Fertilized egg –
zygote
46
92
46
E. Sex Chromosomes
1. Humans have 46 chromosomes, 23 pairs – 1 set
from Mom and 1 from Dad
2. 22 pairs look alike – autosome chromosomes
(not sex chromosomes)
3. 1 pair is the sex chromosomes – determines if you
are male or female
a. females: sex chromosomes are homologous – XX
b. males: sex chromosomes are different – XY
autosome
F. Unique events of meiosis
1. homologous chromosomes pair up
a. look alike
b. code for the same traits
c. one from each parent
2. homologous chromosomes exchange genes – crossing over
a. produces new combination of genes
b. source of genetic variation so offspring are not
genetically identical to parents
No crossing over
– daughter cells
are identical to
parent cells
Crossing over
occurs –causes
genetic variation
(Daughter cells
are NOT identical
to parent cell)
Division 2
Chromosomes
duplicate
Parent
cell
Crossing over
would occur
at this point
Division 1
Daughter cells
have half as
many
chromosomes
as parent cell
Daughter
cells
Chromosome Problems
A. Failure of the homologous chromosomes to
separate normally during meiosis -- nondisjunction
1. missing a chromosome – monosomy (45
chromosomes)
2. having an extra chromosome – trisomy (47
chromosomes)
3. most embryos fail to survive, but some do
a. short, round face, upper eyelids cover the inner
part of the eye, mental retardation – trisomy 21 -three #21 chromosomes
(47 chromosomes) – Down Syndrome
b. male with longer-than-average limbs, sterile – XXY
Klinefelter’s Syndrome
c. female with short stature, webbed neck, sterile – XO
Turner’s Syndrome
B. Parts of chromosomes break off and reattach in
different ways, occurs in meiosis:
deletion / translocation / duplication / inversion
Deletion
Duplication
Inversion
Translocation
C. Detecting chromosome mutations:
1. Picture of individual’s chromosomes – karyotype
2. Amniotic fluid surrounding an embryo is removed for
analysis (done 3½ to 4 months of pregnancy) –
amniocentesis
3. Analysis of chorionic villi which grows between
mother’s uterus and placenta (done 2
months of pregnancy – chorionic villi sampling (CVS)
Comparing Mitosis and Meiosis
1. Unique features of Meiosis
2. Comparison of Meiosis and Mitosis
Mitosis
What kind of cells?
Somatic cells
When does this occur?
Any time
# of Divisions
(Draw picture)
1
Meiosis
Male (XY) = Sperm
Female (XX) = Egg
Sex Cells
Male (XY) = puberty
Female (XX) = before birth
2
Male (XY) = 4 sperm
# of Daughter cells
2
# of Chromosomes
Same as parent cell
diploid or 2N
In humans 46
Type of Reproduction
Asexual
Sexual
Genetic Composition
Daughter cells
identical / not identical
to parent cell
Daughter cells
identical / not identical
to parent cell
Genetic variation
Pairing of Homologous
Chromosomes
YES / NO
YES / NO
Crossing over of genes
Function/Importance
Growth, repair; development of adult
from zygote
Production of gametes:
eggs and sperm
Female (XX) = 1 viable egg
Half as many as parent cell
haploid or N
In humans 23