MEIOSIS NOTES – Pre AP
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
• 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
Males =
1. Males – testes
Females =
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
Zygote develops into
embryo and finally adult
organism by mitosis
N
23
b. without meiosis…………
2N
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
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
Mitosis: No crossing over – daughter cells are identical to parent cells
Meiosis: Crossing over occurs –causes genetic variation
(Daughter cells are NOT identical to parent cell)
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
Mitosis
What kind of cells?
Somatic cells
When does this
occur?
Any time
# of Divisions
(Draw picture)
1
Meiosis
Male (XY) = Testes
Female (XX) = Ovaries
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