Reproduction and Cell Division
• Meiosis takes place in the testes and ovaries
– Produces sperm cells
– Produces egg cells
• Two haploid gametes unite!
– Forms a diploid zygote
Body cells
Gamete-producing cells
• Formation of sex cells during meiosis is called
gametogenesis
• Female eggs go through oogenesis
• Whereas males go through spermatogenesis
• Although both processes produce four cells,
the end results are quite different
Oogenesis
• The cytoplasm does not divide equally after
each nuclear division
• One daughter cell, ootid, receives most of the
cytoplasm
• Polar bodies die and the nutrients is absorbed
• only one ovum is produced from meiosis
Spermatogenesis
• A spermatocyte divide through meiosis to
produce equal size sperm cells
• Much less cytoplasm than egg cells
• Sperm cells are meant to travel and cannot
carry excess weight
• Males can produce one billion sperm cells
every day
• Female babies have approximately two million
primary oocytes in their ovaries
• 300 000 oocytes remain during reproductive
years and only 400-500 will be released
• Primary oocytes have entered meiosis I but
remain in prophase I until first menstrual cycle
Sex Chromosomes
• After several observations on fruit flies it was
found that one chromosome differed in males
and females
• Females: two rod-shaped chromosomes
– XX
• Males: one rod-shaped chromosome and a
smaller hook-shaped chromosome
– XY
• Called sex chromosomes
• Other chromosomes= autosomes
X and Y Chromosomes
Non-disjunction
• https://www.youtube.com/watch?v=nQ3Rjh6
61X8
Normal Meiosis
• In Anaphase I,
homologous
chromosomes split up
and each one goes to a
pole.
• In Anaphase II, the
chromosome break in
two and each chromatid
goes to a pole.
Abnormal Meiosis I
During Anaphase I, if the chromosomes do
not split properly, cell will enter meiosis II
with 2 extra chromosomes
Abnormal Meiosis II
During anaphase
II, if one of the
chromosomes do
not split
properly, one cell
will be left with a
1 chromosome,
and the other
with 3
chromosomes
Non-Disjunction
• Produces: gametes with 22 chromosomes
– Produces chromosomes with 24
• If 24 chromosomes combines with 23
– How many chromosomes will the zygote have?
• If 22 chromosomes combines with 23
– How many chromosomes will the zygote have?
24 Chromosomes
Trisomy 21/Down
Syndrome
• If 24
chromosomes
combine with 23
chromosomes
– 47 chromosomes
– 3 chromosomes in
place of normal
pair
• Called trisomy
Trisomy
13
22 Chromosomes
• If 22
chromosomes
combine with 23
chromosomes
– 45 chromosomes
– 1 chromosomes in
place of normal
pair
• Called monosomy
Down Syndrome
• 3 copies of
chromosome number
21
• Round, full face
• Short height
• Large forehead
• Learning disabilities
• Heart defects
• Shortened life
expectancy
Turner’s Syndrome
• Monosomic disorder
• Produces female with
single X chromosome
• 45 chromosomes in
zygote
• Characteristics:
– Short and thick necks
– Tend to not develop
sexually
– Normal life expectancy
and intelligence
Klinefelter Syndrome
• Inheriting two XX
and Y
• Male at birth
• Produces female
hormones with
maturity
• Sterile
Karyotyping exercise!
• http://www.biology.arizona.edu/human_bio/a
ctivities/karyotyping/karyotyping.html