Meiosis - Reproductive Cell Division
• Meiosis = cell division during which sex cells
(gametes) are produced
• Remember: sex cells contain half the number of
chromosomes as the body cells (haploid)
• Male gametes = sperms (1n)
** n = # of chromosomes
• Female gametes = eggs (1n)
• Fertilization = uniting of male and female
gametes; resulting fertilized egg = zygote (2n)
1n (egg) + 1n (sperm) = 2n (offspring)
Fertilization
n=23
egg
sperm
n=23
2n=46
zygote
Meiosis Cont.
• Occurs only in reproductive organs called gonads
(testes or ovaries)
• Meiosis in males = spermatogenesis
(gives rise to sperms)
• Meiosis in females = oogenesis
(gives rise to eggs)
Karyotype = collection of all organism’s chromosomes
Overview of Meiosis
Sister
chromatids
separate
Homologous
chromosomes
separate
Meiosis I
Meiosis II
Diploid
Prophase I
Metaphase I
Anaphase I
Telophase I
Haploid
Prophase II
Metaphase II
Anaphase II
Telophase II
Haploid
Results of meiosis: 4 haploid cells are produced from one
diploid cell
Meiosis I : homologous chromosomes separate
Spindle
fibers
Nucleus
Nuclear
envelope
Prophase I
(early)
(diploid)
Prophase I
(late)
(diploid)
Metaphase I
(diploid)
Anaphase I
(diploid)
Telophase I
(diploid)
Meiosis II : sister chromatids separate
Prophase II
(haploid)
Metaphase II
(haploid)
Anaphase II
(haploid)
Telophase II
(haploid)
Four
nonidentical
haploid
daughter cells
Why do we have meiosis?
I. To make sperm and eggs
II. To make new combinations of genes (due
to crossing over and independent
assortment of chromosomes) which leads
to variation within a species
Homologous chromosomes
Tetrad
Tetrad
Genetic variation
Random (independent) assortment
CELL DEATH
2 types of cell death:
• Orderly, genetically programmed cell death
= apoptosis
• Cell death due to injury = necrosis
CELL AGING
• Geriatrics = branch of medicine dealing with
medical problems of elderly
As we age:
• Many vital functions slow down
• Telomeres (DNA on tips of chromosomes) shorten
• Glucose is added to proteins and contributes to
stiffening
• Free radicals damage proteins, lipids, and DNA