MEIOSIS
Chapter 10.1
Cell reproduction review
• Cell cycle
Interphase – DNA replicated
2) Mitosis – nuclear division
3) Cytokinesis – cytoplasm divided
1)
• Results in 2 cells that are exact copies
of the original cell.
Sexual Reproduction
• Humans and other animals reproduce
sexually
• Two specialized cells form to make a
new organism.
• Gametes or sex cells
• Human gametes: eggs and sperm
Chromosomes
• Humans have 46 chromosomes
• Each parent contributes 23
chromosomes
• Homologous chromosomes:
Two paired chromosomes, one
from each parent
• 23 sets of homologous
chromosomes
Haploid and Diploid Cells
• Haploid – (N) number
chromosomes
• Diploid – (2N) number
chromosomes
• Gametes are haploid
• Gametes maintain the same
number of chromosomes from
generation to generation
Meiosis
• Cell division to make four gametes
with half the number of chromosomes
• Two divisions : Meiosis I and Meiosis II
• Male: Spermatogenesis
• Female: Oogenesis
Interphase
• Chromosomes replicate
• Each duplicated chromosome consists
of two identical sister chromatids
attached at centromeres.
Prophase I – Meiosis I
• Chromosomes condense
• Synapsis – Homologous chromosomes
come together to form a tetrad.
• Crossing over occurs (variation)
Prophase I – Crossing over
• Segments of non-sister chromatids
break and reattach to the other
chromatid
• Chiasmata – site of crossing over
Prophase I
Metaphase I
• Tetrads align on the metaphase plate
• Independent assortment occurs –
orientation of homologous
chromosomes is random.
• Creates variation
Metaphase I
Anaphase I
• Homologous Chromosomes separate
• Sister chromatids remain attached at
centromere.
Telophase I
• Homologous chromosomes reach the
poles
• The poles have haploid set of
chromosomes
• Cytokinesis – cytoplasm splits. Two
haploid daughter cells are made.
Telophase I
Cytokinesis
Meiosis II
• No Interphase – no replication
• Meiosis II is very similar to mitosis
Prophase II
• Chromosomes condense
• Nucleus disappears
Metaphase II
• Chromosomes line up on metaphase
plate
Anaphase II
• Sister chromatids separate
Telophase II
• Chromosomes arrive at poles
• Nucleus reforms
• Cytokinesis occurs – produces 4
haploid daughter cells
Telophase II
Spermatogenesis
• Male – begins at puberty continues
until death
Oogenesis
• Female – begins before birth until
menopause
Fertilization
• Fusion of Egg and sperm to form a
Zygote.
• Egg – haploid
• Sperm – haploid
• Zygote – diploid
Genetic variation
• Meiosis creates variation within a
population
1) Crossing over (prophase I)
2) Independent assortment (Metaphase I)
3) Random fertilization
• Variation allows for natural selection and
evolution to occur