BIG QUESTION:
Explain why I look like my parents.
Is the DNA equal from each parent?
What role does reproduction play into this process? Why
would that make me look like my mother and father?
Do you get equal amount of genes from each parent?
Cell Division
Meiosis
Definition
• Cell division by which eggs and sperm
are produced.
• The production of sex cells, which are
not genetically identical, through a
series of cell divisions.
● Homologous chromosomes: one of
the matching pair of chromosomes,
one from each parent.
● Haploid cell (n): a single set of
chromosomes
● Gamete: a reproductive cell that
contains the single (haploid) set of
● Diploid cell (2n): 2 sets of homologous
chromosomes
● Almost all human cells are diploid.
● The total number is 46;
○ 23 from your mother
○ 23 from your father
● Ensures that humans have the same
number of chromosomes in each
generation.
● It is a two-step process that reduces the
chromosome number by half—from 46
to 23—to form sperm and egg cells.
● This is a reduction in genetic material.
Why do the gametes
only have 23
chromosomes???????
● When the sperm and egg cells join at
fertilization, each contributes 23
chromosomes
● So the resulting embryo will have the
usual 46.
Meiosis also allows genetic
variation through a process of
DNA shuffling while the cells are
dividing.
Meiosis
• TWO divisions
• ONE DNA replication
Meiosis
• Interphase: Before meiosis begins, genetic material
is duplicated.
• First division of meiosis
– Prophase 1: Duplicated chromatin condenses. Each
chromosome consists of two, closely associated sister
chromatids. Crossing over can occur during the latter part of
this stage.
– Metaphase 1: Homologous chromosomes align at the
equatorial plate.
– Anaphase 1: Homologous pairs separate with sister
chromatids remaining together.
– Telophase 1: Two daughter cells are formed with each
daughter containing only one chromosome of the
homologous pair.
Meiosis
• Second division of meiosis: Gamete formation
– Prophase 2: DNA does not replicate.
– Metaphase 2: Chromosomes align at the equatorial plate.
– Anaphase 2: Centromeres divide and sister chromatids
migrate separately to each pole.
– Telophase 2: Cell division is complete. Four haploid
daughter cells are obtained.
• One parent cell produces four daughter cells.
Daughter cells have half the number of chromosomes
found in the original parent cell and with crossing
over, are genetically different.
http://www.palaeos.com/
Crossing Over
• Where a section of one chromosome
switches places with the same section
from the other chromosome of the pair.
• Recombination
http://www.accessexcellence.org
Animation
http://www.phschool.com/science/biology_pl
ace/biocoach/meiosis/process.html
Animation
http://www.csuchico.edu/~jbell/Biol207/ani
mations/meiosis.html