Process of Meiosis
Section 6.2
Objectives
• SWBAT compare and contrast the two rounds
of division in meiosis.
• SWBAT describe how haploid cells develop
into mature gametes.
Vocabulary
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•
•
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Gametogenesis
Sperm (spermatazoa)
Egg (ovum)
Polar body
Meiosis Basics
• Like mitosis, it is a form of nuclear division.
• Unlike mitosis, meiosis has two rounds of
nuclear division (mitosis has only one).
– The divisions are called meiosis I and meiosis II.
– Each round of cell division has four phases.
• The product of meiosis is four haploid cells
from one diploid cell.
– Thus, meiosis is a reductive process.
Meiosis Basics
Homologous Chromosomes and Sister
Chromatids
• Homologous chromosomes are two separate, yet
similar, chromosomes – one from your mother and
one from your father.
• Remember, homologous chromosomes are very
similar since the have the same length and carry
the same genes.
• They are not copies of one another.
• Sister chromatids, however, are genetically identical
to one another.
• A chromatid is half of a duplicated chromosome
– both halves together are called sister
chromatids.
Sister chromatids
refers to duplicated
chromosomes that
are attached at the
centromere.
Homologous Chromosomes and Sister
Chromatids
Homologous chromosomes are
divided in meiosis I.
Sister chromatids are not divided
until meiosis II.
Phases of Meiosis
• Before meiosis begins, DNA has already been
copied.
• Meiosis I divides homologous chromosomes,
producing two haploid cells with duplicated
chromosomes (i.e. sister chromatids).
Meiosis I
• This figure is from p. 174 of your book
Meiosis II
• Meiosis II divides sister chromatids resulting in
undoubled chromosomes.
• Note: DNA is not copied again between these
two stages.
Meiosis II
• This figure is from p. 175 in your book
Differences between mitosis and meiosis
• Meiosis has 2 cell divisions (mitosis only 1).
• During meiosis, homologous chromosomes pair
up along the cell equator, while homologous
chromosomes never pair up in mitosis.
• In anaphase I sister chromatids remain together
while sister chromatids separate in anaphase of
mitosis.
• Meiosis results in haploid cells – mitosis diploid
cells.
Haploid Cells Become Mature Gametes
• Once the haploid cells are produced by
meiosis, they must go through gametogenesis
(the production of gametes) to become
mature gametes.
• While meiosis is the same in males and
females, the final stages of gametogenesis
differs between the sexes – resulting in Sperm
and Eggs (ova).
Sperm
• Sperm is much smaller
than the egg.
• Its contribution is its DNA
(it does not contribute
nutrients like the egg).
• The haploid cell is altered
to become streamlined
and motile (moving with
its tail).
Ova (eggs)
• The formation of an ovum is a more complex
process than that for a sperm.
• It actually begins inside of the developing body of
a female embryo.
– The process is characterized by periods of active
development and log periods of inactivity.
• An ovum not only gives its share of DNA but also
contributes the organelles, molecular building
blocks (Ex. amino and nucleic acids), and other
materials an embryo needs to begin life.
Ova (eggs)
• Only on cell produced by
each round of mitosis
becomes an egg. The other
cells become polar bodies.
• All of the cytoplasm,
organelles, and other
molecular building blocks go
to only one of the cells. The
other three cells (the polar
bodies) contain little more
than DNA. They eventually
are broken down by the
female’s body.