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Block: ____________
Science 9 – Ch. 3 Sexual Reproduction
Section 3.1 Meiosis
Introduction to Meiosis: Every cell in the body contains a _____________, except red blood
cells.
 Each nucleus contains ___________________
 Cells that contain a complete amount (46 or 23 pairs) are said to be diploid
Diploid = double All cells in the body, with a few exceptions, are ___________.
 Cells that contain only half the amount of chromosomes (23) are said to be __________
 Haploid = single
Gametes are all haploid
Gametes- sex cells (___________ in males; _____________ in females)
 Sexual Reproduction – is when two parents are needed to create an offspring
 For sexual reproduction to occur, organisms need specialized cells to aid in
reproduction called ______________________.
o Gametes : a gamete is a cell that fuses with another cell during fertilization in organisms
that reproduce sexually.
 Gametes can be broken down into sperm and egg
o Sperm : Male gametes - many, small, mobile
o Eggs : Female gametes - are relatively few, large, and do not move.
 To produce the gametes, specific body cells undergo a specific type of cell ivision called
meiosis. This produces a cell that is said to be haploid.
o Regular cells (called __________________) have their chromosomes arranged
in pairs called _____________________________.
Homologous chromosomes:
Pairs of chromosomes are called _______________________________.
Homologous chromosomes are the same size & shape and carry the same genes.
Note that they may carry different versions of the genes (called alleles).
 This split is needed because since sexual reproduction requires two parents,
if there isn’t a reduction of chromosomes, then the offspring would have too
many chromosomes in its cells.
 Steps to meiosis: The steps of meiosis are similar to those of mitosis, except with a
couple of slight differences
Diploid v. Haploid
________________ (2n) carry two copies of each chromosome (homologous pairs).
_________________ (n) carry one copy of each chromosome.
Human diploid cells have 46 chromosomes; haploid cells (gametes) have 23.
Meiosis I: 1st Division
Meiosis I results in two daughter cells, but each has only one of the homologous
chromosomes, with its sister chromatid still attached.
Meiosis II: 2nd Division
Mitosis is the cell division process that occurs in our body and does not change
the number of chromosomes in the cell. It happens in all somatic cells of the body
and helps to make new cells.
In meiosis, the number of chromosomes is halved and therefore, it is also known
as Haploid Division or Reduction Division. It helps in variation in the offspring and
sex determination, as half the chromosomes are from the father and half from the mother
Mitosis vs. Meiosis
Stages
Meiosis
Interphase - begins with a diploid cell
- four chromosomes combine to
form two tetrads
Prophase - the chromosomes in the tetrad
cross over each other, allowing
them to exchange genetic material
Metaphase - the two tetrads line up in the center
- the two tetrads split up into four
Anaphase chromosomes which go to both
poles
- the two sets of chromosomes
Telophase become enclosed by the nuclear
envelope
Mitosis
- begins with a diploid cell
- four chromatids combine to form
two chromosomes linked by a
centromere
- the two chromosomes line up in the
center
- the two chromosomes split up into
four chromatids which move to both
of the poles
- the two sets of chromatids are
enclosed by the nuclear envelope
- two cells are created with two
- two cells are formed with two sets chromatids in each one.
Cytokenesis
of chromosomes in each one
- mitosis is now complete.
Prophase II
Metaphase
II
Anaphase
II
Telophase
II
Cytokenesis
- DNA replication is skipped and
the two cell's nuclear envelope are
dissolved and the spindle reformed
- the four chromatids in each cell
are connected together to form two
chromosomes
- the two chromosomes line up in
the center
- the two chromosomes are split up
into their daughter chromatids and
moved towards opposites poles
- the nuclear envelope is reformed
around the two poles on each cell.
- the cells are split up again and four
haploid cells remain as a result
- meiosis is now complete