Meiosis, Sex Cells, Sexual Reproduction
Skeleton Notes
Introduction:
In higher organisms, plants and animals, individuals have two copies of every chromosome
and are referred two as a Diploid (2n) “double set” individual. In humans the diploid #=46 (23 pairs).
One set of each pair of chromosomes is inherited from the mother and the other set from the father.
In sexual reproduction, the two partners produce gametes which are joined to produce an offspring.
However, two problems must be solved in sexual reproduction. How does the joining of two
reproductive cells (gametes) occur without doubling the amount of chromosomes? How can the
chromosome number be reduced by half so that the number of chromosomes does not increase with
each generation of reproduction? The answer- MEIOSIS.
Objectives:
Differentiate between Mitosis and Meiosis
Model the events of meiosis, accentuating the major differences between Mitosis and Meiosis.
Explain the importance of “Crossing Over” in the context of genetic variability.
Notes Overview:
 Meiosis is a form of cell division in which there are ______successive cell divisions.
 In meiosis a _________ cell with (
) or “double set” of chromosomes is reduced to
four ___________ cells with ( ) or “single set” of chromosomes.
 In humans, the diploid number=______, the haploid number = ______
 One chromosome represents 100’s of 1000’s of genes each gene is specific to an area
on the DNA molecule
 Meiosis occurs in our germative cells which are regular body (diploid) cells that
produce our ____________
 The germative cell in females is the __________ which produces ______.
 The germative cell in males is the ___________ which produces _______.
Why do We need Meiosis?
 The process of meiosis guarantees the chromosomal number remains stable from one
generation to the next during __________ _______________.
from mom
from dad
child
too
much
muc
h!
Meiosis reduces genetic content
by _________
__________
_____________________
___________
____________
_____________________
______________________
Homologous Chromosomes:





The 46 chromosomes are actually _________ pairs of chromosomes.
The members of each pair are called ______________ because they are
similar in genetic content.
The chromosomes are equivalent and contain the same kinds of __________
arranged in the same order.
They may have different versions of genes or “_____________” because they
come from different parents.
In contrast to Mitosis, in Meiosis homologous chromosomes _________ prior
to cell division forming a grouping of four __________________ known as
Tetrad
During meiosis I ____________
__________ separate and therefore
different _________ also separate
During Meiosis II
individual chromatids
separate
MEIOSIS
During meiosis, three important things occur that did not occur in mitotis:
1. _______________ pair up forming _______________ pairs of chromosomes.
Metaphase of Mitosis
Metaphase of Meiosis I
Chromosomes line up
individually along the
equatorial plane
Chromosomes pair up
prior to division forming
Homologous pairs of
chromosomes
2. Homologous chromosomes exchange genetic information in an event called ______________
_______________.
PROPHASE I
Crossing Over



Occurs during _____________ of meiosis I
This process generates __________ by reordering the _____________ of genes within
homologous chromosomes
The resulting chromatids have _______ that is unique or recombinant.
A
A
B
B
C
b
C
c
D D
E
F
a
a
d
E
F
e
f
A
A
B
b
c
C
a
a
B
b
c
b
C
c
D D
d
E
e
f
F
d
E
F
e
d
e
f
f
The result of “Crossing-over” is ______________ DNA. DNA with unique
genetic make-up from two different chromosomes.
3. The newly scrambled chromosomes separate and go into _____ different daughter cells so
that each new cell contains only ______ chromatid of each pair of homologous chromosomes.
Paired Homologous
Chromosomes separate
Meiosis I
Meiosis II
Diploid germ cell
Sister
chromatids
separate
Haploid gametes
Mitosis vs. Meiosis
Mitosis
Meiosis
Responsible for growth, repair, and division in
primitive unicellular life forms.
Occurs only in diploid cells and produces only
haploid daughter cells.
The nucleus divides twice producing four nuclei.
Occurs in body cells and produces identical
daughter cells having the diploid (2n) number
of chromosomes.
The daughter cells are clones (genetically
identical) to the parent cell
Each haploid chromosome is a new combination of old
chromosomes because of crossing over
The chromosomes replicate only once, so each
nucleus contains half of the number of
chromosomes (Haploid Chromosome).
Stages of Meiosis:
Prophase I
Early Prophase:
Describe
Late Prophase:
Describe
Metaphase I
Describe:
Anaphase I
Describe:
Telophase I
Describe:
Cytokinesis:
Meiosis II
Daughter cell
From Meiosis I
Sister chromatids carry the same genes but
may contain different versions
Gene
Homolog- (chromatid of a
homologous chromosome)
Meiosis II produces gametes with
___ copy of each chromosome called a ___________ and thus
____ copy of each gene.
Describe:
Metaphase II of Meiosis
Describe:
Anaphase II of Meiosis
Describe:
Telephase II of Meiosis
Describe:
Results of Meiosis and Cytokinesis
Each of these four cells is called a
_________________
The cells are _______________ because they
contain half the genetic material as the parent cell.
Each cell has _________ copy of each original
________________.
Each chromosome has one ___________ or version
of each gene
There may be different combinations of
___________ for different genes depending the
extent of ___________ ______________ that
occurred in Prophase I of meiosis.