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CH.11.4 MEIOSIS & SEXUAL REPRODUCTION
MEIOSIS
 Also known as _______gametogenesis____, the making of gametes(sex cells)
 Occurs ONLY in the ___gonads_______ (sex glands) to produce __gametes____ (egg &
sperm)
 --- Male gonads = __testes________
---Female gonads=____ovaries ___
---___Spermatogenesis___ production of sperm through meiosis
--- __Oogenesis_______ production of eggs through meiosis
 Meiosis 1 is known as “reduction division” because each gamete produced by meiosis
has ___half___ the normal # of chromosomes
--Therefore, in sexually reproducing organisms, meiosis followed by __fertilization____
maintains the __normal
chromosome___ number
 In HUMANS there are 46
chromosomes (called the __2n___ or
diploid #)
 All _body___ cells have the diploid #
(2n) of chromosomes
 Diploid # is 2n= the number of
chromosomes in each cell of a species
 The haploid # (_n_) is found ONLY in
egg & sperm cells (gametes)
 Hint: diploid =double(2n) &
haploid=half (n)
Meiosis is the process of
equal division.
reduction division.
tetrad formation.
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 MEIOSIS involves __2__ CELL DIVISIONS:
- Meiosis 1____ the reduction stage followed by second division is same as mitosis
called ___Meiosis 2____
Meiosis 1
 Primary or __stem __cell of the sex cell of ovary or testis is the same as a
body cell Replication of each chromosome occurs  Each chromosome is
_copied__
 There are _2___ chromatids for each type of chromosome  This is called
____sister__ _ chromatids__
Prophase 1;
 chromatin winds into chromosomes as nuclear
membrane disappears
 chromosomes find and line up with their
____homolog___ in a process call ____synapsis__
 homologues form ___tetrads___ and touch one another
other
 chromosomes trade pieces in a process called
__CROSSING OVER__
 this particular __stem___ cell has a diploid number of 4
in Figure 1.
Metaphase 1;
 spindle fibers attach to chromosomes as centrioles
appear
 homologs line up in PAIRS along the ___equator___ of
the cell
 INDEPENDENT ASSORTMENT occurs
 Mendel's law of independent assortment, states
that allele pairs separate independently during the
formation of gametes. This means that traits are
transmitted to offspring independently of one another.
Anaphase 1;
 homologous chromosomes __separate___ , moving one
to each side of the cell in a process called segregation
 the process will effectively __ reduce___ the chromosome number from (in
humans) 46 doubles to 23 double chromosomes
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Fig. 1
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Telophase 1;
 cell membrane starts to ___pinch___ inward
 nuclear membrane reappears
Cytokinesis 1 (overlaps with Telophase 1);
 cell membrane continues to pinch in, __dividing__ _cytoplasm__ equally
between daughter cells
 notice the 2 new cells are now two haploid (2 chromosomes instead of 4)
Review Quiz 1: Mcgraw-Hill.com/ unique features of meiosis
Meiosis 2
-EACH of the resulting two haploid daughter cells from Meiosis 1 undergoes a
__SECOND___ cell division that is the same as __Mitosis____
Prophase 2 (like mitosis);
 nuclear membrane __disappears___
 chromosomes remain _condensed_ from Telophase 1
Metaphase 2 (like mitosis);
 chromosomes line up on ___metaphase__ __plate___
 __centrioles appear and move toward poles
 spindle fibers start to___stretch__across cell from centrioles
 spindle fibers attach to __centomere________ of each chromosome
Anaphase 2(like mitosis);
 the __sister__ chromatids now separate & move toward opposite poles
 Each of the resulting 4 cells has the ____haploid__ (n) number of
chromosomes
Telophase 2(like mitosis);
 except the ovum in females produces _one__ __large__ _egg__ and three
discarded "_polar__ _bodies__" by unevenly distributing cytoplasm.
 Spermatogenesis make 4 viable ___sperm___, however
 cell membrane pinches in as ___nuclear__ __membrane__ reforms
Cytokinesis 2 (like mitosis overlaps with Telophase 2);
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 cytoplasm finishes dividing
 creates _4_ DIFFERENT haploid (n) __gamete__ cells (egg or sperm )for
sexual reproduction
HW: Chapter 11 Introduction to Genetics Section Review 11-4 pg. 134
Review Quiz 2: http://highered.mcgrawhill.com/sites/9834092339/student_view0/chapter11/meiosis_with_crossing_o
ver.html
10/18
MITOSIS produces cells with the normal ___diploid____________ # of chromosomes (2n)
WHILE MEIOSIS produces cells with the _haploid_____________ # (n)
 Remember, meiosis only occurs in the ___gonads____________ to produce gametes
The disadvantage of ASEXUAL reproduction is that there is _no__ variation ( __diversity___) all
offspring are identical to the parent
SEXUAL reproduction, however, relies on genetic contributions from __two____ parents
(instead of just one)
 There is also variation in the gametes produced by each parent since “__crossing_____
__over_______” can occur during Meiosis I
 Variation allows a population, as a whole, to ___adapt_________ to a changing
environment  What is this called???
Section 11-4 Self-Assessment Page 5
2 Types of Sexual Reproduction
 __Conjucation____ = the __exchange_____ of genetic material (DNA) between 2
different _organisms______ of the same species
Ex. Paramecium, spirogyra, bacteria
 this is how __antibiotic_ __resistance__ occurs, a BIG PROBLEM in
treating infections such as tuberculosis, MRSA and VRE infections
 __Fertilization____ = egg + sperm  __offspring______
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--- Fertilization occurs when the ____nucleus__ of an egg & sperm fuse, producing a
zygote (= a single _fertilized____ cell)
--- Millions of sperm are attracted to an egg, but only __1__ can fertilize it
2 Types of Fertilization
a) External = gametes fuse ___outside___ the body of the female
 Occurs in many __aquatic_______ animals (fish, amphibians) (water keeps
eggs from drying out)
 Since the eggs have less chance of being fertilized, __many__ are produced
b) Internal = gametes fuse ___inside___ the moist reproductive tract of the female
 Occurs in most _vertebrate____ animals (reptiles, birds , mammals)
 FEWER eggs are produced because they are well __protected___ & have a
greater chance of being fertilized
Section 11-4 Self-Assessment
1. During meiosis, the number of chromosomes per cell is cut in half through the separation of
A. daughter cells.
B. homologous chromosomes.
C. gametes.
2. The formation of a tetrad occurs during
A. anaphase I.
B. metaphase II.
C. prophase I.
3. In many female animals, meiosis I and II result in the production of
A. only 1 egg.
B. 1 egg and 3 polar bodies.
C. 4 eggs.
11.4 Section Assessment
1.
Key Concept Describe the main results of meiosis.
2.
Key Concept What are the principal differences between mitosis and meiosis?
3. What do the terms diploid and haploid mean?
4. What is crossing-over?
5. Critical Thinking Applying Concepts In human cells, 2N = 46. How many chromosomes would you expect to
find in a sperm cell? In an egg cell? In a white blood cell? Explain.
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Sexual and Asexual Reproduction
In asexual reproduction, mitosis occurs, but not meiosis. Which type of reproduction—sexual or asexual—
results in offspring with greater genetic variation? Explain your answer.
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