11.4 Meiosis
 Vocabulary: Homologous, diploid, haploid, meiosis,
tetrad, crossing-over
 Key Concepts:
 What happens during the process of meiosis?
 How is meiosis different from mitosis?
Types of Reproduction
Section 11-4
ASEXUAL
SEXUAL
1. One Parent
1. 2 parents.
2. Binary fission (in prokaryotes)
or mitosis in (eukaryotes).
3. Offspring are a clone of the
parent. Not genetically different.
2. Results in union of sex cells (egg
& sperm or pollen in plants) Sex
cells are called gametes.
3. Offspring not clones of parent.
Genetically different.
METHODS OF ASEXUAL REPRODUCTION
SEXUAL REPRODUCTION
1. Chromosomes come in pairs called HOMOLOGOUS PAIRS.
A. Remember – one of each pair is from each parent.
B. Humans have 23 homologous pair.
2. 1 – 22 pairs are called autosomes. Normal body cells (muscle, nerve,
bone, skin etc) have 23 pairs of chromosomes.
3. 23rd pair (either XX or XY) are called sex chromosomes
4. Normal body cells are Diploid (2N) and sex cells are Haploid (1N) (half
of Diploid)
EXAMPLES IN ORGANISMS OF 1N & 2N
1. A cat has 38 chromosomes (2N) in its normal body cells. Its sex cells (sperm or
egg) has half this # (1N) (19). A chicken has 39 chromosomes in its sex cells (1N)
thus it has 78 in its normal body cells.
2. Thus when in humans sperm (23 chromosomes) meets the egg (23
chromosomes) the chromosome number of the fertilized egg (called a zygote) is
46. Thus 1N & 1N = 2N.The fertilized egg then divides by mitosis (asexual
reproduction) to keep all somatic (body) cells at 46.
3. Thus, their most be another type of cell division that cuts the chromosome
number in half . This division is called MEIOSIS.
Example of how
gametes join
together to make
a fertilized egg
(zygote.)
Figure 11-15 Meiosis
Section 11-4
Meiosis I
Meiosis II
Section 11-4
Meiosis I results in two
haploid (N) daughter cells,
each with half the number of
chromosomes as the original.
Prophase II
The chromosomes line up in a
similar way to the metaphase
stage of mitosis.
Metaphase II
The sister chromatids separate
and move toward opposite
ends of the cell.
Anaphase II
Meiosis II results in four
haploid (N) daughter cells.
Telophase II
Parent or mother cell
Note how crossing over
happens and how the
final 4 cells only have
one of each
homologous pair.
4 Daughter cells
Note that sperm
production usually
produces 4 viable
(usable) sperm while
females only produce
1 usable egg.
MOST IMPORTANT PURPOSES OF MEIOSIS
1. Each sex cell (gamete) receives 1 CHROMOSOME FROM EACH
HOMOLOGOUS (MATCHING) PAIR NEVER BOTH!!
2. Meiosis creates genetic variation in organisms by:
A. Chromosomes assort themselves in gametes independently.
B. Different chromosome combinations possible in a sex cell
C. Crossing over can produce even more genetic variation.
HOW CROSSING
OVER CAN HAPPEN – DURING
Crossing-Over
PROPHASE I
Section 11-4
REMEMBER – This does not have to happen but it can!