Download Meiosis II

Meiosis:
Georgia Performance Standards:
SB2c:Using Mendel’s laws, explain the role of meiosis in reproductive
variability.
SB2e: Compare the advantages of sexual reproduction and asexual
reproduction in different situations.
Essential Questions:
• How are haploid gamete cells produced from diploid cells?
• How does meiosis generate variation in offspring?
Warm-up:
• Normal human body cells each contain 46
chromosomes.
• The cell division process that body cells
undergo is called mitosis and produces
daughter cells that are virtually identical to
the parent cell.
• Working with a partner, discuss and answer
the questions that follow.
Warm-up:
1. How many chromosomes would a sperm or an egg
contain if either one resulted from the process of
mitosis?
2. If a sperm containing 46 chromosomes fused with
an egg containing 46 chromosomes, how many
chromosomes would the resulting fertilized egg
contain? Do you think this would create any
problems in the developing embryo?
3. In order to produce a fertilized egg with the
appropriate number of chromosomes (46), how
many chromosomes should each sperm and egg
have?
Chapter
10
Sexual Reproduction and Genetics
Chromosomes and Chromosome Number
 Human body cells have 46 chromosomes
 Each parent contributes 23 chromosomes
 22 autosomes (undergoes mitosis)
 1 sex chromosome (X or Y) (undergoes
meiosis)
 Homologous chromosomes—one of two
paired chromosomes, one from each
parent
Chapter
10
Sexual Reproduction and Genetics
10.1 Meiosis
Chromosomes and Chromosome Number
 Same length
 Same centromere position
 Carry genes that
control the same
inherited traits
Sexual Reproduction and Genetics
Sexual Reproduction v. Asexual Reproduction
• Sexual reproduction
• Beneficial genes
multiply faster over
time.
 Asexual reproduction
• The organism inherits
all of its chromosomes
from a single parent.
• Genetically different
(variation in genes)
• The new individual is
genetically identical
to its parent.
• EX: Meiosis, and
conjugation
(Prokaryotes)
• Ex: mitosis and
binary fission
(Prokaryotes)
Haploid & Diploid:
• A human has 46
chromosomes in their
somatic cells (body
cells)
– Diploid (2N) - 2
sets of
chromosomes
– One set from mom
and one set from
dad
• Homologous
chromosomes
(same)
• When males and
female produce
gametes (sex cells)
like sperm and egg
cells, then the
chromosome separate
(Law of segregation)
• Each gamete will then
receive one set of
chromosomes
(haploid 1N)
How are haploid (N) gamete cells
produced from diploid (2N) cells?
• Meiosis - a process of reduction division
in which the number of chromosomes per
cell is cut in half through the separation
of homologous chromosomes in a diploid
cell.
– meiosis I, and meiosis II
• By the end of meiosis II, the diploid cell
that entered meiosis has become 4 haploid
cells.
Meiosis I
• Prior to meiosis I, each chromosome is
replicated.
• In prophase of meiosis I, however, each
chromosome pairs with its corresponding
homologous chromosome to form a structure
called a tetrad.
• There are 4 chromatids in a tetrad. This
pairing of homologous chromosomes is the
key to understanding meiosis.
Meiosis I Con’t:
• As homologous
chromosomes pair up and
form tetrads in meiosis I,
they may exchange
portions of their
chromatids in a process
called crossing-over.
• Crossing-over results in
the exchange of alleles
between homologous
chromosomes and
produces new
combinations of alleles.
• Video
• The homologous
chromosomes separate,
and two new cells are
formed.
• The two cells produced by
meiosis I have sets of
chromosomes and alleles
that are different from
each other and from the
diploid cell that entered
meiosis I.
• Video
Crossing-Over
Section 11-4
Go to
Section:
Meiosis I
Interphase I
Prophase I
Metaphase I
Cells undergo
a round of DNA
replication,
forming
duplicate
Chromosomes.
Each
chromosome
pairs with its
corresponding
homologous
chromosome to
form a tetrad.
Spindle fibers
attach to the
chromosomes
that line up in the
middle of the cell.
Go to
Section:
Anaphase I
The fibers pull the
homologous
chromosomes
toward the
opposite ends of
the cell.
Meiosis II
• The two cells produced by
meiosis I now enter a second
meiotic division.
• The chromosomes don’t
replicate again, before
entering meiosis II.
– Thus, each of the cell’s
chromosomes has 2
chromatids.
• During metaphase II of
meiosis, 2 chromosomes line
up in the center of each cell.
• In anaphase II, the paired
chromatids separate. Each
of the four daughter cells
produced in meiosis II
receives 2 chromatids.
• Those four daughter cells
now contain the haploid
number (N)—just 2
chromosomes each.
• Video Clip
• Video Clip
Meiosis II
Prophase II
Meiosis I results in
two haploid (N)
daughter cells, each
with half the number
of chromosomes as
the original.
Go to
Section:
Metaphase II
The
chromosomes line
up in a similar
way to the
metaphase stage
of mitosis.
Anaphase II
The sister
chromatids
separate and
move toward
opposite ends of
the cell.
Telophase II
Meiosis II
results in four
haploid (N)
daughter cells.
Gamete Formation:
• In male animals, the haploid gametes
produced by meiosis are called sperm.
• In female animals, generally only one
of the cells produced by meiosis is
involved in reproduction.
– The other three cells, called polar
bodies, usually are not involved in
reproduction.
Meiosis produces
four genetically
different haploid
cells.
Spermatogenesis
(production of
sperm cells)
Oogenesis
(production of
egg (ova) cells)
Chapter
10
Sexual Reproduction and Genetics
Comparing Mitosis and Meiosis
• Mitosis results in the
production of two
genetically identical
diploid cells
• A diploid cell that divides
by mitosis gives rise to
two diploid (2N) daughter
cells.
• The daughter cells have
sets of chromosomes and
alleles that are identical
to each other and to the
original parent cell.
• Meiosis produces four
genetically different
haploid cells.
• Meiosis, on the other
hand, begins with a
diploid cell but produces
four haploid (N) cells.
• These daughter cells have
sets of chromosomes and
alleles that are
genetically different from
the diploid cell and from
one another.
Ticket Out the Door:
1. Describe the main results of meiosis.
2. What are the principal differences between
mitosis and meiosis?
3. What do the terms diploid and haploid mean?
4. What is crossing-over?
5. 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.