Download Meiosis

Biology
A Guide to the Natural World
Chapter 10 • Lecture Outline
Preparing for Sexual Reproduction: Meiosis
Fifth Edition
David Krogh
© 2011 Pearson Education, Inc.
10.1 An Overview of Meiosis
© 2011 Pearson Education, Inc.
An Overview of Meiosis
• In human beings, nearly all cells have
paired sets of chromosomes, meaning these
cells are diploid.
© 2011 Pearson Education, Inc.
Overview of Meiosis
• Meiosis is the process by which a single
diploid cell divides to produce four haploid
cells—cells that contain a single set of
chromosomes.
© 2011 Pearson Education, Inc.
Overview of Meiosis
• The haploid cells produced through meiosis
are called gametes.
• Female gametes are eggs
• Male gametes are sperm.
• They are the reproductive cells of human
beings and many other organisms.
© 2011 Pearson Education, Inc.
homologous
pairs
Homologous
means the
same in size
and function
Mitosis
1. Both mitosis and meiosis are initiated in cells that are diploid or “2n,”
meaning cells that contain paired sets of chromosomes. The members
of each pair are homologous—the same in size and function. Two pairs
of homologous chromosomes are shown within the cells in both the
mitosis and meiosis figures. In each homologous pair, one chromosome
(in red) comes from the mother of the person whose cell is undergoing
meiosis, while the other chromosome (in blue) comes from the father of
this person.
Meiosis
somatic
cell
gamete
precursor
2n
duplication
2n
2n
duplication
2. Prior to the initiation of both mitosis and meiosis, the chromosomes
duplicate. In both processes, each chromosome is now composed of
two sister chromatids.
2n
2n
3. In mitosis, the chromosomes line up on the metaphase plate, one
sister chromatid on each side of the plate. In meiosis, homologous
chromosomes—not sister chromatids—line up on opposite sides of
the metaphase plate.
2n
4. In mitosis, the sister chromatids separate. In meiosis, the homologous
pairs of chromosomes separate.
2n
2n
division
2n
2n
n
division
5. In mitosis, cell division takes place, and each of the sister chromatids
from step 4 is now a full-fledged chromosome. Mitosis is finished. In
meiosis, one member of each homologous pair has gone to one cell,
the other member to the other cell. Because each of these cells now
has only a single set of chromosomes, each is in the haploid or “n” state.
Next, these single chromosomes line up on the metaphase plate, with
their sister chromatids on opposite sides of the plate.
n
n
n
division
6. The sister chromatids of each chromosome then separate.
division
7. The cells divide again, yielding four haploid cells.
n
n
n
n
© 2011 Pearson Education, Inc.
Figure 10.1
Overview of Meiosis
• When the haploid sperm and haploid egg
fuse, a diploid fertilized egg (or zygote) is
produced, setting into development a new
generation of organism.
© 2011 Pearson Education, Inc.
10.2 The Steps in Meiosis
© 2011 Pearson Education, Inc.
The Steps in Meiosis
• In meiosis, there is one round of
chromosome duplication followed by two
rounds of cell division.
© 2011 Pearson Education, Inc.
The Steps in Meiosis
• There are two primary stages to meiosis,
meiosis I and meiosis II.
© 2011 Pearson Education, Inc.
Meiosis I
• In meiosis I, chromosome duplication is
followed by a pairing of homologous
chromosomes with one another, during
which time they exchange reciprocal
sections of themselves.
© 2011 Pearson Education, Inc.
Meiosis I
• Homologous chromosome pairs then line up
at the metaphase plate.
• One member of each pair is on one side of
the plate, the other member is on the other
side.
© 2011 Pearson Education, Inc.
Meiosis I
• These homologous pairs are then separated,
in the first round of cell division, each of
them becoming part of a separate daughter
cell.
© 2011 Pearson Education, Inc.
Meiosis
Suggested Media Enhancement:
Meiosis
To access this animation go to folder C_Animations_and_Video_Files
and open the BioFlix folder.
© 2011 Pearson Education, Inc.
Meiosis II
• In meiosis II, the chromatids of the
duplicated chromosomes are separated into
separate daughter cells.
© 2011 Pearson Education, Inc.
The Steps of Meiosis
Prior to
meiosis Diploid
Meiosis I
metaphase
plate
End of
interphase
DNA has already
duplicated
Prophase I
Homologous
chromosomes link
as they condense,
forming tetrads.
Metaphase I
Microtubules
move homologous
chromosomes
to metaphase plate.
Crossing over
occurs
(see Figure 10.3)
Independent
assortment occurs
(see Figure 10.4)
© 2011 Pearson Education, Inc.
Anaphase I
Microtubules
separate homologous
chromosomes
(sister chromatids
remain together).
Figure 10.2
The Steps of Meiosis
Haploid
Meiosis II
cytokinesis
cytokinesis
Telophase I
Two haploid
daughter cells
result from
cytokinesis.
Prophase II
Metaphase II
(Brief)
Sister chromatids
line up at new
metaphase plate.
© 2011 Pearson Education, Inc.
Anaphase II
Telophase II
Sister chromatids Four haploid
cells result.
separate.
Figure 10.2
10.3 The Significance of Meiosis
© 2011 Pearson Education, Inc.
The Significance of Meiosis
• Meiosis generates diversity by ensuring that
the gametes it gives rise to will differ
genetically from one another.
© 2011 Pearson Education, Inc.
Meiosis Generates Diversity
• Meiosis is unlike regular cell division, or
mitosis, which produces daughter cells that
are exact genetic copies of parent cells.
• Meiosis generates genetic diversity in two
ways.
© 2011 Pearson Education, Inc.
Meiosis Generates Diversity
• First, in prophase I of meiosis, homologous
chromosomes pair with each other.
• In the process called crossing over or
recombination, they exchange reciprocal
chromosomal segments with one another.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Meiosis Generates Diversity
• Second, in metaphase I of meiosis, there
is a random alignment or independent
assortment of maternal and paternal
chromosomes on either side of the
metaphase plate.
• This chance alignment determines which
daughter cell each chromosome will end
up in.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Meiosis Generates Diversity
• The genetic diversity brought about by
meiosis and sexual reproduction is
responsible, to a significant extent, for the
great diversity of life-forms seen in the
living world today.
© 2011 Pearson Education, Inc.
Meiosis Generates Diversity
• Evolution is spurred on by differences
among offspring, and meiosis and sexual
reproduction ensure such differences.
• By contrast, asexual reproduction, as is seen
in bacteria and other organisms, produces
organisms that are exact genetic copies, or
clones, of the parental organism.
© 2011 Pearson Education, Inc.
10.4 Meiosis and Sex Outcome
© 2011 Pearson Education, Inc.
Meiosis and Sex Outcome
• Human females have 23 matched pairs of
chromosomes—22 pairs of autosomes and
one pair of sex-determining chromosomes,
which in females are X chromosomes.
© 2011 Pearson Education, Inc.
Meiosis and Sex Outcome
• Human males have 22 autosomes, one X
chromosome, and one Y chromosome.
© 2011 Pearson Education, Inc.
The X and the Y
© 2011 Pearson Education, Inc.
Figure 10.6
Meiosis and Sex Outcome
• Each egg that a female produces has a
single X chromosome in it.
© 2011 Pearson Education, Inc.
Meiosis and Sex Outcome
• Each sperm that a male produces has either
an X or a Y chromosome within it.
© 2011 Pearson Education, Inc.
Meiosis and Sex Outcome
• If a sperm with a Y chromosome fertilizes
an egg, the offspring will be male.
© 2011 Pearson Education, Inc.
Meiosis and Sex Outcome
• If a sperm with an X chromosome fertilizes
the egg, the offspring will be female.
© 2011 Pearson Education, Inc.
10.5 Gamete Formation in Humans
© 2011 Pearson Education, Inc.
Sperm Formation
• The starting cells in human male gamete
formation, spermatogonia, are diploid cells
that are stem cells in that they give rise not
only to a set of specialized cells—the
diploid primary spermatocytes—but to
more spermatogonia as well.
© 2011 Pearson Education, Inc.
Sperm Formation
• Primary spermatocytes go through meiosis,
producing haploid secondary
spermatocytes.
• These in turn give rise to the spermatids that
develop into mature sperm cells.
© 2011 Pearson Education, Inc.
Egg Formation
• In adult females, there may be no
counterparts to the male spermatogonia—no
cells that function as reproductive stem
cells.
• This is currently a matter of scientific
controversy.
© 2011 Pearson Education, Inc.
Egg Formation
• Female gamete formation begins with cells
called oogonia, most or all of which are
produced prior to the birth of the female.
© 2011 Pearson Education, Inc.
Egg Formation
• These give rise to primary oocytes—the
cells that will go through meiosis I, thus
producing haploid oocytes.
© 2011 Pearson Education, Inc.
Sperm and Egg Formation in Humans
Oogenesis
Spermatogenesis
oogonium
spermatogonium
1. The diploid spermatogonium
cell produces a primary
spermatocyte.
primary
oocyte
primary
spermatocyte
2. The primary spermatocyte
goes through meiosis I,
yielding two haploid
secondary spermatocytes.
Meiosis I
polar
body
secondary
spermatocytes
3. The secondary spermatocytes
go through meiosis II, yielding
four haploid spermatids,
which will develop into
mature sperm cells.
1. Before the birth of the
female, a cell called an
oogonium develops into
a primary oocyte; this
cell enters meiosis I, but
remains there until it
matures in the female
ovary, beginning at
puberty.
2. On average, one primary
oocyte per month will
complete meiosis I. In
this process, an unequal
meiotic division of
cellular material leads to
secondary
the production of one
oocyte
polar body and one
secondary oocyte, which
enters into meiosis II.
Meiosis II
spermatids
polar bodies
(will be degraded)
© 2011 Pearson Education, Inc.
egg
3. Only secondary oocytes
that are fertilized by
sperm will complete
meiosis II and develop
into an egg. The three
polar bodies that are
produced by meiosis I
and II will be degraded.
Figure 10.8
Egg Formation
• The vast majority of primary oocytes never
complete meiosis, however.
• It is only the single primary oocyte released
each month, in the process of ovulation, that
completes meiosis I.
© 2011 Pearson Education, Inc.
Egg Formation
• Only those ovulated oocytes that are
fertilized by sperm complete meiosis II.
© 2011 Pearson Education, Inc.
Egg Formation
• Only one of the cells produced in meiosis
will have the potential to develop into a
haploid egg.
• The other cells receive very little
cytoplasmic material during the process of
cell division and thus are destined to
become nonfunctional cells called polar
bodies.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
10.6 Life Cycles: Humans and
Other Organisms
© 2011 Pearson Education, Inc.
Life Cycles: Humans and Other Organisms
• Not all reproduction is sexual reproduction,
meaning reproduction that works through a
fusion of two reproductive cells.
© 2011 Pearson Education, Inc.
Asexual Reproduction
• Most types of organisms are capable of
asexual reproduction, although such
reproduction is rare among more complex
organisms and is never carried out by
mammals or birds.
© 2011 Pearson Education, Inc.
Asexual Reproduction
• Asexual reproduction can take several
forms, including binary fission, vegetative
reproduction, and regeneration.
© 2011 Pearson Education, Inc.
Asexual Reproduction
• Bacteria reproduce through one form of
asexual reproduction, called binary fission,
in which a given bacterial cell replicates its
single chromosome and then divides in two.
© 2011 Pearson Education, Inc.
Asexual Reproduction
• Plants can engage in vegetative reproduction.
• Other organisms, such as worms and sea stars,
can carry out regeneration, in which a new,
complete organism can be formed from a
portion of an existing one.
© 2011 Pearson Education, Inc.
Regeneration
© 2011 Pearson Education, Inc.
Figure 10.11