1. Meiosis Overview: sexual reproduction requires special cells made by meiosis.
Asexual reproduction occurs when a single parent produces identical offspring.
Sexual reproduction produces offspring from the fusion of two reproductive cells (gametes: eggs and sperms) in a process called fertilization.
Some species (particularly among plants) can use both methods of reproduction.
There are two ways in which an organism can reproduce. Many organisms, including bacteria, fungi, and even some plants and animals, undergo asexual reproduction, in which a single parent produces identical offspring. Other organisms, including most animals and plants, undergo sexual reproduction, in which offspring are produced by the fusion of two reproductive cells in the process of fertilization. And some species, particularly among plants, can use both methods. In sexual reproduction, because a combination of DNA from two separate individuals is passed on to offspring, the resulting offspring are genetically different from their parents and, for reasons explained later, from one another. 1

2. Chromosome number-Ploidy levels
Diploid: two sets of chromosomes
Humans- 46
Haploid : one set of chromosomes; eggs and sperms
Polyploidy: more than two complete sets of chromosomes

3. Sexual Reproduction and Meiosis
DIPLOID CELLS
have two copies of each chromosome
HAPLOID CELLS
have one copy of
each chromosome
have two copies of
ADULT FEMALE
• 23 chromosome pairs
(46 chromosomes total)
ADULT MALE
EGG (female gamete)
• 23 chromosomes total
SPERM (male gamete)
FERTILIZED EGG
OFFSPRING
MEIOSIS
FERTILIZATION
MITOSIS
Meiosis
Meiosis enables organisms to produce haploid gametes.
It produces gametes that differ from one another with respect to the combinations of alleles they carry and brings about genetic variability
What would happen if sexually reproducing organisms, humans included, produced reproductive cells through mitosis? Both parents would contribute a full set of genes—that is, 23 pairs of chromosomes in humans—to create a new individual, and the new offspring would inherit 46 pairs of chromosomes in all. And when that individual reproduced, if he or she contributed 46 pairs of chromosomes and his or her mate also contributed 46 pairs, their offspring would have 92 pairs of chromosomes. Where would it end? The genome would double in size every generation.
In sexually reproducing organisms, the solution to chromosome overload is meiosis. This process enables organisms to make gametes, special reproductive cells that have half as many chromosomes as the rest of the cells in the organism’s body (the somatic cells). In humans, for example, each gamete cell has only one set of 23 chromosomes, rather than two sets.
In genetics, the term diploid refers to cells that have two copies of each chromosome (in humans, two sets of 23 chromosomes, for 46 chromosomes in total), and the term haploid refers to cells that have one copy of each chromosome. Thus, somatic cells are diploid, and gametes, the cells produced in meiosis, are haploid.
At fertilization, two haploid cells, each with one set of 23 chromosomes, merge and create a new individual with the proper diploid human genome of 46 chromosomes. And this new individual, through meiosis, will also produce haploid gametes that have only a single set of 23 chromosomes. With sexual reproduction, then, diploid organisms produce haploid gametes that fuse at fertilization to restore the diploid state (Figure 8-18). In this way, meiosis maintains a stable genome size in a species.
Although there are some variations on this pattern of alternation between the haploid and diploid states, in most cases, multicellular animals produce haploid cells for reproduction. And after two haploid gametes come together to form a diploid fertilized egg, multiple cell divisions by mitosis produce a diploid, multicellular animal.
Fertilization = fusion of haploid egg and haploid sperm to form the single cell called zygote 3

4. What is meiosis? “Reduction Division”
1 diploid (2n) cell  4 haploid (1n) cells
involves 2 nuclear divisions (meiosis I & meiosis II)
occurs in gonads (reproductive organs)

5. Life Cycle of an Animal

6. Two nuclear divisions (that result in 4 daughter cells)
Meiosis: consists of meiosis I and meiosis II
Two nuclear divisions (that result in 4 daughter cells)
Meiosis I
Meiosis II 46

7. Sperm and egg are produced by meiosis: the details, step by step.
Meiosis only takes place in the gonads.
Meiosis begins with a cell containing 46 chromosomes:
Maternal and paternal copy of each chromosome (homologues)
HOMOLOGUES
The maternal and paternal copies of a chromosome
CENTROMERE
The point at which two sister chromatids are held together
Maternal
chromosome
Paternal
SISTER CHROMATIDS
The two identical copies of a chromosome created during replication
Replication
Mitosis is an all-purpose process for cell division. It occurs all over the body, all the time. Meiosis, on the other hand, takes place in just a single place: the gonads (the ovaries and testes). And it takes place for just a single reason: the production of gametes (sperm and eggs).
Meiosis starts with a specialized diploid cell found in the gonads that is capable of undergoing meiosis. Thus, for humans, meiosis starts with a cell that has 46 chromosomes. These include a maternal copy and a paternal copy of each of 22 chromosomes—each of these pairs is called a homologous pair, or homologues—along with two additional chromosomes (one from each parent), called sex chromosomes (Figure 8-20). 7

8. Homologous pair of chromosomes or homologues
Centromere
the maternal and paternal copies of a chromosome
Sister chromatids

9. Meiosis reduces the genome by half.
The products of meiosis are four haploid cells, each containing just one copy of each chromosome, rather than a homologous pair.
INTERPHASE
Each chromosome in a homologous pair replicates to form two sister chromatids.
(Chromosomes shown condensed here for diagrammatic purposes.)
Homologues
Maternal chromosome
Paternal chromosome
Sister chromatids
MEIOSIS I
In the first division of meiosis, the homologous pairs separate.
MEIOSIS II
In the second division of meiosis, the sister chromatids separate.
Diploid parent cell
Haploid daughter cells (gametes)
Before meiosis can occur (and just as we saw with mitosis), each of these 46 chromosomes is duplicated during the cell’s interphase. This means that when meiosis begins, we have 92 (2 × 46) strands of DNA, after replication.
Unlike mitosis, which has only one cell division, cells undergoing meiosis divide twice. In the first division, the homologues separate. In other words, for each of the 23 chromosome pairs, the maternal sister chromatid pairs and the paternal sister chromatid pairs separate into two new cells. In the second division, each of the two new cells divides again, so that each of the four daughter cells contains a single chromosome from the homologous pair. At the end of meiosis, there are four new cells, each of which has 23 strands of DNA—that is, 23 chromosomes (Figure 8-21). Note that, in animals, none of these four cells will undergo any further cell division—they do not become parent cells for a new cycle of cell division.

10. Skin cells from a dog have a TOTAL of 78 chromosomes
Skin cells from a dog have a TOTAL of 78 chromosomes. How many TOTAL chromosomes do each sperm cell from the dog have? 19 39 78
156
Ans: B

11. Meiosis in detail Meiosis I Prophase I Metaphase I Anaphase I
Telophase I
--Cytokinesis happens here
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis happens again

12. Meiosis I: Reduction Division
1. Prophase I
replicated chromosomes condense
spindle fibers forms
nuclear membrane breaks down
Homologous chromosomes pair
crossing over occurs
Meiosis I is called reduction division because it reduces the number of chromosomes (separates homologous chromosomes). Human germ cells have 46 chromosomes, so at the completion of meiosis I, resulting daughter cells will have only 23 chromosomes.

13. Crossing over and meiosis are important sources of variation.
Crossing over (genetic recombination) occurs when homologous chromosomes swap genetic information.
Chiasmata are the points along the homologues where the genetic information was exchanged.
Crossing over doesn’t create new alleles but it does create new combinations of alleles on a chromatid.
Each of these 4 chromatids
gets packaged into a haploid
gamete (sperm or egg cell).
HOMOLOGOUS CHROMOSOMES
Crossing over between the
sister chromatids of the
homologous chromosomes
Homologous chromosomes after the exchange of genetic information
Chromatids with
recombined DNA
Paternal copy
Maternal copy
Sister chromatids
Genetically speaking, there are two ways to create unique individuals. The obvious way is for an organism to carry an allele that is not present in any other individuals. Alternatively—and equally successful in creating uniqueness—an individual can carry a collection of alleles, no single one of which is unique, that has never before occurred in another individual. Both types of novelty introduce important variation into a population of organisms. The process of crossing over, or genetic recombination (Figure 8-25), which occurs during prophase I in meiosis, creates a significant amount of the second type of variation.
Let’s look at crossing over more carefully. Take, for example, the homologous pair of human chromosome 15. It includes two copies of chromosome 15: one copy from your mother (which you inherited from the egg that was fertilized to create you) and one copy from your father (which you inherited from the sperm that fertilized the egg). Each chromosome in the pair carries the same genes, but because they came from different people, they don’t necessarily have the same alleles.
Once the sister chromatids of the homologous chromosome pairs line up in prophase I (so that there are now four chromatids in two pairs lying very close together), regions that are close together can swap segments. A piece of one of the maternal chromatids—perhaps including the first 100 genes on the strand of DNA—may swap places with the same segment in a paternal chromatid. Elsewhere, a stretch of 20 genes in the middle may be swapped from the other maternal chromatid with one of the paternal chromatids. The points at which chromatids exchange genetic material during recombination are called chiasmata (sing. chiasma).
Every time a swap of DNA segments takes place, an identical amount of genetic material is exchanged, so all four chromatids still contain the complete set of genes that make up the chromosome. The combination of alleles on each chromatid, though, is now different.
Suppose there are genes relating to eye color and height on a particular chromosome. After crossing over, a chromatid that carried instructions for brown eyes and short height may now carry instructions for brown eyes and tall height. All of the alleles from your parents are present on one DNA molecule or another. But the combination of alleles (and the traits they determine) that are linked together on a single chromatid is new. And when a gamete, let’s say it’s an egg, carrying a new combination of alleles is fertilized by a sperm, the developing individual will carry a completely novel set of alleles.
Without creating new versions of any traits (such as yellow eyes or purple hair), crossing over creates gametes with collections of alleles that may never have existed together before. In Chapter 9, we’ll see that this variation is tremendously important for evolution.

14. The image represents ___
A pair of sister chromatids
A pair of unduplicated chromosome
Duplicated homologous chromosomes
All of the above
Ans:C

15. Metaphase I
paired homologous chromosomes line up along cell equator

16. Anaphase I
homologous chromosomes separate & move to opposite poles

17. Telophase I spindle fibers breaks down
chromosomes become long and thin
nuclear membranes reform
Cytokinesis occurs between meiosis I & II

18. Meiosis II (equational division)
Prophase II
chromosomes condense
spindle fibers forms
nuclear membrane breaks down
Metaphase II
chromosomes line up along equator of cell
Fibers attach to chromatids

19. Meiosis II (equational division)
Anaphase II
sister chromatids (now called chromosomes) separate & move toward opposite poles
Telophase II
spindle fibers break down
chromosomes de-condense
nuclear membranes reform
Cytokinesis divides two cells into four nonidentical cells.

20. Meiosis Division 1: Homologues Separate (Stages 1−3)
INTERPHASE
• Chromosomes (uncondensed in this phase) replicate in preparation for meiosis.
Centromere
Replicated
chromosome
Nuclear
membrane
Random assortment of the maternal and paternal sister chromatids at the metaphase plate generates variation.
Crossing over among the sister chromatids generates variation. 2 1 3
PROPHASE I
Replicated chromosomes condense.
Spindle is formed.
Homologous pairs of sister chromatids come together and cross over.
Nuclear membrane disintegrates.
METAPHASE I
• Homologues move toward the center of the cell (at this point, referred to as the metaphase plate) and line up.
ANAPHASE I
• Homologues separate and are pulled to opposite poles. Sister chromatids going to each side are a mix of maternal and paternal genetic material.
Piece of maternal chromatid
now on paternal chromatid
following crossing over
Homologues
Spindle
Spindle fiber
Interphase. Before meiosis begins, every chromosome creates an exact duplicate of itself by replication. The chromosomes that were each a single, long, linear piece of genetic material become a pair of identical long, linear pieces, held together at the centromeres. The first meiotic division takes place in four stages.
1. Prophase I: chromosomes condense and crossing over occurs. This is by far the most complex of all the phases of meiosis. As in mitosis, it begins with all of the replicated genetic material condensing. As the sister chromatids become shorter and thicker, the homologous chromosomes come together. This is where the process diverges from mitosis. The homologous chromosomes (each of which has become a pair of identical chromatids) line up, touching. Under a microscope, the two homologous pairs of sister chromatids appear as pairs of X’s lying one on top of the other. At this point, the sister chromatids that are next to each other do something that makes every sperm or egg cell genetically unique: they swap little segments of DNA. Some of the genes that you inherited from your mother may get swapped onto the strand of DNA you inherited from your father, and vice versa. This possible outcome of the crossing over of portions of the chromatids is called genetic recombination (or, more often, just recombination). It can take place at several spots (up to dozens) on each chromatid. As a result of recombination, every sister chromatid possesses a unique mixture of your genetic material. Note that crossing over takes place only during the production of gametes, in meiosis. It does not occur during mitosis. Following crossing over, the nuclear membrane disintegrates. We explore crossing over in more detail in Section The remaining steps of meiosis are relatively straightforward.
2. Metaphase I: After crossing over, each pair of homologous chromosomes (that is, the pairs of X’s lying one on top of the other) moves to the center of the cell, pulled by the spindle fibers to form the arrangement called the metaphase plate. (Keep in mind that each pair of homologous chromosomes includes the maternal and paternal versions of the chromosome— with crossed-over segments—and the replicated copy of each, making four strands in all.) The maternal and paternal sister chromatid pairs line up at the metaphase plate in a random fashion, called random assortment, so that the pairs of sister chromatids pulled toward each pole are a mix of maternal and paternal sister chromatids. As a result of random assortment, all the products of meiosis are genetically unique.
3. Anaphase I: This phase is the beginning of the first cell division that occurs during meiosis. In anaphase, the spindle fibers pull the homologues apart toward opposite poles of the cell. One of the homologues (consisting of two sister chromatids) goes to one pole, the other to the opposite pole.

21. Meiosis II: Separating the Sister Chromatids4
TELOPHASE I AND
CYTOKINESIS
• Sister chromatids arrive at the cell poles, and the nuclear membrane reassembles around them.
• The cell pinches into two daughter cells.
• Chromosomes may unwind slightly.
Daughter cell 1
Daughter cell 2
There is a brief interphase prior to prophase II. Chromosomes are not replicated again at this stage. 6 5 7 8
PROPHASE II
• Chromosomes in daughter cells condense.
• Spindle forms.
METAPHASE II
• Sister chromatid pairs line up at the center of the cell.
ANAPHASE II
• Sister chromatids are pulled apart by the spindle fibers toward opposite cell poles.
TELOPHASE II
AND CYTOKINESIS
• The nuclear membrane reassembles around the chromosomes.
• The two daughter cells pinch into four haploid daughter cells.
Sister chromatids
Daughter cell 3
Daughter cell 4
4. Telophase I and cytokinesis: After the pairs of chromatids arrive at the two poles of the cell, nuclear membranes re-form, then cytokinesis occurs: the cytoplasm divides, and the cell membrane pinches the cell into two daughter cells. Each daughter cell has a nucleus that contains the genetic material—two sister chromatids for each of the 23 chromosomes in humans.
There is a brief interphase after the first division of meiosis. In some organisms, the DNA molecules (now in the form of chromatid pairs) briefly uncoil and fade from view. In others, the second part of meiosis begins immediately. It is important to note that in the brief interphase before prophase II, there is no replication of any of the chromosomes.
5. Prophase II: The second division of meiosis begins with prophase II. The genetic material in each of the two daughter cells once again coils tightly, making the pairs of chromatids visible under the microscope. (Unlike prophase I, no crossing over occurs during prophase II.)
6. Metaphase II: In each of the two daughter cells, the sister chromatids (each appearing as an X) move to the center of the cell, pulled by thread-like structures in the cytoskeleton attached to the centromere, where the sister chromatids are held together. The congregation of all the genetic material in the center of each daughter cell is visible as a flat metaphase plate.
7. Anaphase II: During this phase, the fibers attached to the centromere begin pulling each chromatid in the sister chromatid pair toward opposite ends of each daughter cell.
8. Telophase II: Finally, the sister chromatids for all 23 chromosomes have been pulled to opposite poles. The cytoplasm then divides, the cell membrane pinches the cell into two new daughter cells, and the process comes to a close.
In humans, the outcome of one diploid cell undergoing meiosis is the creation of four haploid daughter cells, each with a set of 23 individual chromosomes. These chromosomes contain a combination of traits from the individual’s diploid set of chromosomes.

22. Which of the following is most like mitosis?
Which of the following is most like mitosis?
Meiosis I
Meiosis II
Ans: B

23. Male and female gametes are produced in slightly different ways.
Females produce the larger gamete. Males produce the smaller, more motile gamete.
Male and female gametes are produced in slightly different ways.
Genetic material is divided evenly, but nearly all of the cytoplasm goes to just one of the cells.
Diploid female cell
MEIOSIS I
(Telophase I)
Polar
body
REPLICATION
Functional female gamete
(haploid cell)
As in the first division, one cell gets nearly all of the cytoplasm. The net result is one large egg and smaller cells (called polar bodies) that degrade almost immediately.
(Telophase II)
When there are two sexes—as in nearly every sexually reproducing animal and plant species—the females are the sex that produces the larger gamete, and the males produce the smaller, more motile, gamete (Figure 8-23).
Meiosis works differently in males and females. The female gamete is larger than the male gamete because it has more cytoplasm. During the production of sperm, meiosis takes place just as described in Section 8-10, resulting in four evenly sized cells that become sperm. During egg production, the cell divides in telophase I, and the genetic material is evenly divided, but nearly all of the cytoplasm goes to one of the cells and almost none goes to the other. The smaller cell is called a polar body and degrades almost immediately in most animals (although it sometimes goes through a second division, after which, in most animals, both of those polar bodies degrade). Then, in the second meiotic division of the larger cell, there is again an unequal division of cytoplasm.
As in the first division, one of the new cells gets nearly all of the cytoplasm and the other gets almost none, forming another polar body. The net result of meiosis in the production of eggs is one large egg with lots of cytoplasm and two or three small polar bodies with very little cytoplasm that degrade and never function as gametes (Figure 8-24).
Ultimately, whether egg or sperm, each gamete ends up with just one copy of each chromosome. That way, the fertilized egg that results from the fusion of sperm and egg carries two complete sets of chromosomes, so the developing individual will be diploid. The extra cytoplasm carried by the egg contains a large supply of nutrients and other chemical resources to help with initial development of the organism following fertilization. 23

24. Sources of genetic variation
There are multiple reasons why offspring are genetically different from their parents and from one another.
ALLELES COME FROM TWO PARENTS
Each parent donates his or her own set of genetic material.
CROSSING OVER
Crossing over during meiosis produces a mixture of maternal and paternal genetic material on each chromatid.
REASSORTMENT OF HOMOLOGUES
The homologues and sister chromatids distributed to each daughter cell during meiosis are a random mix of maternal and paternal genetic material.
Sexual reproduction leads to offspring that are genetically different from one another and from either parent, through three different processes (Figure 8-26).
1. Combining alleles from two parents at fertilization. First and foremost, with sexual reproduction, a new individual comes from the fusion of gametes from two different individuals. Each of these parents comes with his or her own unique set of genetic material.
2. Crossing over during the production of gametes. Crossing over during prophase I of meiosis causes every chromosome in a gamete to carry a mixture of an individual’s maternal and paternal genetic material.
3. Shuffling and reassortment of homologues during meiosis. When homologues for each chromosome are pulled to opposite poles of the cell during the first division of meiosis (anaphase I), the maternal and paternal homologues are randomly separated. Many different combinations of maternal and paternal homologues could end up in each gamete. 24

25. Random alignment of homologous chromosomes (reassortment of homologs)

26. Random alignment of chromosomes contributes to the genetic diversity of gametes
Possibility 1
Possibility 2
Possibility 3
Possibility 4

27. Crossing over A. creates new alleles (genes).
B. creates new combinations of alleles.
C. creates new chromosomes.
D. recombines alleles from non-homologous chromosomes.
Ans; B

28. Meiosis Summary Occurs in _____ cells
Produces ___________ daughter cells
Gametes (sperm and egg) are made by meiosis
Increases genetic variability