1. CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES
"You're unique. Just like everyone else..."

6. Introduction
Heredity – transmission of traits from one generation to the next
Variation – offspring differ from parent
Genetics – study of heredity and variation
Living organisms are distinguished by their ability to reproduce their own kind.
Offspring resemble their parents more than they do less closely related individuals of the same species.
The transmission of traits from one generation to the next is called heredity or inheritance.
However, offspring differ somewhat from parents and siblings, demonstrating variation.
Genetics is the study of heredity and variation.

7. Karyotypes- ordered displays of an individual’s chromosomes

8. Offspring acquire genes from parents by inheriting chromosomes
23 pairs – Somatic cell
23–Germ cell (sperm)
23–Germ cell (egg)
Parents endow their offspring with coded information in the form of genes.
Your genome is derived from the thousands of genes that you inherited from your mother and your father.
Genes program specific traits that emerge as we develop from fertilized eggs into adults.
Your genome may include a gene for freckles, which you inherited from your mother.
Genes are segments of DNA.
Genetic information is transmitted as specific sequences of the four deoxyribonucleotides in DNA.
This is analogous to the symbolic information of letters in which words and sentences are translated into mental images.
Cells translate genetic “sentences” into freckles and other features with no resemblance to genes.
Most genes program cells to synthesize specific enzymes and other proteins that produce an organism’s inherited traits.
The transmission of hereditary traits has its molecular basis in the precise replication of DNA.
This produces copies of genes that can be passed from parents to offspring.
In plants and animals, sperm and ova (unfertilized eggs) transmit genes from one generation to the next.
After fertilization (fusion) of a sperm cell with an ovum, genes from both parents are present in the nucleus of the fertilized egg.
Almost all of the DNA in a eukaryotic cells is subdivided into chromosomes in the nucleus.
Tiny amounts of DNA are found in mitochondria and chloroplasts.
Every living species has a characteristic number of chromosomes.
Humans have 46 in almost all of their cells.
Each chromosome consists of a single DNA molecule in association with various proteins.
Each chromosome has hundreds or thousands of genes, each at a specific location, it’s locus.
As an organism develops from a zygote to a sexually mature adult, the zygote’s genes are passes on to all somatic cells by mitosis.
Gametes, which develop in the gonads, are not produced by mitosis.
If gametes were produced by mitosis, the fusion of gametes would produce offspring with four sets of chromosomes after one generation, eight after a second and so on.
Instead, gametes undergo the process of meiosis in which the chromosome number is halved.
Human sperm or ova have a haploid set of 23 different chromosomes, one from each homologous pair.
23 pairs – Somatic cell
23 pairs – Somatic cell

9. Sex, at what cost?????
Sex is production of gametes by meiosis (halving the chromosome number), and then syngamy (fusion of gametes)

10. asexual sexual reproduction
Sex, at what cost?????
asexual sexual reproduction
Forfeit half their genes - (in the trash!)
Pass genes on twice as fast!
No forfeiting of genes
BUT SEX HAS BEEN PRESERVED - WHY?
The Red Queen:
Many say that our culture is obsessed with sex, and uses sexual references to sell everything from fragrances to cars. And wouldn't life be dull without it? Yet from the viewpoint of evolutionary theory, sex isn't the most efficient way of reproducing. In fact, scientists have been asking for decades, why does sexual reproduction even exist? If the goal of life is spreading one's genes far and wide, asexually reproducing organisms seem to have the edge. They do not have to invest time and energy finding a mate. Asexual individuals can pass on their genes twice as fast as those reproducing sexually, because they pass on all their genes -- and only their genes. When reproduction is sexual, half of the genes handed on to the next generation are those of the other parent. The offspring may lose some of the genetic traits that made each parent successful. There are many hypotheses but, until recently, little hard evidence on the advantages of sex. They stem from the notion that the genetic variation created in sexual reproduction is worth the cost. And at least in some environments, that variation must give a competitive edge over asexual organisms that can spread their genes efficiently, but vary little from one generation to the next. One explanation is the increasingly popular Red Queen hypothesis, referring to the huffy chess piece in Lewis Carroll's Through the Looking Glass. In Looking Glass Land, the Queen tells Alice, "It takes all the running you can do, to keep in the same place." According to the Red Queen hypothesis, sexual reproduction persists because it enables many species to rapidly evolve new genetic defenses against parasites that attempt to live off them. Scientists from Rutgers University in New Jersey have tested this idea by observing different groups of small fish called topminnow in Mexico. Some populations of the topminnow reproduce sexually, while others reproduce asexually, so they provide the perfect opportunity to test these ideas. The topminnow is under constant attack by a parasite, a worm that causes something called black-spot disease. The researchers found that identical populations ("clones") of the asexually reproducing topminnows harbored many more black-spot worms than did those producing sexually, a finding that fit the Red Queen hypothesis: The sexual topminnows could devise new defenses faster by recombination than the asexually producing clones.

11. Like begets like, more or less: asexual rep. sexual rep.
More variation due to genetic recombination
Increases ‘fitness’
More number of organisms
Less variation
In asexual reproduction, a single individual passes along copies of all its genes to its offspring.
Single-celled eukaryotes reproduce asexually by mitotic cell division to produce two identical daughter cells.
Even some multicellular eukaryotes, like hydra, can reproduce by budding cells produced by mitosis.
Hydra (multicellular eukaryote)
reproduces by budding

12. Fertilization and meiosis alternate in sexual life cycles
A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism.
Fertilization and meiosis alternate in sexual life cycles
A cell with a double chromosome set is diploid.
In humans, each somatic cell (all cells other than sperm or ovum) has 46 chromosomes.
Each chromosome can be distinguished by its size, position of the centromere, and by pattern of staining with certain dyes.
A karyotype display of the 46 chromosomes shows 23 pairs of chromosomes, each pair with the same length, centromere position, and staining pattern.
These homologous chromosome pairs carry genes that control the same inherited characters.
By means of sexual intercourse, a haploid sperm reaches and fuses with a haploid ovum.
These cells fuse (syngamy) resulting in fertilization.
The fertilized egg (zygote) now has two haploid sets of chromosomes bearing genes from the maternal and paternal family lines.
The zygote and all cells with two sets of chromosomes are diploid cells.
For humans, the diploid number of chromosomes is 46 (2n = 46).
A cell with a single chromosome set is haploid.

13. The timing of meiosis and fertilization does vary among species.

14. Most fungi and some protists have a second type of life cycle.
The zygote is the only diploid phase.
After fusion of two gametes to form a zygote, the zygote undergoes meiosis to produce haploid cells.
These haploid cells undergo mitosis to develop into a haploid multicellular adult organism.
Some haploid cells develop into gametes by mitosis.

15. Plants and some algae have a third type of life cycle, alternation of generation.
This life cycle includes both haploid (gametophyte) and diploid (sporophyte) multicellular stages.
Meiosis by the sporophyte produces haploid spores that develop by mitosis into the gametophyte.
Gametes produced via mitosis by the gametophyte fuse to form the zygote which produces the sporophyte by mitosis.

17. Homologous chromosomes- are inherited one from ‘mom’ and one from ‘dad’B B
There are 23 pairs of homologous chromosomes

18. Homologous chromosome pairs carry the genes that control the same inherited trait. How many homologous chromosomes do you have?
In humans, each somatic cell (all cells other than sperm or ovum) has 46 chromosomes.
Each chromosome can be distinguished by its size, position of the centromere, and by pattern of staining with certain dyes.
A karyotype display of the 46 chromosomes shows 23 pairs of chromosomes, each pair with the same length, centromere position, and staining pattern.
These homologous chromosome pairs carry genes that control the same inherited characters.

19. Autosomes and Sex Chromosomes in body cells
22 pairs + XX =
22 pairs + XY =
An exception to the rule of homologous chromosomes is found in the sex chromosomes, the X and the Y.
The pattern of inheritance of these chromosomes determine an individual’s sex.
Human females have a homologous pair of X chromosomes (XX).
Human males have an X and a Y chromosome (XY).
Because only small parts of these have the same genes, most of their genes have no counterpart on the other chromosome.
The other 22 pairs are called autosomes.
The occurrence of homologous pairs of chromosomes is a consequence of sexual reproduction.
We inherit one chromosome of each homologous pair from each parent.
The 46 chromosomes in a somatic cell can be viewed as two sets of 23, a maternal set and a paternal set.
Sperm cells or ova (gametes) have only one set of chromosomes - 22 autosomes and an X or a Y.
Autosomes
Sex Chromosomes

20. Homologous chromosome pairs are separated during MEIOSIS phase 1.
A B C D E F G
Sister chromatids still attached
A B C D E F G
A B C D E F G
2 of each type; so 4 gametes total
But, the story does not end here….
Johnny
a b c d e f g
In humans, each somatic cell (all cells other than sperm or ovum) has 46 chromosomes.
Each chromosome can be distinguished by its size, position of the centromere, and by pattern of staining with certain dyes.
A karyotype display of the 46 chromosomes shows 23 pairs of chromosomes, each pair with the same length, centromere position, and staining pattern.
These homologous chromosome pairs carry genes that control the same inherited characters.
a b c d e f g
a b c d e f g
Sister chromatids will separate during 2nd phase of meiosis.
Johnny’s sperm

21. Homologous chromosome pairs line up and undergo crossing over during meiosis.
A B C D E F G
A B C D
a b c d e f g
Johnny makes many, many ‘recombinations’ of his parental genes in his sperm
a b c d e f g
E F G
Johnny
In humans, each somatic cell (all cells other than sperm or ovum) has 46 chromosomes.
Each chromosome can be distinguished by its size, position of the centromere, and by pattern of staining with certain dyes.
A karyotype display of the 46 chromosomes shows 23 pairs of chromosomes, each pair with the same length, centromere position, and staining pattern.
These homologous chromosome pairs carry genes that control the same inherited characters.
a b c d e f g
Each chromosome can cross over 2-3 times - lots of recombinations of genes!
Johnny’s sperm

22. Meiosis reduces chromosome number from diploid to haploid:
Two consecutive cell divisions, meiosis I and meiosis II, which results in four daughter cells.
Each daughter cell has HALF the number of chromosomes as the parent cell
Many steps of meiosis resemble steps in mitosis.
Both are preceded by the replication of chromosomes.
However, in meiosis, there are two consecutive cell divisions, meiosis I and meiosis II, which results in four daughter cells.
Each final daughter cell has only half as many chromosomes as the parent cell.
Meiosis II
Meiosis I

23. Meiosis reduces chromosome number by copying the chromosomes once, but dividing twice.
The first division, meiosis I, separates homologous chromosomes.
The second, meiosis II, separates sister chromatids.
Hence reduction division

27. Meiosis I : includes prophase I, metaphase I, anaphase I, and telophase I.
Interphase: (before Meiosis I) the chromosomes are replicated to form sister chromatids.
These are genetically identical and joined at the centromere.
Also, the single centrosome is replicated.

28. Prophase I: chromosomes condense and homologous chromosomes pair up to form tetrads.
Synapsis: special proteins -synaptonemal complex attach homologous chromosomes tightly together.
Chiasmata: At several sites the chromatids of homologous chromosomes are crossed (chiasmata) and segments of the chromosomes are traded.
A spindle forms from each centrosome and spindle fibers attached to kinetochores on the chromosomes begin to move the tetrads around.

30. Metaphase I, the tetrads/homologous chromosomes are arranged at the metaphase plate.
Microtubules from poles are attached to homologous chromosomes (at centromeres)

31. Anaphase I, the homologous chromosomes separate and are pulled toward opposite poles.

32. In telophase I, movement of homologous chromosomes continues until there is a haploid set at each pole.
Each chromosome consists of linked sister chromatids.
Cytokinesis occurs
In some species, nuclei may reform, but there is no further replication of chromosomes.

33. Meiosis II is very similar to mitosis.
Prophase II a spindle apparatus forms, attaches to kinetochores of each sister chromatids, and moves them around.
Spindle fibers from one pole attach to the kinetochore of one sister chromatid and those of the other pole to the other sister chromatid.

34. Metaphase II, the sister chromatids are arranged at the metaphase plate.
The kinetochores of sister chromatids face opposite poles.
Fig. 13.7

35. At anaphase II, the centomeres of sister chromatids separate and the now separate sisters travel toward opposite poles.
Fig. 13.7

36. In telophase II, separated sister chromatids arrive at opposite poles.
Nuclei form around the chromatids.
Cytokinesis separates the cytoplasm.
At the end of meiosis, there are four haploid daughter cells.

37. Mitosis produces two identical daughter cells, but meiosis produces 4 very different cells.

39. AT PUBERTY

40. During Spermatogenesis & Mitosis During Oogenesis
Oogenesis occurs by meiosis as well, but with some modifications.   First of all, it would not be good for females to make millions of ova, since a female cannot carry millions of fetuses!  Instead, no more than one ovum per month should be made.  Because of this, females do not need to have constant mitosis of their germ cells occurring-- it is OK that once a germ cell is used it is not replenished.   Secondly, the ova need to contain a lot of nutrients (to get the embryo through its first set of divisions), so these cells need to be big.  Finally, it turns out that meiosis is only completed when producing an egg that is fertilized.  Let's go through these ideas a bit more.
    In an ovary are millions of oocytes.  An oocyte is not the same thing as a germ cell.  The germ cells develop in the fetus and before birth begin meiosis, and then freeze right at the start of the first meiotic division.  These cells are stopped in prophase I of meiosis I, and are called primary oocytes.  They lie within the ovaries surrounded by other cells, the follicular cells.  The primary oocyte plus its follicular cells comprise the primary follicle.
    Only when signalled by hormones, one single primary oocyte (of all those in both ovaries) picks up meiosis where it left off.  It finishes its first meiotic division.  It does this by dividing its genetic material appropriately for meiosis I, but then undergoing cytokinesis differently.
    Cytokinesis is the physical separation of the daughter cells into two completely unattached cells.  It occurs via a cleavage furrow.  During mitosis and spermatogenesis, cytokinesis of the two daughter cells occurs right down the middle of the parent cell (symmetrical cytokinesis); but during oogenesis, cytokinesis occurs unevenly (asymmetrical cytokinesis).  This is shown for you in the following two animations:
During Spermatogenesis & Mitosis
During Oogenesis
    Why is it important that asymmetrical cytokinesis occurs in oogenesis?  The primary oocyte is a very large cell containing many nutrients that will be important for the early mitotic divisions of the zygote (as it grows into an embryo).  When the cell divides during meiosis, only one of the daughter cells can become the ovum-- we only want one ovum per month!  So, the one cell that is destined to become the ovum (out of 4 potential daughter cells) is the only one that should get the nutrients.  And it should get ALL of the nutrients!  We want to start our zygote off well!  This means that the other cells do not need much of the nutrient-rich cytoplasm.  These tiny other cells are called polar bodies, and they do not last long.  Polar bodies die pretty quickly.  Just the ovum survives.
    The large cell in the picture above is either the product of the first or second meiotic division that lives.  If it is the product of the first meiotic division (where the parent cell was the primary oocyte) it is called the secondary oocyte.  If it is the product of the second meiotic division (where the parent cell was the secondary oocyte) it is called the ovum.   The terms secondary oocyte and ovum are NOT interchangeable-- they are different.
    OK.  Now let's get through all of meiosis.   Females start off with millions of primary oocytes.  Every month, one of these primary oocytes is hormonally stimulated to begin to enlarge and to complete the first meiotic division.  (As it enlarges, the entire follicle enlarges-- this is shown in the ovulation web page... but the polar body is not included in the animation there).   The oocyte is now a secondary oocyte, and its polar body will degenerate.  The secondary oocyte is released from the ovary.
    Once the secondary oocyte exits the ovary, it begins to travel down the uterine (fallopian) tube.  If it does not encounter a spermatozoan, it never undergoes the second meiotic division.  The second meiotic division only occurs when fertilization happens.  That means that I, since I have never gotten pregnant, I have never had any of my oocytes finish meiosis!  However, if the secondary oocyte comes in contact with a spermatozoan and fertilization begins, the secondary oocyte undergoes its second meiotic division to form the ovum (and another polar body).  Now the ovum is ready to fuse with the spermatozoan.

41. Sexual life cycles produce genetic variation among offspring
Three mechanisms contribute to genetic variation:
independent assortment
crossing over
random fertilization
The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation during sexual reproduction.

42. Independent Assortment - 50% chance that gamete will get the maternal chromosome and a 50% chance it will get the paternal chromosome
Independent assortment of chromosomes contributes to genetic variability due to the random orientation of tetrads at the metaphase plate.
There is a fifty-fifty chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a fifty-fifty chance that it will receive the paternal chromosome.

43. Independent Assortment - genes on different chromosomes can be assorted or ‘mixed up’ to produce all combinations in gametes A a A a b B B b A A A A a a a a B b b B B
A -Normal
a - Albino
B - Normal
b - Bald B b b
Independent assortment of chromosomes contributes to genetic variability due to the random orientation of tetrads at the metaphase plate.
There is a fifty-fifty chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a fifty-fifty chance that it will receive the paternal chromosome. A A a A a a A a b b b B B B b B
Albino, bald
Normal, bald
Albino, Hair
Normal, Normal

44. Independent Assortment
Independent assortment - due to the random orientation of chromosomes at the metaphase plate.
50%
Chance of getting a chromosome from mom =
50%
Chance of getting a chromosome from dad =

45. The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number of the organism.
n = 23, there are 223 or about 8 million possible combinations of chromosomes from Independent assortment alone
Each homologous pair of chromosomes is positioned independently of the other pairs at metaphase I.
Therefore, the first meiotic division results in independent assortment of maternal and paternal chromosomes into daughter cells.
If n = 3, there are eight possible combinations.
For humans with n = 23, there are 223 or about 8 million possible combinations of chromosomes.

46. Also, any sperm can fuse with any egg - random fertilization.
In crossing over, homologous portions of two nonsister chromatids trade places.
For humans, this occurs two to three times per chromosome pair.
Also, any sperm can fuse with any egg - random fertilization.
Crossing over begins very early in prophase I as homologous chromosomes pair up gene by gene.
One sister chromatid may undergo different patterns of crossing over than its match.
Independent assortment of these nonidentical sister chromatids during meiosis II increases still more the number of genetic types of gametes that can result from meiosis.

47. So all in all variation resuls because -
A zygote produced by mating of a woman and man has a unique genetic identity.
An ovum is one of approximately 8 million possible chromosome combinations (actually 223).
The successful sperm represents one of 8 million different possibilities (actually 223).
The resulting zygote is composed of 1 in 70 trillion (223 x 223) possible combinations of chromosomes.
Crossing over adds even more variation to this

48. The three sources of genetic variability in a sexually reproducing organism are:
Independent assortment of homologous chromosomes during meiosis I and of nonidentical sister chromatids during meiosis II.
Crossing over between homologous chromosomes during prophase I.
Random fertilization of an ovum by a sperm.
All three mechanisms reshuffle the various genes carried by individual members of a population.
Mutations, still to be discussed, are what ultimately create a population’s diversity of genes.

49. Evolutionary adaptation depends on a population’s genetic variation
Darwin recognized the importance of genetic variation in evolution via natural selection.
A population evolves through the differential reproductive success of its variant members.
Those individuals best suited to the local environment leave the most offspring, transmitting their genes in the process.
This natural selection results in adaptation, the accumulation of favorable genetic variations.

50. As the environment changes or a population moves to a new environment, new genetic combinations that work best in the new conditions will produce more offspring and these genes will increase.
The formerly favored genes will decrease.
Sex and mutations are two sources of the continual generation of new genetic variability.
Gregor Mendel, a contemporary of Darwin, published a theory of inheritance that helps explain genetic variation.
However, this work was largely unknown for over 40 years until 1900.

51. Mitosis and meiosis have several key differences.
The chromosome number is reduced by half in meiosis, but not in mitosis.
Mitosis produces daughter cells that are genetically identical to the parent and to each other.
Meiosis produces cells that differ from the parent and each other.

52. Three events, unique to meiosis, occur during the first division cycle.
1. During prophase I, homologous chromosomes pair up in a process called synapsis.
A protein zipper, the synaptonemal complex, holds homologous chromosomes together tightly.
Later in prophase I, the joined homologous chromosomes are visible as a tetrad.
At X-shaped regions called chiasmata, sections of nonsister chromatids are exchanged.
Chiasmata is the physical manifestation of crossing over, a form of genetic rearrangement.

53. 2. At metaphase I homologous pairs of chromosomes, not individual chromosomes are aligned along the metaphase plate.
In humans, you would see 23 tetrads.
3. At anaphase I, it is homologous chromosomes, not sister chromatids, that separate and are carried to opposite poles of the cell.
Sister chromatids remain attached at the centromere until anaphase II.
The processes during the second meiotic division are virtually identical to those of mitosis.