Major Objectives:

1. Understand heredity and genes and basic information on DNA

2. Differences between asexual and sexual life cycles

3. Understand role of meiosis in sexual

life cycles

4. Know the major steps of meiosis

Major Objectives:

5. Know where the major sources of genetic variability come from during meiosis

6. Know when the different phases occur in humans (females and males)

7. Know what can go wrong in meiosis

8. Know major differences between mitosis and meiosis

What is heredity?

The transmission of traits from one generation to the next

How are these traits passed on?

By offspring acquiring genes from parents by inheriting chromosomes

(SEXUAL REPRODUCTION)

Karyotype

Ordered display of an individual's chromosomes Homologous Chromosome

ASEXUAL REPRODUCTION

* A single individual passes along copies of all its genes to offspring via binary fission in prokaryotes

Rod-Shaped Bacterium, E. coli, dividing by binary fission (TEM x92,750). This image is copyright Dennis Kunkel

MITOSIS REVIEW QUESTIONS

1. What type of cells does mitosis occur in?2. How many times does DNA replicate in mitosis?3. How many cellular division(s) occur in mitosis?4. How many daughter cells are formed by mitosis?5. What is the chromosome number in daughter cells formed by mitosis from diploid parent cells?6. In mitosis, are daughter cells identical to or different from parent cells?

SEXUAL REPRODUCTION

Reproduction where two parents give rise to offspring that have unique combinations of genes inherited from the parental gametes

Gametes need 1) motility to be able to meet and unite and 2) food to nourish the developing embryo

Why Meiosis?

Reduces genetic information that is passed on to offspring; fertilization of sperm and ovum recombine genetic information to form new organism (zygote)=GENETIC VARIABILITY

New organism

Gametes

Original organisms

Haploid (n)

Diploid (2n)

Diploid (2n)

Note: The actual process of meiosis is more complicated than this drawing—we will get to that in a bit

Why Meiosis?

New organism

Gametes

Original organisms

Haploid (n)

Diploid (2n)

Diploid (2n)

Genetics Problem—Color of leaves:

G=chlorophyll; g=no chlorophyll

gg GG

g G Gg

Gg

Where does meiosis occur?

Only in Reproductive Organs:

Males-testes-sperm cell(s)

Females-ovaries-egg cell (ovum; ova)

Human gametes only have one set of chromosomes- 22 autosomes and an X (female) or an X or Y (male) sex chromosome= HAPLOID

REVIEW--HAPLOID/DIPLOID

1.  If an organism has 2N=24 chromosomes is this the haploid or diploid number of chromosomes?

2.  How many chromosomes will be found in a gamete produced by this organism?

3.  How many chromosomes will be found in a zygote produced by this organism?

4.  If the zygote of an organism has 12 chromosomes, N= __ chromosomes.

Life Cycle-Generation to generation sequence of stages on the reproductive history of an organism—from conception to production of own offspring

Gametophyte

Sporotophyte

SPOROPHYTE: DOMINANT: In ferns, rhizome with roots & leaves (fronds); sporangia in sori on the underside of fronds

Pteridophytes

GAMETOPHYTE: In ferns, INDEPENDENT, heart-shaped prothallus with antheridia and archegonia

Meiosis-Replicates once, divides twice (reduces amount of genetic material)

Interphase is an important stage preceding meiosis. Without this stage meiosis would not occur.

During this stage, each individual chromatid replicates, similar to mitosis.

Prophase I is one of the most important stages of meiosis.

1) DNA of the chromosomes begin to twist and condense, making the DNA visible to the microscope.

2) Each chromosome actively seeks out its homologous pair (which also has a sister chromatid).

Cells in Prophase I

At Metaphase I, the homologous pairs and their sister chromatids prepare for separation.

During metaphase, the chromosomes are lined by the spindle fibers at the metaphase plate.

Cell in Metaphase I

Anaphase I pulls apart the tetrad, separating each homologous chromosome.

It is by random chance that a certain chromosome is pulled to a certain pole.

                    

                            

Cell in Anaphase I

In Telophase I two nuclear envelopes begin to surround the separate chromosomes.

Interkinesis follows, which is a resting period from Telophase I to Prophase II. This differs from mitosis because DNA replication does not occur.

Cell in Telophase I

MAIN EVENTS OF MEIOSIS I

The number of chromosomes has been cut in half, HOMOLOGOUS CHROMOSOMES SEPARATE. End result is two haploid daughter cells. 

During Prophase II, each dyad (1/2 a tetrad) is composed of a pair of sister chromatids connected by a centromere.

The centrosomes (replicated during Telophase I) produce the spindle fibers and start to move toward the poles of the cell.

Cells in Prophase II

Metaphase II is similar to Metaphase I in that the dyads (sister chromatids) are lined up at a metaphase plate by the spindle fibers.

Cells in Metaphase II

Anaphase II separates the dyads into individual chromatids. Each sister chromatid ends up on one side of the cell.

Cells in Anaphase II

In Telophase II, the nuclear envelopes forms around each set of DNA and the cytoplasm divides.

Four haploid cells have formed from one diploid cell. The chromosomal content is 1/2 the chromosomal content of a diploid cell.

Cells in Telophase II

MAIN EVENTS OF MEIOSIS II

Separate the sister chromatids of the replicated chromosomes.

End result is four haploid daughter cells. GAMETES in animals or SPORES in plants.

MEIOSIS REVIEW QUESTIONS

1. What type of cells does meiosis occur in?2. How many times does DNA replicate in meiosis?3. How many cellular division(s) occur in meiosis?4. How many daughter cells are formed by meiosis?5. What is the chromosome number in daughter cells formed by meiosis from diploid parent cells?6. In meiosis, are daughter cells identical to or different from parent cells?

MECHANISMS FOR GENETIC VARIATION

1. Independent assortment.

2. Crossing over.

3. Random fertilization.

Independent assortment

Independent assortment

*Variation due to the random orientation of homologous chromosomes during meiosis I and nonidentical sister chromatids in meiosis II.# of possible combinations when chromosomes are independently assorted is 2n, where n=haploid #.

HOW MANY POSSIBLE COMBINATIONS FOR HUMANS??

2n,2(23 power)= ~8 MILLION

Crossing over

Occurs in Prophase I; homologous portions of two nonsister chromatids trade spaces.

Crossing over*Occurs in humans, 2-3 times per chromosome pair.

*Crossing over does not create new genes.  It simply recombines existing genes in new ways.

Random fertilization

HOW MANY POSSIBLE COMBINATIONS FOR HUMANS??

2n,2(23 power)= ~8 MILLION in ovum2n,2(23 power)= ~8 MILLION in spermZygote composed of 1 in 70

trillion possible combinations

Crossing over and random fertilization add more diversity

Gametogenesis-process of sperm or ovum production via meiosisWHAT:

WHERE:

WHEN:

END RESULT:

MALESSpermatogenesis-production of sperm

in testes, 100,000,000/hr/teste

puberty until death

4 haploid spermatids that will differentiate into 4 mature sperm cells

Spermatogenesis

Entire process takes ~74 days

FEMALES

WHAT:

WHERE:

WHEN:

Oogenesis-production of ova (eggs)

in ovaries

Initial steps occur PRIOR to birth. By week 25, cells develop into primary oocytes and begin meiosis until late prophase I and then STOP (~700,000-2 million).

FEMALES

WHEN: Oocytes recommence at puberty (~20-40,000 remain). Oocytes have been in prophase I for 12-40 years. During female lifetime ~480 may complete meiosis I (one/month). Meiosis II will NOT be completed unless fertilized by a sperm (cell cycle restarted by sperm).

END RESULT:MEIOSIS I: large secondary oocyte and small polar body; MEIOSIS II: ovum and three polar bodies

ERRORS IN MEIOSIS1. Non-disjunction-When

homologous chromosomes fail to separate or disjoin in Meiosis I. 25-50% of miscarriages result from zygotes with abnormal chromosome numbers. 1 in 118 newborns have chromosomal abnormalities.Aneuploidy-If effect is limited to one or a few chromosomes;

one more=TRISOMY,

one less=MONOSOMY

ChromosomalAbnormality

Number amongSpontaneous

Abortions

Number among

Live BirthsTrisomy for    1 0 02 159 03 53 04 95 05 0 0

6 - 12 561 013 128 1714 275 015 318 016 1,229 017 10 018 223 13

19 - 20 52 021 350 113

Paute's Syndrome

Edward's Syndrome

Down Syndrome

TRISOMY 21-Down Syndrome

*Occurs once in every 650 to 1,000 live births

*Risk increases with maternal age (above 35 for mother): age 30:1 in 1,000 and at age 40:9 in 1,000

*Possible cause due to "suspended" division that female gametes undergo

Egg is in center of pictureMany cumulus cells from the ovary are seen around the egg. This is a low quality, oocyte from a woman 41 years oldEgg is irregularly shaped and dark.

A "good" egg from a 32 year old woman.

Photos from Advanced Fertility Clinic in Chicago

Sex Chromosomes

   

XYY 4 46

XXY 4 44

X0 1,350 8

XXX 21 44

Translocations 239 216

Polyploidy    

Triploidy 1,275 0

Tetraploidy 450 0

Klinefelter Syndrome

Turner Syndrome

2. Alteration of Chromosome Structure

Four Types of Change:1) Deletion2) Duplication3) Inversion4) Translocation

ERRORS IN MEIOSIS

REVIEW CELL DIVISION