Why you are

unique.01/09/12

Document Outline

I. Introduction to MeiosisII. Purpose of Meiosis #1: Decrease the Number of Chromosomes and Copies of Genes

III. Purpose of Meiosis #2: Create Gametes that Vary Genetically IV. Fertilization V. Summary of Genetic Variability due to Meiosis and Sexual Reproduction

VI. X InactivationVII. Reexamine the genes that are inactivated on the Barr Body:

Females vs. Males: Sex-linked PossibilitiesVIII. Sex-influenced and Sex-limited TraitsIX. Gender DevelopmentX. Additional or Missing Chromosomes

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Genetic Diversity I: Why we are uniqueMEIOSIS, SEXUAL REPRODUCTION AND GENDER

Primary Learning Objectives:

1. Understand terms and concepts crucial to comprehending the process of meiosis

2. Obtain a clear understanding of how the process of meiosis decreases the number of chromosomes from two sets of 23 (46) to one set of 23 and the number of copies of each gene to 1.

3. Understand the processes that create the genetic diversity possible in an individual’s gametes. Why are you unique?

4. Know the differences between somatic cell division (mitosis), and gamete cell division (meiosis) and understand the functions of both.

Part 1I. Introduction

A. Karyotype showing Replicated Homologous Chromosomes

1. Replicated Chromosomes are produced through DNA Replication. Each replicated chromosome contains two Sister Chromatids.

2. Sister Chromatids vs. Homologs

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Figure 1

B. Gametic cells vs. Somatic cells

C. Diploid vs. Haploid

1. Diploid: 2n cell, or a cell containing TWO sets of chromosome (one from mother, one from father, the maternal and paternal homologous pairs). Human somatic cells are diploid.

a. Homozygous vs. Heterozygous

- For any given gene, the condition of having two of the same allele for a particular gene

- For any given gene, the condition of having two different alleles for a particular gene

b. Human somatic cells remain diploid after cell division.

2. Haploid: A cell containing ONE set of chromosomes (maternal and paternal homologous pairs. Gametes (sex cells: eggs & sperm) are haploid

D. Functions of Meiosis:

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Part 2II. Purpose of Meiosis #1: Decrease the # of Chromosomes and the # of Copies of each gene in Gametes

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Initial cell in the ovaries (oogonia) or testes (spermatogonia):

1. Diploid or Haploid?

2. Are the Chromosomes Replicated or Unreplicated? What does that mean?

3. Total number of Chromosomes?

4. # of copies of each gene?

After Meiosis I:

1. Diploid or Haploid?

2. Are the Chromosomes Replicated or Unreplicated? What does that mean?

3. Total number of Chromosomes?

4. # of copies of each gene?

After Meiosis II- Final Gametes:

1. Diploid or Haploid?

2. Are the Chromosomes Replicated or Unreplicated? What does that mean?

3. Total number of Chromosomes?

4. Number of copies of each gene?

Fig. 2A.

B.

C.

Part 3

III. Purpose of Meiosis #2: Create Gametes that Vary Genetically

Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced from cells in the ovaries or testes. In meiosis, after only one round of DNA replication, there are two successive cellular divisions. Four stages can be described for each of the two divisions (Prophase I & II, Metaphase I & II, Anaphase I & II, Telophase I & II). The following figures describe a few details of meiosis and illustrate how it creates so many possible genetic variations in an individual’s gametes, that no one can create genetically identical eggs or sperm.

A. Meiosis I (Reduction Division) See Fig. 2 A-B.

1. Prophase I

a. Homologs PAIR together.

b. CROSSING OVER occurs (See Fig.4).

c. Nuclear membrane disappears and chromosomes condense.

d. Spindle fibers form from the centrioles and attach to the centromeres of the chromosomes.

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Fig. 3

Fig. 4Crossing Over

Recombination

2. Metaphase I (See Fig. 3)

a. Homologous pairs line up, together, on the metaphase plate

b. Remember, one member of a homologous pair is from the mother (maternal) and the other member is from the father (paternal)

c. It is completely random on which side of the metaphase plate the maternal and paternal member of each homologous pair align themselves

d. Each homologous pair is INDEPENDENT of all other homologous pairs as far as which side of the metaphase plate the maternal and paternal chromosomes go.

This random, independent alignment of maternal and paternal chromosomes on the metaphase plate during metaphase I of meiosis is called INDEPENDENT ASSORTMENT. (See Fig. 5.)

3. Anaphase I

The two members of each homologous pair are pulled to opposite poles of the cell by the spindle fibers.

The sister chromatids STAY TOGETHER!

4. Telophase I: (See Fig. 2B and Fig. 3)

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DNA uncoils, spindle disappears, nuclear envelopes reform & cytokinesis occurs.Fig. 5

Independent Assortment

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Part 4

B. Meiosis II (See Fig. 2B-C)

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Prophase II: Similar to Prophase I but no crossing over.

Metaphase II-Chromosomes line up on the metaphase plate.-Remember, there are no longer homologous pairs in the cell; only one of each.

Anaphase II-Centromeres separate so that sister chromatids are separated. Each sister chromatid is now a sister chromosome, unattached to each other.- The sister chromatids (now sister chromosomes) are pulled to opposite poles of the cell.

Telophase II Similar to Telophase I, but now with a total of 4 cells.

Fig. 6

IV. Fertilization

"Following fusion of the fertilizing sperm with the oocyte, the sperm head is incorporated into the egg cytoplasm. The nuclear envelope of the sperm disperses. In vertebrates, other sperm components, including mitochondria, are degraded rather than incorporated into the embryo.

Chromatin from both the sperm and egg are soon encapsulated in a nuclear membrane. The image to the right shows a one-cell rabbit embryo shortly after fertilization - this embryo was fertilized by two sperm, and would likely die within a few days. (http://arbl.cvmbs.colostate.edu/hbooks/pathphys/reprod/fert/fert.html)

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Part 5V. Summary: Genetic Variability Due To Meiosis and Sexual Reproduction

A. Crossing Over: An event during prophase I of meiosis when homologs exchange parts.

Four gametes are formed. Therefore, there are four Chromosome #1 possibilities.Chromosome #1 has approximately 263 million base pairs. Crossing over could occur between any of those base pairs. To calculate the number of genetic possibilities through crossing over on the Chromosome #1 pair (If there is only one cross over)…

263,000,000 x 2 + 2 = 526,000,002 (5.26 x 108)(# bp/chromosome #1) x (# of variable chromatids; i.e., 2 variable sister chromatids/cross-over) + 2 nonrecombining sister chromatids.

If we calculate all of the chromosomes: Chromosome # Length (bp in Millions) # of genetic possibilities due to crossing over

(once on that pair)2 255 255,000,000 x 2 + 2 = 510,000,0023 214 214,000,000 x 2 + 2 = 428,000,0024 203 203,000,000 x 2 + 2 = 406,000,0025 194 194,000,000 x 2 + 2 = 388,000,0026 183 183,000,000 x 2 + 2 = 366,000,0027 171 171,000,000 x 2 + 2 = 342,000,0028 155 155,000,000 x 2 + 2 = 310,000,0029 145 145,000,000 x 2 + 2 = 290,000,00210 144 144,000,000 x 2 + 2 = 288,000,00211 144 144,000,000 x 2 + 2 = 288,000,00212 143 143,000,000 x 2 + 2 = 286,000,00213 114 114,000,000 x 2 + 2 = 228,000,00214 109 109,000,000 x 2 + 2 = 218,000,00215 106 106,000,000 x 2 + 2 = 212,000,00216 90 98,000,000 x 2 + 2 = 196,000,00217 92 92,000,000 x 2 + 2 = 184,000,00218 85 85,000,000 x 2 + 2 = 170,000,00219 67 67,000,000 x 2 + 2 = 134,000,00220 72 72,000,000 x 2 + 2 = 144,000,00221 50 50,000,000 x 2 + 2 = 100,000,00222 56 56,000,000 x 2 + 2 = 112,000,002X and Y Don’t use. Cross-overs neglible; only a small area involved

Multiply the results for all pairs to calculate the combined genetic possibilities if each pair crossed over once per meiosis.

= 6.7408 x 10184

*

* Current world population http://www.census.gov/cgi-bin/ipc/popclockw

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B. Independent Assortment: A term used to describe the arrangement of maternal and paternal homologs during metaphase I of meiosis. Each homologous pair lines up on the metaphase plate, with the maternal chromosome on one side and the paternal chromosome on the other side. The maternal and paternal homologs separate during anaphase I and will eventually end up in different gametes. The side maternal and paternal chromosomes are on is random and will not influence other homologous pairs. Therefore, gametes are a mixture of the maternal and paternal chromosomes of the parents.

C. Fusion of egg and sperm during fertilization: Combines a haploid genome of the egg with a haploid genome of the sperm creating a diploid zygote from two individuals with their own genetic ancestry, crossing over, independent assortment, generation after generation.

D. Spontaneous mutation during DNA replication before meiosis begins.

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VI. X inactivation

A. Inactivation occurs at the 16-cell stage of development.

B. It is random and permanent in each cell.

C. Barr body

D. Turner’s syndrome: Turner’s syndrome led researchers to additional information about genes on the Barr body.

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Fig. 7

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VII. Reexamine the genes that are inactivated on the Barr Body (these are the genes that are not found on the Y chromosome: Females vs. Males: Sex-linked Possibilities

FemaleGenotypes

Inactivation Example: Systemic Disease-X-linked SCID

XAXA

XaXa

XAXa

Male Genotypes

No inactivation Example: Systemic Disease-X-linked SCID

XAY

XaY

Example: Color-blindnessMost people can see the primary colors of a traffic light because of a gene that codes for color vision. But an error in that gene's DNA code causes some viewers to see red and green as shades of yellowish brown. This is an X-linked trait. Roughly 10 million men and 550,000 women in the United States are color blind.

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So how many active copies do a female or male have of these genes?

VIII. Sex-influenced and Sex- limited Traits

A. Sex-influenced Traits: Either dominant in one sex and recessive in the other AND/ORaffects one sex earlier in life or more severely.

-Often a result of hormonal influences.

Male Pattern Baldness in 4 Generations of The Adams Family

B. Sex-limited Traits

Only affects one sex because the other gender does not have those parts of functions.

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IX. Gender developmentConception

7th week

After 7th week

Early childhood (earlier? later?)

XX males and XY females. How?

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X. Additional or Missing Chromosomes

A. Aneuploidy: extra or missing chromosomes

-Trisomy vs. Monosomy

-Aneuploids that may survive birth

1. Down syndrome (trisomy 21)

-Most common aneuploidy.

-Symptoms vary greatly: profoundly retarded to functional; kidney, heart, and digestive system defects common; increased risk of leukemia, increased risk of Alzheimer disease.

2. Trisomy 13 (Patau syndrome)

-Few survive to birth; of those that do, most die in the first months of life.

-Defects in all organs, mental retardation, extra fingers and toes.

3. Trisomy 18 (Edward syndrome)

-Few survive to birth; of those that do, most die in the first months of life.

-Defects in all organs, mental retardation.

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Autosomes

4. Turner syndrome (X0)

-99% die before birth.

-Those that live may have subnormal intelligence, underdeveloped secondary sexual characteristics, usually must take hormone supplements, usually sterile.

5. Klinefelter syndrome (XXY)

-Underdeveloped secondary sexual characteristics (often helped with testosterone injections). May develop female secondary sexual characteristics.

-Usually sterile.

-Long arms and legs, large hands and feet.

-May have subnormal intelligence.

-As the number of X's increase, the symptoms become more severe.

6. Triplo-X (XXX)

-Only symptoms: usually very tall, irregular menstration, may have subnormal intelligence.

-May produce eggs with 2 X's.

7. Jacob's syndrome (XYY)

-Tall, acne, may have speech/reading problems.

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Sex chromosomes

B. Nondisjunction: Failure of chromosomes to separate properly during meiosis.

Meiosis I Nondisjunction: Homologous pairs do not separate properly.

Fig. 8

Aneuploid NormalFertilization… Egg Sperm Zygote

With Egg #1 X

Y

With Egg #2: X

Y

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1 2

NormalFertilization… Egg Sperm Zygote

With Gamete #1: X

With Gamete #2: X

With Gamete #3: X

With Gamete #4: X

With Gamete #5: X

With Gamete #6 X

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Gametes3

4

21 3

4 5

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Meiosis II Nondisjunction: Sister chromatids do not separate into different cells properly.Fig. 9

Are any of the zygotes normal?

Post-Test Questions

1. Some definitions are given below. Write the appropriate word for each of the definitions below.a. Random alignment of maternal and paternal homologous chromosomes on the metaphase plate during meiosis I. This creates a huge amount of genetic variation in eggs and sperm.

b. The process of decreasing the number of chromosomes by half to produce gametes; this process also is responsible for much of the great genetic variation between individuals.

c. Process that occurs during Prophase I of meiosis where homologous chromosomes pair together, come in direct contact, and exchange parts. This produces variation (different genetic combinations) in the offspring.

d. Exact copies of one chromosome produced during DNA replication are called… While still attached at the centromere, these copies make up one chromosome; they are separated during anaphase II of meiosis, and only then each become their own chromosome.

e. A cell that contains only one member of each homologous pair; therefore, these cells only have 23 chromosomes.

f. These chromosomes come in pairs, one from the mother (maternal) and the other from the father (paternal). They are the same size, have the same genes for the same traits at the same loci, and have centromeres in the same locations. Each member of the pair undergoes DNA replication to form sister chromatids.

g. A cell (all somatic cells) with both members of all homologous pairs (2 sets of chromosomes); therefore, these cells have 46 chromosomes.

h. A condition of having two of the same alleles for a particular gene.

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i. A condition of having two different alleles for a particular gene. 2. Another name for homologous chromosomes is .

3. State whether the following are diploid or haploid.

1) Cells immediately (in the ovaries or testes) before meiosis:

2) Cells (in the ovaries or testes) after meiosis I:

3) Cells (in the ovaries or testes) after meiosis II:

4) The first cell after fertilization:

4. State the number of copies of each gene that are contained in the following.

1) Cells before meiosis:

2) Cells after meiosis I:

3) Cells after meiosis II:

4) The first cell after fertilization:

5. State the total number of chromosomes in the cells.

1) Cells before meiosis:

2) Cells after meiosis I:

3) Cells after meiosis II (gametes):

4) The first cell after fertilization:

6. What is the difference between maternal and paternal chromosomes in one homologous pair? (Hint: This is too easy.)

maternal:

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paternal:

7. Describe the difference between the separation of chromosomes during anaphase I and anaphase II.

anaphase I:

anaphase II:

8. In humans, how many possible chromosome combinations, due to independent assortment only, are possible in each gamete? What is the formula for calculating this number?

9. List one event in each of the stages of meiosis.Meiosis I

Prophase I:

Metaphase I:

Anaphase I:

Telophase I:

Meiosis II

Prophase II:

Metaphase II:

Anaphase II:

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Telophase II:

10. Write the stage of meiosis where the following takes place.

a. Homologous chromosomes are pulled to opposite poles of the cell

b. Sister chromatids are pulled to opposite poles of the cell

c. Homologous pairs line up on the metaphase plate together, the maternal member on one side and the paternal member on the other side

d. Crossing over

e. Cells undergo cytokinesis resulting in 4 daughter cells

f. Nuclear membrane disappears in one cell

h. Chromosomes line up on the metaphase plate in single file

i. Homologous chromosomes pair together

j. Cells containing one copy of each homologous chromosome and one copy of each gene are produced

k. Cells containing one copy of each homologous chromosome and two copies of each gene are

produced

l. Spindle fibers attach to the centromeres of each chromosome

m. DNA replication

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11. List the four processes that produce genetic variation.

12. Clearly describe in complete sentences what happens during crossing over. At what stage of meiosis does crossing over occur? DRAW representative chromosomes undergoing crossing over (you may want to use colored pencils). How does crossing over contribute to genetic variability?

13. Describe fertilization and how it restores the human genetic complement (number of chromosomes and ploidy-the number of maternal and paternal chromosomes in each pair).

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14. Clearly describe INDEPENDENT ASSORTMENT during metaphase I of meiosis. How does this increase genetic variability in gametes (and therefore offspring)?

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15. Are the chromosomes in the karyotype below replicated or unreplicated? How do you know?

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Write the correct terms for the descriptions below.

16. Traits that only affect one sex because the other sex doesn't have those parts or functions.

17. Traits that are either dominant in one sex and recessive in the other AND/OR affect one sex earlier in life of more severely.

18. What sex an individual "feels" like.

19. Sex-determining region of the Y chromosome; if present, the embryo will form testes and will become a male.

20. The inactivated X chromosome in females.

21. "Feeling" like the opposite sex.

22. In the 16-cell stage XX embryos, the random ‘turning off’ of genes in one of the X chromosomes.

23. Trait controlled by a gene on the X chromosome.

24. Area of the 7-week embryo that eventually will become the mature ovaries or testes.

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25. What syndrome led researchers to believe that at least some of the genes on the inactivated X chromosome are necessary for proper functioning in females? Why?

26. One of the X chromosomes in each female cell is inactivated. Males only have one X chromosome. Theoretically, males and females both only have one functioning copy of all of the genes on the X chromosome. Why, then, are males disproportionately affected by recessive X chromosome genetic disorders?

27. List one sex-influenced trait and one sex-limited trait.

28. List factors responsible for the development of testes or ovaries in males and females.

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29. Draw the chromosomes in the diagram below depicting nondisjunction during meiosis I.

30. Draw the chromosomes in the diagram below depicting nondisjunction during meiosis II.

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31. Observe the karyotype.

Female or male?

Does this individual have a normal chromosome complement? If not, explain. If an aneuploidy is present, what is it called?

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32. Observe the karyotype.

Female or male?

Does this individual have a normal chromosome complement? If not, explain. If an aneuploidy is present, what is it called?

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33. Observe the karyotype.

Female or male?

Does this individual have a normal chromosome complement? If not, explain. If an aneuploidy is present, what is it called?

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http://www.biology.arizona.edu/human_bio/activities/karyotyping/karyotyping.html

34. Go to the above website.

Read the introduction and complete the patient karyotypes for patient A, B and C. Use the back arrow to return to the activity if you place the wrong chromosome. Remember to observe banding patterns along with size when placing the chromosomes.

When you have successfully completed the karyotypes, answer the following:

I. Patient A

What notation would you use to characterize Patient A's karyotype?

Diagnosis:

II. Patient B

What notation would you use to characterize Patient B's karyotype?

Diagnosis:

III. Patient C

What notation would you use to characterize Patient C's karyotype?

Diagnosis:

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