MEIOSIS

Making Sex Cells

Get the Lingo Down!!!

• Somatic cell = body cell

Skin Nerve Blood

Human Chromosomes

• How many chromosomes?

• 46

• How many pairs of chromosomes?

• 23

Homologous Chromosomes

• Two sister chromatids joined at the

centromere From MOTHER plus• Two sister chromatids joined at the

centromere From FATHER =• All Four Chromatids carrying genes

controlling the same inherited characteristics

Loci• The place on a chromosome where a specific gene is located

• The plural is "loci," not "locuses."

• EXAMPLE: gene for hair color

• Gene for height

Autosomes

• Any chromosome

that is not a sex chromosome

• Humans - #1 through #22

Sex Chromosomes

• Female = XX

• Male = XY

• Chromosome #23

• Is this a male or female?

Is this a male or female?

Where do we get the pair of chromosomes?

• One set from our father

• One set from our mother

Human Cells

• Diploid number

• 2n

• 46

• somatic cells

• Haploid number

• n

• 23

• sex cells

Sex Cells

• “Gametes”

• EX: Egg and Sperm

Are Gametes n or 2n?

• N

• Single set of chromosomes

Are Gametes haploid or diploid?

-Haploid = 23

-n

23

23 chromosomes

sperm

egg

Fertilization

• Fusing of Egg and Sperm

• Woo-Hoo!• What’s the Fertilized egg

called?Zygote

Where are the sex cells made?

• Testes make sperm

• Ovaries make eggs

Only in reproductive organs

What is the process…

• In which sex cells are made?

• MEIOSIS

How many cells

formed?Haploid

or Diploid?

4 haploid 2 diploid

MEIOSIS INTERPHASE

• Yes, the chromosomes duplicate

• Still chromatin, loose

Meiosis I: Prophase I

90% of meiosis time

Meiosis I: Prophase I

Synapsis: formation of a tetrad (4 homologous chromosomes)

Let’s Review: All DNA and Protein

With Jonathan Edwards…whatever happened to him, anyway?

Let’s See a Close-up: Crossing Over

Chiasma

• Sites of crossing over

• Appear as X-shaped regions

• Site where two homologous chromosomes are attached to each other

Genetic Recombination

• The production of gene combinations different from those carried by the original chromosomes

• ADVANTAGE: increases diversity

What else happens Prophase I?

• What dissolves?

• Nuclear Membrane and Nucleolus

What else happens Prophase I?

• What also forms?

• spindles

Meiosis I: Metaphase I

Where are the tetrads lined up?Metaphase plate

Meiosis I: Anaphase I

Where are the sister chromatids going?

Apart to the poles

NOTE: the double strands

Meiosis I: Telophase I

Note the double strands at the poles

Cytokinesis begins

Interkinesis/Cytokinesis

• Some cells will go into a temporary chromatin-forming interphase (like in humans) called

interkinesis

Interkinesis/Cytokinesis

• Some will immediately go into Prophase II

• NOT ALL CELLS DO THIS AT THE SAME TIME…BUT ALL WILL EVENTUALLY GO INTO PROPHASE II

Interkinesis/Cytokinesis

• (Different than mitosis interphase, however)

• NO NEW DUPLICATION OF CHROMOSOMES…

• FURTHER DIVISION OF THE CHROMOSOMES WILL OCCUR

Meiosis II: Prophase II

How many cells are there?

2

What is dissolving?

Nuclear membrane

What’s forming?New spindles

Meiosis II: Metaphase II

How many cells?

2

Where are the chromatids lined up?

Metaphase (equatorial plate)

Meiosis II: Anaphase II

How many cells?

2

What are the single chromosomes doing?Moving to poles

Meiosis II:Telophase II

How many cells will result at the end?

4

Will the cells be haploid or diploid?Haploid

Name the Phase (Meiosis I)

Name the Phase (Meiosis II)

Cytokinesis will yield 4 haploid cells

Sperm MeiosisAll cells same size

Sperm maturing in a seminiferous tubule

Egg MeiosisOne large egg, 3 small polar bodies

• “oogenesis”

Follicle and egg

Contrast

• MITOSIS• 1 division(PMAT)• Results in 2

daughter cells

• MEIOSIS• 2 divisions(PMAT) I and

(PMAT) II• Results in 4

daughter cells

Contrast

• MITOSIS

• Diploid offspring

• Makes somatic cells

• MEIOSIS• Haploid

offspring

• Makes sex cells

Contrast

• MITOSIS• Makes

identical cells• Same

number of chromosomes

• MEIOSIS• Makes

similar cells• Half the

number of chromosomes

Independent Assortment

• This means that traits are transmitted to offspring independently of one another.

• Independent Assortment Animation

Independent Assortment

• In other words,

• RANDOM COMBINATION OF ALLELES APPEAR IN THE GAMETES

How many possible combinations are there?

•2n

•n = haploid number

•22 = ?

•8

How many combos?

• When one man (223) combines with one woman (223)?

• 246

How many combos from independent assortment:

• HUMANS

• 223 = ?

• About 8 million

• (8,388,608)

If only one crossover occurs, how many possible combinations?

• 423 combinations

• (70,368,744,000,000)

With fertilization and crossing-over, how many combos possible?

• (4 23) 2 = 4,951,760,200,000,000,000,000,000,000

Wait a second…

•What are alleles?

Alleles

• Different Versions of Genes• FOR EXAMPLE: B = brown eye color

b = blue eye color

• FOR EXAMPLE: T = Tall• t = short

More Allele Examples (fruit)

• Hi = Hard rind inhibitor

• Hr = Hard rind

• l = light fruit color

• Rd = dominant = Red skin color of fruit

• St = stripped fruit lengthwise

Rd = dominant = Red skin color of fruit

Dog Breeds: Lots of Alleles

Dog Color Alleles• in order of decreasing

dominance:

• Ay - agouti "red" (black, if any, appears in restricted areas only)Aw - "white-bellied" agouti

• A - solid agouti at - black & tana – non agouti (recessive black)

• Ea - dominant black

• Em - black mask

Why are the cells formed in meiosis different from the

parents?

• Random Fertilization (who ya have sex with)

• Independent Assortment

• Crossing Over

So an advantage of Meiosis…

• A tremendous amount of genetic variation is possible!

Let’s Review

• Three sources of genetic variability:

• 1. Crossing over during prophase I of meiosis

• 2. Independent orientation of chromosomes at metaphase I

• 3. Random fertilization

Karyotype

• An orderly display of magnified images of the individual’s chromosomes

• Shows the chromosomes as they appear in metaphase

What is a Normal Karyotype?

• We are supposed to have 46 total chromosomes in each cell (22 pairs of autosomes = 44, + 2 sex chromosomes).

Preparing a Karyotype

• 1. Use lymphocytes (white blood cells)

• 2. Chemical to stimulate division• 3. Chemical to stop in metaphase• (stop spindle fibers

forming)

Preparing a Karyotype (cont)

• 4. Centrifuged to remove white blood cells

• 5. Chromosomes spread out in hypotonic solution.

• 6. Drop on a slide.• 7.Sort by size and shape.

Resulting Photograph

• Sort by size and shape

• Largest to smallest

Amniocentesis

•Take fluid from amniotic fluid around the baby

Spectral Karyotype

Normal Karyotype

•WHY?

Down Syndrome Karyotype

•Trisomy 21

Down Syndrome

• Trisomy 21

• Folds over eyes

• Sluggish muscles

• Mental Problems

Down Syndrome

• The most common chromosome number abnormality

• Round face

• flattened nose bridge

• small, irregular teeth

Down Syndrome

• Short Stature

• heart defects

• susceptibility to respiratory infection , leukemia and Alzheimer’s

Does the mother’s age matter?

• As the age of the mother increases above 30, the frequency of Trisomy 21 also increases

Abnormal Sex Chromosomes• 47 XXY syndrome

• male

• testes small (sterile)

• breast enlargement

• feminine body contours

• also XXYY, XXXY, XXXXY

• Klinefelter’s

TURNER SYNDROME• XO (only one X)• short• often web of skin

between neck and shoulders

• sterile• poor breast

development

Turner Karyotype

Why are the cells formed in meiosis different from the

parents?

• Random Fertilization (who ya have sex with)

• Independent Assortment

• Crossing Over

How many possible combinations are there?

•2n

•n = haploid number

•22 = ?

•8

DELETION

Fragment of the chromosome is lost

Duplication

Fragment of one chromosome attaches to a homologous chromosome

TranslocationFragment reattaches in reverse direction (less likely to produce harm)

INVERSION

• The chromosome breaks in two places, a piece of the chromosome is removed and the chromosome pieces remaining rejoin.

• Less likely to remove harm

INVERSION 46,XY,inv(16)

• The left one is normal and the right one is inverted near the centromere.Inversions, by definition, do not involve loss or gain of chromosomal material.

45,XX,rob(13,14) A Robertsonian

translocation

(an end to end fusion of #13 and #14 There is no net gain or loss of genetic material in this person so they would have a normal phenotype.

Locus

Broccoli + Cauliflower = Broccoflower

Male Lion X Female Tiger

•Liger