AP BiologyModified from: Kim Foglia,

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Chapter 13.

Meiosis &Sexual Reproduction

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How about the rest of us? What if a complex multicellular organism

(like us) wants to reproduce? joining of egg + sperm

Do we make egg & sperm by mitosis?

46 46+ 92

egg sperm zygote

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Human female karyotype

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Human male karyotype

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How do we make sperm & eggs? reduce 46 chromosomes 23 chromosomes

half the number of chromosomes

23

2346

egg

sperm

46

meiosis 46

fertilization

23

23

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Meiosis: production of gametes

Alternating processes,alternating stages chromosome number

must be reduced diploid haploid 2n n

humans: 46 23 meiosis reduces

chromosome number fertilization restores

chromosome number haploid diploid n 2n

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Homologous chromosomes Paired chromosomes

both chromosomes of a pair carry genes control same inherited characters homologous = same information

diploid2n

homologouschromosomes

double strandedhomologous chromosomes

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2nd division of meiosis separates sister chromatids

1st division of meiosis separates homologous pairs

Double divisionof meiosis

DNA replication

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Steps of meiosis Meiosis 1

interphase prophase 1 metaphase 1 anaphase 1 telophase 1

Meiosis 2 prophase 2 metaphase 2 anaphase 2 telophase 2

2nd division of meiosis separates sister chromatids

(1n 1n)

* just like mitosis *

1st division of meiosis separates homologous pairs

(2n 1n)

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Crossing over During Prophase 1

homologous pairs swappieces of chromosome

sister chromatids intertwine crossing over

tetrad

synapsis

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Crossing over 3 steps

cross over breakage of DNA re-fusing of DNA

New combinations of traits

What are theadvantages of

sexual reproduction?

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Genetic variation Meiosis & crossing over introduce

great genetic variation to population drives evolution

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The value of meiosis Meiosis introduces genetic variation

gametes of offspring do not have same genes as gametes from parents

genetic recombination random assortment in humans produces 223

(8,388,608) different combinations

from Mom from Dadnew gametes

made by offspring

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And more variation… Crossing over

creates completely new combinations of traits in next generation

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Random fertilization Any 2 parents will produce a zygote

with over 70 trillion (223 x 223) diploid combinations

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Sources of genetic variability Genetic variability in sexual reproduction

independent assortment homologous chromosomes in Meiosis 1

crossing over between homologous chromosomes in

prophase 1 random fertilization

random ovum fertilized by a random sperm

metaphase1

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Sexual reproduction creates variabilitySexual reproduction allows us to maintain both genetic similarity & differences.

Martin & Charlie Sheen, Emilio Estevez

Michael & KirkDouglas

Baldwin brothers

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Cell A has 4 chromosomes and Cell D has 2 chromosomes. What process created Cell D?

1. Mitosis

2. Asexual Reproduction

3. Fusion

4. Meiosis Cell A Cel

l B Cell C Cel

l D1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 2410

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Cell A is considered _______ while Cell D is considered ________.

Cell A

Cell B Cel

l C Cell D

1 2 3 4

0% 0%0%0%

1. Haploid; Diploid

2. Tetrad; Diploid

3. Diploid; Haploid

4. Homologous; Haploid

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 24

10

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Cell A is considered _______ while Cell D is considered ________.

A. somatic cell; gamete

B. gamete; somatic cell

C. stem cell; somatic cell

D. stem cell; gamete

somatic c

ell; gamete

gamete; s

omatic cell

stem ce

ll; so

matic cell

stem ce

ll; gamete

0% 0%0%0%

30

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How can we best describe the diagram below?

1 2 3 4

0% 0%0%0%1. Two sister chromatids2. Homologous chromosomes3. One replicated chromosome4. Haploid chromsomes

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 2410

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What process is best demonstrated by the diagram below?

1 2 3 4

0% 0%0%0%

1. Anaphase I2. Segregation of alleles3. Crossing over 4. Independent assortment

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 2410

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Which of the following leads to genetic variation during meiosis?

1 2 3 4 5

0% 0% 0%0%0%

A.Crossing Over

B.DNA replication

C.Independent assortment

D.Mitotic spindle formation1. All of the above (A, B, C, D)2. A & B3. A & D4. A, B, & D5. A & C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 2410

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Mitosis vs. Meiosis Mitosis

1 division daughter cells

genetically identical to parent cell

produces 2 cells 2n 2n produces cells for

growth & repair no crossing over

Meiosis 2 divisions daughter cells

genetically different from parent

produces 4 cells 2n 1n produces gametes

crossing over

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Mitosis vs. Meiosis

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Changes in Chromosomes Number Euploidy = correct # of chromosomes Aneuploidy = a change in

chromosomes number due to non-disjunction during meiosis

Monosomy- only 1 copy of an individual chromosome

Trisomy- 3 copies of an individual chromosome

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Non-disjuction Primary nondisjuction = Meiosis I

Secondary nondisjuction = Meiosis II

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Trisomy 21 : Down Syndrome Delayed mental and social skills Decreased muscle tone at birth Asymmetrical or odd-shaped skull Small skull Small mouth with protruding tongue Broad short hands Increased risk of developing Leukemia and Alzheimer’s later in life

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Trisomy 18 : Edward’s Syndrome Most children die in the first year of life, some have lived 10

years Growth deficiency Feeding difficulties Breathing difficulties Developmental delays Mental Retardation Overlapped, flexed fingers Webbing of the second and third toes Clubfeet Structural heart defects at birth

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Trisomy 13 : Patau Syndrome Mental retardation, severe Seizures Small head Scalp defects Cleft lip and/or palate Eyes close set (hypotelorism) –may fuse Extra digits (polydactyl) Hernias Undescended testicle Children die in the first year of life

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Karyotype A visual display of the chromosomes arranged by

size, shape, and banding pattern Used to identify aneuploid conditions

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Procedure: Amniocentesis and Karyotyping

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Karyotyping

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Changes in Sex Chromosome # Turners Syndrome (XO) – missing Barr Body Kleinfelter’s Syndrome (XXY) Swyer Syndrome (XY female) La Chapelle Syndrome (XX male) Poly-X Females (XXX, XXXX) Jacob’s Syndrome (XYY males)

-SRY gene (located on short arm of Y chromosome)

-hormone= testis-determining factor

-Barr Body – Inactive X chromosome (XX)

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Chromosomal Mutations

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Deletion Syndromes Williams Syndrome (deletion of a piece

of chromosome 7) Cri du chat (cat’s cry) (deletion of a

piece of chromosome 5)

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Translocation Syndromes Alagille syndrome – Chromosomes 2

and 20 exchange segments Cancers

Chronic myelogenous leukemia (2 and 9)

Burkitt lymphoma (8 and 14)

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Any Questions??

What are theDISadvantages of

sexual reproduction?