Ch. 7 Meiosis & Sexual Reproduction

What if…

What if… A HUMAN sperm & egg each had 46 chromosomes, like other human body cells, how many chromosomes would a zygote have?

Why might an increase in the number of chromosomes a human cell has cause problems?

WHITEBOARDS

Draw the gametes for males and females

Inside each sex cell, give the number of chromosomes

Below your drawing, write whether the sex cells are haploid or diploid

If an organism’s sex cells have 12 chromosomes, how chromosomes will a body cell have?

Meiosis- Formation of Haploid Cells

Meiosis 1. DNA in the original cell replicated

2. Meiosis halves the number of chromosomes when forming gametes or spores

Still occurs in the nucleus!

Draw it out!

Meiosis Look at your books (p. 144) and your notes for pictures of Meiosis I & II

Draw your assigned stage on the board – be ready to explain it!

Meiosis I

Meiosis

Meiosis II

Meiosis

Hold them up!

Meiosis Hold up the number of cells produced at the end of Meiosis I

Hold up the number of cells produced at the end of Meiosis II

Hold up the number of sets of chromosomes in one haploid cell at the end of Meiosis II– 2 Sets = 46 chromosomes– 1 Set = 23 chromosomes

Meiosis- Formation of Haploid Cells

Meiosis 4 haploid cells result at the end of Meiosis II

Think – Pair - Share

Genetic

Variation

What does the term “genetic variation” mean?

How important is genetic variation to the survival of our species?

Importance of Genetic Variation

Importance of genetic variation

1. Meiosis and the joining of gametes are essential to evolution

No genetic process generates variation more quickly

2. Evolution appears to increase with increases in genetic variation

Meiosis & Genetic Variation

Genetic Variation

1. Independent Assortment-– In humans, each gamete receives 1

chromosome from each of 23 pairs of homologous chromosomes ( = 23 total chromosomes = 1n)

– Which of the 2 homologues each offspring receives is a matter of chance

– 8 million different gamete combinations can result

Meiosis & Genetic Variation

Genetic Variation

2. Crossing Over- – DNA exchange during Prophase I

in Meiosis– Portions of a chromatid on 1

homologous chromosome are broken & exchanged with the chromatid portions of the other homologous chromosome

This recombination GREATLY increases genetic variation

-How is this different from translocation? -Crossing over occurs between

homologouschromosomes-Translocation is with non-homologous chromosomes

Meiosis & Genetic Variation

Genetic Variation

3. Random Fertilization- two gametes are randomly joined to form a zygote– 64 trillion possible outcomes

Genetic Variation - Importance

Importance of genetic variation

Breeding larger or faster animals can be limited until enough genetic variation is eventually generated to continue the breeding

Genetic Variation Importance

Importance of genetic variation

4. Natural selection doesn’t always favor genetic change– Existing conditions may be

favorable, slowing evolution

Meiosis- Formation of Haploid Cells

Meiosis Pages 148 – 149

Boys do Spermatogensis

Girls do Oogenisis

Meiosis- Formation of Haploid Cells

Meiosis in Males

SPERMATOGENESIS- produce sperm in the testes of male animals– A. Diploid cell increases in size to

germ cell– B. Undergoes meiosis I– C. Undergoes meiosis II to form 4

haploid cells– D. 4 cells develop tail (sperm)

Meiosis- Gamete Production

Meiosis in females

OOGENESIS- produces eggs in the ovary of female animals– A. Diploid cell increases in size to

a germ cell– B. Undergoes meiosis I- cytoplasm

splits unevenly into each egg cell– C. Undergoes meiosis II- more

uneven cytoplasm distribution– D. One large egg cell develops, 3

polar bodies that do not survive

Meiosis vs. Mitosis

Meiosis vs. Mitosis

Mitosis –– Division of somatic (body) cells– Diploid (2n)– Creates identical cells

Meiosis – •Division of chromosomes to

form reproductive cells•Haploid (n) sex cells

produced•Genetic variation

Sexual Life Cycles in Eukaryotes

Life Cycle Entire span in the life of an organism from one generation to the next

Sexual Life Cycles in Eukaryotes

Sexual Haploid Life Cycle

1. Zygote is diploid but undergoes meiosis immediately to make new haploid cells (any DNA damage is repaired)

Sexual Life Cycles in Eukaryotes

Sexual Haploid Life Cycle

2. Haploid cells create haploid individuals

3. Gametes are produced by mitosis

Sexual Life Cycles in Eukaryotes

Sexual Haploid Life Cycle

4. Gametes fuse to produce diploid zygote, cycle continues

Occurs in protists, fungi, algae, chlamydomonas

Sexual Life Cycles in Eukaryotes

Sexual Diploid Life Cycle

1. Adults are diploid, each inheriting characteristics from both parents

2. Diploid reproductive cells undergo meiosis to produce gametes

Sexual Life Cycles in Eukaryotes

Sexual Diploid Life Cycle

3. Fertilization- joining of gametes (sperm and egg)

4. Resulting zygote divides by mitosis

Sexual Life Cycles in Eukaryotes

Sexual Diploid Life Cycle

5. Cells of adult also end up diploid

6. All cells involved are diploid, except gametes which are haploid

Ex: Humans/ Animals

Sexual Life Cycles in Eukaryotes

Alternation of generations

1. Often in plants, alternates between haploid and diploid phases

Sexual Life Cycles in Eukaryotes

Alternation of generations

2. Sporophyte- diploid phase in plant life cycle that produces spores– Spore forming cells undergo

meiosis to produce spores (haploid reproductive cell that become an adult without fusing with another cell)

Sexual Life Cycles in Eukaryotes

Alternation of generations

3. Gametophyte- haploid phase in plant life cycle that produces gametes through mitosis– Gametes fuse and give rise to

diploid phase

Sexual Life Cycles in Eukaryotes

Alternation of generations

4. Ex: Moss– Stalk tip’s capsule pops off scattering

haploid spores– Spores germinate by mitosis and form

gametophytes– Male gametophyte releases sperm that

swim through film of moisture to eggs in female gametophyte

– Diploid zygote develops as sporophyte within the gametophyte

– Cycle continues

Cloning by parthenogenesis

Cloning by parthenogenesis

1. A new individual develops form an unfertilized egg

2. No male involved, offspring is genetic clone of mother

3. Mother’s own chromosomes are copied in place of the male’s

4. Occurred in females without male companionship for long periods of time

5. Dandelions, hawkweeds, fish, lizards, frogs, snakes, male drone honeybees, NO MAMMALS

Assessment

Identify the type of reproduction that results in offspring that are genetically identical to their parent

Describe two different types of eukaryotic asexual reproduction

Compare the haploid life cycle found in chlamydomonas with a diploid life cycle

Summarize the process of alternation of generations

Evaluate the significance of mutations and repair of mutations to the evolution of sexual reproduction