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Chapter 7Meiosis & Sexual
Reproduction
Do you remember… (mitosis)
This chapter deals with making cells that are genetically different genetically different
through meiosisthrough meiosis!
Meiosis is
different from
mitosis
What we are learning about… Steps of Meiosis
What causes genetic variation
Difference between asexual and sexual reproduction
The Process of Meiosis
Meiosis cell division that halves the number of chromosomes Nucleus divides twicetwice (meiosis I, meiosis II) Genetic variation occurs – new cells made are
NOT identical to the parent
cell you started with
The Steps to Meiosis Meiosis is a 2 part process
Meiosis I Prophase I, Metaphase I, Anaphase I,
Telophase I Meiosis II
Prophase II, Metaphase II, Anaphase II, Telophase II
Meiosis I Step 1: Prophase 1
Chromosomes visible, nuclear envelope breaks, crossing over occurs
Meiosis I Step 2: Metaphase I
Homologous chromosomes move to equator
Meiosis I
Step 3: Anaphase I Homologous chromosomes move to opposite poles
Meiosis I Step 4: Telophase I & Cytokinesis
Chromosomes gather at poles, cytoplasm divides
The END of Meiosis I In meiosis I – homologous
choromosomes separate into different cells.
At the end of meiosis I you end up with 2 different cells with half the number of chromosomes you started with.
But it isn't done yet…
Meiosis II Step 5: Prophase II
New spindles & microtubules form (move chromosomes)
Meiosis II Step 6: Metaphase II
Chromosomes line up at equator (middle)
Meiosis II Step 7: Anaphase II
Centromere divide, chromatids pulled to poles
Topic 5: Meiosis II Step 8: Telophase II & Cytokinesis
Nuclear envelope forms, cytoplasm divides
The END of Meiosis II In meiosis II sister chromatids
separate into different cells. You end up with 4 genetically
different cells with half the number of chromosomes you started with.
These cells are sex cells (sperm and ovum).
Meiosis Animation
Meiosis & Genetic Variation
MeiosisMeiosis gives rapid generation of new genetic combinations Independent assortment Crossing over Random fertilization
Independent Assortment Random distribution of homologous
chromosomes during meiosis Each of the 23 pair of chromosomes
separate independently
There are 8 million possible combinations for human gametes
n= 8n= 8
Crossing Over & Random Fertilization
During crossing over DNA is exchanged Also adds to genetic variation
Random fertilization No sperm and egg carry same genetic info Fertilization of egg and sperm is random
Square the outcomes If n=23 (n=23) * (n=23) 64 trillion possibilities
Topic 4: Importance of Genetic Variation
Essential to evolutionEssential to evolution Pace of evolution sped up by genetic recombination
Happens quicklyHappens quickly
Not all genetic variation is “favored”Not all genetic variation is “favored” Reason why some species show little to no change
Spermatogenesis vs. Oogenesis (in humans)
7.2 Asexual & Sexual
Reproduction
Topic 1: Sexual & Asexual Reproduction
Organisms can look Identical to parents & siblings Similar to parents & siblings Different than parents &
siblingsHOW?
Type of reproduction & genetic variation
Asexual Reproduction
Asexual Reproduction Single parent passes
copies of ALL its genes No fusion of haploid
gametes Offspring are clones
Occurs in both prokaryotic and eukaryotic organisms
Types of Asexual Reproduction
1. Fission- parent separates (splits) into 2 organisms of equal size
Types of Asexual Reproduction
2. Fragmentation- parent breaks into several pieces
1. Later develop into complete adults
Types of Asexual Reproduction
3. Budding- new individuals split off existing ones
Sexual Reproduction 2 parents give ½ their chromosomes (23)
In their haploid gametes (sperm or egg)
Fusion of haploid gametes form diploid offspring Genetically different
Traits from both parents
Occurs in eukaryotic organisms
Genetic Diversity Occurs with sexual reproduction Allows for adaptation to changing
environments Evolution
Allowed for early organisms to repair DNA Form diploid cells in response to stresses
Sexual Life Cycles in Eukaryotes:Haploid Adult Life Cycle
Sexual Life Cycles in Eukaryotes:Diploid Adult Life Cycle
33
This alternation of generation refers to the alternation of two phases: a multicellular diploid phase alternating with a multicellular haploid phase.
Sexual Life Cycles in Eukaryotes:Alternation of Generation
34
Sporophyte- Diploid phase in the life cycle where spores are produced.
Gametophyte- Haploid phase where gametes are produced.
Two Phases of Alteration of Generation
A multicellular haploid generation alternates with a multicellular diploid generation.
Alternation of generations in a flowering plant
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