1. Biology, 9th ed,Sylvia Mader
Chapter 10
Chapter 10
Meiosis & Sexual Reproduction
Meiosis & Sexual Reproduction

2. Biology, 9th ed,Sylvia Mader
Outline
Chapter 10
Meiosis & Sexual Reproduction
Reduction in Chromosome Number
Meiosis Overview
Homologous Pairs
Genetic Variation
Crossing-Over
Independent Assortment
Fertilization
Phases of Meiosis
Meiosis I
Meiosis II
Meiosis Compared to Mitosis
Human Life Cycle

3. Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis & Sexual Reproduction
Meiosis:
Used only for sexual reproduction
Halves the chromosome number prior to fertilization
Parents diploid
Meiosis produces haploid gametes
Gametes fuse in fertilization to form diploid zygote
Becomes the next diploid generation

4. Homologous Pairs of Chromosomes
Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis & Sexual Reproduction
Homologous Pairs of Chromosomes
In diploid body cells chromosomes occur in pairs (two of each type)
Humans have 23 different types of chromosomes
Chromosomes of the same type are said to be homologous
They have the same length
Their centromeres are in the same position
One came from the father (the paternal homolog) the other from the mother (the maternal homolog)

5. Homologous Chromosomes
Biology, 9th ed,Sylvia Mader
Homologous Chromosomes
Chapter 10
Meiosis & Sexual Reproduction

6. Homologous Pairs of Chromosomes
Biology, 9th ed,Sylvia Mader
Chapter 10
Homologous Pairs of Chromosomes
Meiosis & Sexual Reproduction
Homologous chromosomes have genes controlling the same trait at the same position
Many genes exist in several variant forms in a large population
The variants that exist for a gene are called alleles
An individual may have:
Identical alleles for a specific gene on both homologs (homozygous for the trait), or
A maternal allele that differs from the corresponding paternal allele (heterozygous for the trait)

7. Biology, 9th ed,Sylvia Mader
Overview of Meiosis
Chapter 10
Meiosis & Sexual Reproduction

8. Phases of Meiosis I: Prophase I
Biology, 9th ed,Sylvia Mader
Chapter 10
Phases of Meiosis I: Prophase I
Meiosis & Sexual Reproduction
Meiosis I (reductional division):
Prophase I
Each chromosome internally duplicated (consists of two identical sister chromatids)
Physically align themselves against each other end to end
End view would show four chromatids – Tetrad

9. Phases of Meiosis I: Metaphase I
Spindle is fully formed
Homologous pairs arranged onto the metaphase plate
Kinetochores are seen
Protein complex right outside the centromere that attach to spindle fibers

10. Phases of Meiosis I: Anaphase I & Telophase I
Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis & Sexual Reproduction
Phases of Meiosis I: Anaphase I & Telophase I
Meiosis I (cont.):
Anaphase I
Synapsis breaks up
Homologous chromosomes separate from one another
Homologues move towards opposite poles
Each is still an internally duplicate chromosome with two chromatids

11. Phases of Meiosis I: Telophase I
Spindle disappears
Daughter cells have one internally duplicate chromosome from each homologous pair
One (internally duplicate) chromosome of each type (1n, haploid)
May or may not form a nuclear envelope or undergo cytokinesis before meiosis II

12. Phases of Meiosis I: Cytokinesis I & Interkinesis
Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis & Sexual Reproduction
Phases of Meiosis I: Cytokinesis I & Interkinesis
Meiosis I (cont.):
Cytokinesis I
Two haploid daughter cells
Both with one internally duplicate chromosome of each type
Meiosis I is reductional (halves chromosome number)
Interkinesis
Similar to mitotic interphase, but shorter
No replication of DNA

13. Genetic Variation: Crossing Over
Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis & Sexual Reproduction
Genetic Variation: Crossing Over
Meiosis brings about genetic variation in two key ways:
Crossing-over between homologous chromosomes, and
Independent assortment of homologous chromosomes
1. Crossing Over:
Exchange of genetic material between nonsister chromatids during meiosis I
At synapsis, a nucleoprotein lattice (called the synaptonemal complex) appears between homologues
Holds homologues together and aligns DNA of nonsister chromatids (allows crossing-over)
Then homologues separate and are distributed to different daughter cells

14. Biology, 9th ed,Sylvia Mader
Chapter 10
Crossing Over
Meiosis & Sexual Reproduction

15. Genetic Variation: Independent Assortment
Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis & Sexual Reproduction
Genetic Variation: Independent Assortment
2. Independent assortment:
When homologues align at the metaphase plate:
They separate in a random manner
The maternal or paternal homologue may be oriented toward either pole of mother cell
Causes random mixing of blocks of alleles into gametes

16. Independent Assortment
Biology, 9th ed,Sylvia Mader
Independent Assortment
Chapter 10
Meiosis & Sexual Reproduction

17. Biology, 9th ed,Sylvia Mader
Chapter 10
Recombination
Meiosis & Sexual Reproduction

18. Genetic Variation: Fertilization
Biology, 9th ed,Sylvia Mader
Chapter 10
Genetic Variation: Fertilization
Meiosis & Sexual Reproduction
When gametes fuse at fertilization: Chromosomes donated by the parents are combined
In humans, (223)2 = 70,368,744,000,000 chromosomally different zygotes are possible
If crossing-over occurs only once
(423)2, or 4,951,760,200,000,000,000,000,000,000 genetically different zygotes are possible

19. Genetic Variation: Significance
Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis & Sexual Reproduction
Genetic Variation: Significance
Asexual reproduction produces genetically identical clones
Asexual reproduction is advantageous when environment is stable
Sexual reproduction causes novel genetic recombinations
If environment changes, genetic variability introduced by sexual reproduction may be advantageous

20. Phases of Meiosis II: Similar to Mitosis
Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis & Sexual Reproduction
Phases of Meiosis II: Similar to Mitosis
Metaphase II
Overview
Unremarkable
Virtually indistinguishable from mitosis of two haploid cells
Prophase II – Chromosomes condense
Metaphase II – chromosomes align at metaphase plate
Anaphase II
Centromere dissolves
Sister chromatids separate and become daughter chromosomes
Telophase II and cytokinesis II
Four haploid cells
All genetically unique

21. Meiosis I & II in Plant Cells
Biology, 9th ed,Sylvia Mader
Meiosis I & II in Plant Cells
Chapter 10
Meiosis & Sexual Reproduction

22. Meiosis versus Mitosis
Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis versus Mitosis
Meiosis & Sexual Reproduction
Meiosis
Requires two nuclear divisions
Chromosomes synapse and cross over
Centromeres survive Anaphase I
Halves chromosome number
Produces four daughter nuclei
Produces daughter cells genetically different from parent and each other
Used only for sexual reproduction
Mitosis
Requires one nuclear division
Chromosomes do not synapse nor cross over
Centromeres dissolve in mitotic anaphase
Preserves chromosome number
Produces two daughter nuclei
Produces daughter cells genetically identical to parent and to each other
Used for asexual reproduction and growth

23. Meiosis Compared to Mitosis
Biology, 9th ed,Sylvia Mader
Meiosis Compared to Mitosis
Chapter 10
Meiosis & Sexual Reproduction

24. Meiosis I Compared to Mitosis
Biology, 9th ed,Sylvia Mader
Meiosis I Compared to Mitosis
Chapter 10
Meiosis & Sexual Reproduction

25. Meiosis II Compared to Mitosis
Biology, 9th ed,Sylvia Mader
Meiosis II Compared to Mitosis
Chapter 10
Meiosis & Sexual Reproduction

26. Life Cycle Basics: Plants
Biology, 9th ed,Sylvia Mader
Chapter 10
Life Cycle Basics: Plants
Meiosis & Sexual Reproduction
Haploid multicellular “individuals” alternate with diploid multicellular “individuals”
The haploid individual:
Known as the gametophyte and may be larger or smaller than the diploid individual
In plants, algae, & fungi, gametes produced by haploid individuals
Mosses are haploid most of their life cycle
The diploid individual:
Known as the sporophyte and may be larger or smaller than the haploid individual
Ferns & higher plants have mostly diploid life cycles
In fungi and most algae, only the zygote is diploid

27. Life Cycle Basics: Animals
Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis & Sexual Reproduction
Life Cycle Basics: Animals
In familiar animals:
“Individuals” are diploid; produce haploid gametes
The products of meiosis are always gametes
Meiosis occurs only during gametogenesis
Production of sperm
Spermatogenesis - all four cells become sperm
Production of eggs
Oogenesis - only one of four nuclei get cytoplasm and becomes the egg or ovum
(Others wither away as polar bodies)

28. Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis & Sexual Reproduction
The Human Life Cycle
Sperm and egg are produced by meiosis
A sperm and egg fuse at fertilization
Results in a zygote
The one-celled stage of an individual of the next generation
Undergoes mitosis
Results in multicellular embryo that gradually takes on features determined when zygote was formed
All growth occurs as mitotic division
As a result of mitosis, each somatic cell in body
Has same number of chromosomes as zygote
Has genetic makeup determined when zygote was formed

29. Biology, 9th ed,Sylvia Mader
Chapter 10
Meiosis & Sexual Reproduction
Review
Reduction in Chromosome Number
Meiosis Overview
Homologous Pairs
Genetic Variation
Crossing-Over
Independent Assortment
Fertilization
Phases of Meiosis
Meiosis I
Meiosis II
Meiosis Compared to Mitosis
Human Life Cycle