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
Essential Knowledge
Chapter 10
Meiosis & Sexual Reproduction
3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.
c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually reproducing organisms.
1. Meiosis ensures that each gamete receives one complete haploid (1n) set of chromosomes.
2. During meiosis, homologous chromosomes are paired, with one homologue originating from the maternal parent and the other from the paternal parent. Orientation of the chromosome pairs is random with respect to the cell poles.

4. Biology, 9th ed,Sylvia Mader
Essential Knowledge
Chapter 10
Meiosis & Sexual Reproduction
3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.
c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually reproducing organisms.
3. Separation of the homologous chromosomes ensures that each gamete receives a haploid (1n) set of chromosomes composed of both maternal and paternal chromosomes.
4. During meiosis, homologous chromatids exchange genetic material via a process called “crossing over,” which increases genetic variation in the resultant gametes.

5. Biology, 9th ed,Sylvia Mader
Essential Knowledge
Chapter 10
Meiosis & Sexual Reproduction
3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.
c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually reproducing organisms.
5. Fertilization involves the fusion of two gametes, increases genetic variation in populations by providing for new combinations of genetic information in the zygote, and restores the diploid number of chromosomes.

6. Meiosis: Halves the Chromosome Number
Biology, 9th ed,Sylvia Mader
Meiosis: Halves the Chromosome Number
Chapter 10
Meiosis & Sexual Reproduction
Special type of cell division
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

7. Homologous Pairs of Chromosomes
Biology, 9th ed,Sylvia Mader
Homologous Pairs of Chromosomes
Chapter 10
Meiosis & Sexual Reproduction
In diploid body cells chromosomes occur in pairs
Humans have 23 different types of chromosomes
Diploid cells have two of each type
Chromosomes of the same type are said to be homologous
They have the same length
Their centromeres are positioned in the same place
One came from the father (the paternal homolog) the other from the mother (the maternal homolog)
When stained, they show similar banding patterns
Because they have genes controlling the same traits at the same positions

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

9. Homologous Pairs of Chromosomes
Biology, 9th ed,Sylvia Mader
Homologous Pairs of Chromosomes
Chapter 10
Meiosis & Sexual Reproduction
Homologous chromosomes have genes controlling the same trait at the same position
Each gene occurs in duplicate
A maternal copy from the mother
A paternal copy from the father
Many genes exist in several variant forms in a large population
Homologous copies of a gene may encode identical or differing genetic information
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)

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

11. Phases of Meiosis I: Prophase I & Metaphase I
Biology, 9th ed,Sylvia Mader
Phases of Meiosis I: Prophase I & Metaphase I
Chapter 10
Meiosis & Sexual Reproduction
Meiosis I (reductional division):
Prophase I
Each chromosome internally duplicated (consists of two identical sister chromatids)
Homologous chromosomes pair up – synapsis
Physically align themselves against each other end to end
End view would show four chromatids – Tetrad
Metaphase I
Homologous pairs arranged onto the metaphase plate

12. Phases of Meiosis I: Anaphase I & Telophase I
Biology, 9th ed,Sylvia Mader
Phases of Meiosis I: Anaphase I & Telophase I
Chapter 10
Meiosis & Sexual Reproduction
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
Telophase I
Daughter cells have one internally duplicate chromosome from each homologous pair
One (internally duplicate) chromosome of each type (1n, haploid)

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

14. Genetic Variation: Crossing Over
Biology, 9th ed,Sylvia Mader
Genetic Variation: Crossing Over
Chapter 10
Meiosis & Sexual Reproduction
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
Aligns DNA of nonsister chromatids
Allows crossing-over to occur
Then homologues separate and are distributed to different daughter cells

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

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

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

18. Genetic Variation: Fertilization
Biology, 9th ed,Sylvia Mader
Genetic Variation: Fertilization
Chapter 10
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
Genetic Variation: Significance
Chapter 10
Meiosis & Sexual Reproduction
Asexual reproduction produces genetically identical clones
Sexual reproduction cause novel genetic recombinations
Asexual reproduction is advantageous when environment is stable
However, 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
Phases of Meiosis II: Similar to Mitosis
Chapter 10
Meiosis & Sexual Reproduction
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 versus Mitosis
Biology, 9th ed,Sylvia Mader
Meiosis versus Mitosis
Chapter 10
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

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

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

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

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

26. Life Cycle Basics: Animals
Biology, 9th ed,Sylvia Mader
Life Cycle Basics: Animals
Chapter 10
Meiosis & Sexual Reproduction
In familiar animals:
“Individuals” are diploid; produce haploid gametes
Only haploid part of life cycle is the 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
Becomes the egg or ovum
Others wither away as polar bodies

27. Biology, 9th ed,Sylvia Mader
The Human Life Cycle
Chapter 10
Meiosis & Sexual Reproduction
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

28. Gametogenesis in Mammals
Biology, 9th ed,Sylvia Mader
Gametogenesis in Mammals
Chapter 10
Meiosis & Sexual Reproduction

29. Biology, 9th ed,Sylvia Mader
Chapter 10
Ending Slide Chapter 10
Meiosis & Sexual Reproduction
Meiosis & Sexual Reproduction