1. 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

2. Essential knowledge 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. a. The cell cycle is a complex set of stages that is highly regulated with checkpoints, which determine the ultimate fate of the cell. b. Mitosis passes a complete genome from the parent cell to daughter cells. c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually reproducing organisms.

3. Essential knowledge 3.C.2:
Biological systems have multiple processes that increase genetic variation. a. The imperfect nature of DNA replication and repair increases variation. b. The horizontal acquisitions of genetic information primarily in prokaryotes via transformation (uptake of naked DNA), transduction (viral transmission of genetic information), conjugation (cell-to-cell transfer) and transposition (movement of DNA segments within and between DNA molecules) increase variation. c. Sexual reproduction in eukaryotes involving gamete formation, including crossing-over during meiosis and the random assortment of chromosomes during meiosis, and fertilization serve to increase variation. Reproduction processes that increase genetic variation are evolutionarily conserved and are shared by various organisms.

4. Essential knowledge 3.A.4:
The inheritance pattern of many traits cannot be explained by simple Mendelian genetics.
Many traits are the product of multiple genes and/or physiological processes.
b. Some traits are determined by genes on sex
chromosomes.
c. Some traits result from nonnuclear inheritance.

5. Essential knowledge 3.B.2:
A variety of intercellular and intracellular
Signal transmissions mediate gene expression.
Signal transmission within and between cells mediates gene expression.
b. Signal transmission within and between cells mediates cell function.

6. Essential knowledge 3.A.1:
DNA, and in some cases RNA, is the primary source of heritable information.
Genetic information is transmitted from one generation to the next through DNA or RNA.
b. DNA and RNA molecules have structural similarities and differences that define function.
c. Genetic information flows from a sequence of nucleotides in a gene to a sequence of amino acids in a protein.
d. Phenotypes are determined through protein activities.
e. Genetic engineering techniques can manipulate the heritable information of DNA and, in special cases, RNA.

7. 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

8. 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
22 pairs of autosomes
1 pair of sex chromosomes
Diploid cells have two of each type
Chromosomes of the same type are said to be homologous
They have the same length
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

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
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. 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 prophase of meiosis I
At synapsis, a nucleoprotein lattice (called the synaptonemal complex) appears between homologues
Holds homologues together forming a tetrad
Aligns DNA of nonsister chromatids
Chiasmata are points where crossing over occurs
Allows crossing-over to occur

11. Crossing Over Crossing over
Produces recombinant chromosomes that carry genes derived from two different parents

12. 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

13. In independent assortment
Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs

14. Genetic Variation: Random Fertilization
Biology, 9th ed,Sylvia Mader
Genetic Variation: Random Fertilization
Chapter 10
Meiosis & Sexual Reproduction
3. Random Fertilization
When gametes fuse at fertilization:
Chromosomes donated by the parents are combined (genetic recombination) in a random manner
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

15. 4. Mutation produces genetic variation
Crossing over, independent assortment and random fertilization produce only new combinations of existing alleles

16. 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
However, if environment changes, genetic variability introduced by sexual reproduction may be advantageous

17. 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 and crossing over occur
Physically align themselves against each other end to end
Metaphase I
Homologous pairs arranged onto the metaphase plate

18. 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

19. 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
No replication of DNA

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
Meiosis 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

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
Halves chromosome number (haploid)
Produces four daughter nuclei
Produces daughter cells genetically different from parent and each other
Used only for sexual reproduction
Produces gametes
Mitosis
Requires one nuclear division
Chromosomes do not synapse nor cross over
Centromeres dissolve in mitotic anaphase
Preserves chromosome number (diploid)
Produces daughter cells genetically identical to parent and to each other
Used for asexual reproduction and growth, repair, regeneration
Produces somatic cells

22. 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

23. 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
Production of eggs
Oogenesis
Only one of four nuclei get cytoplasm
Becomes the egg or ovum
Others wither away as polar bodies

24. Oogenesis begins prior to birth
At birth, primary oocyte cells are resting in prophase of meiosis I
During ovulation one primary oocyte completes meiosis I to become a secondary oocyte

25. Males produce primary spermatocytes by mitosis throughout their reproductive years, females are born with all the primary oocytes they will ever have (100,000+), oocytes stopped in prophase I

26. 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

27. Biology, 9th ed,Sylvia Mader
Review
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