Chapter 7 Meiosis & Sexual Reproduction (Stem Cells) Section 1 – Meiosis Section 2 – Sexual Reproduction

Stem Cells Definition: Cells that have three specialties: They have the capability to differentiate into a specialized cell or remain as a stem cell Provide repair to tissues or organs after long periods of inactivation Can continuous divide when needed

Stem Cell Research Embryonic Stem Cells Rare cells found in adult organs or tissues that are undifferentiated Usually limited capacity to original origin Adult Stem Cells Only derived from eggs fertilized In Vitro Growth is performed in a laboratory for research purposes (not in a mother’s womb) In vitro fertilization—A technique that unites the egg and sperm in a laboratory instead of inside the female body.

Ethical Question Is it ethically right to possibly destroy a human embryo to remove stem cells for future medical treatment?

Stem Cells

Meiosis A form of cell division that produces haploid cells – gametes or spores that contain half the amount of chromosomes Before meiosis – DNA must be replicated Homologous chromosomes are used for meiosis End result: Four genetically different cells

Prophase i Chromosomes condense – nuclear envelope breaks down Homologous chromosomes pair up lengthwise Crossing-over: exchanging of parts of the chromatids

Metaphase i The pairs of chromosomes are moved by spindle fibers They line-up on the “equator” or metaphase plate of the cell No part of chromosome is separated

Anaphase i Whole pairs are pulled to opposite poles of the cell by the fibers Chromatids DO NOT separate Half are pulled to the North Half are pulled to the South

Telophase i Chromosomes gather at their respective poles Nuclear envelope develops around each pole Cytoplasm divides through cytokinesis

Prophase ii New spindle fibers form Nuclear envelope breaks down

Metaphase ii Same as Metaphase I The pairs of chromosomes are moved by spindle fibers They line-up on the “equator” or metaphase plate of the cell No part of chromosome is separated

Anaphase ii NOT the same as Anaphase I Centromeres are divided – separates the chromatids from their pair Chromatids are pulled to separate poles and now named chromosomes One half of pair moves East and other moves West

Telophase ii Same as Telophase I Chromosomes gather at their respective poles Nuclear envelope develops around each pole Cytoplasm divides through cytokinesis End Product: Unpaired chromosomes – genetically different from one another

Independent assortment Random distribution of chromosomes during meiosis Randomness is based on starting position, one switch can cause a whole different outcome

Crossing-over The process that occurs before independent assortment that creates an unlimited number of possibilities Randomness starts when the sperm fertilizes the egg

Importance of Genetic Variation Genetic variation allows for faster evolution As genes are used up – the process becomes more difficult Natural selection however does not always favor genetic variation which can slow evolution down

Meiosis in Males Spermatogenesis: sperm production in the testes in male organisms (occurs daily) The sperm cell growths through meiosis I and produces four functional sperm through meiosis II Sperm looks like a tadpole – small body with a long tail

Meiosis in females Oogenesis: egg production in the ovaries in female organisms (happens once in the womb) Only one functional egg is produced during one full cycle of meiosis Cytoplasm is split unevenly during meiosis I

Sexual & asexual reproduction a process of where a single parent passes on an exact copy of its genes to its offspring Clone: offspring that is genetically identical to its parent Sexual Reproduction: a process where two parents participate to develop genetically different offspring

Genetic diversity Asexual Reproduction Low genetic diversity with a high rate of reproduction Disadvantage for adaptive purposes Examples: Budding, Fragmentation, Fission Organisms: Hydra, Bacteria

Sexual Reproduction Many different possible gene combinations Allows for diversity to be the foundation for evolution

Haploid life cycles Starting cells: haploid Haploid individuals produce gametes through meiosis Fusion: gametes come together to produce a diploid zygote Diploid zygote will proceed through a type of diploid life cycle to correct genetic damage

Diploid life cycles Genetic information is from two parents Production of gametes is produced through meiosis rather than mitosis Fertilization: joining of a sperm and egg to produce an zygote Zygote: a diploid cell

Alternation of Generations Plants, algae and protists usually alternate between haploid and diploid cycles – very complex Plants in Diploid Phase: Sporophyte: cells within go through meiosis to produce spores Spores: growth of an adult without fusion Plants in Haploid Phase: Gametophyte: production of gametes through mitosis – which must fuse to become adult