Chapter 7 Meiosis & Sexual Reproduction

Do you remember… (mitosis) This chapter deals with making cells that are genetically different through 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 twice Nucleus divides twice (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 & Genetic Variation Meiosis 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= 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 evolution Pace of evolution sped up by genetic recombination Happens quickly Not all genetic variation is “favored” Reason why some species show little to no change

7.2 Asexual & Sexual Reproduction

Topic 1: Sexual & Asexual Reproduction Organisms can look Identical to parents & siblings Similar to parents & siblings Different than parents & siblings HOW? 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