Chapter 7Meiosis & Sexual

Reproduction

Do you remember… (mitosis)

This chapter deals with making cells that are genetically different through genetically different through

meiosismeiosis!

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 Nucleus divides twicetwice (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 MeiosisMeiosis 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= 8n= 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 evolutionEssential to evolution Pace of evolution sped up by genetic recombination

Happens quicklyHappens quickly

Not all genetic variation is “favored”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 &

siblingsHOW?

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