Meiosis & Reproduction

Chapter 7

objectives• Recognize that during meiosis, the formation of sex cells,

chromosomes are reduced to half the number present in the parent cell

• Explain how fertilization restores the diploid number of chromosomes

• Recognize cells both increase in number and differentiate, becoming specialized in structure and function, during and after embryonic development

• Identify factors (e.g. Biochemical, temperature) that may affect the differentiation of cells and the development of an organism

• Distinguish between asexual (i.e. binary fission, budding, cloning) and sexual reproduction

• Describe the advantages and disadvantages of asexual and sexual reproduction with regard to variation within a population

VI. Meiosis

A. Two divisions of the nucleus

B. Results in the formation of 4 gametes (egg, sperm)

C. Reduces the number of chromosomes by half (in humans from 46 to 23)

Stages of Meiosis I

1.*Prophase I- crossing over occurs, portions of one chromosome are broken and exchanged with portions of the other homologue

**results in genetic variability

2. Metaphase I- homologous chromosomes pair up together at the equator

3. Anaphase I- spindle fibers shorten and homologous chromosomes separate and move toward opposite poles

4. Telophase I and Cytokinesis

Meiosis II- same as mitosis

1. Prophase II

2. Metaphase II

3. Anaphase II

4. Telophase II & Cytokinesis

Females- meiosis is called oogenesis, forms 1 ovum and 3 polar bodies, occurs in ovaries

Males- meiosis is called spermatogenesis, forms 4 sperm cells, occurs in testes

Meiosis video

Sexual Reproduction- requires egg and sperm

1. Zygote- fertilized egg, first cell of new offspring

2. Zygote divides by mitosis many times to produce a blastula (hollow ball of cells)

Zygote

1 division

Blastula24 hours after

conception

5 days after conception

3. Gastrula- one side of the blastula collapses and cells begin to change into 3 layers

Blastula Gastrula

4. The 3 gastrula cell layers begin to differentiate (change) into specific kinds of cells

Asexual Reproduction- does not require egg and sperm3 Types:

1. Budding

2. Regeneration

3. Binary Fission

Advantages/Disadvantages

Sexual reproduction-

*provides genetic variability to the population

Asexual reproduction-

*produce many offspring in a short time

*offspring have the exact same DNA as parent

*all mutations in DNA get passed to offspring