Chapter 12

Meiosis

Mitosis Review

• Cell division

• Duplicates chromosomes

• Takes one diploid cell and makes two more diploid cells

Asexual Reproduction

• Single parents copies self to make offspring

• Genetically identical offspring

• Uses mitosis only

Pros

• Uses less energy

• Not reliant on finding mates

Cons

• Low genetic variation

• Difficult to adapt and evolve

Sexual Reproduction

• Reproduction w/ offspring from two parents

• Genetically unique offspring

• Uses both mitosis and meiosis

Pros

• High genetic variation

• Facilitates adaption and evolution

Cons

• Uses more energy and resources

• Have to find mate

Alleles

• Different forms of the same gene

• Slight difference in nucleotides

• Ex: gene - eye color. Allele - the different eye colors

Human genome is 3 billion nucleotides

• All humans have the same genes(same species) but different alleles (diversity among same species)

• Human genome is 99.9% identical

• 0.1% is still a lot of room for diversity

Sexual reproduction doesn’t use diploid cells

• Uses gametes such as egg and sperm

• Haploid: cell with one copy of chromosomes

****

Sexual reproduction uses meiosis

• Meiosis: cell division that halves the number of chromosomes

• Diploid cells undergo meiosis to make haploid cells

• gamete/ haploid Ex: egg and sperm, fungal/plant spores

Fertilization restores chromosome number

• Fertilization: fusion of 2 gametes during sexual reproduction

• Zygote

  • First cell of new offspring (from fusion of gametes)

  • Now has 46 chromosomes (diploid)

Steps of Meiosis

• Runs in 2 stages

• Meiosis I

  • Starts with a diploid cell

• Meiosis II

  • Starts with a haploid cell

Meiosis I

• Prophase I - chromosomes thicken, swap segments

• Metaphase I - chromosomes line up in middle

• Anaphase I - chromosomes split and move to opposite ends of the cell

• Telophase I - cell splits, forms 2 haploid cells

Meiosis II

• Prophase II - chromosomes thicken

• Metaphase II - chromosomes line up in middle

• Anaphase II - chromosomes split and move to opposite ends of cell

• Telophase II - cell splits again, forms 2 more haploid cells

• All the same steps as Meiosis I except that telophase II makes 2 more cells

One diploid cell becomes 4 haploid in meiosis