Chapter 11.4 (Pg. 323-329): Meiosis

Chromosome Number- Is how many chromosomes a cell has- Is represented by the letter N- Can be either haploid or diploid- Changes during S phase of

interphase, and during cell division

Chromosome Number- Diploid

- A cell has two copies of each chromosome

- Is written as 2N- These two copies are

homologous- One set came from each parent- Have the same genes in the same

locations

- Diploid cells are created by mitosis

Chromosome Number- Haploid

- A cell has one copy of each chromosome

- Is written as N- Most often occurs in gametes- Haploid cells are created by

meiosis

- Haploid sperm can combine with a haploid egg to make a diploid zygote, or fertilized egg

Phases of Meiosis- Occur in two stages, Meiosis I and

Meiosis II

- Each contains prophase, metaphase, anaphase, telophase, and cytokinesis

- Interphase occurs before Meiosis I, but not before Meiosis II

Meiosis I

- Interphase- DNA is copied during S phase of

Interphase

- Organelles and molecules are copied to prepare for cell division

Meiosis I- Prophase I

- Spindle forms- Nuclear membrane disappears- Chromosomes condense- Homologous chromosomes pair up

- Forms a tetrad

- Homologous chromosomes cross over- Chromatid on one chromosome

swaps a section with another- This creates genetic variation

Meiosis I- Metaphase I

- Homologous chromosomes line up across the center of the cell

Meiosis I- Anaphase I

- Spindle fibers pull each homologous chromosome pair towards opposite ends of the cell

Meiosis I

- Telophase I and Cytokinesis- A nuclear membrane forms

around each cluster of chromosomes

- Spindle disappears

- The cell membrane pinches in to separate the cytoplasm into two haploid daughter cells

Meiosis II

- Prophase II- Chromosomes condense- Nuclear membranes disappear- Spindles form

Meiosis II

- Metaphase II- Chromosomes line up along the

middle of the cell

Meiosis II

- Anaphase II- Individual chromatids move to

opposite sides of the cell

Meiosis II

- Telophase II and Cytokinesis- Nuclei re-form- The cell membrane pinches in to

separate the cytoplasm into four genetically unique haploid daughter cells

Differences Between Mitosis and Meiosis

Mitosis1. Makes genetically identical cells2. Makes diploid cells3. Maintains the original chromosome

number4. Makes two daughter cells5. Uses only one cell division6. No pairing of homologous chromosomes7. Only separates sister chromatids8. No crossing over occurs9. Used for many cellular processes

Meiosis1. Makes genetically unique cells2. Makes haploid cells3. Reduces the chromosome number by

half4. Makes four daughter cells5. Uses two cell divisions6. Pairs homologous chromosomes

together7. Separates homologous chromosomes,

then sister chromatids8. Crossing over occurs9. Only makes gametes for reproduction

Linked Genes- Alleles of different genes tend to

be inherited together from one generation to the next when those genes are located on the same chromosome

- Can be shown with a gene map- Indicates the location of each gene

on a chromosome