Section 11.4

Chromosome Number

• An organism with 2 parents must inherit a single copy of every gene from each parent.

• When gametes are made, the 2 sets of genes must be separated so that each gamete has 1 set of genes.

Chromosome Number

• Diploid Cells (2)– When a cell has both sets of homologous

chromosomes.• Body cells

• Haploid Cells (1)– When a cell has one set of chromosomes• Gametes

Phases of Meiosis

• Meiosis is when the number of chromosomes is cut in half.

• Has two divisions.

Meiosis I

• Chromosomes replicate prior to meiosis I.• Each chromosome has 2 sister chromatids,

joined at the center by a centromere.

Meiosis I• ***Prophase I– Each replicated chromosome pairs with its

homologous chromosome.– Pair forms a tetrad– Crossing-over occurs between tetrads.• Makes new combos of alleles.

Meiosis I

• Metaphase I: paired homologous chromosomes line up across the middle.

• Anaphase I: chromosomes pulled to opposite ends of the cell.

• Telophase I:nuclear envelope reforms • Cytokinesis I: cytoplasm divides, forms 2

diploid cells.

Meiosis I

Meiosis II

• Each daughter cell undergoes a second meiotic division.

• Chromosomes are NOT replicated before this.

Meiosis II

• Prophase II– Chromosomes become visible– DO NOT form tetrads• Homologous pairs were separated during meiosis I.

Meiosis II

• Metaphase II, Anaphase II, Telophase II, Cytokinesis II.

• Stages just like meiosis I.• Results in 4 haploid cells

Gametes to Zygotes

• 4 haploid cells are gametes– Sperm in males– Eggs in females

• When an egg is fertilized (by a sperm), it is called a zygote.– Zygote forms a new organism

Comparing Meiosis and Mitosis

• Replication and Separation of Genetic Material– Mitosis: each daughter cell has one complete set of

chromosomes. (Body Cells)– Meiosis: daughter cells contain 1/2 the number of

chromososmes. (Gametes)

Comparing Meiosis and Mitosis

• Changes in Chromosome Number– Mitosis: Normally doesn’t change chromosome #• Diploid

– Meiosis: Chromosome number is halved• Haploid

Comparing Meiosis and Mitosis

• Number of Cell Divisions:– Mitosis• 2 cells; genetically identical; diploid

– Meiosis• 4 cells; genetically different; haploid

Gene Linkage

• Thomas Morgan used fruit flies.• He determined that genes located on the

same chromosome segregate together.

Gene Maps

• If genes are farther apart, the more likely it is that they will cross-over.

• If genes are close together, cross-overs are rare.

• Use frequency of crossing-over to determine distance.