The word "meiosis" comes from the Greek meioun, meaning � "to make small," since it results in a � reduction in chromosome number in the gamete cell.

Start with 4 chromosomes

End with 2 chromosomes

� 1. How many chromosomes would a human sperm or an egg

contain if either one resulted from the process of mitosis?

�a. 46 chromosomes

� 2. If a sperm containing 46 chromosomes fused with an egg

containing 46 chromosomes, how many chromosomes would

the resulting fertilized egg contain? Do you think this would

create any problems in the developing embryo?

�b. 46 + 46 = 92; a developing embryo would not survive if

it contained 92 chromosomes.

� 3. In order to produce a fertilized egg with the appropriate

number of chromosomes (46), how many chromosomes

should each sperm and egg have?

� c. Sperm and egg should each have 23 chromosomes.

Consider these Questions …

*Meiosis � how you make sperm and egg cells with only 23 chromosomes*

� 1. � Number varies by organism› a. Ex: turkey � 82, fruit fly � 6, human �46

� 2. Organisms that reproduce sexually have pairs of similar chromosomes› a. Called � HOMOLOGOUS CHROMOSOMESHOMOLOGOUS CHROMOSOMESHOMOLOGOUS CHROMOSOMESHOMOLOGOUS CHROMOSOMES

› b. Similar in size, structure, and carry genes for the � same traits (but may be different forms of the gene)

› c. Each member of a pair comes from � each parent

› d. Ex: humans have � 23 pairs of homologous chromosomes, or � 46 total

� i. � 23 in sperm, � 23 in egg, pair up

� 3. Diploid vs. Haploid

a. Diploida. Diploida. Diploida. Diploid

› i. � Double set of chromosomes

› ii. � “2n”

› iii. Humans – 23 sets/pairs, � 2n = 46

b. Haploidb. Haploidb. Haploidb. Haploid

› i.. � Single set of chromosomes

› ii. � “n”

› iii. Humans – one chromo. from each set, � n = 23

� 4. Chromosome numbers & cell types

a. Somatic cells

› i. �“Regular” body cells

› ii. � Always diploid

› iii. Humans � 46 chromo.

b. Gametes

› i. �“sex cells”

› ii. Usually egg & sperm (or

pollen/spores)

› iii. � Always haploid

› iv. Humans � 23 chromo.

Meiosis I

� 1. Interphase I

› a. Cells undergo a round of � DNA replication,

forming duplicate chromosomes

� 2. Prophase I

› a. Each chromosome pairs with its

corresponding homologous chromosome to form a � tetrad

� i. tetrad � contains 4 chromatids

› b. Homologous chromosomes exchange

portions of their chromatids

� ii. � Called crossing over

tetrad

crossing-over

Crossing-Over

� 3. Metaphase I

› a. � Spindle fibers attach to the chromosomes

› b. Tetrads line up in the � middle of the cell

� 4. Anaphase I

› a. Fibers pull homologous chromosomes toward � opposite ends of the cell

› b. Centromere � stays intact

� 5. Telophase I and Cytokinesis

› a. � Nuclear membranes form

› b. Cell separates into � 2 cells

Meiosis II

� 1. Prophase II

› a. Meiosis I results in � 2 haploid (N)

daughter cells each with � half the

number of homologous chromosomes as

the original cell

� 2. Metaphase II

› a. Chromosomes line up in the � center of the

cell

� 3. Anaphase II

› a. Paired � sister chromatids separate and

move towards � opposite ends of the cell

› b. Centromere � is broken

� 4. Telophase II and Cytokinesis

› a. Meiosis II results in � 4 haploid (N) daughter

cells

› b. Each cell contains � half the number of

chromosomes as the original cell

Tetrad = 4 chromatids

Homologous chromosome

Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase II

The chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

1. Males � 4 sperm formed2. Females � 1 egg formed (larger) and 2 polar bodies (smaller)a. � only the egg is used in reproduction

Unequal division of cytoplasm

Mitosis� 1. Known as � cell division

� 2. Consists of � 1 division

� 3. Forms � 2 genetically identical daughter cells

� 4. Daughter cells have �same amount of chromosomes as parent cell (humans = 46)

� 5. Occurs only in � body cells› somatic cells

� 6. Purpose = � make more body cells for growth and repair

Meiosis� 1. Known as � cell reduction

division� 2. Consists of � 2 divisions

� 3. Forms � 4 genetically different daughter cells

� 4. Daughter cells have � half the amount of chromosomes as the parent cell (humans = 23)

� 5. Occurs only in � sex cells› gametes

� 6. Purpose = � form egg and sperm cells for fertilization