MEIOSIS makes gametes

Meiosis produces gametes • Meiosis was first observed by the Belgian cytologist Pierre-Joseph van

Beneden in 1887• 60 years before we knew that DNA contained the genetic material• Gametes (eggs and sperm) contain half the complement of chromosomes found

in other cells• The fusion of gametes is called fertilization

– It creates the zygote, which contains two copies of each chromosome

• Meiosis only occurs in the gonads (ovaries and testes).

• All other cell divisions are mitotic

The Sexual Life Cycle • The life cycles of all sexually-reproducing organisms

follows the same basic pattern– Haploid cells or organisms alternate with diploid cells

or organisms

Figure 9.4

The Stages of Meiosis• Meiosis consists of two successive divisions, but only one DNA

replication– Meiosis I

• Separates the two homologs– Meiosis II

• Separates the two sister chromatids• Meiosis halves the number of chromosomes see Figure 9.5

Prophase I• Chromosomes condense (coil)

• Nuclear membrane breaks down

• Homologous chromosomes undergo synapsis (Pair up )

• Cross over occurs (Recombination) and chromosomes exchange segments newly forming

microtubules

Metaphase I

• Homologous chromosome pairs line up at random at the equatorial midline

spindle equator

one pair of homologous chromosomes

Anaphase I

• Centromeres do not divide

• Homologs separate and move

to opposite poles

Telophase I • Nuclear membrane

reforms • Chromosomes uncoil

After Cytokinesis• Two haploid cells are produced

• Meiosis II– After meiosis I there is a brief interphase

• No DNA synthesis occurs

– Meiosis II is similar to mitosis, but with two main differences

• 1. Haploid set of chromosomes

• 2. Sister chromatids are not identical because of cross over

Prophase II begins with haploid cells

• Chromosomes coil

• Spindle forms

• Nuclear membrane breaks down

• Each chromosome is composed of two sister chromatids attached at the centromere

Metaphase II

• Chromosomes line up on the midline and attach to spindle fibers

Anaphase II

• Centromeres divide

• Sister chromatids move to opposite poles

Telophase II • Nuclear membrane

reforms • Chromosomes uncoil

After Cytokinesis • Four unique haploid

cells are produced

see Figure 9.6

see Figure 9.6

Meiosis I has two unique features1. Synapsis

• Homologous chromosomes pair all along their lengths in prophase I• While paired homologs cross over

2. Reduction division• Homologs separate in Anaphase I reducing the chromosome

number in 1/2 (2n to 1n)• This produces haploid gametes

Evolutionary Consequences of Sex

• Sexual reproduction increases genetic diversity through three key mechanisms

–1. Crossing over

–2. Independent assortment

–3. Random fertilization

• DNA exchanges between maternal and paternal chromatid pairs

Crossing over

see Figure 9.7

Independent Assortment

Label these SEE process!

• In humans, a gamete receives one homolog of each of the 23 chromosomes– Humans have 23 pairs of chromosomes

• Independent assortment gives 223 combinations in an egg or sperm– 8,388,608 possible kinds of gametes

• Random fertilization of two independently-produced gametes – Therefore, the possible combinations in an offspring – 8,388,608 X 8,388,608 = 70,368,744,177,664– More than 70 trillion!

• And this number does not count crossing-over

Independent assortment

Three chromosome pairs23 combinations

Practice this with colored pens so that you can SEE the difference

Compare and Contrast Mitosis and Meiosis*

*guaranteed to be on quiz and exam