Lesson 9.3: Meiosis: The Life Cycle of Sex Cells

• Goals: • Identify male and female gametes• Compare chromosome numbers

between somatic cells and gametes• Describe the stages of meiosis• Compare meiosis and mitosis

Meiosis• Somatic cell = body cell which is

DIPLOID (2n), - it has 2 copies of each chromosome (one from mom and one from dad) these are called HOMOLOGOUS CHROMOSOMES– In humans, somatic cells have 46

chromosomes (skin cells, liver cells etc.)

Homologous Chromosomes

• Homologous chromosome – one of a matching pair of chromosomes that comes from each parent

MomDad

• Gametes, or sex cells do not perform mitosis• Human sex cells have 23 chromosomes…

HOW DOES THIS HAPPEN?• Gametes are involved in sexual reproduction

and are HAPLOID (n) (1 set of chromosomes)– Male gamete = sperm– Female gamete = egg– In humans the sperm and egg each have 23

chromosomes

The Phases of Meiosis

• Consists of 2 separate cell divisions named Meiosis I and Meiosis II

• Starts with one diploid cell and ends with FOUR haploid cells called gametes (sex cells)

• During Meiosis I the number of chromosomes is reduced by ½ therefore meiosis I is known as reduction

• Meiosis II is the same as mitosis (division of the nucleus)

Meiosis I – PROPHASE I• Preceded by Interphase

(therefore chromosomes have replicated sister chromatids)

• Spindle fibers appear and attach to centromeres

• In meiosis I HOMOLOGOUS chromosomes pair up in a process called SYNAPSIS which results in TETRADS

• While chromosomes are paired, they trade DNA pieces with each other CROSSING OVER

Meiosis I – Metaphase I

• Tetrads line up randomly along the metaphase plate (tetrads meet in the middle)

Meiosis I – Anaphase I

• Homologous chromosome separate – move Away/Apart from one another towards opposite ends of the cell

• *Sister chromatids are still together!

Meiosis I – Telophase I and Cytokinesis

• Each end of the cell now has a haploid nucleus (nucleus only has one set of chromosomes)

• Nuclei and cytoplasm divide in half

• 2 haploid cells form• Each contains 2

copies of the same set of chromosomes

Meiosis II• Does not begin with DNA

replication

• Prophase II: Spindle fibers attach to sister chromatids

• Metaphase II: spindle fibers move sister chromatids to the middle/center

• Anaphase II: Centromeres divide, sister chromatids separate and move apart to the poles

• Telophase II: Nuclear envelope reforms, cytoplasm divides

• Result: 4 haploid (1/2 the number of chromosomes as the original parent cell), genetically different cells

Lesson 3 Review

• Please read text pages 256-258 and answer the following questions on page 259

• 1. __________• 2. __________• 3. __________• 4. __________• 5. __________• 6. __________

Check Mitosis, Meiosis or Both Mitosis Meios

isBoth

No pairing of homologous chromosomes

Two divisions

Four daughter cells produced

Associated with growth and asexual reproduction

Associated with sexual reproduction

One division

Two daughter cells produced

Involves duplication of chromosomes

Chromosome # maintained

Chromosome # is halved

Crossing over between homologous may occur

Daughter cells are identical to parent cell

Daughter cells are not identical to parent cell

Produces gametes

Synapsis occurs in prophase

State the PhaseState the Phase (for meiosis - indicate meiosis I or II)

• Mitosis– Nucleolus reappears– Sister chromatids line up

at equator– Cleavage furrow forms– Cell elongates– Chromosomes move

AWAY from each other– Spindle fibers stretch

across the cell– Nucleolus and Nuclear

membrane disappear

• Meiosis• Tetrads form• Crossing over occurs• Chromatin condenses• Tetrads line up at equator• Sister chromatids separate

and chromosomes move AWAY…

• Centromere divides• 2 cells are formed• 4 haploid cells are formed• Centrioles move to opposite

ends of the cell

Review: A comparison of mitosis and meiosiscomparison animation