Meiosis and Sexual Reproduction Notes To be used with Meiosis and Sexual Reproduction Guided Notes

From poppa From momma

Gametogenesis Gametogenesis: the formation of gametes (eggs and sperm) Each species has a characteristic number of chromosomes Homologous Chromosomes: the two members of a pair of chromosomes contain genes for the same traits

Diploid vs. Haploid Diploid Number: the number of chromosomes found in all body cells (2n) Haploid Number: 1/2 the diploid number of chromosomes (n) found in the egg and sperm cells of a species DIPLOID HAPLOID

Gametogenesis Terms Gametes: eggs or sperm Gonads: specialized organs in higher animals where the gametes are made Ovaries: female gonads Testes: male gonads

Meiosis Meiosis: process which involves a 1/2 reduction division of the chromosome number Haploid nuclei are formed which contain only one chromosome of the original pair Meiosis ONLY occurs in the gamete producing cells of the gonads (testes and ovaries) The production of sex cells

First Meiotic Division- Interpahse1 DNA and chromosomes replicate

Prophase 1 Each chromosomes pairs with its matching homologous chromosome forming a tetrad Crossing-over occurs here

Metaphase 1 Spindle fibers attach to the homologous chromosomes

Anaphase 1 Spindle fibers pull homologous chromosomes to opposite ends of the cell

Telophase 1 Nuclear membrane forms. Cytokinesis occurs forming 2 new daughter cells having half the # of chromosomes.

Meiosis Terms Synapsis: the meeting of two homologous chromosome pairs Tetrads: the two pairs of homologous chromosomes Crossing Over: the exchange of chromosome pieces in the tetrad during crossing over Crossing over increases the variability of the offspring This is why the offspring of sexual reproduction show many variations

Crossing Over

Meiosis 2- Very similar to Mitosis

Prophase 2 Nuclear membrane disappears and spindle fibers form

Metaphase 2 Chromosomes line up in the middle

Anaphase 2 Chromosomes are divided in half (centromere is split)

Telophase 2 Cells divide in half again (cytokinesis) Produced 4 haploid (n) daughter cells

Meiosis, continued Disjunction: Homologous pairs of chromosomes separate and move toward opposite poles of the cell Nondisjunction: the failure of homologous chromosomes to separate

Nondisjunction

Nondisjunction Down's syndrome: results from the nondisjunction of chromosome 21 individual has an extra 21st chromosome polyploidy: having an entire extra set of chromosomes Will result in the 3n or 4n number of chromosomes

Down’s Syndrome Rates

Cyclopia Nondisjunciton of chromosome 13-15 or 17-18 Two eyes within one orbit Proboscis-like appendage above the eye Low-set or missing ears Missing facial bones Congenital heart disease (do not survive)

Turner’s syndrome XO sex chromosomes 1/2000 female births Short stature Wide-spaced underdeveloped nipples Webbed neck Swelling of hands & feet Sterile

Klinefelter’s Syndrome XXY sex chromosomes 1/800-1000 male births Greatly reduced male genitalia No sperm production & no sex drive Some breast production Wide curvy hips

Jacob’s Syndrome XYY sex chromosomes 1/1000 male births Taller than average Normal to subnormal intelligence Worse than normal acne Dense body hair (2% of maximum security prison population)

Superwoman Syndrome XXX sex chromosomes 1/2000 female births Normal to slightly retarded intelligence Extra development of female secondary sex characteristics Fertile

Polyploidy

Meiosis and Fertilization Meiosis and fertilization are necessary for sexual reproduction Egg (n) + Sperm (n) =Zygote (2n)

Spermatogenesis Spermatogenesis: The production of sperm one successful meiotic division produces 4 sperm

Sperm Sperm Structure: smaller than the egg but capable of locomotion with its flagellum.

Oogenesis Oogenesis: the production of ova (eggs) One meiotic division produces 1 egg and 3 polar bodies.

The Meiosis Square Dance

Homologous chromosomes

Prophase I

Synapsis and crossing over

Metaphase I -- Notice the synpasis

Anaphase I

Prophase 2 – similar to mitosis Replication does not occur between meiosis 1 and meiosis 2

Metaphase 2

Anaphase 2 –

Telophase 2 – chromosome number is now ½ of the original

Cytokinesis – meiosis results in 4 daughter cells

Answers to Homework

Summary The diploid number (2n) of chromosomes is reduced by 1/2 to the haploid number (n) Eggs and sperm are formed Crossing Over occurs—lots of variation Variations rarely occur in MITOSIS Variations often occur in MEIOSIS

Results of Meiosis Mitosis is associated with growth and asexual reproduction Meiosis is associated with sexual reproduction Spermatogenesis: the production of sperm Produces 4 sperm Oogenesis: the production of ova or eggs Produces one egg and three polar bodies