Sexual Reproduction

Asexual Reproduction Problems only one parent offspring are genetically identical = clones lack of genetic variation threatens species survival

Comparison of Asexual vs Sexual Reproduction one parent offspring clones use mitosis good for large # of offspring produces somatic cells Sexual two parents offspring unique use meiosis good for genetic variation produces sex cells

Sexual Reproduction Occurs in sex organs or gonads TESTES in  in Seminiferous Tubules Ovary in  in Follicle

Sex Organs in Plants  

Produces sex cells called gametes in animals

or sex cells called spores in plants.

Chromosomes in eukaryotes homologous chromosomes are grouped in pairs called homologues or homologous chromosomes

Homologous Chromosomes Blue eyes Blue eyes …chromosomes that carry the same kind of information at the same locus Red hair Blonde hair Freckles No Freckles

Diploid vs Haploid Diploid – 2n chromosomes in homologous pairs 1 of each pair from  and 1 of each pair from Haploid – n chromosomes single, no pairs chromosome from either  or parent

Meiosis

Meiosis… occurs only in sex cells carries out 2 divisions produces 4 cells cells produced have ½ normal # of chromosomes cells are genetically unique

Two Divisions… Meiosis I – reduces chromosome # produces 2 haploid cells Meiosis II – divides the 2 cells into 4 cells

Overview of Meiosis

Meiosis I

2 significant events occur During Prophase I… Synapsis – the pairing of homologous chromosomes Crossing Over – the exchange of genetic material between two nonsister chromatids

All 4 chromatids are now different. Crossing Over is 1st major cause of GENETIC VARIATION

Metaphase – chromosomes line up in pairs Anaphase – centromeres do not divide; pulled to poles

2nd major cause of genetic variation occurs… During anaphase, the 2nd major cause of genetic variation occurs… INDEPENDENT ASSORTMENT OF CHROMOSOMES

At the end of telophase I we have 2 haploid cells. Each cell has 1 of each chromosome pair, but each chromosome still has 2 sister chromatids – double DNA.

Meiosis I is followed by a Interkinesis is just like resting stage called INTERKINESIS Interkinesis is just like Interphase except there is no S stage… no replication of DNA

All events in Meiosis II are just like Mitosis.

Note that centromeres divide in Anaphase II.

Meiosis vs Mitosis Mitosis… Meiosis… 4 haploid cells 2 diploid cells 2 divisions sex cells only genetic variation interkinesis crossing over independent assortment Mitosis… 2 diploid cells 1 division somatic cells clones

Reproduction in Humans

Overview of Meiosis I

Note that… Meiosis I produces two “same size” cells in spermatogenesis Meiosis I produces two different size cells in oogenesis - one is small and called a POLAR BODY

Oogenesis Fertilization triggers Meiosis II 1 ovum + 3 polar bodies Cells are unequal size

Spermatogenesis 4 = sized sperm produced

Spermatogenesis Oogenesis Begins at puberty Continues thru life Continuous process 4 = sized cells Daily production Oogenesis Begins at puberty Ends at menopause Fertilization required to trigger Meiosis II 4 unequal sized cells Monthly production