Sex Cells Gametes (germ cells) Eggs and Sperm Somatic Cells All other cells

Autosomes – same number in males and females Sex Chromosomes – X or Y Human females: XX Human males: XY Testosterone and other hormones from the Y make a male

46 chromosomes (23 pairs)

Describes how many sets of different chromosomes a cell has. n = number of chromosomes in 1 set (haploid number) Haploid Cells: 1n Diploid Cells: 2n In humans, n = 23 (the number of different chromosomes we have) 32/16 62

Key Haploid (n) Diploid (2n) Egg (n) Haploid gametes (n  23) Sperm (n) Ovary Testis Mitosis and development Diploid zygote (2n  46) Multicellular diploid adults (2n  46) MEIOSIS FERTILIZATION

Key Haploid (n) Diploid (2n) Gametes MEIOSISFERTILIZATION Zygote Mitosis Diploid multicellular organism (a) Animals n n n 2n2n 2n2n 2n2n 2n2n 2n2n n n n n n n n n n n MEIOSIS FERTILIZATION Mitosis Gametes Spores Gametes Zygote Haploid multi- cellular organism (gametophyte) Diploid multicellular organism (sporophyte) Haploid unicellular or multicellular organism (b) Plants and some algae (c) Most fungi and some protists Plants and some algae exhibit an alternation of generations.

In crossing over, nonsister chromatids exchange DNA segments. Each pair of chromosomes forms a tetrad, a group of four chromatids. Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred.

Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs

Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM

Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I

Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II

Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughter cells Recombinant chromosomes