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Meiosis &
Sexual Life Cycles
Heredity Information
 Genes – units of heredity
 Section of DNA
 Unique sequence of nucleotides
 Program cells to synthesize particular proteins
 Thousands per chromosome
 Location of gene on DNA - locus
Reproduction Comparison
 Asexual
Sexual
 One parent
Two parents
 Identical offspring
 Non-identical
 Mitosis
offspring

Meiosis
Genetic Info
 Chromosomes
 46 in humans (23  Mom, 23  Dad)
 22 are present in homologous pairs
 Pair up based on length, centromere
position, genes
 Known as autosomes
A Karotype
Genetic Info
 Last pair – sex chromosomes (X and Y)
 Sperm & egg cells have 23 chromosomes
 22 autosomes, 1 sex
 Haploid number (n)
 Made by meiosis
 Fertilization (syngamy) makes zygote
(fertilized egg) – now 46 chromosomes
 Diploid (2n)
Life Cycle Varieties
 Animals –
picture A
 Fungi &
some algae –
picture B
Life Cycle Varieties
 Plants & some algae – picture C
 Alternation of Generations
 Multicellular diploid stage – sporophyte
 Meiosis makes haploid spores
 Spore goes through mitosis – makes haploid
multicellular – gametophyte
 Gametophyte makes gametes by mitosis
 Fertilization makes zygote, which becomes
sporophyte
Meiosis – Let’s Investigate
 One replication,
two consecutive
divisions
 Result – 4 haploid
daughter cells
 Figure of meiosis
steps in text –
STUDY
Terms to know
 Synapsis – pairing up of homologous
chromosomes
 Tetrad – pair of homologous chromosomes
 Crossing over of
non-sister chromatids
– chiasmata (chiasma)
 Results in variations
How Do We Get Variety?
 Independent Assortment of Chromosomes
 Before meiosis, how chromosomes line up on
metaphase plate is random
 Each pair is independent
 Metaphase I for tetrad; metaphase II for
chromatids
 Results in many combinations of
chromosomes in gametes (8 million
possibilities)
How Do We Get Variety?
 Crossing Over
 Prophase I
 Homologous chromosomes exchange
segments of information from non-sister
chromatids
 Humans average 2-3 crosses per pair
How Do We Get Variety?
 Random Fertilization
 Random sperm – 8 million possibilities
 Random egg – 8 million possibilities
 Result – 1 zygote of over 70 trillion without
any crossing over