Meiosis and Sexual Life Cycles

Unit 3 – Chapter 13

Diploid vs. Haploid

• A cell that has both members of a homologous pair of chromosomes in the nucleus, is considered to be Diploid (2n)

• A cell that has only one homologue of a chromosome pair (so either the maternal or paternal homologue is missing) is considered Haploid (n)

Karyotypes are made from cells arrested at metaphase

KARYOTYPE OF A DIPLOID CELL KARYOTYPE OF A HAPLOID CELL

Importance of Meiosis

• Reduces the number of chromosomes by half – the cell goes from diploid (2n) to haploid (n) – humans go from 23 pairs to 23 single chromosomes

• Prevents polyploidy (multiple chromosomes) – and maintains the chromosome number of each species

• Produces genetic variation and contributes to evolution

The Human Life Cycle

Spermatogenesis

OVULATION

Egg and Fallopian Tube Lining

Egg and Fertilization

Zygote and First Mitotic Division

Sexual Life Cycles Produce Genetic Variation Among Offspring

• Independent Assortment

• Crossing Over

• Random Fertilization

Independent Assortment

• The orientation of the homologous chromosomes at the metaphase plate in metaphase I is completely random

• The maternal and paternal homologues can be on either side of the plate

• There is a 50% chance that a daughter cell of meiosis I will get a maternal homologue of a particular chromosome pair and a 50% chance that it will get the paternal homologue

Independent Assortment

• The number of combinations possible for gametes formed by meiosis can be determined by 2n, where 2 is the number of chromosomes per homologous pair and n is the haploid number of that organism.

• So for human – 223 = ~8 million chromosomal combinations possible in gametes

Independent Assortment

Crossing Over• This adds even more

variation to gametes

• Each time the homologous chromosomes swap alleles, the possible number of chromosomal combinations in gametes is increased even further

Random Fertilization

• A human egg that represents one of ~8 million possible chromosomal combinations, is fertilized by a sperm that represents one of ~8 million chromosomal possible combinations

• So even without crossing over occurring, the zygote will contain one of 64 trillion (8 million x 8 million) chromosomal combinations!!