-
Diploid Cells
Two copies of each chromosome (one from each parent)
Human cells are diploid
-
Meiosis
- Making sex cells
- (egg and sperm...often referred as "gametes")
- Gametes have half the genetic information,
- they are haploid (one copy of each chromosome)
-
-
Zygote
The diploid result when a haploid sperm and haploid egg fuse together
-
Steps of Meiosis
- one round of chromosome duplication
- followed by two rounds of cell division (Meiosis I and Meiosis II)
-
Meiosis I
- Cells start off as diploid & Chromosomes have been duplicated:
- -Prophase I
- -Metaphase I
- -Anaphase I
- -Telephase I
Result: 2 haploid cells
-
Tetrads
- homologous chromosomes exhange reciprocal sections
- Takes place in Prophase I
-
Metaphase I
Microtubles move homologuous chromosomes to metaphase plate ( different than metaphase in mitosis!!)
-
Anaphase I
- Microtubles separate homologous chromosomes
- --Sister chromatids remain together!
-
Telophase I
- Nuclear envelope reforms
- spindle dissasembles
- 2 cells result from cytokinesis
-
-
Meiosis II
- Another Round of division
- -2 daugher cells made in Meiosis I divide again
- -this time, no DNA replication in between
This time, sister chromatids separate
Result: 4 haploid daughter cells
-
-
-
How to have Genetic Diversity
Gametes created by meiosis are genetically different from each other
- 2 Events:
- Crossing Over (formation of tetrads)
- Independent Assortment
-
Independent Assortment
Random distribution of homologous chromosome pairs during Metaphase I
-
Importance of genetic diversity
- ensures offspring will be unique
- more genomes for natural selection to choose from
- depedent on mutation or "horizontal" gene transfer (bacteria clonal strains)
-
Which diploid cells undergo division to form gametes?
- Males: Spermatogonium
- Females: Ooogonium
-
Sperm formation in males
Spermatogenesis starts at puberty and continues through life
- Spermatogonium - "stem" sex cells:
- -- can go on to make sperm
- OR
- --can go on to make more spermatogonia
-
-
About Oogenesis
- All or most potential gametes produced before birth
- meiotic divisions unequal, leading to formation of polar bodies
- puberty releases the "arrested state" of primary oocytes, leading to ovulation
- Meiosis II does not occur until fertilization of 2nd oocyte
- Ovulation and reproduction naturally end during menopause
-
-
Asexual Reproduction
reproduction that does not require sex
NOTE: only birds and mammals are ALL strictly sexually producing types of organisms..
-
Forms of asexual reproduction
Parthenogenesis: embryo develops from unfertilized egg
- Binary Fission: cell division resulting in 2 single-celled organisms
- --Different from meosis, two new cellls, not similar cells
- Vegetative reproduction: in plants, new individuals are made without seeds.
- **in animals: regeneration
-
Hermaphroditism
- Have both male and female reproductive parts
- Variation on sexual reproduction
- Example:
- Pea plants
- Earthworms
-
Differences between Mitosis and Meiosis
- Mitosis:
- diploid
- all the same
- all other cells
- Meiosis:
- haploid
- sex cells
- all different
|
|