What are the phases of the cell cycle?
- Interphase
- G1
- S
- G2
- M
- Mitosis
- Cytokinesis
What happens in G1
- replicate mitochondria, robosomes, ER
- increase cell size
- Includes a restriction point
What happens in S
Clone DNA to create 2 chromatids joined at the centromere
What happens in G2
- Checkpoint
- enough organelles?
- correctly replicated DNA?
What happens in M
- Prophase
- Metaphase
- Anaphase
- Telophase
Prophase
- Chromatin condenses into chromosomes
- centrioles go to poles
- spindle fiber and aster formation
- nuclear membrane dissolves
- kinetochore fibers attach to kinetochores
Metaphase
microtubles align the chromosomes at the metaphase plate
Anaphase
sister chromatids are pulled apart by shortening of the kinetochore fibers
Telophase
- spindle apparatus disappears
- nuclear membranes form
- chromosomes relax
- cytoplasm and organelles are split in cytokinesis
What are the phases of meiosis
- Prophase I and II
- Metaphase I and II
- Anaphase I and II
- Telophase I and II
General discription of meiosis I
reductional division: homologous chromosomes are separated creating 2 haploid daughter cells
prophase I
- chromatin condenses into chromosomes
- spindle apparatus forms
- nuclear membrane breaks down
- synapsis
-homologous chromosomes (maternal and parental) form a tetrad held together by the synaptonemal complex
-crossing over can occur at chiasmata
Why is crossing over important
- can unlink linked genes which increases genetic variability of gametes
- explains Mendel’s 2nd law of independent assortment
metaphase I
- tetrads align at the metaphase plate
- each homologous pair attaches to its own kinetorchore fiber
anaphase I
- disjuction: the maternal and paternal chromosomes separate
- accounts for Mendel’s 1st law of segregation
telophase I
- nuclear membranes form
- cells now haploid
interkinesis
- may have a brief intermission between meiosis I and II
- chromosomes may uncoil a little
general description of meosis II
equational division: sister chromatids are separated which does not chance ploidy
prophase II
- nuclear membranes dissolve
- centrioles go to the poles
- spindle apparatus forms
metaphase II
chromosomes align at the metaphase plate
anaphase II
sister chromatids are pulled apart
telophase II
nuclear membranes form
differences between mitosis and meiosis
- mitosis
-2n to 2n
-somatic cells
-homologous chromosomes do not pair
-no crossing over - meiosis
-2n to n
-sex cells only
-homologous chromosomes pair
-can have crossing over
