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What are the 4 phases in the cell cycle?
- -G1 phase
- -S phase
- -G2 phase
- -M phase
Explain G1 phase
- First gap
- The time cell grows in size and carries out normal functions.
Explain S phase
- DNA synthesis phase
- All the DNA in the nucleus are duplicated, so that all chromosomes have replica of themselves.
Explain G2 phase
- Second gap phase
- The cell prepares to divide by checking for errors in DNA replication, and multiplying important organelles like mitochondria.
Explain M phase
- Mitosis, or the division phase
- The nucleus divides, and the cell divides to make two new cells.
What are the two ways cells in eukaryotic organisms divide?
- This is the most common type of cell division, by which cells are added to the growing body, to replace lost cells, to heal wounds etc.
- The purpose of mitosis is to divide a cell in to two new cells that are identical to each other and to the original cell.
- This happens only during the production of gametes.
- The purpose of meiosis is to divide the hereditary material in the nucleus by half, and to produce unique gametes.
- Important for evolution because it adds to the variations among individuals in a population.
What are the stages of Mitosis?
- chromosomes condense, and sister chromatids become visible.
- Centrosomes divide and migrate to opposite poles of the cells, forming a spindle of microtubles.
nuclear envelope disappears, spindle fibers from opposite poles attach to kinetochores of sister chromatids
Chromosomes align at the center of the cell, sister chromatids attached to spindle fibers.
Spindle fibers shrink, pulling sister chromatids of each chromosome apart from each other, towards opposite poles
- new nuclear envelopes form at opposite ends of the cell.
- Chromosomes stretch again to threadlike form.
What follows nuclear division?
- Dividing the cell into two new cells that carry exactly the same hereditary materials in both.
They are the matching pairs of chromosomes in all diploid (2n) organisms like humans.
What are the stages in Meiosis I?
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
- Most unique stage of meiosis.
- Homologous chromosomes come together (synapsis), and exhange parts from each other's sister chromatids.
- Crossing over
- The crossing over pts look like an "X"
- Called chiasma
- Spindle fibers from opposite poles of the cell attach to each homolog.
The homologs align at the cell center
- Homologous chromosomes are pulled by spindle to the opposite poles of the cell.
- Sister chromatids are still attached to each other at centromeres.
Nuclear envelopes may reform in some species, the spindle disappears.
What follows meiosis I?
- New cells are haploid (1n)
The purpose of meiosis II?
To separate sister chromatids that are still attached. Therefore, meiosis II can ve compared to mitosis.
Spindle reappears, and spindle fibers from opposite poles connect to kinetochoes of sister chromatids.
Chromosomes are lines up at the cell center
Sister chromatids are separated from each other, and are pulled to the opposite poles of the cell due to shrinking of spindle fibers.
Nuclear envelope reforms, chromosomes stretch.
What is Meiosis II followed by?
- -Second cytokinesis.
- -Resulting in 4 haploid cells.