Cell Cycle, Mitosis and Meiosis Flashcards
What is the cell cycle?
A highly ordered sequence of events, which results in the division of a cell and two genetically identical daughter cells.
What are the two phases of the cell cycle?
Interphase and Mitotic phase
What is the first stage of interphase?
G1 (First Growth Phase)
What happens during G1?
Proteins that produce organelles are synthesised and organelles are replicated. The cell increases in size.
What is the second stage of interphase?
S (Synthesis Phase)
What happens during S phase?
DNA is replicated in the nucleus
What is the third stage of interphase?
G2 (Second Growth Phase)
What happens during G2?
The cell continues to increase in size, energy stores are increased and duplicated DNA is checked for errors
What are the two stages of cell division?
Mitosis and Cytokinesis
What happens during mitosis?
The nucleus divides
What happens during cytokinesis?
The cytoplasm divides and two cells are produced
What is the Go phase?
Phase when the cell leaves the cycle, either temporarily or permanently
Why might a cell leave the cycle?
- Differentiation: Cell becomes specialised and can no longer divide (permanent)
- DNA of cell may be damaged: Cell can no longer divide and enters a permanent period of cell arrest (Go)
- Majority of cells only divide a limited number of times
- Very few types of cells that enter Go can be stimulated again to go back into the cycle. For example, LYMPHOCYTES can begin to divide again in an immune response
What are checkpoints?
Stages at various points around the cycle that monitor and verify whether the processes at each phase have been accurately completed.
What are the three cell cycle checkpoints?
G1 checkpoint, G2 checkpoint and Spindle Assembly checkpoint (metaphase checkpoint)
What does the G1 checkpoint allow?
If all the requirements are met then the cell is able to begin DNA replication, if not it enters Go
What does the G2 checkpoint check?
Checks whether the DNA has been replicated without error (and other factors). If it is without error then the cell initiates the molecular processes that signal the start of mitosis
What does the metaphase checkpoint check?
Checks whether all of the chromosomes are attached to spindles and have aligned, if not then mitosis cannot proceed.
What is the first phase of mitosis?
Prophase
What happens in prophase?
- Chromatin condenses and chromosomes become visible
- Centrioles move to opposite poles of the cell
- Nuclear envelope breaks down
What is the second phase of mitosis?
Metaphase
What happens in metaphase?
- Chromosomes have aligned at the metaphase plate
- Spindle fibres have attached to the centromere of the chromosomes
What is the third phase of mitosis?
Anaphase
What happens in anaphase?
- Spindle fibres shorten
- Centromeres divide
- Chromatids are separated and pulled to opposite poles of the cell
What is the fourth phase of mitosis?
Telophase
What happens in telophase?
- Nuclear envelope reforms around each set of chromosomes
- Chromatin reforms
What happens in cytokinesis in animals?
- A cleavage furrow is made by the cytoskeleton pulling in the cell surface membrane
- The cell splits
- Chromatin is reformed in the nucleus
What happens in cytokinesis in plants?
- Vesicles from Golgi form where the metaphase plate was and produced a new cell wall and cell surface membrane
- This allows the cell to always have a cell wall around it so it doesn’t lyse from osmotic or turgor pressure problems
How to tell the number of chromosomes?
Count the centromeres! (beware of the diagram that looks like skis)
What is the first stage of meiosis I?
Prophase I
What happens during Prophase I?
- Chromatin has condensed and formed visible chromosomes
- Homologous chromosomes pair up to form bivalents
- Centrioles move to poles of cells
- Nuclear envelope disappears
What is the second stage of meiosis I?
Metaphase I
What happens during Metaphase I?
- Centrioles produce spindle fibres (microtubules made from tubulin) that grow towards the bivalents and attach to the centromere
- Bivalents align on the metaphase plate
What is the third stage of meiosis I?
Anaphase I
What happens during Anaphase I?
- Homologous chromosome pairs are separated
- Sister chromatid remain attached to centromeres
- Spindle fibres shorten
What is the fourth stage of meiosis I?
Telophase I
What happens during Telophase I?
- Nuclear membrane reforms
- 2 daughter nuclei are haploid
- Centrioles will replicate
What happens during Middle Prophase I?
- Crossing over process occurs
- The chromatids in the bivalent are broken and rejoined in a random fashion to make new chromatids
What happens during Late Prophase I?
- Centrioles moved to the poles of the nucleus
- Nuclear envelope starts to dissolve
What is the first stage of Meiosis II?
Prophase II
What happens in Prophase II?
- Chromosomes condense and become visible
- Nuclear envelope breaks down
What is the second phase of Meiosis II?
Metaphase II
What happens during Metaphase II?
- Chromosomes align at the metaphase plate
- Spindle fibre attach to centromere of chromosomes
What happens in both metaphase I and II
Independent Assortment
What is Independent Assortment?
Bivalents line up on the metaphase plate in a random process that is independent of each other. This causes GENETIC VARIATION
What is the the third phase of Meiosis II?
Anaphase II
What happens during Anaphase II?
- Centromere divides and the sister chromatids are pulled a part by the spindle fibre contracting
- Spindle fibres shorten
What is the fourth phase of Meiosis II?
Telophase II
What happens during Telophase II?
- Nuclear membrane reforms
- The four daughter nuclei are haploid and genetically different from each other
What is crossing over?
- Occurs in middle prophase I
- Chromatids in bivalent are broken and rejoined in a random fashion to make new chromatids
- Causes GENETIC VARIATION
What occurs in anaphase I to cause genetic variation?
Sections of DNA which became entangled during crossing over now break off and rejoin
This sometimes results in an exchange of DNA
When exchange occurs, this forms recombinant chromatids, with genes being exchanged between chromatids
Genes being exchanged may be different alleles of the same gene, which genetic variation arises from this
