Cell Division, Cell Diversity and Cellular Organisation Flashcards
Cell cycle
Before dividing, cells must grow and synthesise new organelles and molecules - these processes occur in a particular order, which is known as the cell cycle
Which part of the cell cycle does the most growth and synthesis occur?
Interphase
What percentage of the cell cycle does mitosis typically occupy?
5-10%
What happens at G1?
First growth phase - organelles are synthesised and bio chemicals are produced
What happens during the S phase?
DNA is replicated in the nucleus
What happens during G2?
Second growth phase - energy stores are increased and duplicated DNA is checked for errors
What happens during G0?
- Some mature, differentiated cells such as neurones, no longer divide
- The cycle can be halted in cells with damaged DNA
- These cells are said to have entered G0
Checkpoints of the cell cycle
- The cycle has checkpoints at the end of G1, S, and G2 that verify whether each phase of the cycle has been completed correctly
- These checkpoints are controlled by proteins called cyclins and cyclin-dependent kinases
- The cell cycle can be halted when errors are detected at a checkpoint
The importance of mitosis - processes
Growth, repair (of damaged cells), replacement (of cells, such as red blood cells, that have limited lifespans), asexual reproduction (in eukaryotes)
Chromatids
Two identical DNA molecules (a chromosome) held together at a centromere)
Prophase in mitosis
- Chromatin (uncondensed DNA in a complex with histones) condenses - chromosomes become visible
- Nucleolus disappears
- Centrioles move to the poles of the cell
- Nuclear envelope breaks down (towards the end of prophase)
Metaphase in mitosis
- Spindle fibres (organised by the centrioles) attach to centromeres (towards the end of prophase/beginning of metaphase)
- Chromosomes line up along the centre (equator) of the cell
Anaphase in mitosis
- Spindle fibres shorten
- Centromeres divide
- Chromatids are separated and pulled to opposite poles of the cells
Telophase in mitosis
- Chromatids are at the poles of the cells (and can be referred to as daughter chromosomes)
- Nuclear envelopes reform around each set of chromosomes
- Chromosomes uncoil (and are no longer visible)
- Cell division (cytokinesis) begins
Cytokinesis
- The division of a cell (cytokinesis) begins in telophase, resulting in two genetically identical cells
- Each cell receives approximately half of the organelles and cytoplasm from the original cell
- In animals: a cleavage furrow forms (i.e. cell surface membranes are pulled together by the cytoskeleton)
- In plants: the cell wall prevents cleavage furrows. The two daughter cells are instead separated by new cell wall production down the centre of the original cell
Meiosis
Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells
Prophase I of mitosis
- The key events of mitotic prophase occur (i.e. nuclear envelope disintegrates, nucleolus disappears, spindles form, chromosomes condense)
- Homologous chromosomes pair up to form bivalents
- Crossing over occurs
Metaphase I of mitosis
- Homologous pairs (bivalents) line up at the cell equator
- Independent assortment of chromosomes