M2C6 - Cell cycles Flashcards
What is interphase?
long periods of growth and normal working
What happens during G1?
G1 is the first growth phase. During this phase the cell grows in size, protein synthesis occurs and organelles duplicate. Largest number of cells.
What happens during S?
S is the synthesis phase. During this phase DNA is replicated. Each chromosome is duplicated.
What happens during G2?
G2 is the second growth phase. During this phase the cell grows in size, energy stores increase and the replicated DNA is checked.
What happens during G0?
This is the phase when a cell has left the cell cycle. It can occur because:
-An issue has occurred during a phase of the cell cycle e.g. DNA is damaged.
-If the cell has differentiated to perform a specialised function (it will now no longer divide).
-Can also occur due to the age of a cell - cells will only replicate a limited number of times before entering G0.
What is the role of checkpoints?
Control mechanisms that monitor and verify whether the processes at each phase of the cell cycle have been accurately completed before the cell is allowed to progress into the next phase.
What happens at G1 Checkpoint?
Checks for DNA damage (that DNA has been replicated correctly), cell size, nutrients, growth factors.
What happens at G2 Checkpoint?
Checks for cell size, DNA replication.
What happens during Prophase?
- Chromosomes condense
- Chromosomes have become visible (but are
unordered)
-Nuclear envelope and nucleolus break down and have disappeared.
-Centriole divides in 2 and each daughter centriole moves to opposite ends of the cell.
-Each centriole begins to make a spindle.
What happens during Metaphase?
Chromosomes move to the middle of the cell (the equator) and become attached to the spindle at the centromere (crossover point of chromosome).
What happens during Anaphase?
- Spindle fibres are shortening
- Centromere splits and chromatids are separating and are being
pulled to opposite sides of the cell.
What happens during Telophase?
Chromatids have been pulled to opposite sides
of the cell
* A new cell membrane is visible down the
centre of the cell
-nuclear envelope reforms around each individual set of chromatids.
-Chromosomes uncoil
-Cytokinesis / the cell is beginning to divide
What happens during Cytokinesis in animals?
-A cleavage furrow forms in the middle of the cell
-The plasma membrane is pulled inwards by the cytoskeleton until it fuses in the middle forming 2 separate cells.
What happens during Cytokinesis in plants?
-The cell walls prevent a cleavage furrow from forming.
-Vesicles from the Golgi apparatus assemble where the metaphase plate was and fuse together, dividing the cell down the middle.
-Cell walls form along these sections of membrane forming
a cell plate.
What happens during Prophase 1?
Chromosomes condense, nuclear envelope disintegrates, spindle forms.
-Unlike mitosis, the homologous chromosomes pair up to form bivalents. The homologous pairs then cross over.
What happens during Metaphase 1?
Homologous pairs line up at the centre rather than individual chromosomes.
-Because the pairs randomly line up, each chromosome in the pair can end up facing either pole. This is called independent assortment. And allows for genetic variation.
What happens during Anaphase 1?
-Homologous pairs are pulled to opposite poles but chromatids stay attached.
-The sections of sister chromatids that had crossed over during prophase 1 now break off and swap places.
-This is called genetic recombination.
>Formation of chiasmata (links non sister chromosomes in genetic recombination).
>Exchange of genetic material
-This swapping can lead to a new combination of alleles which was not present in either of the original chromatids
-This mixing up of alleles leads to further variation.
What happens during Telophase 1?
-Similar to telophase in mitosis:
-Chromosomes assemble at each pole, the nuclear envelope reforms and the chromosomes uncoil.
-Cytokineses then occurs forming 2 daughter cells. These cells are haploid.
What happens during Prophase 2?
Chromosomes condense, nuclear envelope disintegrates, spindle forms.
-Only 1 chromosome from each homologous pair is present.
What happens during Metaphase 2?
The individual chromosomes line up at the centre, like in mitosis.
There is more independent assortment so further genetic variation here.
What happens during Anaphase 2?
Chromatids rather than chromosomes are pulled to opposite poles (like in mitosis)
What happens during Telophase 2?
Chromatids assemble at each pole, the nuclear envelope reforms and the chromosomes uncoil. Like mitosis.
Cytokineses then occurs forming 2 daughter cells, each with half the number of chromosomes (haploid).
What are homologous chromosomes?
Two chromosomes in a pair – normally one inherited from the mother and one from the father. Carry the same genes at same loci.
How are erythrocytes specialised for their function?
Flattened biconcave shape, which increases SA:V, essential role is to transport oxygen round the body. I
-Don’t have nuclei or many other organelles which increases space available for haemoglobin.
-Also flexible to squeeze through capillaries.
