Cell Cycle (Mitosis)

Cell Cycle - 06/10/22

The Cell Cycle is made up of 2 phases: interphase and mitosis. Interphase is the main part of the cell cycle and it takes longer than mitosis. The 4 phases of interphase are: G0 phase, G1 phase, S Phase and G2 phase. For example, if the cell cycle were to take up 20 minutes of time to fully complete, 18 of those minutes would be interphase and 2 minutes would be mitosis.

Key Words Important things to mention

G0 phase

This phase is a resting phase for cells. Apoptosis/ Control Cell: Programmed Cell Death (A cell can only be copied around 50 times, after this many times of copying, the DNA in the cell becomes old and cannot be used so the cell will self destruct) Differentiation: Where cells become specialised for their function. (Unspecialised cells do not have a specific function.) Senescence: Where cells grow old (Refer back to the brackets of apoptosis for explanation) Some cells may not have this phase at all (Eg, epithelial cells in the gut lining).  Some cells remain in this phase for a long amount of time. (Eg, neurons) The difference in how long certain types of cells stay in this phase depends on how many subcellular structures need to be copied. 

G1 Phase AKA Growth Phase

The cells begin to increase in size so that they are big enough to split into 2.  The organelles duplicate - subcellular structures are all replicated except chromosomes.  Transcription of genes to make RNA occurs - RNA will be needed in the new cell for protein synthesis. Protein Synthesis Occurs - Enzymes (proteins) will be needed to control the next phase.

S Phase

This is the phase where DNA replication occurs. Every molecule of DNA is replicated in this stage, the genes are replicated in a specific order.  First, housekeeping genes are replicated.  Housekeeping genes: genes required for basic cellular function which are expressed in all cells under normal conditions. Secondly, inactive genes of DNA are replicated.  Each resulting chromosome of DNA replication consists of a pair of identical sister chromatids. Once a cell has entered this phase, there's no turning back.

G2 Phase

This phase is responsible for preparing the cell for mitosis.  The cell continues to grow in this stage as well. The G2 phase is not a necessary part of the cell cycle, as some cell types (Eg, xenopus embryos and some cancers) proceed directly from DNA replication to mitosis.

Mitosis

This is the stage where the chromosomes are split. Cell growth stops now. This phase is made up of 4 stages: Prophase Metaphase Anaphase Telophase

Prophase

Prophase (binary/octet-stream)

Caption: : Prophase

Chromatin condenses into chromosomes The nuclear envelope breaks down The centrioles near the nucleus separate and move to the opposite sides of the pole. 

Metaphase

Metaphase (binary/octet-stream)

Caption: : Metaphase

The spindle captures all the chromosomes and lines them up along the middle of the cell ready to divide. The spindle checkpoint is here, the cell checks to see if all chromosomes are attached to the spindles correctly before splitting them apart so that the daughter cells have the correct numbers of chromosomes. The cell will halt the cycle if any errors are found and will fix it. 

Anaphase

Anaphase (binary/octet-stream)

Caption: : Anaphase

The sister chromatids are separated and pulled to opposite sides of the cell. This process is driven by motor proteins.

Telophase

Telophase (binary/octet-stream)

Caption: : Telophase

The cell is nearly done dividing and tries to establish its normal structures as cytokinesis occurs. The mitotic spindle is broken down. 2 new nuclei form. Nuclear membranes re-appear Chromosomes return to their stringy form

Cytokinesis

The cells are pinched and 2 cells are made.  The pinch crease is known as the cleavage furrow.  2 new cells are formed at the end of cytokinesis.