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IB Biology - Cell Cycle and Mitosis

Chapter 1.6 - Cell Division

May 6, 2020
  • Mitosis is division of the nucleus into two genetically identical daughter nuclei.
  • Chromosomes condense by supercoiling during mitosis.
  • Cytokinesis occurs after mitosis and is different in plant and animal cells.
  • Interphase is a very active phase of the cell cycle with many processes occurring in the nucleus and cytoplasm.
  • Cyclins are involved in the control of the cell cycle.
  • Skill: Identification of phases of mitosis in cells viewed with a microscope or in a micrograph.

In previous IB biology articles, we always mention the cell replication. The cell undergoes cell contents replication and divides into two daughter cells via cell cycle.

The Cell Cycle

  • A series of events that take place during cell replication and division 
  • Consisted of 
  • Interphase, Mitosis, Cytokinesis
  • And each of the phases contains sub-phases which will be discussed later 

Interphase 

  • The cell 

prepares to undergo cell division, hence all the cell contents are replicated actively 

Interphase = the DNA and cell contents are replicated to prepare cell division 

  • Consisted of 
  • G

1, S, and G2 sub-phases

  • G

1 phase:

  • - Cell grows

- Protein is produced via DNA transcription and translation (you will learn in later IB biology syllabus)

  • S phase:
  • the chromosome is duplicated
  • G

2 phase:

- Everything is doubled, so the cell is well-prepared for the division

  • the nucleolus (the production site of ribosome inside the nucleus) duplicates itself 
  • an organelle centrosome (built up by two centrioles) duplicates itself
  • the cell required a lot of energy for division, so 
  • mitochondria increases 
  • Since the replication of mitochondria and chloroplast is not linked to the cell, these two organelles replicate themselves by binary fission independently to ensure each of the daughter cells receive one (IB Biology chapter 1.5) 
  • Under the microscope, no change can be observed, some time may observe two nucleolus 

Mitosis

  • in this phase, the cell is 

divided into two genetically identical cells 

Mitosis = division of the nucleus into two genetically identical daughter nuclei.

  • since the nucleus of 
  • two daughter cells
  •  is derived from the same mother cell, so they are genetically identical 
  • mitosis is important to 
  • repair the dead or damaged cell 
  • it is essential to 
  • embryo development

. The embryo replicates itself to give multiple of cells which differentiate into different cell types (IB biology chapter 1.1 Differentiation and Stem cells )

  • for 

unicellular eukaryotes, mitosis is the way to produce offspring which is called asexual reproduction. The offsprings thus are genetically identical to the mother (IB biology chapter 1.1 The Unicellular and Multicellular organism)

  • consists of 5 sub-phases including, 

prophase, metaphase, anaphase, telophase and cytokinesis 

  • the phases take place continuously 

Prophase 

  • the chromosome condenses themselves by supercoiling 
  • the condensation reduce the size of the chromosome, increasing the ease in the following sub-phases 
  • since the chromosome is replicated, these 

two genetically identical chromosomes are called sister chromatids 

  • sister chromatids are joined at a site called the centromere 
  • the microtubules grow out from the centrosomes, separating the two centrosomes apart and pushing them to the two opposite poles of the cell 

(some eukaryotes do not have centrosomes, the organization of microtubules depends on other organelles) 

  • the nuclear envelope breaks down, allowing the spindles to attach the centromere and giving more space for chromosome to move in the following sub-phases
  • under the microscope, the chromosome becomes visible, and the nuclear envelope disappears 

Metaphase

  • begins when the nuclear envelope disappears 
  • the sister chromatids are moved by microtubules to align themselves in the middle of the cell spindles

 from centrosomes attach to the centromere of the sister chromatids 

  • under the microscope, the chromosomes are aligned in the middle of the cell

Anaphase 

  • the 
  • spindles shorten themselves
  •  so the sister chromatids are separated from each other 
  • as the spindles keep shortening, the 

separated chromosomes are pulled towards the centrosomes at the opposite poles

  • the sister chromatids are separated 
  • under the microscope, the chromosomes are pull towards the two opposite poles 

Telophase 

  • when the chromosomes reach to the opposite poles, the 
  • spindles disappear 
  • nuclear envelope formed

 around each set of chromosome 

  • chromosome uncoiled 
  • under the microscope, the nuclear envelope formed and the chromosome becomes invisible again 
  • it is different from the interphase as the 

two nuclei are still within one cell, the cytoplasm is not yet separated while in interphase, each cell only possess one nucleus 

Cytokinesis

  • Animal and plant cells are different in the cytokinesis
  • Animal cells:
  • the 
  • cell membrane moves inwards at the equator of the cell. Once the cell membranes are formed, the nuclei are separated into two independent cells 
  • Plant cell: 

- a cell plate grows outwards from the equator of the cell. separating the cytoplasm into two 

- vesicle accumulates at the cell plate, releasing cellulose and pectin for cell wall formation

- cell wall is built along with the cell plate and the nuclei are separated once the cell wall is complete

Whole process of cell cycle 

Cyclin-CDK Complex 

  • there are some
  •  checkpoints
  •  in the sub-phases G1, S, G2, and Mitosis to guard the cell cycle 
  • G

1 checkpoint is the main guard of the cell cycle, once the cell passes the G1 checkpoint, it has to go through the whole cell cycle 

  • if not, the cell stop and back to 
  • G

0 phase, which is the resting phase of the cell. 

  • actually most of the cells are in the resting state or else tumour is developed (will be discussed in a later article)
  • there is a protein called 

CDK (cyclin-dependent kinase), it only activates when cyclin bind to it 

  • the presence of 

cyclin-CDK complex control whether the cell is "qualified" to pass the checkpoint or not

  • the complex also 

directs the specific events in the cell cycle 

e.g. the formation of microtubules, alignment of sister chromatids 

  • there are 4 types of cyclins in 4 checkpoints 
  • G

1 cyclin:

- the concentration of G1 cyclin is affected by cell growth signal from the outside. 

  • - it increases throughout the interphase and drops at the end of mitosis 

- it gives the signal to the cell whether or not passing the G1 checkpoint 

  • G

1/S cyclin:

  • - initiate the

 starts of the chromosomes, centrosomes replication 

- the concentration peaks at the middle of G1 and S phase 

  • S cyclin

:

- promotes chromosome replication and early events of mitosis 

- the concentration rises and peaks at G2 phase and drops at the early mitosis 

  • M cyclin

:

- promote the microtubules and spindles formation 

- peak at mitosis 

  • when the cell cycle ends, the cyclin-CDK complex breaks down, the cell rest in G

0 phase 

** Some concepts you have to be clear,

  • homologous chromosome
  • : the pair of chromosomes, one inherited from mother, another is from father, the genetic materials are different between the two pairs but their length, size, and genes locations are the same
  • (they don't join together! unlike sister chromatids) 
  • sister chromatids
  • : two genetically identical chromosomes stick together after duplication 
  • centromere

: the connecting point of the two identical chromosomes 

  • centrosome

: an organelle produces the microtubules and spindle 

  • centrioles

: the building block of centrosome

  • not all eukaryotes possess centrosome 

In this IB Biology article, you have to remember :

  1. the characteristics of the different phases, especially the sub-phases in the mitosis phase 
  2. able to identify which stage is it based on the cell diagram 
  3. don't mess up the sister chromatids, chromosome, centromere, and centrosomes
  4. the function and importance of cell cycle checkpoint and cyclin

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