Topic 1.6 Cell Division

Question 1

Cell Cycle - Interphase

Answer 1

G1 – First intermediate gap stage in which the cell grows and prepares for DNA replication

S – Synthesis stage in which DNA is replicated

G2 – Second intermediate gap stage in which the cell finishes growing and prepares for cell division

Question 2

Cell Cycle - M phase

Answer 2

Mitosis – Nuclear division, whereby DNA (as condensed chromosomes) is separated into two identical nuclei

Cytokinesis – Cytoplasmic division, whereby cellular contents are segregated and the cell splits into two

Question 3

Key processes of Interphase

Answer 3

D - DNA replication – DNA is copied during the S phase of interphase
O - Organelle duplication – Organelles must be duplicated for twin daughter cells
C - Cell growth – Cytoplasmic volume must increase prior to division
T - Transcription / translation – Key proteins and enzymes must be synthesised
O - Obtain nutrients – Vital cellular materials must be present before division
R - Respiration (cellular) – ATP production is needed to drive the division process

Question 4

Chromosome vs Chromatid

Answer 4

Chromatin:

• DNA is usually loosely packed within the nucleus as unravelled chromatin
• In this unravelled form, the DNA is accessible to transcriptional machinery and so genetic information can be translated
• DNA is organised as chromatin in all non-dividing cells and throughout the process of interphase

Chromosome:

• DNA is temporarily packaged into a tightly wound and condensed chromosome prior to division (via supercoiling)
• In this condensed form, the DNA is able to be easily segregated however is inaccessible to transcriptional machinery
• DNA is organised as chromosomes during the process of mitosis (condense in prophase, decondense in telophase)

Question 5

Mitosis

Answer 5

Prophase:

• DNA supercoils and chromosomes condense (becoming visible under microscope)
• Chromosomes are comprised of genetically identical sister chromatids (joined at a centromere)
• Paired centrosomes move to the opposite poles of the cell and form microtubule spindle fibres
• The nuclear membrane breaks down and the nucleus dissolves

Metaphase:

• Microtubule spindle fibres from both centrosomes connect to the centromere of each chromosome
• Microtubule depolymerisation causes spindle fibres to shorten in length and contract
• This causes chromosomes to align along the centre of the cell (equatorial plane or metaphase plate)

Anaphase:

• Continued contraction of the spindle fibres causes genetically identical sister chromatids to separate
  Once the chromatids separate, they are each considered an individual chromosome in their own right
  The genetically identical chromosomes move to the opposite poles of the cell

Telophase:

Once the two chromosome sets arrive at the poles, spindle fibres dissolve
Chromosomes decondense (no longer visible under light microscope)
Nuclear membranes reform around each chromosome set
Cytokinesis occurs concurrently, splitting the cell into two

Question 6

Mitotic Index Calculation

Answer 6

Cells in mitosis / Total number of cells