Cell division

Question 1

What is a cell cycle?

Answer 1

• precise sequence of events which results in a somatic cell (ie not a gamete) splitting into two identical cells.
• 46 chromosomes split into 23 homologous pairs. Somatic cells have 2 sets of chromosomes and are referred to as “diploid cells”

Question 1

What happens in interphase?

Answer 1

• Genetic DNA is replicated.
• Additional organelles & cystosolic components are synthesised.
• State of high metabolic activity
• Consists of 3 phases:
  1. G1 – interval between mitotic phase and S phase
  2. Cell replicates most of its organelles and cytosolic components, but not DNA.
  3. Cells that remain in G1 for long periods or never replicate are described as in G0 (eg nerve cells).

Question 2

What happens in the “S” Phase?

Answer 2

• DNA replication occurs.
• The helical structure largely uncoils and pairs with new base nucleotides and bonds are formed.
• 2 identical new DNA molecules are produced.

Question 3

What happens in the G2 Phase?

Answer 3

• The second gap phase.
• The cell continues to grow.
• More proteins and organelles are manufactured
• The cell prepares to reorganise its contents in preparation for mitosis.
• G2 ends when mitosis begins

Question 4

What happens in the prophase?

Answer 4

• The chromatin fibres organise into chromosomes (a pair of identical, double stranded chromatids). The pair is held together by a centromere.
• The mitotic spindle (an assembly of microtubules) forms. This will push the chromatids to opposite ends of the cell.
• The nuclear envelope dissolves and the nucleolus is broken down.

Question 5

What hapens in the metaphase?

Answer 5

• The microtubules line up the centromeres (the centre point of the chromosomes) in the exact centre of the mitotic spindle (the metaphase plate).

Question 6

What happens in Anaphase?

Answer 6

• Centromeres split.
• Identical pairs of chromosomes migrate to opposite poles of the cell.

Question 7

What hapens in Telophase?

Answer 7

• Chromosomal movement stops.
• New, identical sets of chromosomes at opposite ends of the cell uncoil and revert to chromatin form (thread like).
• Nuclear envelope forms around each chromatin mass.
• Nucleoli reappear.
• Mitotic spindle breaks up.

Question 8

What happens in cytokenesis?

Answer 8

• Begins in late anaphase.
• Indentation forms in plasma membrane – “cleavage furrow”.
• Microfilaments inside the plasma membrane form a contractile ring.
• Plasma membrane pulled progressively inward.
• Cell ultimately pinched into two identical cells.
• When cytokinesis is complete, cell returns to inter.

Question 9

What happens when the cell cycle goes wrong?

Answer 9

• Causes problems with dysregulation of cells which may die too much, not enough, or reproduce with a fault. Diseases and disorders that can result include:
  1. * Cancers.
  2. Neurodegenerative diseases eg Alzheimers, Huntington’s disease, Parkinson’s disease.
  3. Retinal disorders.
  4. Amyotropic lateral sclerosis.
  5. Lupus.