1.6 Cell Division

Mitosis

- Is the division of the nucleus into two genetically identical daughter nuclei
- Chromosome condense by supercooling during mitosis
- Interphase is a very active phase of the cell cycle with many processes occurring in the nucleus and cytoplasm

Cell Division - Prokaryotes

Binary Fission;
1. Copy DNA
2. Separate DNA

Cell Division - Eukaryotes

Mitosis
- Eukaryotic cells divide their nuclear material in a process called mitosis
- Produces genetically identical daughters

Reason for cell division

Growth
- Multicellular organisms increase their size by increasing their number of cells through mitosis
- Large cells have reduced SA:Vol ration - more efficient

Heal/ Repair
- Damaged tissue can recover by replacing dead or damaged cells

Reproduce
- Asexually and sexually

Embryonic Development
- A fertilised egg (zygote) will undergo mitosis and differentiation in order to develop into an embryo

Chromosomes

Condense by supercoiling during mitosis
- Need to be stored inside the nuclei of cells
- Human chromosomes are 15mm to 85mm
(15,000um to 85,000um)

Why supercoil chromosomes?

- DNA is usually loosely packed within the nucleus as unravelled chromatin prior to division
- 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-diving cells and throughout the process interphase
- During mitosis, this becomes a problem
( Chromosomes need to be short and compact enough)
- In the condensed form, the DNA is able to be easily segregated however is inaccessible to transcriptional machinery

Chromatid

One of two identical "sister" parts of a duplicated chromosome

Centromere

- The part of a chromosome that links sister chromatids

Centrioles

Organise spindle microtubules

Spindle microtubules

Also referred to as spindle fibers

Important points

- After anaphase, when the sister chromatids separate they should be referred to as chromosomes
- In animal cells, two centrioles are held by a protein mass referred to as a centrosome

The Cell cycle

- The series of events through which cells pass to divide and create two identical daughter cells
- Nucleus divides first
- Cytoplasm then divides

Interphase

- Consists of the part of the cell cycle that doesn't involve cell division
- A metabolically active phase
- DNA is replicated
- Chromosomes contain 2 identical chromatids

Interphase - G1

- Cell growth (SA:Vol ration decreases)
- Preparation for mitosis
- Organelles produced
- Proteins synthesised

Interphase - S

- Synthesis
- DNA is replicated

Interphase - G2

- Cell growth
- Preparation for mitosis
- Organelles produced
- Protein synthesised

Interphase - G0

- Resting phase
- The cell as left the cycle and has stopped dividing
- Still carry out normal functions

MR POD

MR - metabolic reactions

P- Proteins synthesis
O - Organelles numbers increase
D - DNA is replicated

Metabolic Reactions

Metabolic reactions (e.g. respiration to produce ATP) are necessary for the life of the cell

Protein synthesis

Proteins and enzymes are necessary to allow cell growth

Organelles

Organelles numbers are increased to first support the enlarged cell

DNA

DNA is replicated to ensure a second copy is available to enable mitosis

Prophase

DNA supercoils and becomes visible under the microscope
- Chromatin condenses and becomes inter chromatids
Centrioles move apart & spindle begins to form
- Centrosome move to opposite poles of the cell
- Spindle fibres begin to form between them
Nuclear membrane is broken down and disappears
- So that sister chromatids can be pulled apart

Metaphase

- Microtubules attacked to each centromere move to the chromatids so that each centromere lies on the equator
- Spindle fibres from each two centrosome attach to the centromere of each pair of sister chromatids
- Contraction of the microtubules spindle fibres cause the sister chromatids to line up along the Center of the cell

Anaphase

- Microtubules pull on the centromere moving the chromosome to the pole
- Continued contraction of the microtubules spindle fibres cause the separation of the sister chromatids
- Chromatids are now referred to as chromosomes
- Chromosomes move to the opposite poles of the cell

Telophase

- Chromosome arrive at the poles
- Chromosome uncoils, decondense to chromatin
- No longer visible
- Microtubules spindle fibers disappear
- Nuclear envelope is formed around each group of chromosomes

PMAT

Pizza - Prophase
Makes - Metaphase
Anaelle - Anaphase
Talk - Telophase

Cytokinesis and the control of the cell cycle

- Cytokineses occurs after mitosis and is different in plant and animal cells
- Cycling are involved in the control of the cycle
- Mitosis is the division of the nucleus
- Cytokinesis is the division of the cytoplasm and hence, the cell

Cytokinesis

- The division of the cytoplasm and hence, the cell
- The division of the cell into 2 daughter cells occurs concurrently with telophase
- Though mitosis is similar for animal and plant cells, cytokinesis is very different
(due to cell wall in plant cells)

Cytokinesis in animal cells

1. A ring of contractile protein (microfilaments) immediately inside the pm at the equator pulls the pm inward
2. The inward pull on the pm produces the characteristic cleavage furrow
3. When the cleavage furrow reaches the centre of the cells, it is pinched apart to form two daughter cells

Cytokinesis in plant cells

1. During telophase, the membrane-enclosed vesicles derived from the Golgi apparatus migrate to the Center of the cell
2. Vesicles fuse to form tubular structures which form two layers of pm (e.g. cell plate)
3. The cell plate continues to develop until it connects with the existing cell's pm
4. This complete the division of the cytoplasms and the formation of the 2 daughter cells