Cell Cycle

Interphase — G1

Cell growth, committed to divide

Interphase — S phase

Each chromosome is replicated to form a pair of sister chromatids joined at the centromere

Interphase — G2 phase

Synthesis of proteins for cell division

Prophase

• Chromatin condenses to form chromosomes. Each chromosome comprises 2 sister chromatids joined at the centromere.

• Centrioles move to opposite poles and spindle fibres start to form.

• Nucleolus disappears & nuclear envelope disintegrates into vesicles.

Metaphase

• Chromosomes align at the metaphase plate via kinetochore microtubules

• Sister chromatids of metaphase chromosomes are on either side of the equator

Anaphase

• Centromere of each chromosome divides, sister chromatids separate to form daughter chromosomes

• Kinetochore microtubules shorten, pulls daughter chromosomes centromere-first to opposite poles

• Cell elongates as non-kinetochore microtubules lengthen

Telophase

• Mitotic spindle disassembles

• Nuclear envelope reforms

• Mitotic chromosomes decondense

• Nucleoli reappear

Cytokinesis

ANIMAL CELLS

• Cleavage furrow occurs (cell membrane invaginates)

• Deepens and divides the cell along a plane at the spindle equator, forming 2 diploid cells

PLANT CELLS

• Fluid-filled vesicles appear in the middle of the cell and coalesce to form a cell plate, separating the 2 daughter cells

Importance of mitotic cycle

1. Allows for a multicellular organism to grow

   1. Enables the growth of new genetically identical cells 

2. Allows for replacement of damaged cells

   1. Produces new genetically identical cells to replace cells that die from normal wear and tear

3. Allows for asexual reproduction to occur

4. Maintains the chromosome number of the organism

Need for regulation of mitotic cycle

1. The cell cycle is tightly regulated as it is important for normal growth & development

2. Cancer occurs when the dysregulation of checkpoints of cell division occur or cells escape the cell cycle control mechanism that normally regulates their growth. This leads to uncontrolled division of cells (i.e. tumour formation) and possibly cancer

Checkpoints in cell cycle

G1 phase

• Cell size, nutrient level, growth factors, DNA damage

G2 phase

• DNA replication — if there is single stranded DNA, cell division will not occur

M phase

• Whether kinetochore is properly attached to spindle, whethere chromosomes are properly separated

Prophase I

• Chromatin condenses to form chromosomes. Each chromosome comprises 2 sister chromatids joined at the centromere

• Synapsis occurs → homologous chromosomes pair up to form bivalents

• Crossing over occurs between non-sister chromatids of homologous chromosomes, forming chiasmata. Exchange of corresponding alleles on non-sister chromatids occurs

• Centrioles move to opposite poles and spindle fibres start to form

• Nucleolus disappears & nuclear envelope disintegrates into vesicles

Metaphase I

• Homologous chromosomes align in pairs at the metaphase plate.

  • Independent assortment occurs (pairs of sister chromatids are randomly aligned in a double row)

• Each chromosome is attached to the kinetochore microtubules from the pole it faces

Anaphase I

• Homologous chromosomes separate to opposite poles

• Each chromosome is pulled by a shortening kinetochore microtubule 

  • NO division of centromere yet

Telophase I

• Each pole now has a haploid set of chromosomes

• Chromosomes decondense to form chromatin

• Mitotic spindle disassembles

• Nuclear envelope reforms & nucleolus reappears

Prophase II

• Chromatin condenses to form chromosomes. Each chromosome comprises 2 sister chromatids joined at the centromere

• Centrioles move to opposite poles and spindle fibres start to form.

• Nucleolus disappears & nuclear envelope disintegrates into vesicles

Metaphase II

• Chromosomes align at the metaphase plate in 1 row

• Each chromosome is attached to kinetochore microtubules from both poles

Anaphase II

• Centromere of each chromosome divides, each (non-identical) sister chromatid now known as daughter chromosome

• Kinetochore microtubules shorten, pull daughter chromosomes, centromere-first to opposite poles.

• Cell elongates as non-kinetochore microtubules elongate

Telophase II

• Chromosomes decondense to form chromatin.

• Spindle fibres disintegrate.

• Nuclear envelope reforms & nucleolus reappears

Importance of Meiotic Cycle

1. Produces haploid gametes for sexual reproduction

   1. Helps to preserve the diploid number of chromosomes 

2. Produces genetic variation

   1. Crossing over between non-sister chromatids of homologous chromosomes

   2. Independent assortment of chromosomes

   3. Random fertilisation

3. Meiotic nondisjunction

   1. Occurs when chromosomes fail to separate during meiosis, can occur in the first stage or the second stage

   2. Examples: Down syndrome (trisomy 21), Turner syndrome (XO)