D2.1 Cells & Nuclear Division

Define cell division

The process by which a parent cell divides into two daughter cells

Define cytokinesis

The final stage of cell division, a parent cell’s cytoplasm is split into two daughter cells 

Describe the process of cytokinesis in animal cells (4)

1. A ring of contractile proteins (mainly actin & myosin) forms beneath the cell membrane around the cell’s middle

2. These proteins contract, creating a cleavage furrow that pinches the membrane inward

3. This process continues until cell membrane splits completely, forming two separate daughter cells

→ cell membrane can be pinched as animal cells don’t have a rigid cell wall

Describe the process of cytokinesis in plant cells (4)

1. Vesicles from the Golgi apparatus carry cell wall materials & assemble at cell’s centre 

2. These vesicles fuse to form a cell plate between the two nuclei

3. The cell plate grows outward until it fuses with the existing cell wall, dividing the parent cell into two daughter cells each w/ own cell wall

→ cell membrane can’t be pinched as plant cells have a rigid cell wall

Describe equal cytokinesis (4)

• Usually, the cytoplasm divides equally, giving each daughter cell the same amount of organelles. This includes:

  • Mitochondria

  • Chloroplasts 

• This ensures both daughter cells are viable & can function independently

Describe unequal cytokinesis (1)

Sometimes, cytoplasm is divided unequally → producing daughter cells w/ different sizes & contents

Describe examples of unequal cytokinesis (7)

1) Oogenesis (Humans) aka egg formation - 

• Produces one large egg cell w/ most cytoplasm & small polar bodies that degenerate

  • This cell has all nutrients & organelles required for early embryo growth 

2) Budding (Yeast) -

• A small bud forms on the parent cell

• It grows & eventually detaches, receiving some cytoplasm & organelles but less than parent 

  • This allows a new organism to form but keeps parent largely intact 

Describe roles of nuclear division (2)

1) Prevents formation of anucleate cells (cells w/o nucleus) → non-functional

2) Ensures each daughter cell gets correct set of chromosomes

Describe the roles of mitosis (2)

1) Maintains chromosome number 

2) Produces clones for growth & tissue repair

Describe the roles of meiosis (2)

1) Halves chromosome number to preserve parent’s chromosomes after fertilisation

2) They generate genetic diversity through crossing over + independent assortment of chromosomes → allows evolution & adaptation

What is the role of DNA replication?

DNA replication ensures each daughter cell receives a complete, identical set of genetic information

Describe the process of DNA replication (3)

1. Each chromosome duplicates its DNA, forming two identical sister chromatids 

2. The sister chromatids are joined together at a specific region, called the centromere

3. Chromatids remain attached until they are separated later in anaphase (stage of cell division) 

Describe the role of chromosome condensation during nuclear division (2)

• DNA wraps around histone proteins to form nucleosomes

  • Nucleosomes coil further to form chromatin fibres → supercoils into compact chromosomes

Describe the role of movement in nuclear division (4)

Spindle formation - 

1. Microtubules made of tubulin proteins assemble into a spindle, which extends across two poles of cell

Attachment - 

2. Spindle microtubules attach to chromosomes at a special protein structure on the centromere (called kinetochore) 
   

Separation - 

3. Microtubules shorten & pull sister chromatids (in mitosis) / homologous chromosomes (in meiosis I) toward opposite poles 

4. Motor proteins use ATP to help move chromosomes along microtubules → regulating chromosome number in daughter cells, in meiosis