Why are new cells needed? 1. Growth: Cells grow in volume and mass – Inside of the cell is growing in volume – Cell membrane is growing in surface area – Over time, the cell membrane cannot compensate for the volumetric growth of the inside – Cell division is required

Organisms grow and Cells differentiate to form many structures such as limbs, organs, tissues

2. Response to a new need (ie. Repair or environmental) Cells need to regenerate new tissue in response to damage I.e. Growth of muscle in response to exercise or damage I.e. A cut on your finger will require new skin cells to form

Certain organisms will regenerate an entire limb if lost I.e. Tail of a lizard I.e. The arm of a starfish taken by a predator

Cells divide as well, based on their environment I.e. – A person ascends to a higher altitude (Less oxygen) – Red blood cells divide to increase in number to capture more oxygen – Compensates for reduced amount of oxygen

3. Maintenance of function by replacing cells that – do not function properly or – die (programmed cell death) I.e. Millions of red blood cells die everyday they need to be replaced to maintain their function

The Life cycle of a cell Throughout life, cells proceed through a cycle called the ‘Cell Cycle’

The Cell cycle is composed of two main phases: 1. INTERPHASE 2. MITOTIC PHASE

1. INTERPHASE – Period when cell is growing – The cell carries out normal functions – Prepares for cell division DNA replicates

2. MITOTIC PHASE – Mitosis: the division of the nucleus resulting in two exact copies. – Cytokinesis: Division of the cell into two daughter cells

Chromosomes: Linear structures composed of DNA molecules Found in nuclei of eukaryotic cells Total DNA in the chromosomes of an organism is referred to as its genome

DNA is wound around Proteins called histones. Histones help keep the DNA organized. A series of histones and DNA is called a nucleosomes. A Chromosome is composed of many nucleosomes.

Each chromosome consists of 2 identical sister Chromatids (Each consisting of one double- stranded DNA molecule) Chromatids are attached at a specific region called a centromere. (The point at which the spindle fibres attach to is called the kinetochore. )

Note: When cell division is not happening, chromosomes are uncoiled and may be called chromatin.

MITOSIS Consists of 4 stages: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase

1. Prophase Chromosomes condense and become visible Nucleolus breaks down Centrioles move apart creating spindle fibres (Spindle fibres are microtubules)

2. Metaphase Spindle fibres are forming Spindle fibres attach to the ‘kenetochore’ of a centromere of a chromosome Chromosomes move toward the equator of the cell (Equatorial plane)

3. Anaphase Spindle fibres contract and become shorter. They pull apart the centromere and the sister chromatids The chromatids are moving toward opposite poles

4. Telophase Spindle fibres breakdown Chromosomes elongate as they uncoil Nuclear envelope and nucleoli form Result: Two identical nuclei

At the end of mitosis… In animal cells – The cell membrane pinches near the middle of the cell creating a cleavage furrow – This divides the cytoplasm – This process is called Cytokinesis

At the end of mitosis… In a plant cell: – A cell plate forms across the centre of the cell resulting in a new cell wall

The daughter cells are now ready to enter Interphase once again The cell cycle is repeated

Let us see how it really looks… Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

In a plant cell…

Mitosis