Why are Why are newnew cells needed? 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. 2. Response to a new need (ie.

RepairRepair or or environmentalenvironmental)) 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 function3. 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 cellThe 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: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.

MITOSISMITOSIS

Consists of 4 stages:

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

1. Prophase1. Prophase

Chromosomes condense and become visible

Nucleolus breaks down

Centrioles move apart creating spindle fibres (Spindle fibres are microtubules)

2. Metaphase2. 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. Anaphase3. Anaphase

Spindle fibres contract and become shorter.

They pull apart the centromere and the sister chromatids

The chromatids are moving toward opposite poles

4. Telophase4. Telophase

Spindle fibres breakdown

Chromosomes elongate as they uncoil

Nuclear envelope and nucleoli form

Result: Two identical nuclei

At the end of mitosis…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…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 The daughter cells are now ready to enter Interphase once againenter Interphase once again

The cell cycle is repeated

Let us see how it really Let us see how it really looks…looks…

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

In a plant cell…

MitosisMitosis