The cell cycle and cell growth, death and differentiation

binary fission

the method by which prokaryotic cells (e.g., bacteria) reproduce asexually. It is simpler and faster than mitosis in eukaryotes because prokaryotes:

• Have no nucleus

• Contain a single circular chromosome

• Lack membrane-bound organelles

process of binary fission

1. DNA Replication: The cell's genetic material (DNA) is copied, forming two identical chromosomes. 

2. Chromosome Segregation: The duplicated chromosomes are separated to opposite ends of the cell. 

3. Cell Elongation: The cell elongates to prepare for division. 

4. Cytokinesis: A new cell wall (septum) forms in the middle of the cell, dividing the cell into two daughter cells. 

5. Daughter Cells: Each daughter cell receives a complete copy of the DNA and other cellular components. 

binary fission key points

• Rapid process (can take 20 minutes in some bacteria).

• Produces clones (no genetic variation unless mutation occurs).

• Is how bacterial colonies grow.

eukaryotic cell cycle: 3 main stages

interphase

mitosis

cytokinesis

eukaryotic cell cycle

• interphase – cell grows and prepares for division

  • G₁ phase (Gap 1): Cell grows, organelles replicate.

  • S phase (Synthesis): DNA is replicated.

  • G₂ phase (Gap 2): Cell checks DNA and prepares for mitosis.

• Mitosis (M phase) – nucleus divides

• Cytokinesis – cytoplasm and organelles divide → two daughter cells

interphase pt 1: G1 stage

• pre DNA synthesis

• normal function performed

• cell almost doubles in size

• large increase in the number of organelles

interphase pt 2: S stage

• DNA synthesis

• chromatin (DNA strands) chromosomes duplicated

interphase pt 3: G2 stage

• post DNA synthesis, where the cells prepare to go into mitosis

• cell growth

• normal function performed

prophase

• first mitosis stage

• chromatin (DNA strands) coils into chromosomes

• nucleus dissolves spilling the chromosomes

• spindle fibres form

metaphase

• second stage of mitosis

• spindle fibres attach to centromeres of each chromosome

• spindle fibres pull the chromosomes to the cells equator

anaphase

• third stage of mitosis

• spindle fibres rip apart the chromosomes

• 1 chromatoid pulled to each direction

telophase

fourth stage of mitosis

spindle fibres dissolve

cytokinesis occurs (splitting of cytoplasm)

nucleus reforms

chromatids unwind into chromatin

end result: 2 identical diploid cells

cytokinesis (plant vs. animal cells)

animal cells:

The plasma membrane pinches inward at the cleavage furrow to split the cytoplasm.

plant cells:

A cell plate forms in the middle, which becomes the new cell wall between daughter cells.

What is Apoptosis?

Apoptosis is a highly regulated process of programmed cell death that occurs:

• When a cell is damaged, infected, or no longer needed.

• During development (e.g. removal of webbing between fingers).

• There are two main pathways that trigger apoptosis: extrinsic or mitochondrial pathway and the extrinsic or death receptor pathway

apoptosis pathways

Apoptosis is a regulated, controlled process that removes unwanted, damaged, or harmful cells without damaging surrounding tissue.

There are two main pathways that trigger apoptosis:

• intrinsic (mitochondrial pathway): the signal comes from within the cell

• extrinsic (death receptor pathway): the signal for this pathway is initiated outside the cell

Key Stages of Apoptosis:

• Signalling: Internal (e.g. DNA damage) or external (e.g. immune signals) triggers.

• Shrinking: The cell shrinks and chromatin condenses.

• Blebbing: The membrane forms small protrusions (blebs).

• Fragmentation: The nucleus and cytoplasm break into apoptotic bodies.

• Phagocytosis: Apoptotic bodies are engulfed and removed by nearby cells.

importance of apoptosis

• Prevents tumour formation

• Removes damaged or infected cells

• Essential in tissue sculpting during development

What is Cancer?

Cancer results from disruption of the cell cycle and failure of apoptosis, causing uncontrolled cell division.

Characteristics of Cancer Cells:

• Lose contact inhibition: keep dividing even when overcrowded

• Immortal: continue dividing indefinitely (e.g. HeLa cells)

• Angiogenesis: stimulate blood vessel growth to supply tumour

• Metastasis: can spread to other tissues

• Do not undergo apoptosis when damaged

How Disruption Occurs:

Mutations in proto-oncogenes:

• proton-oncogenes become oncogenes, driving uncontrolled cell division.

Mutations in tumour suppressor genes (e.g. p53):

• Tumour Suppressor Genes normally stop division or trigger apoptosis. When damaged, cells divide unchecked.

Apoptosis fails:

• Damaged cells survive and may become cancerous.

necrosis

Necrosis is a form of accidental and uncontrolled cell death that occurs when cells are damaged by external factors such as:

• Infection

• Toxins

• Trauma (e.g. injury)

• Lack of blood supply or oxygen (ischaemia)

It is not regulated like apoptosis and often leads to inflammation and damage to surrounding tissues.

What Happens During Necrosis?

• External damage (e.g. physical injury or lack of oxygen) causes the cell membrane to become leaky or rupture.

• The cell swells and the organelles break down.

• The cell bursts open and releases its contents into surrounding tissue.

• This triggers an inflammatory response, which may lead to more tissue damage.

What are Stem Cells?

unspecialised cells that:

• Can self-renew (make more stem cells)

• Can differentiate into one or more specialised cell types

Types of Stem Cells by Potency

Totipotent: Can become any cell type, including placenta and embryo. e.g. Zygote (1–3 days after fertilisation)

Pluripotent: Can become any body cell, but not placenta/ e.g. Embryonic stem cells (blastocyst stage)

Multipotent: Can become a limited range of cells. e.g. Adult stem cells (e.g. bone marrow → blood cells)

Unipotent: Can become only one type of cell, but still self-renew. e.g. Skin stem cells

stem cells role in the body

• Replace damaged cells (e.g. blood, skin)

• Crucial in embryonic development

• Potential for therapeutic use (e.g. spinal cord injury, diabetes, Parkinson’s)

differentiation:

the biological process by which an unspecialised stem cell becomes a specialised cell with a specific structure and function.

• It involves changing gene expression — some genes are switched on, others are switched off, depending on what type of cell is needed.

• occurs during embryonic development, and also in tissue growth and repair throughout life.

example:

A pluripotent stem cell can differentiate into a nerve cell, muscle cell, or blood cell, depending on the signals it receives.

specialisation

Specialisation is the outcome of differentiation. It refers to the final form of the cell — once it has acquired:

• A specific shape

• Specific organelles

• A specific function in the body

example:

A red blood cell is specialised to:

• Have no nucleus

• Be disc-shaped for oxygen transport

• Contain lots of haemoglobin

How Does Differentiation and Specialisation Happen?

Stem cell receives signals

• Signals come from the internal environment (like molecules in the cytoplasm) or the external environment (e.g. chemicals, hormones, growth factors).

Gene expression is modified

• Specific genes are activated or deactivated.

• This causes the cell to make certain proteins and not others.

Cell structure changes

• The cell develops specialised features (e.g. flagella, chloroplasts, contractile fibres).

Cell becomes specialised

• It performs a specific role and often loses the ability to divide or change into other cell types.

Example: Blood Cell Differentiation

Multipotent stem cell (in bone marrow)
↓

Can differentiate into either:

• Red blood cell (oxygen transport)

• White blood cell (immune defence)

• Platelet (clotting)

Each has:

• Different gene expression

• Different structure

• Different function

What Is Cell Renewal?

Renewal is the ability of stem cells to divide and produce new cells throughout life.

• Some stem cells divide to self-renew (make more stem cells).

• Others divide and differentiate into specialised cells to replace damaged or old tissue.

Example:

• Skin cells are constantly worn away and replaced.

• Blood cells are replenished every day by stem cells in bone marrow.

What is Tissue Renewal?

Tissue renewal is the ongoing process of replacing old, damaged, or dead cells in tissues with new, functional cells, in order to maintain healthy tissue structure and function.

This process is made possible by stem cells, which can:

• Divide repeatedly to produce more cells (self-renewal)

• Differentiate into specific, specialised cell types

Why is tissue renewal important?

• Maintains homeostasis (a stable internal environment)

• Repairs injuries (e.g. cuts, burns, internal damage)

• Replaces worn-out cells (e.g. skin, blood, lining of the gut)

• Prevents the accumulation of damaged or non-functioning cells

capsases

the enzymes responsible for apoptosis

they are stored in the cell in an inactive precursor form

embryonic stem cells

• (also known ad pluripotent stem cells) are the undifferentiated or relatively undifferentiated cells of embryos

adult stem cells

• (also known as somatic stem cells)

• repair and regenerate damage

• are present in small numbers in some adult tissues such as: hair follicles, bone marrow, the spinal cords, and germ cells, and remain as stem cells throughout an individuals life

what is potency

refers to a stem cells ability to differentiate into different cell types

Non-vascular plants

e.g. moss, do not contain vascular tissue and only

require simplified tissues to function.

leaves

• The plant organs that are responsible for photosynthesis.

• Site of gas exchange and photosynthesis. In most species, leaves are organised to increase sunlight exposure.

flowers

The sexual reproductive organs of flowering plants (angiosperms).

fruits

The seed bearing structures that are responsible for the protection of developing seeds and seed dispersal.

animal tissues

• Muscle tissue: Contracts to exert a force. The three major types are skeletal, cardiac, and smooth muscle tissue.

• nervous tissue: Detects stimuli and transmits electrical signals.

• connective tissue: Connects and supports other tissues and organ structures of the body.

• epithelial tissue: These tissues assist in protection, secretion, and absorption.