2.1.6 Cell Division, Cell Diversity and Cellular Organisation

What is the cell cycle?

A regular sequence of events between one cell division & the next; 3 phases:

1. Interphase (G₁+S+G₂)

2. Mitosis (nuclear division)

3. Cytokinesis (cell division)

What happens during the G₁ phase of interphase?

G₁ phase (GROWTH):

• Cells produce RNA, enzymes & other proteins required for cell growth

• Cells receive signal to divide controlled by cyclins

What happens during the S phase of interphase?

S phase:

• DNA in nucleus replicates → each chromosome contains 2 identical sister chromatids

What happens during the G₂ phase of interphase?

G₂ phase (FURTHER CELL GROWTH):

• Newly synthesised DNA is checked for any errors

• Tubulin is made - protein needed for mitosis

Why is mitosis important?

Mitosis is essential for:

• Growth of multicellular organisms

• Replacement of cells & repair of tissues

• Asexual reproduction

What are the 4 stages of mitosis?

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

What happens during prophase?

• Chromatin condenses, forming chromosomes that are joined together at the centromere

• Centrosomes move to opposite sides → spindle fibres emerge

• Nuclear envelope breaks down into vesicles & nucleolus disappears

What happens during metaphase?

• Chromosomes line up at the equator of the spindle (AKA metaphase plate)

• Spindle fibres attach to centromeres

What happens during anaphase?

• Sister chromatids separate at centromere

• Spindle fibres shorten & pull the separated sister chromatids to opposite poles

What happens during telophase?

• Chromosomes arrived at opposite poles & decondense, reforming chromatin

• Nuclear envelope reforms around each set of chromosomes

• Spindle fibres break down

What is cytokinesis?

The physical separation of the parent cell into two genetically identical daughter cells

How does cytokinesis occur in animals?

The cytoskeleton pulls the cell surface membrane inwards, causing it to invaginate (pinch in)

→ Causes cleavage furrow to forms, until cell membrane is pinched off to form 2 new cells

How does cytokinesis occur in plants?

Vesicles from the Golgi apparatus line up along the metaphase plate, then fuse with each other & the cell surface membrane, to form a new cell surface membrane in the middle of the cell (cell plate)

→ New sections of the cell wall are produced, separating the new daughter cells

How can the cell cycle be regulated?

Includes checkpoints where proofreading & repair enzymes check & correct any errors, to avoid any mutations

What are the 4 types of checkpoints in the cell cycle?

• G₁ checkpoint

• S phase checkpoint

• G₂ checkpoint

• Metaphase checkpoint

What happens at the G₁ checkpoint?

Checks chromosomes for damage before entering S phase

What happens at the S phase checkpoint?

Ensures all DNA has been successfully replicated. If not, cell cycle stops

What happens at the G₂ checkpoint?

Checks replicated DNA for any damage; cycle pauses until repairs occur

What happens at the metaphase checkpoint?

Confirms chromosomes are correctly attched to spindle fibres before anaphase

What is the G₀ phase?

A resting state where cells exist outside of the active cell cycle, so are not dividing. They are still metabolically active, just not replicating DNA

Why might cells enter the G₀ phase?

• Lack of nutrients

• They are fully differentiated

• If errors after each checkpoint are identified

Why is meiosis important?

• It has serveral mechanisms that increase genetic diversity of gametes produced

• Crossing over & independent assortment result in different combinations of alleles in gametes

What is the process of crossing over?

1. Homologous chromosomes pair up, very closely to each other

2. Non-sister chromatids cross-over & get entangled

3. Section of chromatid from one chromosome breaks & rejoins the chromatid from the other chromosome

What happens during independent assortment?

Homologous pairs randomly align along equator of the spindle (metaphase I & II)

• Each pair is arranged with either chromosome on top (random)

• Homologous chromosomes are separated & pulled apart to different poles

What happens during Prophase I?

• DNA condenses, becoming visible chromosomes → 2 sister chromatids arranged side by side in homologous pairs (bivalents)

• Centrioles migrate to opposite poles & spindle is formed

• Nuclear envelope breaks down & nucleolus disappears

• Crossing-over occurs in this stage

What happens during Metaphase I?

• Spindle fibres attach to centromeres, & bivalents line up along equator

• Independent assortment occurs

• Proportion of maternal or paternal chromosomes that end up on each side of equator is due to chance → contributes to genetic variation

What happens during Anaphase I?

• Homologous pairs of chromosomes are separated as microtubules pull while chromosome to opposite ends of spindle

• Centromeres do not divide

What happens during Telophase I?

• Chromosomes arrive at opposite poles

• Spindle fibres break down

• Nuclear envelope forms around groups of chromosomes & nucleoli reform

What happens during Prophase II?

• Nuclear envelope breaks down, chromosomes condense

• Spindle forms

What happens during Metaphase II?

Chromosomes line up in a single file along the equator of the spindle

What happens during Anaphase II?

• Centrosomes divide & individual chromosomes are pulled to opposite poles

• 4 groups of chromosomes remain

What happens during Telophase II?

Nuclear membranes form around each group of chromosomes

How are erythrocytes adapted for their function?

To transport oxygen around the body & carbon dioxide to the lungs

• Biconcave shape → increases SA over which O2 can be absorbed

• Cytoplasm contains high amounts of haemoglobin

• No nucleus → more space for haemoglobin

• Elastic & thin membrane → shorter diffusion distance & allows cell to be flexible, to squeeze through narrow capillaries

How are neutrophils adapted for their function?

Destroy pathogens by phagocytosis & secretion of enzymes

• Flexible shape with multi-lobed nucleus → allows them to squeeze through cell junctions in capillary wall

→ form pseudopodia (cytoplasmic projections) that engulf microorganisms

• Lots of lysosomes → digestive enzymes help digest & destroy invading cells

• Ability to change shape (amoeboid movement)

How are squamous epithelial cells adapted for their function?

Provide a surface covering or outer layer

• Single layer of flattened cells on a basement membrane → forms a thin cross-section, reducing diffusion distance

• Permeable → easy diffusion of gases

How are ciliated eptihelial cells adapted for their function?

Moving substances across the surface of a tissue

• Have cilia → beats in a coordinated way to shift material along surface of epilthelium tissue

• Goblet cells secrete mucus → helps trap dust, dirt & microorganisms, preventing them from entering vital organs where they might cause infection

How are sperm cells adapted for their function?

Reproduction - to fuse with an egg, initiate the development of an embryo & pass on father’s genes

• Head contains nucleus → contains hlaf the normal number of chromosomes

• Acrosome in head contains digestive enzymes → breaks down outer layer of egg cell so haploid nucleus can enter to fuse with egg’s nucleus

• Lots of mitochondria → release energy (via respiration) for tail movement

How are palisade cells adapted for their function?

Carry out photosynthesis to produce glucose & oxygen

• Lots of chloroplasts in cytoplasm → maximise absorption of light for photosynthesis

• Tall & thin shape → allows light to penetrate deeper before encountering another cell wall. Cells are densely packs together

How are root hair cells adapted?

Absorption of water & mineral ions from soil

• Root hairs → increases SA so rate of water uptake by osmosis is greater

• Thinner walls → water can move through easily (shorter diffusion distance)

• Permanent vacuole contains cell sap → more concetrated than soil water, maintaining water potential gradient

• No chloroplasts

• Mitochondria for active transport of mineral ions

How are guard cells adapted for their function?

Controls the opening of the stomata to regulate water loss & gas exchange

• Inner cell walls are thicker, whilst outer cell walls are thinner → difference in thickness allows cell to bend when turgid

• Cytoplasm has high density of chloroplasts & mitochondria

How is xylem adapted for its function?

Transport tissue for water & dissolved ions

• No top & bottom walls between cells → forms continuous hollow tubes through which water is drawn upwards towards leaves by transpiration

• Cells are dead, without organelles or cytoplasm → allows free movement of water

• Outer cells are thickened with lignin → strengthens tubes, helping to support the plant

How is phloem adapted for its function?

Transport of dissolved sugars & amino acids

• Made of living cells, supported by companion cells

• Cells are joined end-to-end & contain holes in the cell walls (sieve plates) → forms tubes that allow sugar & amino acids to flow easily through (by translocation)

How are muscles adapted for their function?

Contraction for movement

• Layers of protein filaments → can slide over each other, causing muscle contraction

• High density of mitochondria → to provide sufficient energy (via respiration) for contraction

• Skeletal muscles fuse together during development to form multinucleate cells that contract in unison

What are some features of stem cells?

• Can divide (by mitosis) an unlimited number of times

• Each new cell has the potential to remain a stem cell or develop into a specialised cell (by differentiation)

What is potency?

Ability of stem cells to differentiate into more specialised cell types

What are the three types of potency?

• Totipotency

• Pluripotency

• Multipotency

What are totipotent stem cells?

Stem cells that can differentiate into any cell type found in an embryo, as well as extra-embryonic cells (cells that make up the placenta).

What are pluripotent stem cells?

Embryonic stem cells that can differentiate into any cell found in an embryo but are not able to differentiate into extra-embryonic cells.

What are multipotent stem cells?

Adult stem cells that have lost some potency associated with embryonic stem cells and are no longer pluripotent. Can differentiate into limited range of cell types

How are erythrocytes and neutrophils produced?

They are differentiated cells derived from a common stem cell within bone marrow

→ stem cells in bone marrow are multipotent

How are xylem and phloem produced?

They are differentiated cells that derive from a common stem cell within meristems

What could stem cells be used for in terms of treatment of disease?

• Repair tissue that has been damaged

• Treat neurological conditions

• Research developmental biology

How might tissue damage occur?

• Accidental damage

• Degenerative disease

• Autoimmune condition

How can stem cells repair damaged tissue?

Stem cells could be encouraged to differentiate into a damaged cell type to repair the damaged tissue e.g:

• skin cells to treat burn patients

• neurones to repair damaged spinal cord

• pancreas cells to treat type 1 diabetes

• retina cells to treat macular degeneration in the eye

How can stem cells treat neurological conditions?

Stem cells could be used to generate new neurones to treat symptoms e.g.

• Replacing damaged brain cells in Alzheimer’s & Parkinson’s disease

How can stem cells be used in developmental biology?

Embryonic stem cells are able to differentiate into embryos, allowing scientists to study the developmental stages of the early embryo.

Research can provide information about:

• Developmental problems

• Effects of medicines on embryos

Evaluate the use of stem cells in medicine

• Ethical concerns around use of embryonic stem cells → potential to develop into an adult human

• Any adult stem cells used in medical treatment could cause an immune response unless they are a close tissue match

• Stem cells can divide indefinitely → if division becomes uncontrolled, it can lead to cancers