2.6: Cell division, cell diversity and cell differentiation

What is the purpose of the cell cycle?

To produce genetically identical daughter cells for growth, repair, replacement, and asexual reproduction.

What are the main stages of the cell cycle?

Interphase (G1, S, G2), mitosis, cytokinesis.

What is interphase?

a phase of the cell cycle where the cell is not dividing; it is subdivided into growth and synthesis phases

What happens in the G1 phase?

growth of cell; growth of organelles; growth in the number of organelles; protein synthesis - longest phase

What happens in the S phase?

DNA replication occurs - so daughter cells have identical DNA and a full copy of DNA; chromosomes become sister chromatids and DNA content doubles

What happens in the G2 phase?

growth of cell; growth of organelles; growth in the number of organelles; protein synthesis; spindle fibres for mitosis form (proteins)

What is the G0 phase?

A resting phase where cells leave the cycle cells are metabolically active but temporarily or permanently cease dividing due to factors like differentiation, DNA damage. It happens after mitosis

What is the purpose of checkpoints?

To monitor and control the cycle, ensure damaged DNA is not passed on, prevent uncontrolled division.

What is checked at the G1 checkpoint and when does it occur?

It occurs between the G1 and S phases. It checks cell size (if it is large enough); proteins and organelles (if it has required ones to undergo replication); whether DNA is d

What happens if any of the G1 checks fail?

cells delay the S phase or enter the G0 phase

What is checked at the G2 checkpoint and when does it occur?

DNA replication accuracy (+any mutations). It occurs after G2

What is checked at the metaphase checkpoint?

Chromosomes properly attached to spindle fibres and aligned at equator.

What is mitosis?

the process of nuclear division that produces two genetically identical daughter nuclei

Why is mitosis important?

• some organisms asexually reproduce using mitosis

• all multicellular organisms grow by mitosis, by producing more cells

• used in tissue repair when wounded

What happens in prophase?

the nuclear envelope breaks down; chromosomes condense (wrap around histone proteins); spindle fibres form

What happens in metaphase?

Chromosomes line up at the equator, spindle fibres attach to centromeres

What happens in anaphase?

Centromeres split, spindle fibres shorten (cytoskeleton contracts), sister chromatids pulled apart to poles

What happens in telophase?

Chromosomes uncoil, nuclear envelope reforms, spindle fibres break down

What happens in cytokinesis?

the cytoplasm divides and the cell splits into 2 forming two new genetically identical daughter cells

What is meiosis?

a type of nuclear division that results in the formation of cells containing half the number of chromosomes of the parent cell

Why is meiosis important in life cycles?

Produces haploid gametes so chromosome number is maintained at fertilisation.

How does meiosis introduce genetic variation?

Through independent assortment of chromosomes and crossing over between non-sister chromatids

Why is genetic variation important?

It drives evolution and adaptation through natural selection

What are homologous chromosomes?

A pair of chromosomes, one maternal and one paternal, same genes at same loci but possibly different alleles.

What happens in Prophase I?

Chromosomes condense, homologous pairs form bivalents, crossing over occurs

What are bivalents?

a pair of homologous chromosomes that have paired up and are aligned together

What is crossing over?

when the maternal chromosome exchanges genetic information with the paternal chromosome (non-sister chromatids) at the chiasma resulting in genetic variation between gametes

What happens in Metaphase I?

Homologous pairs line up at equator, spindle fibres attach to the centromere, independent assortment happens

What is independent assortment?

the random arrangement of chromosomes along the equator (e.g maternal on one side, paternal on the other or mixed) resulting in genetic variation. There are 2 ways chromosomes can line on an equator and 23 chromosomes and so there are 2²³ ways the chromosomes could arrange themselves along the equator

What happens in Anaphase I?

Homologous chromosomes separated to opposite poles

What happens in Telophase I?

chromosomes decondense, nuclear envelope reforms and cytokinesis occurs forming 2 haploid cells

What happens in Prophase II?

Chromosomes condense, spindle reforms, nuclear envelope breaks down

What happens in Metaphase II?

Chromosomes line up at equator, spindle fibres attach to centromere, independent assortment

What happens in Anaphase II?

Centromeres split, sister chromatids pulled apart.

What happens in Telophase II?

chromosomes decondense, nuclear envelope starts to reform, cytokinesis will occur

What does meiosis produce?

4 genetically different gamete cells

Why do gamete cells need to be haploid?

so that when the sperm cell and egg cell fuse, they form a diploid zygote

What is differentiation?

the process by which stem cells become specialised into different types of cells

Why do multicellular organisms need specialised cells?

to carry out particular functions

What are erythrocytes specialised for?

oxygen transport

How are red blood cells specialised?

• Small, biconcave shape → large SA:V for diffusion.

• No nucleus → more space for haemoglobin (carry more oxygen).

• Flexible (cytoskeleton) → fit through narrow capillaries.

What are neutrophils specialised for?

the immune response

How are neutrophils specialised?

• Multi-lobed nucleus → flexible, easier to squeeze through tissues and small gaps to reach the site of infection.

• Many lysosomes containing hydrolytic enzymes → digest and destroy pathogens.

What are sperm cells specialised for?

reproduction

How are sperm cells specialised?

• Tail (flagellum) → movement towards egg cell

• Many mitochondria → provide ATP for swimming.

• Acrosome (lysosome-like) with digestive enzymes → penetrate egg cell

• Haploid nucleus

What do epithelial cells do?

protect body surfaces and aid diffusion and absorption

How are squamous epithelial cells specialised?

• Very thin, flat (one cell thick) → short diffusion distance.

• Found in alveoli, blood vessels.

How are ciliated epithelial cells specialised?

• Have cilia that beat rhythmically → move mucus along airway and egg to fertilisation site

• Goblet cells nearby secrete mucus (trap dust/bacteria).

What are root hair cells specialised for?

water and mineral ion absorption

How are root hair cells specialised?

• Long hair-like projection → large SA for absorption of water/mineral ions.

• Thin cell wall → short diffusion pathway.

• Lots of mitochondria → active transport of minerals.

• Vacuole containing cell sap  → maintains concentration gradient

What are palisade cells specialised for?

photosynthesis

How are palisade cells specialised?

• Many chloroplasts → absorb light in photosynthesis.

• Thin cell walls → easy diffusion of CO₂.

• Large vacuole pushes cytoplasm/chloroplasts to edges → short diffusion distance + turgidity

What are guard cells specialised for?

to control the opening of the stomata

How are guard cells specialised?

• thick outer walls and thin inner walls → allows the cell to bend to open and close the stomata

Define tissue

a group of cells working together to perform a specific function

What is the role of squamous epithelial tissue and how do the tissues work together?

Role: allows for a short diffusion pathway

Work together: to aid diffusion in the lungs

What is the role of ciliated epithelial tissue and how do the tissues work together?

Role: waft mucus to the back of the throat

Work together: to clear the airways

What is the role of cartilage tissue and how do the tissues work together?

Role: a connective tissue that holds the trachea open under negative pressure

Work together: in the lungs to control ventilation

What is the role of muscle tissue and how do the tissues work together?

Role: contract to constrict the airways

Work together: to control ventilation in the lungs

What are the 3 types of muscle tissue and what do they do?

• skeletal muscles - allow rapid, powerful contractions

• cardiac muscles - allow continuous, rhythmic contractions → pumping blood. Does not fatigue

• smooth muscles - allow slow, sustained contractions e.g vasoconstriction

What is the role of xylem tissue and how do the tissues work together?

Role: transports water and minerals and provides support

Work together: provides mechanical support because of lignified cell walls. ; Made of dead cells (hollow vessels with no end walls) to form a continuous tube for water flow

What is the role of phloem tissue and how do the tissues work together?

Role: to transport assimilates in translocation

Work together: Sieve tube elements → allow sap to flow; Companion cells → provide ATP for active transport.

Define organ

a group of tissues working together to perform the same function e.g heart

Define organ system

a group of different organs working together e.g circulatory system

What do organ systems make up?

an organism

What are stem cells?

undifferentiated cells that can differentiate into any type of cell by mitosis

What are totipotent stem cells?

cells that can differentiate into all cell types including placenta

What are the only totipotent cells?

the zygote and those in the very early embryo

What are pluripotent stem cells?

cells that can differentiate into all body cell types but not placenta

What are multipotent stem cells?

cells that can differentiate into a limited range of cells (e.g., blood cells)

What type of stem cells are adult stem cells?

multipotent

What type of stem cells are embryonic stem cells?

pluripotent

Where are adult (multipotent) stem cells found?

in the bone marrow

What can adult stem cells differentiate into?

erythrocytes and neutrophils

How are erythrocytes formed?

Stem cell → erythroblast → loses nucleus, fills with haemoglobin → erythrocyte.

How are neutrophils formed?

Stem cell → myeloblast → develops lobed nucleus and lysosomes → neutrophil.

What are meristems?

undifferentiated tissue where stem cells are found

Where in a plant is meristem found?

shoot, roots, in between the phloem and the xylem

How are xylem vessels formed?

Stem cell → elongates, lignin deposited, cell contents die, end walls break down → hollow tube.

How are phloem sieve tubes formed?

Stem cell → loses nucleus/organelles, end walls become sieve plates → sieve tube element.

What are companion cells, and why are they important?

Cells that support sieve tube elements by providing ATP and proteins to keep them alive.

What are the 2 ways stem cells are mainly used?

medicine and research

How are stem cells used in medicine?

• repair of damaged or lost tissues → treat diseases e.g Parkinsons

• bone marrow transplants → can be used to restore a patient’s blood system after treatment for specific cancers

How are stem cells used in research?

• drug research → drugs are tested on stem cells rather than people to indicate how effective it is and if there are any side-effects

• developmental biology → can learn how each cell functions and what goes wrong when they are diseased

What are the issues with using stem cells?

• stem cells are often taken from embryos, this process destroys the embryo → moral objections as some people believe that embryos are living beings

• harvesting bone marrow is painful for the donor

• there is a risk that the patient’s immune system will reject the donated cells