(2.16) Cell division, Cell diversity and cellular organisation

What is interphase?

When each chromosome is copied to form two sister chromatids.

What are the 3 stages of interphase?

1. The first growth phase

2. Synthesis phase

3. The second growth phase

What is the first growth phase?

This is when proteins are synthesised and organelles replicate. The cell increases in size.

What is the synthesis phase?

DNA is replicated in the nucleus.

What is the second growth phase?

The cell increases in size again, energy stores are increased and the duplicated DNA is checked for errors.

What needs to be controlled during the cell cycle?

• Whether chromosomes are in correct positions

• Whether replicated DNA is error free or needs repairing

What is the cell cycle comprised of?

Cell growth, DNA replication, and cell division (nuclear and cytoplasmic)

What does the G1 checkpoint check?

Nutrients, Growth factor and DNA damage

When does an arrest occur in the G1?

If there is DNA damage

What does the G2 checkpoint check?

Cell size and DNA replication

When does an arrest occur in the G2?

If DNA replication didn’t occur correctly (DNA damage)

What does the metaphase checkpoint check?

Chromosome spindle attachment

When does an arrest occur in the metaphase checkpoint?

If chromosomes haven’t aligned at the mitotic plate and are under bipolar tension

What is the G0 stage?

The resting stage

Why might cells enter the G0 phase?

• Fully differentiated cells become fully specialised to a particular function and are no longer able to divide e.g adult neurones

• Senescent cells are damaged DNA that can no loner divide so enters a period of permanent rest

• Some cells are there temporarily e.g b memory cells and can be stimulated to go back into cell cycle and start dividing again

What would happen if damaged DNA is detected at a checkpoint?

The cell will halt the cycle and try to repair it (it can’t go back to earlier in the cycle and ‘undo’ growth/synthesis etc)

What happens if damage is severe and can’t be repaired?

Apoptosis will occur (when the cell self destructs)

Why is mitosis important?

It is vital for life such as growth, tissue repair and asexual repoduction.

What process occurs before mitosis can take place?

Interphase

What happens to chromosomes during interphase?

We cannot see chromosomes in the nucleus. They are there but they are not visible as distinct structures, instead we see dark materials called chromatin. In chromatin the chromosomes have a lose open structure meaning DNA is accessible for transcription and application

What are the stages of mitosis and what does each process do?

1. Prophase: nuclear membrane breaks down, DNA condenses, spindle fibres start to form

2. Metaphase: spindle fibres attach to the centromeres, chromosomes line up along centre of the cell

3. Anaphase: spindle fibres shorten, sister chromatids pulled to opposite poles of cells

4. Telophase: chromosomes uncoil and decidedness, nuclear membrane reforms, cytokinesis occurs

What happens during cytokinesis?

This is the actual division of the cell into two separate cells

How do animal cells divide by cytokinesis?

Cleavage furrow forms around the middle of the cell to divide cell membrane and cytoplasm

What happens when a plant cell undergoes cytokinesis?

It has a cell wall so it can’t form a furrow, instead vesicles from the golgi apparatus form membrane structures down the centre of the cell. These then fuse together to form a central cell membrane dividing the cytoplasm into two. A new cellulose cell wall also form down the centre

What is meiosis?

This only takes place in sex organs as it is only ever used to produce gametes. It starts with a diploid cell and end with 4 haploid gametes, containing individual chromosomes. It is known as reduction division

What is a homologous pair of chromosomes?

Each nucleus in body cells has two sets of chromosomes (diploid). One maternal and the other paternal. The same maternal and paternal chromosomes is called a homologous pair of chromosomes and they contain the same genes at the same loci

What characteristics would be in a homologous pair of chromosomes?

• Same genes

• Same/ different alleles

• Same length

• Centromere in the same position

• Same branding (can identify by staining with chemicals)

What happens in division 1 of meiosis?

Reductive division occurs as one diploid cell is divided to form two haploid cells in the first division

What happens in division 2 of meiosis?

The pair of chromatids present in each daughter cell are separated, forming two more cells. Four daughter cells are produced in total

Why is it important that gametes are haploid?

• Enables variation - each receives a random selection of alleles from each parent

• Essential for evolution by natural selection

What are 3 ways of producing variation in meiosis?

1. Crossing over (prophase 1)

2. Independent assortment (metaphase 1)

3. Reduction and fusion of gametes (random fertilisation)

What is crossing over?

During prophase 1 non sister chromosomes can ‘swap’ genetic material by crossing over resulting in recombinant DNA. Where they crossed was called the chiasma

What is independent assortment?

During metaphase 1 the arrangement of each bivalent is random.

What is the reduction and fusion of gametes?

Meiosis reduces the number of chromosomes to form a haploid cell. Any one of the cells produced during meiosis can form a gamete. One haploid gamete will fuse with another gamete to form a zygote with 50% different chromosomes than the original parent.

What happens during meiosis division 1?

1. Prophase I - DNA condenses, nuclear envelope breaks down, homologous pairs form bivalent and crossing over occurs

2. Metaphase I - centrioles reach poles, spindle fibres attach to centromeres, bivalents arranged on equator (spindle fibres attached to centromeres), independent assortment

3. Anaphase I - Spindle fibres shorten, homologous pairs separate, chromosomes move to opposite poles

4. Telophase I - Chromosomes reach opposite poles, nuclear envelope reforms, cell may complete cytokines

What happens during meiosis division 2?

1. Prophase II - Same as prophase 1 + centrioles duplicate and no cross over

2. Metaphase II - Independent assortment of sister chromatids along the centre of cells, spindle fibres attach

3. Anaphase II - Same as anaphase I but 2 distinct regions with chromatids being separates and pulled towards the poles

4. Telophase II - Same as telophase I, cell completes cytokinesis and four haploid gametes are formed

Define tissue:

A collection of similar cells performing a common function

Define organ:

A collection of tissues working together to perform a specialised function

Define organ system:

A collection of organs working together to perform an overall life function

Define cell:

The basic structural building block of living organisms

What are specialised cells?

Cells within an organism that are differentiated, meaning they are specialised to carry out specific functions

List 3 specialised animal cells:

• Erythrocytes

• Neutrophils

• Sperm cells

What are the function of erythrocytes?

It carries oxygen around the body and is involved in transport of carbon dioxide

How and why are erythrocytes specialised?

• It contains no nucleus, RER, golgi or mitochondria so there is more space for haemoglobin.

• It has a biconcave shape for a larger SA:V ratio.

• It also has lots of haemoglobin for respiratory pigment

What are the function of neutrophils?

They are white blood cells. Main phagocyte in body (what you see when you have puss)

How and why are neutrophils specialised?

• Have multi lobed nucleus which makes it easier for them to squeeze through small gaps to get to site of infections

• Granular cytoplasm contains multi

• Lots of lysosomes produced for digestion of engulfed materials

What is the function of sperm cells?

They contain genetic material from males. They carry DNA from testes to ovum for fertilisation

How and why are sperm cells specialised?

• It is long and thin so its streamline

• Has a tail (flagella) so it can move to ovum

• It’s haploid, so it becomes diploid at fertilisation

• The acrosome contains digestive enzymes to break through ovums outer layer and penetrate it

• It has many mitochondria to produce ATP for movement of flagella

List 3 specialised plant cells:

• Palisade cells

• Root hair cells

• Guard cells

What is the function of palisade cells?

It’s the main cell involved in photosynthesis

How and why are palisade cells specialised?

• Its long and thin so many can squeeze together for maximum light exposure

• Tightly packed with chloroplasts for faster rate of diffusion

• Thin cell walls chloroplasts can move so theres a higher rate of photosynthesis

What are the functions of the root hair cells?

It takes up water and minerals from soil

How and why are root hair cells specialised?

• Elongated cell wall and membrane which increases SA:V ratio to maximise water and mineral uptake

• Thin cell wall

What are the functions of guard cells?

To open and close the stomata for gas exchange

How and why are guard cells specialised?

• Thick inner cell wall and thin outer cell wall, so when it becomes turgid stomata is open and when flacid closed

List 4 specialised animal tissues:

• Squamous epithelial cells

• Ciliated epithelial cells

• Cartilage

• Muscle

What is the function of squamous epithelial cell?

Layering and linign e.g other exchange surfaces

How and why are squamous epithelial cells specialised?

• Squamous - flattened cells are very thin which forms smooth, flat surfaces (good for blood vessels)

• Thin to allow faster rate of diffusion

• Secretes basement membrane which attaches squamous epithelial cells to connective tissue (e.g bone)

What is the function of ciliated epithelial cells?

Cilia wave to move mucus

How and why are ciliated epithelial cells specialised?

• Ciliated-column shaped

• Has tiny projections called cilia, the cilia wave to move mucus found in the lungs to moved trapped dirt

List three specialised plant tissues?

• Epidermis

• Xylem tissue

• Phloem tissue

What are phloem tissues?

Vascular tissue made of sieve tube elements, companion cells, phloem parenchyma and fibres

What are xylem tissues?

Vascular tissue made of vessels elements, tracheids, parenchyma and fibres

What are squamous epithelial tissue?

Flattened & very thin (1 cell thick). Exchange surface

What are ciliated epithelial cells?

Columnar-shaped cells. Comprises of ciliated cells (cilia) and goblet cells (secrete mucus)

What are cartilage?

A connective tissue, used as structural support or protection (i.e. between bones). A firm tissue, but softer and more flexible than bone. Made from chondrocytes (chondros = cartilage, kytos = cell) in an extracellular matrix

What are muscles?

Consists of bundles of fibres composed of cells which can contract (shorten). Three types: skeletal, smooth and cardiac

What are stem cells?

They are undifferentiated cells, capable of mitosis (so rewind source of undifferentiated cells) are able to find differentiated and become specialised

What are the types of embryonic stem cells?

• Totipotent - differentiate into any cell found in the species

• Pluripotent - produce any cells in the organisms except extra-embryonic tissue e.g placenta

What type are adult stem cells?

• Multipotent which differentiate into multiple, but a limited number of cell lineages e.g blood steam cll

How do stem cells become specialised?

Through differentiation - where changes that occur in cells of multicellular organism so that each different type of cell becomes specialised to perform a specific function

What are things that can change in specialisation?

• Number if particular organisms

• Shape of cell

• Contents cell

Where do erythrocytes, neutrophils and haematopoietic cells differentiate from?

Erythroctes and neutrophils come from adult stem cells. Haematopoietic cells give rise to three blood cells found in the bone marrow

Where are plant cells found?

In the meristem, simple undifferentiated cells within meristematic tissue divide and then differentiate into xylem vessels (inside) and phloem sieve tubes (outside)

List 8 uses of stem cells:

• Heart disease: muscle tissue in the heart

• Type 1 diabetes

• Parkinson disease

• Alzheimers disease

• Muscular degenerate

• Birth defects

• Spinal injuries

• Treatment of burns

Which diverse areas are stem cells already used in?

• Treatment of burns

• Drug trials

• Developmental biology

Name two potential sources of human stem cells and describe ethical issues with the use of stem cells?

• Human stem calls are harvested from adult bone marrow - painful to harvest

• The use of the umbilical cord is debated as they are more likely to get mutations\Embryos also need to be harvested from the embryo so there is a debate about when life begins/ embryo cant give consent