OCR A-Level Biology: 2.1.6 Cell division, cell diversity and cellular organisation.

What is the cell cycle?

The regular sequence of growth and division that cells undergo

What are the stages in the cell cycle?

Interphase
Mitosis and cytokinesis

What are the different stages in interphase?

G₁ phase, S phase, G₂ phase

What are the different checkpoints in the cycle?

G1/S, G2/M, M

What happens during the G₁ phase?

• cell functions normally

• protein and organelle synthesis occurs

• therefore the cell doubles in size.

What is checked at the G₁/S checkpoint?

Checks for:
-Cell size: is the cell large enough to divide
-Nutrients: does the cell have enough nutrients to divide
-Growth factors: Is the cell receiving positive cues from neighbours
-DNA damage

What happens during the S phase?

The DNA replicates, producing pairs of identical sister chromatids.

What happens during the G₂ phase?

Cells grow

What is checked at the G₂/M checkpoint?

DNA replication complete

replicated DNA isn’t damage

this is before cell enters mitosis

What is G0?

• phase where cell is inactive

• may undergo apoptosis (programmed cell death) or differentiation

What happens if a cell fails one of the checkpoints?

-The cell can halt the cycle and attempt to remedy the problematic condition
-Or the cell can advance into G₀ (inactive) phase and await further signals when conditions improve.

What is the purpose of mitosis?

Mitosis produces two genetically identical cells which are used for:
-Asexual reproduction

-Growth

-Repair

What are the main stages in mitosis?

Prophase, metaphase, anaphase, telophase, cytokinesis

What is important to remember about how cells in interphase look like?

-Individual chromosomes are not visible

What happens during prophase?

• Chromosomes condense to become visible in light microscope

• sister chromatids attached by centromere

• spindle fibres emerge from centrosomes

• nuclear membrane breaks down

What is a centromere?

The centromere holds the sister chromatids together (made of protein)

What happens during metaphase?

• Chromosomes line up in the middle of the cell along metaphase plate (equator) and are held on the spindle

• Each chromosome is attached to the spindle on either side of its centromere

What happens during anaphase?

• sister chromatids separated at centromere (centromere divides into 2)

• chromosomes pulled to opposite poles by spindle fibres

What happens during telophase?

• Chromosomes arrive at opposite poles and begin to decondense

• Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes

• The spindle fibres break down

identifying mitosis in plant cells

• Growth in plants occurs in specific regions called meristems

• The root tip meristem can be used to study mitosis

• The root tip meristem can be found just behind the protective root cap

• In the root tip meristem, there is a zone of cell division that contains cells undergoing mitosis

• Pre-prepared slides of root tips can be studied or temporary slides can be prepared using the squash technique (root tips are stained and then gently squashed, spreading the cells out into a thin sheet and allowing individual cells undergoing mitosis to be clearly seen)

What is meiosis?

-An alternative form of cell division.
-It produces four cells that are:
• not genetically identical
• gametes
• haploid (contain half the normal number of chromosomes)

What happens before meiosis starts?

Interphase: (same as mitosis)
DNA unravels and replicates so that each chromosome consists of two identical copies called sister chromatids which are held together by a centromere.

What are the main stages of meiosis?

Prophase 1, Metaphase 1, Anaphase 1, Telophase 1, Prophase 2, Metaphase 2, Anaphase 2 and Telophase 2.

What are homologous chromosomes?

Two chromosomes with the same set of genes, in the same sequence, sometimes with different alleles, one from each parent.

What happens during prophase 1?

• DNA condenses and becomes visible as chromosomes

• DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere

• The chromosomes are arranged side by side in homologous pairs.

  As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur. The point at which the crossing over occurs is called the chiasma (chiasmata; plural)

• In this stage centrioles migrate to opposite poles and the spindle is formed

• The nuclear envelope breaks down and the nucleolus disintegrates

What happens during metaphase 1?

• The homologous chromosome pairs line up along the equator of the spindle, with the spindle fibres attached to the centromeres

• The maternal and paternal chromosomes in each pair position themselves independently of the others; this is independent assortment

  • This means that the proportion of paternal or maternal chromosomes that end up on each side of the equator is due to chance

What happens during anaphase 1?

• The homologous pairs of chromosomes are separated by spindle fibres to opposite ends of cell

• The centromeres do not divide

What happens during telophase 1?

• The chromosomes arrive at opposite poles

• Spindle fibres start to break down

• Nuclear envelopes form around the two groups of chromosomes and nucleoli reform

cytokinesis:

• In animal cells: the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half

• In plant cells, vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The vesicles merge with each other to form the new cell surface membrane.

What happens during prophase 2?

• The nuclear envelope breaks down and chromosomes condense

• A spindle forms at a right angle to the old one

What happens during metaphase 2?

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

What happens during anaphase 2?

• Centromeres divide and individual chromatids are pulled to opposite poles

• This creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell

What happens during telophase 2?

• Nuclear membranes form around each group of chromosomes

cytokinesis

• Cytoplasm divides to produce four genetically different haploid cells

What are sister chromatids?

2 identical copies of DNA held together by a centromere

What are stem cells?

Cells that are not differentiated or specialised.
-They maintain the capacity to undergo mitosis and differentiate into a range of cell types

What are 6 examples of specialised cells?

1. Erythrocytes

2. Neutrophils

3. Sperm cells

4. Palisade cells

5. Guard cells

6. Root hair cells

What is the function and adaptations of erythrocytes?

Erythrocytes carry oxygen in the blood.
-They are small and flexible to fit through small blood vessels and capillaries
-Full of haemoglobin to bind to the oxygen
-No nucleus to allow for more space for the haemoglobin
-Biconcave shape to provide a large surface area to take up oxygen more quickly.

What is the function and adaptations of neutrophils?

They engulf and digest foreign matter or old cells.
-Flexible shape to enable movement through tissues
-Lobed nucleus and granular cytoplasm to help movement through membranes
-Contain lysosomes filled with lysozyme
-Made from stem cells in bone marrow

What is the function and adaptations of sperm cells?

Sperm carry paternal chromosomes to the egg.
-Tail (flagellum) to enable rapid movement
-Acrosome that contains digestive enzymes that help digest egg surface
-Small to make movement easier
-Many mitochondria needed to release the energy needed for rapid movement

What is the function and adaptations of guard cells?

Guard cells control the stomatal opening.
-Flexible walls so cells bend when turgid to open stomata and closing when flaccid - control water loss.

What is the function and adaptations of root hair cells?

Root hair cells absorb water and mineral ions from the soil.
-Long extension (hair) to increase surface area
-Active pumps in cell surface membrane to absorb mineral ions by active transport.
-Thin cell wall to reduce barrier to movement of ions and water

What are tissues?

A collection of cells that work together to perform a particular function. They may be similar to each other or they may perform slightly different roles.

What are 6 examples of specialised cells?

1. squamous epithelia

2. ciliated epithelia

3. cartilage

4. muscle

5. xylem

6. phloem

adaptations of squamous epithelia

• flat and single layer of cells in contact with membrane of epithelium = short diffusion distance

adaptations of ciliated epithelia

• hairlike projection - sway to move substances (eg mucus), goblet cells located in epithelium and they release mucus to trap molecules (eg dust in trachea)

adaptations of cartilage

• firm and flexible (in outer ear, nose, end of bones)

• provides structural support and prevents bones from rubbing together

• made up of elastin and collagen fibres

adaptations of muscle

• composed of tissue that can contract and relax to create movement

• have multiple fibres connecting with connective tissues in between

• mitochondria for energy

adaptations of xylem

• elongated

• hollow dead cells - one continuous tube

• lignin in walls - strengthen and waterproof

• made from stem cells in meristem

adaptations of phloem

• sieve tube/pores in end walls

• lacks most organelles - provide resources for sieve tube elements

• made from stem cells in meristem

What is an organ?

A group of different tissues that work together to perform a certain function.

What are organ systems?

An organ system is made up of two or more organs working together to perform a life function such as excretion or transport.

What is differentiation?

The ability of a cell to specialise to form a particular type of cell

Why are stem cells important?

They are a renewing source of undifferentiated cells for the growth and repair of tissues and organs.
-During growth and repair, stem cells divide to produce new cells, which then differentiate to become specialised to their function.
-Stem cells have the ability to use all their genes.
-Differentiation occurs by switching on or off appropriate genes.

Where do erythrocytes and neutrophils come from?

Stem cells in the bone marrow divide and differentiate to form both red and white blood cells.

How are xylem and phloem cells produced?

New cells are produced by mitosis in the meristem. These cells are expanded by the uptake of water and the development of a vacuole. They then differentiate into xylem and phloem.

How are xylem vessels produced?

-Once the xylem cells have been produced from meristem cells:
-Lignin is deposited in their cell walls to strengthen and waterproof the wall.
-The cells die and the contents are removed as the end walls break down, forming continuous columns of cells.
-These form tubes with wide lumens to carry water and dissolved minerals.
-The lignification is incomplete in some places, forming bordered pits.

How are phloem sieve tubes produced?

-Phloem cells produced from differentiation of meristem cells
-Sieve tube elements lose their nucleus and most of their organelles.
-The end walls develop numerous sieve pores to form sieve plates between the elements.
-Companion cells retain their organelles and can carry out metabolism to obtain and use ATP to actively load sugars into the sieve tubes.
-Sieve tube elements and companion cells are linked by numerous plasmodesmata.

What is a major potential use of stem cells?

For medicine and research

Where do the stem cells used for medicine/research come from?

1. Embryonic stem cells, which are present in a young embryo (toti - normally stem cells are pluri)
2. Blood from the umbilical cord
3. Adult stem cells found in developed tissues such as bone marrow
4. Scientists can also induce certain tissue cells to become stem cells (known as induced pluripotent stem cells or iPS cells).

What are some uses of stem cells in medicine?

• tissue repair - skin cells for burn patients, neurones for damaged spinal chord, pancreas cells for type 1 diabetes

• treating neurological conditions (alzheimer’s and parkinsons)

How are stem cells used in developmental biology (research)?

• Embryonic stem cells have the ability to differentiate into embryos, allowing scientists to study the developmental stages of the early embryo

Research on developmental biology can provide important information about:

• developmental problems

• the effects of medicines on embryos