StemUp: OCR A A level Biology 2.1.6 Cell division, cell diversity and cellular organisation

What are the two main phases of the eukaryotic cell cycle?

1. Interphase - Cells are not actively dividing and prepare for cell division.

2. M (Mitotic) Phase - The period of cell division, including mitosis and cytokinesis.

What happens during interphase? (2)

- Cell carries out normal functions

- Prepares for cell division

What are the three stages of interphase? (3)

1. G1 Phase - Proteins are produced, and organelles replicate

2. S Phase - DNA is replicated, doubling the amount of DNA

3. G2 Phase - The cell increases in size, energy stores rise, and duplicated DNA is checked for errors

What are the stages of the M phase? (2)

1. Mitosis - Division of the nucleus

Cytokinesis - Division of the cytoplasm.

What is the G0 phase? (1)

Phase where the cell leaves the cycle, either temporarily or permanently

What are reasons a cell enters the G0 phase? (3)

1. Differentiation - The cell becomes specialized and can no longer divide.

2. DNA Damage - If the DNA is damaged, the cell may enter G0 permanently.

3. Senescence - Cells eventually stop dividing and become senescent. This increases with age and is linked to age-related diseases like cancer.

Why is it important to regulate the cell cycle? (2)

- To ensure that two identical daughter cells are created from the parent cell

- To ensure the cell divides only when it has grown to the right size, the DNA is error-free, and chromosomes are in the correct positions during mitosis

What are checkpoints in the cell cycle? (1)

Control mechanisms that ensure the cell cycle proceeds correctly

What does the G1 checkpoint check for? (4)

- Cell size

- Nutrients

- Growth factors

- DNA damage

What happens if the cell passes the G1 checkpoint? (2)

- DNA replication is triggered

- The cell enters the S phase

What happens if the cell doesn't meet the requirements of the G1 checkpoint? (1)

The cell enters the G0 phase

What does the G2 checkpoint check for? (3)

- Cell size

- DNA replication

- DNA damage

What happens if the cell passes the G2 checkpoint? (1)

The cell initiates mitosis (M phase)

What does the spindle assembly checkpoint check for? (1)

Checks whether all chromosomes are attached to spindle fibers and aligned

What happens if the spindle assembly checkpoint is passed? (1)

Mitosis proceeds to completion

What happens during prophase? (4)

1. Chromosomes condense and shorten

2. Nucleolus disappears.

3. Centrioles move towards cell ends, forming spindle fibers

4. Nuclear envelope breaks down, and chromosomes lie free in the cytoplasm.

What happens during metaphase? (2)

1. Chromosomes line up in the middle of the cell and attach to spindle fibers via their centromere.

2. A metaphase plane is formed in the centre of the cell

What happens during anaphase? (2)

1. Centromeres divide, separating sister chromatids

2. Spindles contract, pulling chromatids to opposite ends of the cell, giving them a v-shape

What happens during telophase? (3)

1. Chromatids uncoil and become chromosomes again

2. Nuclear envelope forms around each set of chromosomes, creating two nuclei

3. Nucleolus reappears

What happens during cytokinesis? (2)

1. The cytoplasm divides

2. Creating two genetically identical diploid daughter cells

What type of cells are produced by mitosis? (1)

Two genetically identical diploid cells (each with two copies of each chromosome)

How does cytokinesis differ in plant cells? (3)

1. Vesicles from the Golgi apparatus assemble at the metaphase plate location

2 Vesicles fuse with each other and the cell surface membrane, dividing the cell in two

3. New sections of the cell wall form along the new membrane

How can chromosomes be observed in plant cells under a microscope? (2)

- Chromosomes can be stained with a dye like toluidine blue O

- To make them visible

Draw an image of how each of the 'PMAT' stages look in a microscope of a cell (4)

Why is mitosis necessary? (3)

- Growth - development of organism

- Tissue repair - to replace damaged or dead cells

- Asexual reproduction

How do prokaryotic organisms (e.g., bacteria) divide? (1)

By binary fission

What is the significance of meiosis? (2)

- Occurs in sexually reproducing organisms to produce gametes

- It results in 4 genetically different, haploid daughter cells

What is the difference between haploid and diploid cells? (2)

- Diploid = Contains 2 copies of each chromosome

- Haploid: Contains 1 copy of each chromosome

What type of cells are produced by meiosis? (2)

- 4 genetically different

- Haploid daughter cells

Why is meiosis important for sexual reproduction? (2)

- Meiosis produces haploid cells

- When fertilisation occurs, the diploid number is restored

What are homologous chromosomes? (3)

- Pair of chromosomes

- One maternal and one paternal

- Same size and have the same genes at the same loci but may have different alleles

What are sister chromatids? (2)

- Identical copies of each other formed during DNA replication

- They remain connected at the centromere until separated during cell division

What is the difference between homologous chromosomes and sister chromatids? (2)

- Homologous chromosomes are not identical and may have different alleles

- Sister chromatids are identical copies of a duplicated chromosome.

What happens before meiosis starts? (2)

1. DNA unravels and replicates, producing two copies of each chromosome

2. DNA condenses to form double-armed chromosomes, each consisting of two sister chromatids joined by a centromere

What happens in the first division of meiosis (meiosis I)? (2)

1. Homologous pairs of chromosomes are separated, halving the chromosome number

2. Independent segregation occurs, creating random combinations of chromosomes in each daughter cell

What happens during prophase I of meiosis? (4)

1. Chromosomes condense and arrange into homologous pairs

2. Crossing-over occurs

3. Centrioles move to opposite poles, forming spindle fibers

4. The nuclear envelope breaks down.

What happens during metaphase I of meiosis? (3)

1. Homologous pairs line up at the center of the cell and attach to spindle fibers by their centromeres

2. Independent assortment occurs, where the orientation of each homologous pair is random

3. Leading to a mix of maternal and paternal chromosomes at either pole.

What happens during anaphase I of meiosis? (2)

1. Spindles contract, separating homologous pairs

2. One chromosome from each pair moves to opposite ends of the cell

What happens during telophase I of meiosis? (2)

1. Chromosomes uncoil

2. The nuclear envelope forms around each group of chromosomes.

What happens during cytokinesis after meiosis I? (2)

1. The cytoplasm divides

2. Resulting in two haploid daughter cells

What happens during the second division of meiosis (meiosis II)? (4)

1. Pairs of sister chromatids are separated

2. The centromeres are divided

3. Producing 4 haploid daughter cells (gametes)

4. That are genetically different from each other

What happens during prophase II of meiosis? (3)

1. Chromosomes, still consisting of two sister chromatids, condense and become visible

2. The nuclear envelope breaks down

3. Spindle formation begins

What happens during metaphase II of meiosis? (1)

Individual chromosomes (not homologous pairs) assemble on the metaphase plate

What happens during anaphase II of meiosis? (2)

1. Sister chromatids are separated and pulled to opposite poles after division of the centromeres

2. Just like in anaphase of mitosis

What happens during telophase II of meiosis? (2)

1. Chromosomes uncoil

2. The nuclear envelope reforms, and the nucleolus becomes visible

What happens during cytokinesis after meiosis II? (2)

1. The cytoplasm divides

2. Producing 4 genetically different haploid daughter cells

Why are the daughter cells produced in meiosis genetically different? (1)

Due to crossing over and independent assortment during meiosis

Draw a diagram of the process of meiosis (6)

What are two ways meiosis leads to genetic variation? (2)

1. Crossing over

2. Independent segregation

How does random fertilisation contribute to genetic variation? (2)

1. Any sperm can fuse with any egg

2. Which increases genetic variation within a species

What is crossing-over and how does it lead to genetic variation? (4)

1. Homologous chromosomes come together during meiosis

2. Chromatids twist and swap parts

3. The chromatids contain different combinations of alleles

4. Each daughter cell gets chromosomes with unique allele combinations.

What is independent assortment and how does it lead to genetic variation? (3)

1. When homologous chromosomes are separated in meiosis I

2. Which chromosome from each pair ends up in each daughter cell is random

3. This results in daughter cells with different combinations of alleles

How is meiosis different from mitosis? (3)

- Meiosis produces cells with half the number of chromosomes, while mitosis produces cells with the same number as the parent cell

- Meiosis produces genetically different daughter cells, while mitosis produces genetically identical cells

- Meiosis produces 4 daughter cells, while mitosis produces 2 daughter cells

Why do meiosis and mitosis produce different outcomes? (2)

- Mitosis involves one division, while meiosis involves two divisions

- Mitosis does not involve crossing-over or independent segregation, so daughter cells are genetically identical

How do you calculate the number of possible combinations of chromosomes for each daughter cell in meiosis? (2)

- 2ⁿ

- Where n is the number of pairs of homologous chromosomes (haploid number)

How are erythrocytes specialised for carrying oxygen? (3)

- Biconcave shape increases surface area for gas exchange

- No nucleus allows more space for haemoglobin to carry oxygen

- Flexible to fit through narrow capillaries

How are neutrophils specialised to fight infections? (3)

- Multi-lobed nucleus helps them squeeze through gaps to reach infection sites

- Flexible shape allows them to engulf foreign pathogens

- Granular cytoplasm contains lysosomes with digestive enzymes to break down engulfed particles

What is the function of epithelial cells? (2)

- Epithelial cells cover organ surfaces

- Are joined by interlinking cell membranes with a base membrane

How are squamous epithelial cells specialised? (3)

- Found in lungs

- They are very thin

- Providing a short diffusion pathway for gas exchange

How are ciliated epithelial cells specialised? (2)

- Found in airways

- They have cilia that beat to move particles away from the lungs

How are sperm cells specialised for delivering genetic information to the female gamete? (3)

- Flagellum: Enables swimming towards the egg

- Lots of mitochondria: Provides energy to swim

- Acrosome: Contains digestive enzymes to penetrate the egg's surface for fertilisation

How are palisade cells specialised for photosynthesis? (4)

- Many chloroplasts: for maximise sunlight absorption

- Chloroplasts can move: to absorb more light as needed

- Rectangular shape: allows close packing to form a continuous layer

- Thin walls: has a short diffusion distance for carbon dioxide

How are root hair cells specialised for absorbing water and minerals? (3)

- Long extensions: Large surface area for absorption.

- Thin, permeable cell walls: Allows easy entry of water and ions.

- Lots of mitochondria: Provides ATP for active transport of minerals.

How are guard cells specialised for regulating gas exchange? (3)

- Found in pairs: Form a stoma (gap between them).

- Become turgid in light: Opens stomata for gas exchange

- Thin outer walls and thick inner walls: Cause bending outward, opening the stomata to allow gas exchange for photosynthesis.

How are cells organised into tissues, organs, and organ systems? (4)

- Specialised cells group together to form tissues.

- A tissue is a group of cells working together to perform a particular function

- Different tissues work together to form organs

- Different organs make up an organ system.

What is squamous epithelium? (2)

- A single layer of flat cells lining a surface

- Found in places where rapid exchange is essential, such as the alveoli in the lungs

What is ciliated epithelium? (3)

- A layer of cells covered in cilia

- Found in places where substances need to be moved

- Such as the trachea to move mucus away from the lungs.

What are muscle tissues made of? (1)

Bundles of elongated cells called muscle fibres

What are three different types of muscle tissues? (3)

- Smooth (e.g., stomach wall)

- Cardiac (found in the heart)

- Skeletal (used for movement)

What is cartilage? (3)

- A type of connective tissue found in joints, firm and flexible

- It also shapes and supports the ears, nose, and windpipe

- Formed when chondroblasts secrete an extracellular matrix and become trapped inside

What is xylem tissue? (3)

- A type of vascular tissue that transports water and supports the plant

- Contains dead, hollow xylem vessels and living parenchyma cells

- Xylem walls are strengthened with lignin, making them waterproof.

What is phloem tissue? (3)

- A type of vascular tissue that transports sugars around the plant

- Consists of sieve cells and companion cells

- Sieve cells have end walls called sieve plates to allow sap to move through easily

What is a stem cell? (1)

An undifferentiated cell that has the potential to form different types of cells

What is potency in stem cells? (1)

Stem cell's ability to differentiate into different cell types

What are totipotent stem cells? (2)

- Stem cells that can develop into any type of body cell, including extra-embryonic tissue

- Found in the early embryo (first 4 days post-fertilisation), e.g., zygote

What are pluripotent stem cells? (2)

- Stem cells that can develop into any body cell except extra-embryonic cells (e.g., placenta)

- Present in early embryos, and can form all tissue types but not whole organisms

What are multipotent stem cells? (2)

- Stem cells that can differentiate into a limited number of cell types

- E.g., haematopoietic stem cells in bone marrow give rise to various blood cells

What are unipotent stem cells? (2)

- Stem cells that can only differentiate into one cell type

- E.g., unipotent stem cells in the heart can replace damaged cardiomyocytes

How are stem cells used in animals? (1)

Replace damaged cells, e.g., new skin cells

How are stem cells used in plants? (2)

- Plants grow continuously

- So stem cells are needed to produce new shoots and roots throughout their life

Why can a skin cell produce keratin but not myosin, and a muscle cell produce myosin but not keratin? (3)

- In skin cells, the gene for keratin is expressed,

- While the gene for myosin is inactive.

- In muscle cells, the opposite is true

Why do skin cells retain the ability to divide, while cells in some other organs do not? (2)

- Skin cells are prone to wear and tear, requiring frequent replacement

- Other organs experience less damage and need fewer new cells

How are erythrocytes derived from stem cells? (2)

- Erythrocytes lack a nucleus and organelles, giving them a short lifespan (about 120 days).

- Adult stem cells in the bone marrow divide and differentiate to replace worn-out erythrocytes.

How are neutrophils derived from stem cells? (2)

- Neutrophils have a very short lifespan (around 6 hours)

- Adult stem cells in the bone marrow divide and differentiate to replace worn-out neutrophils

How are xylem vessels and phloem sieve tubes produced in plants? (2)

- Stem cells in the vascular bundle (a type of meristem)

- Divide and differentiate to become xylem vessels and phloem sieve tubes

How could stem cells be used to treat neural diseases? (2)

- Can be used to regrow healthy nerve cells

- Replacing those that die in e.g. Alzheimer's, which causes memory loss

How are stem cells used in developmental biology? (1)

- Stem cells' ability to divide indefinitely and differentiate

- Makes them useful for studying how organisms grow and develop

What are some positives of using stem cells in research? (2)

1. Stem cell therapies could save lives, e.g., by preventing people from dying while waiting for an organ transplant

2. They can improve the quality of life, e.g., by restoring sight for blind individuals

What are some negatives of using stem cells in research? (3)

1. Obtaining embryonic stem cells involves destroying an embryo, which some people believe is unethical.

2. Some believe that embryos have a right to life, and using them for stem cell research is morally wrong.

3. Others argue that only adult stem cells should be used, although they are not as flexible as embryonic stem cells.