2.6 Cell Division , Cell Diversity And Cellular Organisation

Question 1

State what the cell cycle is and outline its stages

Answer 1

Regulated cycle of division with intermediate growth periods

1. Interphase
2. Mitosis or meiosis (nuclear division)
3. Cytokinesis (cytoplasmic division)

Question 2

Outline what happens during interphase

Answer 2

G1: cell synthesises proteins for replication e.g.tubulin for spindle fibres and cell size doubles
S: DNA replicates = chromosomes consist of 2 sister chromatids joined at a centromere
G2: organelles divide

Question 3

What is the purpose of mitosis ?

Answer 3

Produces 2 genetically identical daughter cells for:

• growth
• Cell replacement/ tissue repair
• asexual reproduction

Question 4

Name the stages of mitosis

Answer 4

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

(PMAT)

Question 5

Outline what happens during prophase

Answer 5

1. Chromosomes condense, becoming visible. (X-shaped: 2 sister chromatids joined at centromere)
2. Centrioles move to opposite poles of cell (animal cells) and mitotic spindle fibres form
3. Nuclear envelope and nucleolus break down = chromosomes free in cytoplasm

Question 6

Outline what happens during metaphase

Answer 6

Sister chromatids line up at cell equator , attached to the mitotic spindle by their centromeres

Question 7

Outline what happens during anaphase

Answer 7

Requires energy from ATP hydrolysis

1. Spindle fibres contract = centromeres divide
2. Sister chromatids separate into 2 distinct chromosomes and are pulled to opposite poles of cell ( looks like ‘V’ shapes facing eachother )
3. Spindle fibres break down

Question 8

Outline what happens during telophase

Answer 8

1. Chromosomes decondense , becoming invisible again
2. New nuclear envelope form around each set of chromosomes = 2 new nuclei , each with 1 copy of each chromosome

Question 9

What happens during cytokinesis ?

Answer 9

1. Cell membrane cleavage furrow forms
2. Contractile division of cytoplasm

Question 10

How is the cell cycle regulated ?

Answer 10

Checkpoints regulated by cell-signalling proteins ensure damaged cells do not progress to next stage of cycle

Question 11

Describe what happens at each checkpoint in the cell cycle

Answer 11

• Between G1 and S, cell checks for DNA damage. After restriction point, cell enters cycle
• between G2 and M, cell checks chromosome replication
• at metaphase checkpoint, cell checks that sister chromatids have attached to spindle correctly.

Question 12

What is meiosis?

Answer 12

A form of cell division that produces four genetically different haploid cells (cells with half the number of chromosomes found in the parent cell) known as gametes

Question 13

What happens during meiosis (I) ?

Answer 13

• Matching chromosomes from each parent pair up.
• They swap bits of DNA at points called chiasmata (crossing over).
• The cell splits in two, and each new cell gets either the mum’s or dad’s version of each chromosome—randomly.

Question 14

What are homologous chromosomes?

Answer 14

• A pair of chromosomes (one from mum, one from dad) have genes in the same places (same loci).
• These genes can have the same or different versions, called alleles.

Question 15

What happens during meiosis (II) ?

Answer 15

1. Independent segregation of sister chromatids
   2.Each cell divides again , producing 4 haploid cells

Question 16

How does meiosis produce genetic variation ?

Answer 16

• Crossing over in meiosis I swaps DNA between paired chromosomes.
• Independent assortment means chromosomes and chromatids are separated randomly.
• This creates new combinations of alleles in the gametes (sperm or egg).

Question 17

How do cells become specialised ?

Answer 17

• During cell differentiation, only some genes are expressed while others are switched off.
• Transcription factors control gene expression.
• This leads to production of specific proteins, which determine the structure and function of the cell.
• As a result, the cell becomes specialised for a particular role.

Question 18

What is a transcription factor ?

Answer 18

A protein that controls the transcription of genes so that only certain parts of the
DNA are expressed, e.g. in order to allow a cell to specialise.

Question 19

How do transcription factors work ?

Answer 19

• Transcription factors move from the cytoplasm into the nucleus.
• They bind to the promoter region of the target gene.
• This either makes it easier or harder for RNA polymerase to attach, increasing or decreasing the rate of transcription.

Question 20

What is a stem cell ?

Answer 20

an undifferentiated cell that has the potential to differentiate into a variety of specialised cell types.

Question 21

Name and define the 4 types of stem cell

Answer 21

• Totipotent: can develop into any cell type including the placenta and embryo.
• Pluripotent: can develop into any cell type excluding the placenta and embryo.
• Multipotent: can only develop into a few different types of cell.
• Unipotent: can only develop into once type of cell.

Question 22

Suggest some uses of stem cells

Answer 22

• repair of damaged tissue
• drug testing on artificially grown tissues
• treating neurological diseases e.g.Alzheimer’s and Parkinson’s
• Researching developmental biology e.glfoundation of organs , embryos

Question 23

Describe the 2 groups of specialised cells in blood

Answer 23

Erythrocytes (red blood cells): biconcave, no nucleus , lots of haemoglobin to carry oxygen

Leukocytes (white blood cells): involved in the immune response , lymphocytes are responsible for producing antibodies and managing immune responses. Neutrophils and eosinophils are involved in the phagocytosis of foreign material and pathogens.Monocytes differentiate into macrophages that also engulf and digest foreign material

Question 24

How do the specialised cells in blood form ?

Answer 24

Multipotent stem cells in the bone marrow differentiate into:
- erythrocytes, which have a short lifespan and cannot undergo mitosis since they have no nucleus
- leucocytes, including neutrophils

Question 25

State the relationship between a system and specialised cells

Answer 25

• Specialised cells are adapted to perform a specific function. • These specialised cells group together to form tissues, which carry out particular functions. • Organs are made up of several types of tissues working together to perform complex functions. • Organ systems are groups of organs that work together to carry out vital processes in the body.

Question 26

Describe the structure of squamous and ciliated epithelia

Answer 26

• Simple squamous epithelium consists of a single layer of thin, flat squamous cells. These cells have a round nucleus and are attached to the underlying tissue by a basement membrane. • Ciliated epithelium is made up of column-shaped epithelial cells that have surface projections called cilia. The cilia beat in a synchronised pattern to move substances across the surface.

Question 27

Describe the specialised structure of a spermatozoon

Answer 27

• The spermatozoon (sperm cell) has a streamlined shape for efficient movement. • It contains a head with a haploid nucleus, which holds the genetic material. The head is covered by an acrosome, which contains enzymes that help the sperm penetrate the egg. • The midpiece contains mitochondria that provide energy for the sperm’s movement. • The tail (flagellum) enables the sperm to swim towards the egg.

Question 28

Describe the structure and function of palisade cells and guard cells in plants

Answer 28

• Palisade cells are specialised for photosynthesis. They contain numerous chloroplasts to absorb light energy efficiently. These cells are packed closely together to maximise light absorption. • Guard cells surround the stoma (small pores on the leaf surface). They control the opening and closing of the stoma: when turgid, the stoma opens, and when flaccid, the stoma closes. The cell walls are thickened in areas by spirals of cellulose, which helps control the shape change of the guard cells.

Question 29

Describe the structure and function of root hair cells

Answer 29

• Root hair cells are specialised for absorbing water and minerals from the soil. • The long, hair-like projections increase the surface area, aiding osmosis and the uptake of minerals through active transport via carrier proteins. • These cells contain many mitochondria, which produce the necessary ATP to fuel the active transport processes.

Question 30

What are meristems ?

Answer 30

• Meristems are regions of undifferentiated, totipotent plant cells that can divide and differentiate into various specialised cell types, such as xylem vessels and phloem sieve tubes. • They are classified by location as: • Apical meristems – found at root and shoot tips, responsible for lengthwise growth. • Intercalary meristems – located in stem regions, allowing growth between mature tissues. • Lateral meristems – found in vascular areas, responsible for widening (secondary growth) of roots and stems.

Question 31

Describe the structure of a vascular bundle (search up what it looks like)

Answer 31

• A vascular bundle contains xylem (transports water) and phloem (transports sugars), sometimes with cambium in between.

Question 32

Describe the structure of phloem tissues

Question 33

Describe the structure of phloem tissues

Answer 33

• Phloem tissue is made up of sieve tube elements and companion cells. • Sieve tube elements are long, hollow cells that form tubes to transport sucrose in solution (sap). They have sieve plates at their ends and lack a nucleus. • Companion cells lie alongside sieve tube elements and provide ATP for active loading of sucrose. • Plasmodesmata are microscopic channels between the cells that allow the movement of substances and communication between sieve tube elements and companion cells.

Question 34

Describe the primary cell types in xylem tissue

Answer 34

• Xylem tissue is made up of vessel elements and tracheids, specialised for water transport and support. • Vessel elements have lignified secondary walls for strength and waterproofing, with perforated end walls to allow efficient water flow between cells. • Tracheids are elongated cells with tapered ends, allowing close packing. They have pits in their walls for lateral water movement. Both cell types are dead, with no cytoplasm or nucleus, forming hollow tubes.

Question 35

Describe the additional cell types in xylem tissue

Answer 35

• Xylem parenchyma: Living packing tissue with thin walls, involved in storage and maintaining turgidity. • Sclerenchyma fibres: Long, narrow cells with heavily lignified walls, providing mechanical strength and supporting the vessel under negative pressure. • Sclereids: Shorter, irregular-shaped cells with thick lignified walls that add extra support and protection to xylem tissue.

Question 36

Describe the structure of cartilage

Answer 36

• Cartilage is a smooth, elastic, avascular connective tissue made of chondrocytes (cartilage cells). • Chondrocytes produce a large amount of extracellular matrix (ECM), which is rich in collagen fibres and proteoglycans.

Question 37

What are the three types of cartilage ?

Answer 37

• Hyaline cartilage – strong and flexible, found in joints and the nose. • Yellow elastic cartilage – more elastic fibres, found in the ear. • White fibrous cartilage – tough and dense, found in intervertebral discs.

Question 38

Name the 3 types of muscles I’m the body and where they are located

Answer 38

• Cardiac muscle – found only in the heart; responsible for continuous, rhythmic contraction. • Smooth muscle – found in the walls of blood vessels, intestines, and other hollow organs; controls involuntary movements. • Skeletal muscle – attached to the skeleton by tendons; responsible for voluntary movement of the body.

Question 39

Describe the gross structure of skeletal muscle

Answer 39

• Skeletal muscle is made of bundles of long, parallel muscle fibres called myofibrils, formed by the fusion of individual muscle cells. • This fusion creates multinucleate fibres with no weak points between cells, increasing strength. • Each bundle is surrounded by endomysium, a layer of loose connective tissue that contains capillaries to supply oxygen and nutrients.