2.6 Cell Division, Cell Diversity and Cellular Organisation

Cell Cycle

Regulated cycle of division with intermediate growth periods.

Stages of Cell Cycle

1. interphase 2. mitosis or meiosis (nuclear division) 3. cytokinesis (cytoplasmic division)

Interphase

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

Purpose of Mitosis

Produces 2 genetically identical daughter cells for: ●growth ●cell replacement/ tissue repair ●asexual reproduction

Stages of Mitosis

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

Prophase

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

Metaphase

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

Anaphase

Requires energy from ATP hydrolysis. 1. Spindle fibres contract = centromeres divide. 2. Sister chromatids separate into 2 distinct chromosomes & are pulled to opposite poles of cell. (looks like 'V' shapes facing each other). 3. Spindle fibres break down.

Telophase

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

Cytokinesis

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

Regulation of Cell Cycle

Checkpoints regulated by cell-signalling proteins ensure damaged cells do not progress to next stage of cycle. Cyclin-dependent kinase enzymes phosphorylate proteins that initiate next phase of reactions.

Key Checkpoints in Cell Cycle

Between G1 & S, cell checks for DNA damage (e.g. via action of p53). After restriction point, cell enters cycle. Between G2 & M, cell checks chromosome replication. At metaphase checkpoint, cell checks that sister chromatids have attached to spindle correctly.

Meiosis

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.

Meiosis I

1. Homologous chromosomes pair to form bivalents. 2. Crossing over (exchange of sections of genetic material) occurs at chiasmata. 3. Cell divides into two.

Homologous chromosomes

Pair of chromosomes with genes at the same locus. 1 maternal & 1 paternal. Some alleles may be the same while others are different.

Meiosis II

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

Genetic variation in meiosis

Crossing over during meiosis I. Independent assortment (random segregation) of homologous chromosomes & sister chromatids. Result in new combinations of alleles.

Cell specialization

Some genes are expressed while others are silenced due to cell differentiation mediated by transcription factors. Cells produce proteins that determine their structure & function.

Transcription factor

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.

Mechanism of transcription factors

1. Move from the cytoplasm into nucleus. 2. Bind to promoter region upstream of target gene. 3. Makes it easier or more difficult for RNA polymerase to bind to gene. This increases or decreases rate of transcription.

Stem cell

Undifferentiated cells that can divide indefinitely and turn into other specific cell types.

Types of stem cells

●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.

Uses of stem cells

●Repair of damaged tissue e.g. cardiomyocytes after myocardial infarction. ●Drug testing on artificially grown tissues. ●Treating neurological diseases e.g. Alzheimer's & Parkinson's. ●Researching developmental biology e.g. formation of organs, embryos.

Specialised cells in blood

Erythrocytes (red blood cells): biconcave, no nucleus, lots of haemoglobin to carry oxygen. Leucocytes (white blood cells): lymphocytes, eosinophils, neutrophils to engulf foreign material, monocytes.

Formation of specialised blood cells

Multipotent stem cells in the bone marrow differentiate into: ●Erythrocytes, which have a short lifespan & cannot undergo mitosis since they have no nucleus. ●Leucocytes, including neutrophils.

Relationship between system and specialised cells

specialised cells → tissues that perform specific function → organs made of several tissue types → organ systems.

Structure of squamous epithelium

Flat cells that form a single layer, allowing for easy diffusion and filtration.

Structure of ciliated epithelium

Cells with hair-like structures (cilia) on their surface that help move substances across the epithelial surface.

Simple squamous epithelium

Single smooth layer of squamous cells (thin & flat with round nucleus) fixed in place by basement membrane.

Ciliated epithelium

Made of ciliated epithelial cells (column-shaped with surface projections called cilia that move in a synchronised pattern).

Spermatozoon

Specialised to fertilise an ovum during sexual reproduction in mammals.

Palisade cells

Specialised to absorb light energy for photosynthesis, so contain many chloroplasts. Pack closely together.

Guard cells

Form stoma. When turgid, stoma opens; when flaccid, stoma closes. Walls are thickened by spirals of cellulose.

Root hair cells

Specialised to absorb water and low-concentration minerals from soil. Hair-like projections increase surface area for osmosis / carrier proteins for active transport. Many mitochondria produce ATP for active transport.

Meristems

Totipotent undifferentiated plant cells that can develop into various types of plant cell, including xylem vessels & phloem sieve tubes. Classified as apical (at root & shoot tips), intercalary (stem) or lateral (in vascular areas).

Vascular bundle

Contains phloem tissue, cambium (meristematic tissue), and xylem tissue.

Phloem tissue

Sieve tube elements: form a tube to transport sucrose in the dissolved form of sap. Companion cells: involved in ATP production for active loading of sucrose into sieve tubes. Plasmodesmata: gaps between cell walls where the cytoplasm links, allowing substances to flow.

Xylem tissue

Vessel elements: lignified secondary walls for mechanical strength & waterproofing; perforated end walls for rapid water flow. Tracheids: tapered ends for close packing; pits for lateral water movement; no cytoplasm or nucleus.

Xylem parenchyma

Packing tissue with thin walls that transmit turgidity.

Sclereids

A type of cell found in xylem tissue.

Sclerenchyma fibres

Heavily lignified to withstand negative pressure.

Cartilage

Avascular smooth elastic tissue made of chondrocytes, which produce extensive extracellular matrix (ECM). ECM mainly contains collagen & proteoglycan. 3 categories: hyaline, yellow elastic, white fibrous (depends on ratio of cells: ECM).

Cardiac muscle

Exclusively found in heart.

Smooth muscle

Walls of blood vessels and intestines.

Skeletal muscle

Attached to incompressible skeleton by tendons.

Skeletal muscle structure

Muscle cells are fused together to form bundles of parallel muscle fibres (myofibrils). Arrangement ensures there is no point of weakness between cells. Each bundle is surrounded by endomysium: loose connective tissue with many capillaries.