Cell division/diversity/differentiation

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Cell Division - The Cell Cycle

The cell cycle = the sequence of events that take place as a parent cell duplicates its contents and divides, to produce 2 new daughter cells

Cell cycle stages = INTERPHASE (G1, S, G2 and G0 phases) and the M PHASE (prophase, metaphase, anaphase, telophase, cytokinesis)

G1 = Cell growth, transcription of genes to make RNA, biosynthesis eg. protein synthesis, organelles duplicate, enzymes for DNA replication made, p53 controlled

G0 = when cells undergo apoptosis/differentiation/senescence eg. neurones always and if DNA errors. Lymphocytes = special case

G2 = Cell growth, energy stores are increased (for anaphase), special chemicals made that stimulate condensation of chromatin + spindle formation

CHECKPOINTS ensure chromosomes in right place, DNA error-free, tumour formation prevented

Tumour supressor genes eg. p53, oncogenes to proto-oncogenes

G1/S checkpoint = checks chemicals needed for DNA replication are there + DNA is error free

G2/M checkpoint = checks DNA has been replicated with no errors

M checkpoint = checks all chromosomes attached to spindle

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Mitosis

Reasons for mitosis: Asexual reproduction (eg. bacteria = binary fission, yeast = budding), Growth and Repair

Mitotic Index = Number of cells with visible chromosomes/ Total number of cells x100

Asexual reproduction = less genetic variation + a reduced gene pool

PROPHASE: replicated chromosomes from interphase supercoil (shorten + thicken), nuclear envelope breaks down, daughter centrosomes containing centrioles move to opposite poles of cell + tubulin threads form a spindle between centrioles

METAPHASE: replicated chromosomes line up down the cell equator, each chromosome becomes attached to the spindle by its centromere, via the kinetochore, chromatids face opposite poles

ANAPHASE: replicas of each chromosome are pulled apart to opposite cell poles, centromere divides which separates the sister chromatids, each chromatid now becomes an individual chromosome, motor proteins walk along tubulin threads when pulling + spindle fibres shorten, lead by centromere = V shape

TELOPHASE: two new nuclei form as nuclear envelopes form around each new set of chromosomes, spindle fibres break down, the chromosomes uncoil so no longer visible.

CYTOKINESIS: the whole cell splits into 2 forming 2 daughter cells

ANIMALS = plasma membrane folds inwards + 'nips' in cytoplasm

PLANT = end plate forms where the equator was by vesicles holding cell wall material lining up + fusing

Meiosis

Meiosis = sexual reproduction producing GAMETEs w/ a HAPOID no. of chromosomes (23) (n). Normal body (germ) cells have a DIPLOID no. of chromosomes (46) (2n) - diploid produced when egg an sperm fuse, -> zygote

Meiosis occurs in gonads (ovaries + testes) - 23 chromosomes from egg and sperm form matching pairs

HOMOLOGOUS CHROMOSOMES = MATCHING PAIRS (1 from egg, 1 from sperm) containing the same genes at the same places on the same chromosome, but different alleles (versions)

Heterozygous = dominant shows, Homozygous dominant = dominant shows, homozygous recessive = recessive shows

INTERPHASE = In S phase, chromosomes are duplicated, G1, G2, G0

PROPHASE I = DNA supercoils, nuclear envelope breaks down, spindle starts to form, chromosomes come together in homologous pairs (bivalents), crossing over occurs

METAPHASE I = Homologous chromosomes attach along cell equator by spindle fibres attaching to centromeres, independent assortment of chromosomes

ANAPHASE I = Members of each pair of homologous chromosomes are pulled apart by motor proteins (centromeres DON'T divide), most plants go from anaphase I to prophase II

TELOPHASE I = (animal), nuclear envelopes form (2), cytokinesis, short interphase where chromosomes uncoil

PROPHASE II = nuclear envelopes disintergrate if formed, chromosomes (no longer identical) condense, spindle forms

METAPHASE II = chromosomes attach by centromere to spindle (at equator), independent assortment of chromatids

ANAPHASE II = centromeres divide, chromosomes pulled apart

TELOPHASE II = nuclear envelopes form around haploid nuclei, animals + plants = different

HOW GENETIC VARIATION IS PRODUCED = crossing over, forming chiasmata (allele shuffling), independent assortment of chromosomes/chromatids, mutations, random fertilisation

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Diversity in animal cells

EPITHELIAL CELLS: make up epithelium tissue, found in alveoli walls, skin, capillaries (in + out the body) SQUAMOUS epithelium have flattened shapes (thin for diffusion of gases), CILIATED (SIMPLE COLUMNAR) have cilia to move mucus along in synchronised manner

ERYTHROCYTES: carry O2 from lungs to respiring cells. Very SMALL so large SA:VOL ratio for fast O2 diffusion, BICONCAVE disc (increases SA), FLEXIBLE (good cytoskeleton allows change of shape as moves through capillaries), most organelles are LOST at differentiation (more space for Hb)

NEUTROPHILS: ingest invading pathogens by PHAGOCYTOSIS. TWICE the size of erythrocytes (lots of space), contains a MULTILOBED NUCLEUS (DNA packaged in spheres), attracted to infection sites by CHEMOTAXIS

SPERM CELLS: many MITOCHONDRIA for respiration (movement of undulipodium), SMALL, LONG and THIN, contains ENZYMES IN HEAD (released from the acrosome (special lysosome), digest outer ovum covering, LITTLE CYTOPLASM IN HEAD (room for nucleus)

Diversity in animal tissues

EPITHELIAL TISSUE: covers internal/external body surfaces eg. skin, digestive system for PROTECTION/ABSORPTION/SECRETION/EXCRETION/FILTRATION. Made of cells close together, forming continuous sheets. Adjacent cells bound by LATERAL CONTACTS. no blood vessels (nutrients obtained from TISSUE FLUID), short cell cycles (divide often)

MUSCULAR TISSUE: allows movement by contraction, many blood vessels, muscle cells are FIBRES (elongated + contain myofilaments made of actin and myosin, allowing muscle tissue to contract - needs to shorten in length to move bones). Types = SKELETAL (packaged by connective tissue sheets, joined to bones by tendons so they move), CARDIAC (walls of heart), SMOOTH (blood vessels - propels stuff)

CONNECTIVE TISSUE: holds tissues together + supports eg, ligaments, tendons. Consists of a NON LIVING EXTRACELLULAR MATRIX containing proteins + polysaccharides eg. elastin, hyaluronic acid. Matrix separates living cells in tissue + enables to withstand forces

Example of connective tissue = CARTILAGE = immature CHONDROBLASTS secrete the extracellular matrix. After this, they become mature CHONDROCYTES + maintain matrix. Types: HYALINE (embryonic skeleton, reduced friction), FIBROUS (knee, compression) and ELASTIC (suppot + distortion eg, outer ear)

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Diversity in plant cells

PALISADE CELLS = photosynthesis. LONG and CYLINDRIC (pack closely but space for air), LARGE VACUOLE so chloroplasts on OUTSIDE (smaller diffusion distance for CO2), many CHLOROPLASTS, cytoskeleton threads + motor proteins so move chloroplasts according to height.

ROOT HAIR CELLS = HAIR like projections (increased SA for absorption), mineral ions are ACTIVELY TRANSPORTED in so water follows, carrier proteins in membrane for mineral ions, produced ATP

XYLEM = carries water + mineral ions. Dead cells aligned end to end, LIGIN waterproofs, NARROW TUBES (water column doesn't break + capillary action), BORDERED PITS in lignified walls (water can move sideways). lignin in ANNULAR/RETICULATE/SPIRAL patterns - flexibility

Parenchyma = packing tissue, collenchyma = strengthens vascular bundles w/ cellulose, schlerenchyma = lignified

PHLOEM = assimilates transport - sieve tubes made of sieve tube elements + companion cells - elongates sieve tubes, NO NUCLEUS + LITTLE CYTOPLASM (space for sap MASS FLOW), SIEVE PLATES (sap movement)

GUARD CELLS = open + close stomata. Light energy used for ATP, K+ ions actively transported in, water enters by osmosis, swell (cell wall flexible at tip and rigid in inner wall) - turgid - stomata open for gas exchange

Plant organs = stem, xylem + phloem, root

CAMBIUM CELLS in the vascular bundle differentiate into xylem and phloem

MERISTEM CELLS: thin walls w/ little cellulose, no chloroplasts/small vacuole

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Diversity in plant tissues

EPIDERMAL: flattened, closely packed cells that LACK CHLOROPLASTS (except guard cells), protective covering over leaves, stems and roots, some walls have a waxy cuticle to prevent water loss

MERISTEMATIC: contains stem cells, where all other plant cells are derived from, found at TIPS OF SHOOTS + ROOTS + in CAMBIUM of vascular bundle

VASCULAR: transport, xylem + phloem

Stem Cells

Stem cell = an unspecialised cells that is able to express ALL genes, and divide by mitosis

Once stem cells have specialised, they enter G0

Meristematic stem cells can CLONE plants (eg. disease-resistance)

POTENCY = a stem cell's ability to differentiate into different cell types

TOTIPOTENT = can differentiate into ANY type of cell (zygote + first 8-16 cells from mitosis)

PLURIPOTENT = can form ALL TISSUES in the body (human early embryos) - ethics

MULTIPOTENT = can only form a range of cells in CERTAIN tissue types (bone marrow, umbilical cord)

Embryonic stem cells = found in the inner cell mass of the blastocyst (embryo 5-6 days after fertilisation)

Induced pluripotent stem cells = made in labs - generated from adult cells that have REGAINED the capacity to be pluripotent

Main uses of stem cells: BONE MARROW TRANSPLANTS, DRUG TRIALS/RESEARCH, DEVELOPMENTAL BIOLOGY, REPAIR/REPLACEMENT OF DAMAGED TISSUES, MEDICAL (type 1 diabetes, Parkinson's Alzheimer's, spine injuries, grow transplant organs)

Therapeutic cloning = stem cell made to have same genes as patient to reduce rejection. PROCESS: nucleus taken out of human egg cell, nucleus from patient into egg cell, egg cell stimulated to develop into embryo, stem cells grown, stem cells treated