The Cell Cycle

The Cell Cycle

• This is made up of two phases: interphase and mitosis.
• Interphase is the longest stage in the cycle - it is comprised of many stages and check points. During interphase a cell is carrying out its major functions. Such as producing enzymes or hormones.
• During interphase:
• DNA is replicated and checked for errors.
• Protein synthesis in the cytoplasm.
• Mitochondria grow and divide.
• Chloroplasts grow and divide.
• Metabolic process.
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• Bildunterschrift: This is the cell cycle

The stages of interphase

• G1: This is the first growth phase of the cell. During this period the cell grows in size. Also the proteins and organelles are synthesised and replicated. Once it has passed this stage it is committed to cell division.
• G1 Check Point: Here the cell is checked for its size, nutrients, growth factors and DNA damage.
• G0 - It is a stage in the cell cycle which a cell may enter temporarily or permanently. There are a number of reasons for this - differentiation means a cell is no longer able to divide as it has specialised. It will not re-enter. Secondly the DNA may be damaged so it enters the cycle either to be repaired. If irreparable then the cell will undergo apoptosis.
• S: Here DNA is replicated in the nucleus.
• G2: This is the second growth phase of the cell. Here it continues to grow in size.
• G2 Checkpoint: This is the last check before mitosis. To pass a cell must have correctly replicated DNA.

Mitosis

Mitosis stages

• Prophase: Chromatin coils and condenses. This means the chromosomes become visible. The nuclear membrane breaks down and the nuclear envelope disappears., The centrioles move to the poles.
• Metaphase: The spindle fibres form. Chromosomes are moved by the spindles to line up on the metaphase plate.
• Anaphase: Spindles shorten separating the sister chromatids and dividing the centromere. The sister chromatids are pulled to the poles.
• Telophase: Chromatids reached the poles. They are now called chromosomes and the chromosomes uncoil. Cytokinesis can begin. The nuclear membrane reforms.

Meiosis stages

• Meiosis 1
• - Prophase 1: Chromosomes condense, the nuclear envelope disintegrates. The bivalents form and crossing over occurs. The spindle fibres also form.
• - Metaphase 1: Bivalents line up on the metaphase plate. They can orientate in any way - this is independent assortment. This creates different alleles.
• - Anaphase 1: Spindle fibres shorten. The bivalents separate. DNA exchange occurs forming recombinant chromatids. The homologous chromosomes are separated and pulled to the poles.
• - Telophase 1: Chromosomes assemble at the poles The nuclear membrane reforms. Chromosomes uncoil.
• Meiosis 2
• - Prophase 2: The chromosomes condense and become visible again. The nuclear envelope breaks down and the spindle fibres begin to form.
• - Metaphase 2: The individual chromosomes line up on the metaphase plate. There is independent assortment again and genetic variation.
• - Anaphase 2: The chromatids are pulled to the poles after the centromeres divide.
• - Telophase 2: The chromatids assemble at the poles. The chromosomes uncoil forming chromatin again. The nuclear envelope reforms. Cytokinesis results in 4 haploid daughter cells due to reduction division. They are genetically different due to crossing over and independent assortment.

Plant cell division

• Due to the presence of their cell walls, cleavage furrow cannot form.
• Instead, the vesicles from the Golgi apparatus assembles in the same place as the metaphase plate.
• The vesicles fuse with each other, dividing the cell in two.
• Then, new sections of cell wall form along the membrane.

Specialised cells

• Specialised cells --> Tissues --> Organs --> Organ systems --> Whole organisms
• Differentiated cells = They are the cells found in a multicellular organism which are specialised to carry out a particular function.
• Examples:
• Erythrocytes: Flattened biconcave shape to increase their surface area = good for oxygen transport. No nuclei = maximise space for haemoglobin. They are flexible = fit through narrow capillaries.
• Neutrophils: Multi-lobed = squeeze through small gaps to get to infection site. Granular cytoplasm contains lysosomes = these contain enzymes for attacking pathogens.
• Sperm cells: Tail = capable of movement. Contain mitochondria = supply them with the energy they need to survive. Head of sperm contains digestive enzymes = can digest the protective layers around the ovum.
• Palisade cells: Contain chloroplasts to absorb light. Can be closely packed due to shape to form a continuous layer. Thin cell walls to increase diffusion of CO2. Large vacuole to maintain turgor pressure. Chloroplasts can move to absorb more cytoplasm.
• Root hair cell: Have root hairs to increase surface area.
• Guard cell: Stomata to allow gas exchange. Can change shape to prevent water loss. Thicker cell wall on one side so it does not change shape symmetrically as the volume changes.

Tissues

• A tissue is made up of a collection of differentiated cells that have a specialised function or functions. There are 4 types: nervous tissue (transmission of electrical impulses), epithelial (cover body surfaces), muscle (contraction), connective (to hold tissues together or as a transport medium.
• Squamous epithelium: very thin due to flat cells and only one cell thick. Good for rapid diffusion due to it being thin.
• Ciliated epithelium: Have cilia which more to shift mucus. Also they have goblet cells which produce mucus to trap bacteria.
• Cartilage: connective tissue. Contains fibres of elastin and collagen. It is firm and prevents bones rubbing against each other.
• Muscle: Needs to be able to contract.
• Epidermis: covers the surface of plants. Usually covered by a waxy cuticle. Contain stomata which allows CO2, H2O and O2 in and out.
• Xylem: composed of dead vessels which allow a continous water flow. The walls are strengthened by lignin.
• Phloem: Composed of perforated sieve plates, sieve tubes and parenchyma cells.

Organ systems

• Contains a number of organs coordinated for a function.
• Examples:
• Digestive system: breaks down food and water into their monomers.
• Cardiovascular: moves blood around the body.
• Gaseous exchange: brings air into the body so oxygen can be extracted for respiration and CO2 can be expelled.

Stem Cells

• They are undifferentiated cells with no function.
• Potency: The ability to differentiate into different cell types.
• Totipotent: they can differentiate into any cell type. E.g. those in a zygote or fertilised egg.
• Pluripotent: they can form all tissue types by not whole organisms. These are present in early embryos.
• Multipotent: they can only form a range of cells within a certain tissue type. E.g. haematopoetic stem cells in the bone marrow which can form many different blood types.
• Sources of animal stem cells
• Embryonic stem cells: present in early embryo development and are totipotent.
• Adult stem cells: present from brith. They are typically found in specific areas such as the bone marrow. They can be harvested from the umbilical cords of newborn babies (pluripotent). Invasive surgery isn't needed.
• Sources of plant stem cells
• Meristematic tissue, e.g. in roots and tips.

Folie 11

• Uses
• Heart disease
• Type 1 diabetes
• Parkinsons
• Alzheimers
• Macular degeneration
• Birth defects
• Spinal injuries
• Burn treatment
• Drug trials
• Ethics
• Particularly against the use of embryos as its unethical. People see it as murder.
• Plant stem cells don't raise the same ethics.