topic 2

• chromosomes are coiled up lengths of DNA molecules
• body cells normally have two copies of each chromosome making them diploid cells
• when cells divide by mitosis it makes two identical cells
• multicellular organisms use mitosis to grow or replace cells that have been damaged
• Interphase: The first phase of the cell cycle, during which the cell grows and copies its DNA and increase the amount of subcellular structures
• mitosis stages: prophase, metaphase, anaphase, telophase
• prophase: the chromosomes condense and the nuclear membrane breaks down so the chromosomes lie free in the cytoplasm
• metaphase: the chromosomes line up at the centre of the cell
• anaphase: spindle fibres pull the chromosomes apart then the chromatids are pulled to opposite ends
• cytokinesis: the division of the cytoplasm into two daughter cells
• telophase: the final stage of mitosis, in which the nucleus divides into two nuclei as the membranes form around each set of chromosomes
• number of cells after division= 2^n (where n is the number of divisions)
• Growth is an increase in size or mass
• Cell differentiation: The process by which a cell changes to become specialised for its job
• Cell elongation: The process of cells becoming longer and wider as they grow.
• Cell differentiation in animals is lost at an early stage
• plants often grow continuously and continue to differntiate to develop new parts
• the rate at which cells divide is controlled by genes
• if a gene is mutated it can cause a mass of abnormal cells causing a tumour
• if the tumour invades and destory surrounding tissue it is called cancer
• stem cell are undifferentiated cells that can divide to produce more cells which then differentiate
• stem cells are found in early human embryos
• adults have stem cells in bone marrow only create certain cells or replace damaged cells in animals
• adult stem cells aren't as versatile as embryonic stem cells
• meristem: a region of plant tissue, found chiefly at the growing tips of roots and shoots, consisting of actively dividing cells forming new tissue
• meristems produce unspecialised cells that can divide and differentiate to make any type of cell for as long as the plant lives
• the unspecialised cells created by meristems go on to form specialised tissues like xylem and phloem
• doctors already use adult stem cells to cure some diseases
• risks with stem cells: tumour development, disease transmission, rejection
• tumour development: stem cells divide very quickly and can form a tumour
• disease transmission: viruses live inside cells and can be passed on if donated
• rejection: the body may recognise the cells as foreign and trigger an immune response
• the nervous system is made up of neurones(nerve cells)
• sensory receptors: group of cells that can detect a stimulus
• when a stimulus is detected by receptors the information is converted to a nervous impulse and transmitted to the CNS
• The CNS coordinates the response and impulses travel through the CNS along relay neurones
• The CNS sends information to an effector along a motor neurone then the effector responds accordingly
• the time it takes to respond to a stimulus is called your reaction time
• all neurones have a cell body with a nucleus
• the cell body has extensions that connect to other neurones called dendrites and dendrons which carry nerve impulses towards the cell body