Zusammenfassung der Ressource
B5
- Cell Organisation in animals
- Cells of the same type are organised into tissues
- Different tissues work together and are grouped together to form organs
- Organs work together as body systems
- Organisms begin life as a zygote - a fertilised egg
- The zygote divides by mitosis to form an embryo
- In humans up to and including the 8 cell stage all the cells are identical. These cells are embryonic stem cells
- After the 8 cell stage the cells become specialised and different tissues form
- In adults stem cells remain in certain parts of the body. They can only differentiate into certain cell types
- In specialised cells, only the genes are needed to enable that cell to function, as that type of cell is switched on
- Cell specialisation in Plants
- Specialised plant cells from tissues such as the xylem, which transports water and mineral salts, and phloem, which transports the products of photosynthesis
- Tissues are organised into organs eg stems flowers etc...
- Cells in regions called meristems are unspecialised
- When meristem cells divide into 2 the new cell produced can differentiate into different cell types (the other stays a meristem cell)
- Meristems produce growth and height and width
- Plant Clones
- New plants can be grown by placing the cut end of a shoot in water or soil
- Roots grow at the base of the stem, while the shoot continues to grow
- Pieces of plants that have meristems and are used to produce clones are called cuttings
- Cuttings
- Can be used to produce new plants with the same desirable features as the parent
- Produce clones that are genetically identical to the parent plant
- Root growth in cuttings is promoted by plant hormones
- Another method of cloning is called tissue culture - a small piece of tissue or a few cells are placed on agar jelly containing nutrients and plant hormones, this makes it grow into a small plant
- Plant hormones called auxins are included in the agar for tissue culture and in hormone rooting powder
- Auxins increase cell division and cell enlargement, promoting growth of the plant tissue
- Plant growth and development
- Plant growth and development is affected by the environment
- Plants response to the direction of light is called phototropism
- Plants grow towards the light so they are positively phototropic
- By growing towards light plants increase their chances of survival
- The plant hormone called auxin is produced in the growing tip of plant shoots. It moves down the shoot and produces growth below the tip
- If a plant is illuminated by one side
- The auxin produced in the tip is distributed towards the shaded side
- The auxin produces growth on the shaded side
- The shoot grows towards the lights
- Mitosis
- Is the type of cell division that makes something grow
- Mitosis results in 2 daughter cells coming from the original cell which are genetically identical
- Process
- 1. Before mitosis the DNA in each chromosome is copied, each chromosome doubles itself
- 2. The chromosomes line up in the middle of the cell
- 3. One chromosome of each pair go into each daughter cell
- BEFORE mitosis the cell increases in size, the number of organelles increase
- Meiosis
- Is the cell division used to produce gametes (sex cells) sperm and egg cells
- In humans, gametes only contain 23 chromosomes not 46
- Meiosis produces 4 daughter cells
- Process
- 1. The chromosomes are duplicated and they pair up (like in mitosis)
- 2. The cell divides into 2
- 3. Those cells again divide into 2 to produce 4 daughter cells
- Chromosomes, genes and DNA
- Chromosomes
- Are thread-like structures found in the nucleus
- Are made from a DNA molecule
- Can be grouped into pairs (pairs of 23 in humans)
- DNA
- A DNA molecule is a double helix
- The DNA molecule is 2 strands facing eachother
- The strands of DNA are made up of units linked by chemicals called bases
- There are 4 bases A, T, G, C
- T links with A - AT TA
- G links with C - GC CG
- The order of the bases in a gene makes up the genetic code. Which gives instructions for the creation of proteins
- Protein Synthesis
- It occurs in the cytoplasm
- Messenger RNA (mRNA) is produced in the nucleus, using DNA as the template
- mRNA carries the instructions for the assembly of proteins into the cytoplasm
- Proteins are assembled on organelles in the cytoplasm called ribosomes
- The number and the sequence of amino acids determines the type of protein and its properties
- The sequence of amino acids in the protein is determined by its genetic code
- The bases work in threes (base triplets) to code for an amino acid
- mRNA is a copy of the base sequence of the DNA that makes up a gene
- The mRNA leaves the nucleus and attaches to a ribosome
- Transfer RNA (tRNA) ferry amino acids to the ribosome where they are bonded together to form proteins
- Switching genes on and off
- The cell only produces the proteins it needs to carry out its function
- The genes to make these proteins are switched on; the others are switched off
- Up to the 8 cell stage of the embryo the cells are the same
- The cells produced in the division of embryonic stem cells undergo differentiation to produce specialised cells
- Specialised cells begin to make specific proteins. They usually change shape and structure. eg muscle cells must produce the proteins needed to contract
- In embryonic stem cells any gene can be switched on so it can produce any type of cell
- Embryonic stem cells (and adult stem cells) therefore have the potential to replace cells needed to replace damaged tissues
- Stem cell research and therapy
- Stem cells are used to produce new cells to replace damaged or diseased cells
- Using embryonic stem cells raises ethical issues because in removing cells the embryo is destroyed
- According to some, embryos have a right to life from when they're conceived
- Embryonic stem cells are normally removed in - in vitro fertilisation
- The creation of embryos produced with the intention of destroying them would be even more controversial
- Therapeutic cloning
- It overcomes some ethical issues of using embryonic stem cells
- It involves replacing the nucleus of an egg by the nucleus of a body cell
- And stimulating the egg cell to divide to produce an embryo
- This technique doesnt require fertilisation and the cells will be genetically identical to the patient so it won't be rejected by the immune system. But the embryo is still destroyed
- Using chemical treatment, scientists have managed to transform mammal body cells to stem cell