B5 - Revision Cards in GCSE Science

How Organisms Develop

Cells are grouped into tissues e.g. muscle cells ---->Muscular tissue
Different tissues are grouped together and work together in organs
Organsims begin life as a zygote (fertilised egg) whcih divides by mitosis to form an embryo
In humans, cells are identical upto the eight cell stage. These are embryonic stem cells which produce any cell type in the body
After eight cell stage, cells become specialised (differentiation), and different tissues form.
In specialised cells, only the genes are needed to enable the cell to function, as that type of cells is switched on.
Specialised plant cells form tissues such as xylem (transports water) and phloem (transports glucose)
Meristems are unspecialised
When meristem cells divide into two, new cell can differentiate into different cell types
In plants, the only cells that divide are in meristems

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New plants can be grown by placing the cut end of a shoot in water or soil
Pieces of plants that have meristems are used to produce clones are called cuttings:
They can be used to produce new plants wiht the same desirable features as the parent
Or produce clones that are genetically identical to the parent plant.
Root growth in cuttings is promoted by plant hormones
Tissue Culture - small piece of tissue placed on agar jelly containing nutrients and plant hormones. Each grows into a small plant
Plant hormones called auxins are included in agar for tissue culture and hormone rooting powder
Auxins increase cell division and enlargement, promoting growth of the plant tissue
Plants grow towards light (positive phototropism), roots grown away from light (negative phototropism) - Increases chance of survival
Auxins produced at the tip and diffuses down and produces growth below the tip
When a shoot tip is exposed to light, auxins distributed on shaded side
Auxin causes the cells to elongate faster on shaded side so shoots grow towards light

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Type of cell division that takes place when an organism grows and cells divide to repair tissues
Mitosis results in production of two daughter cells that are genetically identical
Before mitosis, the DNA in each chromosome is copied.
Each double chromosome separates so that 2 nuclei and two cells are produced.
Cell cycle consists of:
Cell growth: cell increases in size; number of organells increase; DNA in each chromosome is copied
Mitosis: two daughter cells, each identical to parent cell and containing identical set of chromosomes, are produced as the strands of each double chromosome separate and two nuclei are formed.
1) Each chromosome makes an identical copy of itself
2) Chromosomes line up at the centre of the cell and cell fibres pull them apart
3) Membranes form around each of the sets of the chromosomes, becoming nuclei of two new cells
4) Cytoplasm divides to form two new cells

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Used to produce gametes (sex cells-egg and sperm in animals; eggs and pollen grains in flowering plants)
Gametes contain half the number of chromosoms (23) as body cells (46)
At fertillisation, gametes join to form a zygote with 46 chromosome
Zygote contains a set of chromoses from each parent
1) Before cell divides, it duplicates DNA
2) In first division, chromosome pairs line up in centre of cell
3) Pairs are then pulled apart so each new cell has one copy of each chromosome
4) In second division, chromosomes line up again in centre of cell. Arms of the chromosomes are pulled apart.
5) Produces four gametes with half the number of chromosomes
Mitosis produces two daughter cells with an identical number of chromosomes as the parent cell
Meiosis producues four daughter cells with half the number of chromosomes

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DNA molecule is a double helix which are made of bases - A,T,C and G
Order of bases in a gene makes up genetic code which gives instructions for the assembly of a protein (amino acids and the order in which they're orranged)
Each set of three bases (triplet) codes for one amino acid
Protein Synthesis occurs in the cytoplasm
Genes do not leave the nucleus so mRNA is produced in the nuclues, using DNA as a template
1) The two DNA strands unzip. mRNA is made using one strand of DNA as a template
2) mRNA moves out of the nucleus and joins with a ribosome in the cytoplasm
3)Ribosome sticks amino acids together in a chain to make a protein, following the order of bases in the mRNA.
Number and sequence of amino acids determines the type of protein
Sequence of amino acids is determined by the genetic code
Bases work in threes (triplets) to code for an amino acid

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A cell only produces the proteins it needs to carry out its function
The genes to make these proteins are switched on whilst others are switched off
Up to the 8 cell stage, the embryonic stem cells are identical
Specialised cells begin to make specific proteins
In embryonic stem cells, any gene can be switched on so they can produce any cell type.
Embryonic stem cells have the potentical to replace cells need to replace damaged tissues
Adult stem cells are found in the bone marrow and can be used to produce a limited number of cell types
Embryonic stem cells raises ethical issues:
Embryo is destroyed
Embryos have a right to life from when they're conceived
Creation of embryos with intent to destroy would be more controversial
Therapeutic cloning overcomes some ethical issues:
Involves replacing the nucleus of an egg by the nucleus of a body cell
Involves stimulating the egg cell to divide to produce an embryo

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