growth and development

growth and development

1. how organisms develop

   • cell specialisation
     • A zygote is a structure that forms when a sperm fertilises an egg
       • The zygote then divides many times by mitosis to form an embryo
     • Up to the eight-cell stage, all of the cells are identical
       • They are called embryonic stem cells
     • It is possible for embryonic stem cells to develop into any other specialised type of cell that the growing embryo needs
       • for example, nerve cells, blood cells and muscle cells
       • However, once the embryonic stem cells become specialised, they can't change into any other type of cell
     • The specialised cells can form all the different types of tissue that the embryo needs
     • Groups of different types of tissues are arranged together to form organs
     • switching genes on and off
       • Cells become specialised because the genes that are not required are switched off
       • Only the genes needed to make a particular type of cell work are switched on
       • So muscle cells only have the genes needed to make muscle cell proteins switched on
       • All the other genes, such as those needed to make blood cell proteins and nerve cell proteins, are switched off
   • cell specialisation in plants
     • Unspecialised stem cells also exist in plants
       • hey can become specialised into the cells of roots, leaves or flowers
     • cloning plants
       • almost all plants are clones as well as fruit and veg
         • his is because the plants they come from have been grown from cuttings
       • Cuttings develop much bigger root systems if they are dipped in hormone rooting powder or planted in rooting compound containing growth hormone
         • These hormones cause unspecialised stem cells to grow and develop
         • They turn into tissues such as xylem and phloem, and organs such as roots, leaves, stems and flowers, thereby forming a complete new plant
     • meristems
       • Plants cells are different to animal cells in another way
       • Unspecialised stem cells in plants are grouped together in structures called meristems
       • Cells produced by meristems ensure plants continue to grow in height and width throughout their life
       • Plant meristems divide to produce cells that increase the height of the plant, length of the roots and girth of the stem
       • They also produce cells that develop into leaves and flowers
     • phototropism
       • Phototropism is a plant's growth response to light
       • When the stem grows towards the light, the plant can photosynthesise more
       • More food is produced, so the plant can grow faster. This increases the plant's chances of survival
   • auxin
     • Auxins are plant hormones that make some parts of a plant stem grow faster than others
       • he result is that the plant stem bends towards the light
     • Auxin is produced in the tip of growing shoots
     • If the tips are removed, they cannot produce auxin, so phototropism cannot occur
     • If the tips are covered, light cannot break down the auxin, so phototropism cannot then occur either
     • Phototropism increases the plant's chances of survival

2. producing new cells

   • mitosis
     • Mitosis occurs wherever more cells are needed
     • t produces two new cells that are identical to each other, and to the parent cel
     • The cycle starts as the number of organelles - the different parts of the cell - increases
     • Before a cell divides, its chromosomes are copied exactly. The DNA molecule is made of two strands. As each of the two strands separates, new strands are made alongside each of them, thereby making two new copies
   • meiosis
     • It is used to produce male and female gametes
     • The gametes are sperm or eggs, and only contain half as many chromosomes (23)
     • At fertilisation, the nuclei of the sperm and an egg join to form the zygote
     • he zygote contains 23 pairs of chromosomes - 23 single chromosomes from the sperm, and 23 single chromosomes from the egg, thereby creating the correct number of 46 chromosomes for all body cells

3. Controlling growth and development

   • DNA
     • DNA is a very large molecule shaped like a twisted ladder. The shape is a double helix.
     • Long strands of DNA make up chromosomes. These are found in the nucleus of a cell
   • The genetic code
     • DNA is a chemical code, or set of instructions

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