B5- Growth and development

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Cell specialisation in animals

  • In organisms that are multicellular, cells are specialised to do different jobs.
  • Cells of the same type are grouped into tissues, e.g. muscle cells= muscular tissue.
  • Different tissues are grouped together, and work together, in organs. E.g. the heart has muscular tissue, epithelial (lining and covering) tissue, blood and nervous tissue.
  • Organs work together as body systems, e.g. the circulatory system.
  • Organisms begin life as a zygote- a fertilised egg.

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Cell specialisation

  • The zygote divides by mitosis- into 2,4,8, (exponentially)- to form and embryo.
  • In humans up and including the eight-cell stage, the cells are identical. These cells are embryonic stem cells- they will produce any cell type in the body.
  • After the eight-cell stage, cells become specialised (differentiation), and different tissues form.
  • As adults stem cells remain in certain parts of the body. Adult stem cells can differentiate into a limited number of cell types. e.g. bone marrow cells into different types of blood cell.
  • In specialised cells only the genes are needed to enable the cell to function, as that type of cell is switched on. In embryonic stem cells, any gene can be switched on.

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Cell specialisation in plants

  • Specialised plant cells form tissues such as xylem, which transports water and mineral salts, and phloem, which transports the products of photosynthesis.
  • Tissues are organised into organs e.g. stems, leaves, roots and flowers.
  • Cells in regions called meristems are unspecialised.
  • When meristem cells divide into two, the new cell produced can differentiate into different cell types (the other stays as a meristem cell).
  • In plants the only cells that divide are in meristems.
  • Meristems produce growth in height and width (by division of meristem cells, followed by enlargement of one of the daughter cells).


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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.
  • Plants grown in this way include garden plants and houseplants.
  • Pieces of plants, e.g. plants stems, 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 (using hormone rooting powder).


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Tissue culture

  • Another mehod of cloning.
  • A small piece of tissue, or a few cells are placed on agar jelly containing nutrients and plant hormones. Each will grow into a small plant or plantlet. 
  • 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.


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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.
  • Light is essential for photoynthesis, so by gowing towards a source of light they increase their chance of suvival.
  • The plant hormone 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 from one side:
    • The auxin produced in the tip is distributed towards the shaded area.
    • The auxin produces growth on the shaded side.
    • The shoot grows towards the light.
    • (http://ts1.mm.bing.net/th?id=HN.607991043901885394&w=192&h=144&c=7&rs=1&pid=1.7)
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  • Mitosis is the type of cell division that takes place when an organism grows and cells divide to repair tissues.
  • Mitosis results in the production of two daughter cells that are genetically identical, i.e. have the same number of chromosomes as the parent cell.
  • Before mitosis, the DNA in each chromosome is copied. Each chromosome is now a double chromosome, ith two DNA molecules.
  • During mitosis, each double chromosome separates, so that two nuclei and two cells are produced.
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The cell cycle

  • The events between and leading up to cell division itself, are called the cell cycle.The main processes are:
    • Cell growth: the cell increases in size, the numbers of organelles increase, the DNA in each chromosome is copied.
    • Mitosis: the two daughter cells, each identical to the parent cell and containing an identical set of chromosomes, are produced as strands of each double chromosome seperate and two nuclei are formed.
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  • Meiosis is the type of cell division used to produce gametes (sex cells- eggs and sperm in humans; eggs and pollen grains in flowering plants).
  • In humans, gametes contain half the number of chromosomes (23) as body cells (46 or 23 pairs), i.e. only one chromosome from each pair.
  • At fertilisation, gametes (sperm and eggs) join to form a zygote with 46 chromosomes.
  • It's important that gametes contain 23 chromosomes (1 chromosome from each pair), otherwise the zygote would end up with 92 chromosomes.
  • The zygote contains a set of chromosomes from each parent.
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Chromosomes, genes and DNA

  • Chromosomes:
    • are thread-like structures found in the nucleus.
    • are made from a DNA molecule.
    • Can be grouped into pairs (jumans have 23)
  • The DNA molecule is a double helix.
  • The DNA molecule is 2 strands facing each other.
  • The 2 strands of DNA are made up of units linked by chemicals called bases.
  • There are four bases- A, T, C and G, wich allways pair up A with T and C with G.
  • The order of bases in a gene makes up the genetic code. This is the code that gives instructions for the assembly of a protein (the amino acids that are in the protein and the orde that they are arranged.


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Protein synthesis

  • In plant and animal cells, the genetic code that carries instructions for protein synthesis ison the DNA in the nucleus.
  • Protein synthesis occurs in the cytoplasm.
  • Genes do not move out of the nucleus, so in order to carry the genetic code to the cytoplasm, messenger RNA (mRNA) is produced in the nucleus, using the DNA as the template (during which time the 2 strands of DNA seperate).
  • mRNA carries the instructions for the assembly of proteins (protein synthesis) into the cytoplasm.
  • Proteins are assembled on organelles in the cytoplasm called ribosomes. Proteins are assembled from amino acids according to the instructions provided by a gene.
  • The number and sequence of amino acids determines the type of protein and its properties.
  • The sequence of amino acids in the protein is determined by the 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 RNAs (tRNA) ferry amino acids to the ribosome, where they are bonded together.
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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 (embryonic stem cells) are identical.
  • The cells produced by the division of embryonic stem cells undergo differentiation to produce specialised cells.
  • Specialised cells begin to make specific proteins. They usually change shape or structure, e.g. muscles cells produce the proteins that enable them to contract.
  • In embryonic stem cells, any gene can be switched on, so they can produce any type of cell.
  • Embryonic stem cells (and adult stem cells) therefore have the potential to replace cells neede to replace damaged tissues.
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Stem cell research and therapy

  • Stem cells are used to produce new cells to replace damaged or diseased cells.
  • Adult stem cells are found at various locations in the body, e.g. in the bone marrow.
  • These cells can be used to produce a limited number of cell types, e.g. bone marow cells will differentiate to produce types of blood cells.
  • Using embryonic cells raises ethical issues, because in removing cells, the embyo is destroyed.
  • According to some, embryos have a right to life from when they are conceived.
  • Embryonic stem cells are usually removed from surplus embryos from in vitro fertilisation (IVF).
  • The creation of embryos produced with the intention of destroying them would be even more controversial.
  • Work with stem cells is therefore the subject to government regulation.


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Therapeutic cloning

Overcomes some ethical issues of using embryonic stem cells, it involves:

  • Replacing the nucleus of an egg by the nucleus of a body cell.
  • Stimulating the egg to divide to produce an 'embryo'.

The technique does not require fertilisatiom, and the cells will be genetically identical to the patients (so will not be rejected by the immune system).

The 'embryo' produced is still destroyed after stem cells are extracted.

Using chemical treatment, scientists have managed to transform mammalian body cells into stem cells.

Using this technique, inactive genes in the nuclei of body cells have been reactivated. The hope is that the transformed cells will be able to form cells of all cell types.

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