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B5
How do organisms develop?
• How are multicellular organisms different to
single celled organisms?
• Cells in multicellular organisms are specialised
to do particular jobs
• Groups of specialised cells are called....
• Tissues
• Groups of tissues form.......
• Organs
How do organisms develop?
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What is a fertilised egg cell called?
Zygote
By what process does the zygote divide?
Mitosis
What does the dividing zygote form?
Embryo
What are the cells called at the embryonic stage?
Embryonic stem cells
What is unique about these cells?
Identical and unspecialised, can develop into any type of cell required by
the organism
• What happens after the 8 cell stage?
• Most of the embryo cells become specialised and form different types of
what?
• Tissue
How do organisms develop?
• What happens to some of the adult cells as
the foetus develops?
• Remain unspecialised (adult stem cells) and
can become specialised at a later stage
• What is a limitation with adult stem cells?
• Can only develop into a limited number of
cell types
How do organisms develop? - Plants
• What are the regions in a plant called where cells
are mitotically active?
• Meristems
• What does “mitotically active” mean?
• Cells can divide by mitosis
• What are meristem cells?
• Unspecialised plant stem cells
• What do meristem cells and embryonic stem cells
have in common?
• Both can specialise into ANY cell type
How do organisms develop? - Plants
• What type of tissue can meristem cells turn into?
• Any – xylem, phloem, flowers, stem, leaves,
roots
• Why does the presence of meristem cells make
cloning plants easy?
• Plant tips contain meristem cells and so you can
cut a tip and re-grow the cutting as it will
contain meristem cells that can produce any
tissue type
How do organisms develop? - Plants
• What do we call the cuttings from the parent
plant?
• Clones
• Why is the process of taking cuttings an
advantage for farmers?
• Grow plants quickly with DESIRABLE
FEATURES
How do organisms develop? - Plants
• What hormone can promote the growth of roots on a
cutting?
• Auxin
• Explain PHOTOTROPISM
• Growth towards light
• When directional light falls on a plant stem
• Meristem cells in the tip will allow growth
• Auxins in the tip will diffuse to the darker side of the stem
• Diffuse along the darker side
• Auxin promote rapid cell growth along the darker side
• Stem grows quicker along the darker side
• Stem grows/bends towards the light
How do organisms develop? - Plants
• Why is phototropism so important to a plant?
• Ensures a plant’s survival as it maximises the
amount of sunlight the plant absorbs for
photosynthesis – increases growth
• Auxin diffuses away from the light
How does an organism produce new
cells?
• Mitosis:
• What type of cell is produced from mitosis?
• Cells that are genetically identical to the parent
cell
• What two things occur during cell growth?
• Number of organelles (structures in cells e.g.
Mitochondria) increase
• Chromosomes are copied – two strands of DNA
separate and new strands form alongside them
How does an organism produce new
cells?
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What two things occur during mitosis?
Copies of the chromosomes separate
Nucleus divides
What type of cell is produced from meiosis?
Cells that are gametes – have half the chromosomes
of the parent cell and so are not genetically identical
• Explain how a zygote obtains a full set of chromosomes
• The zygote has a full set of chromosomes as the
zygote has chromosomes from each parent (from
each gamete)
How does an organism produce new
cells?
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Why does an organism produce new cells?
Growth and repair
Meiosis is a type of cell division that takes place in humans.
Use ideas about chromosomes to describe how cells made
by meiosis are different from cells made by mitosis, and
why this is important
• Cells made by meiosis contain half the number of
chromosomes of the parent cell but cells made by mitosis
have the same number of chromosomes.
• This is important because meiosis produces gametes,
which fuse with other gametes during fertilisation to
make a cell/zygote with the correct number of
chromosomes
How do genes control growth and
development within the cell?
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What structure does the DNA molecule have?
Double helix
What are the four DNA bases and how do they pair up?
A with T
G with C
Why is the order of the DNA bases so important?
Genetic code for the production of a protein (contains
instructions for producing proteins)
• Where is the genetic code in a cell?
• Nucleus
How do genes control growth and
development within the cell?
• What is a gene?
• Section of DNA that contains the instructions to produce a
particular protein in a cell
• What is an amino acid?
• Chemical unit that makes up a protein
• How many different amino acids are there?
• 20
• How are there so many different proteins produced in a
cell?
• The specific order of the amino acids produces different
proteins
• Where are proteins made in the cell?
• Cytoplasm
How do genes control growth and
development within the cell?
• Explain how the order of bases in a gene
produces a specific sequence of amino acids to
make a particular protein
• Every three bases is called a triplet
• Each triplet codes for a specific amino acid
• So the order of the bases decides which amino
acid is produced
• The order of the triplets decides the order of the
amino acids
• Called triplet code
How do genes control growth and
development within the cell?
• How does the triplet code in the nucleus produce a protein
in the cytoplasm?
• Two DNA strands unzip
• A copy of the gene is made as a molecule of mRNA
• Molecule of messenger RNA (mRNA) is made using one
strand of the DNA as a template
• mRNA molecule is smaller than the DNA molecule so can
move out of the nucleus through the nuclear membrane
• Joins onto a ribosome in the cytoplasm
• The ribosome joins amino acids together, in the order
shown on the mRNA molecule
• This amino acid chain forms a protein
How do genes control growth and
development within the cell?
• Why aren’t the same proteins produced in ALL
cells?
• Genes are switched off in certain cells. The cells
produces only the specific proteins they need
• Only the genes that are needed for the cell to
function are switched on
• What is unique about embryonic stem cells?
• Any gene can be switched on during
development to produce any type of specialised
cell
How do genes control growth and
development within the cell?
• How can these stem cells be useful in
medicine?
• Can be manipulated to produce cells to
replace damaged tissue
• People with blood diseases e.g. Leukaemia
can be given bone marrow transplants
• Bone marrow contains adult stem cells that
can produce new blood cells to replace
cancerous ones
How do genes control growth and
development within the cell?
• How can cloning be used to make stem cells?
• Take an egg cell and remove the nucleus
• Take a body cell from the adult patient and remove the
nucleus
• Place the nucleus from the body cell into the empty egg
cell
• Under the right conditions, inactive genes can be switched
on in the nucleus to form an embryo
• Stem cells from the embryo can then be used to form new
tissue
• Can switch on inactive genes in the stem cells to form
different tissue types