Download B2 Topic 1: The building blocks of cells Light microscope Light

B2 Topic 1: The building blocks of cells
Light microscope
o Light microscopes shine light on the
specimen (i.e the cell to be studied)
o The image is then passed through
lensesis magnified (i.e made bigger)
o the different parts of a cell can be seen
Electron microscope
 See the cell in more detail
Animal cell
Plant cell
Bacterial cell
have two types of DNA:
 Chromosomal DNA – giant loop of DNA
containing most of the genetic material
 Plasmid DNA –carries extra information
have a cell wall:
 different to the cell wall in plants – not
made of cellulose, and it is more flexible
 provides support and shape for bacterium
(some) have flagella on the outside:
 These are long, whip-like structures that
bacteria can use to move themselves along.
Tail like structure.
DNA
Sections of DNA are called genes.
1 gene codes for 1 protein
Structure of DNA

Double helix – two strands coiled
together
 Two strands linked by weak hydrogen
bonds between complementary base
pairs. Base Pairs:
 Adenine
 Thymine
 Guanine
 Cytosine
 A always matches to T
 G always matches to C
 Sugar-phosphate back bone
B2 Topic 1: The building blocks of cells
DNA discovery
o Wilkins and Frankin directing X-ray
beams at DNA and creating images.
o Watson and Crick building 3D models of
DNA using data from other scientists.
o Wilkins gave Watson and Crick one of
Franklin’s detailed images of DNA.
o This led to Watson and Crick building
their correct DNA model
o When Watson and Crick published their
paper Franklin’s role was barely
mentioned
o Eventually all 4 recognised for work.
All but Franklin (she had died) awarded Nobel
prize.
Genetic engineering
Removing gene from one organism and
inserting it into DNA of another.
How it’s done:
1. Required gene located i.e. insulin gene
2. Restriction enzymes cut this gene out
3. Vector found i.e. plasmid DNA of
bacteria
4. Vector cut open using enzymes
5. Required gene inserted and stuck in
place using enzymes
6. The genetically engineered organism
(bacteria) can reproduce and would
produce the protein in question i.e.
insulin
Producing GM insulin
Benefits
Drawbacks
No longer removed
Bacteria produces
from pigs/cattle so
insulin slightly
vegetarians and
different – may not
vegans can use.
suit everyone
Large supply can be
made quickly and
cheaply
Producing beta-carotene rice
o Beta-carotene needed by humans to
make vitamin A
o Gene for Beta-carotene inserted into rice,
rice produces beta-carotene in grains.
Golden rice made.
Benefits
Drawbacks
Humans eating the
GM rice could
rice will make
crossbreed with wild
vitamin A. Lack of
rice plants and
vit.A can cause
contaminate wild
blindness.
DNA
Locations where rice Unsure of effects of
is easily grown (less
eating GM foods –
economically
could be harmful
developed countries) Levels of betahave access to vit.A.
carotene not high
enough to make
difference
GMO can be
expensive
Producing herbicide resistant crops
o Herbicides kill weeds
Benefits
Drawbacks
Farmers can spray
Cross-pollination can
crops once with very take place between
strong herbicide to
GM crop and weed,
kill weeds and their
making the weed
crops won’t die.
resistant too
Reduces amount of
Reduce biodiversity
crop spraying Less shelter for
cheaper
animals as weeds are
dying.
B2 Topic 1: The building blocks of cells
Mitosis and Meiosis
o Processes of cell division.
o Both begin with diploid (two copies of
each chromosome) cells
Divisions
Daughter cell
number
Daughter cells
diploid/
haploid
Genetically
identical to
parent?
Use?
Process
Mitosis
1
2
Meiosis
2
4
Diploid
Haploid (one copy of
each chromosome)
Yes
No
Growth
Repair
Asexual
reproduction
Cloning
1. DNA
replication
2. Copies of
DNA
separate and
cell divides
3. 2 diploid
daughter
cells form
How to clone mammal:
1. A diploid nucleus is removed from a body cell
of the animal that is going to be cloned
2. The diploid nucleus is inserted into an
enucleated egg cell (i.e a cell that has had its
nucleus removed)
3. The egg cell is stimulated, by electric pulse, to
start dividing by mitosis
4. It is then implanted into the uterus (womb)
of a surrogate mother where it will develop
into a new individual
5. Surrogate will give birth to clone of animal
Producing gametes
(Sex cells/ sperm/
egg)
1.
2.
3.
4.
5.
DNA replication
Copies of DNA
separate and cell
divides
2 diploid
daughter cells
form
Cells divide again
4 haploid
daughter cells
form
Fertilisation
2 haploid gametes (sperm and egg) fuse.
Their nuclei fuse
Diploid cell forms
Diploid cell undergoes mitosis to form zygote
Clones
Clones are genetically identical to parent
Example of Asexual reproduction
Benefits
Risks
Keep desirable traits Very few embryos
e.g. bulls whose
produced during
sperm produces high cloning develop
quality calves
successfully
Keep desirable GE
Susceptible to
traits – cows
disease
engineered to
produce human
insulin
STEM CELLS
Stem cells are unspecialised cells (not yet
differentiated). These could be very useful to
treat many medical problems.
Once a cell has specialised it cannot go back to
being a stem cell.
TWO TYPES IN HUMANS:
o Embryonic stem cell – develop into nearly any
type of cell.
o Adult stem cell – already slightly
differentiated and so can only develop into
particular cell types.
Problems associated with extracting embryonic
stem cells:
Use leftover embryos created for couples having
fertility treatment…
o However, extracting the embryonic stem cells
kills the embryo
o some people think that because embryos go
on to develop into people, destroying
embryos is the same as murder
Two ways scientists are trying to solve this issue:
1. Use adult stem cells to make cloned embryos the embryonic stem cells could then be extracted
from the clones without any natural embryos
having to be killed
2. Turn specialised body cells into stem cells by
reprogramming them – if this works, it will help
to completely avoid the ethical problem of using
embryos
B2 Topic 1: The building blocks of cells
Age quicker
Human genome project (HGP)
Finding the base pair sequence of human DNA.
Involved scientists in 18 countries and took 13
years.
Everyone has at least 99.9% DNA in common.
BENEFITS:
o Improved testing for genetic disorders
o Gene therapy – replace faulty genes
o Personalised medicines
o Looking more closely at evolution
DRAWBACKS:
o Life insurance could cost more if you
had genetic disorder
o Stress would increase
Protein manufacture/synthesis
The order of base in DNA decide the order of
amino acids, which make up the protein.
Protein synthesis happens in two stages:
Transcription
Takes place inside the nucleus
1. DNA unzips – weak hydrogen bonds
between strands broken by enzymes
2. One stand of DNA acts as a template
3. mRNA bases which are complementary
to template are linked, forming a mRNA
strand
4. mRNA leaves nucleus
5. DNA zips back up
RNA vs DNA:
o RNA only has one strand
o RNA has a base called uracil (U) instead of
thymine (T)
 in RNA: adenine (A) bases pair with
uracil (U) bases
 in DNA: adenine (A) bases pair with
thymine (T) bases
Translation
Takes place at the ribosome in the cytoplasm
At the ribosome there are tRNA molecules which
have specific amino acids attached.
1. mRNA goes to ribosome
2. ribosome reads mRNA 3 base pairs at a
time (3bp= codon)
3. As the ribosome reads each codon it
matches it to a tRNA with a
complementary anticodon
4. Each tRNA brings a specific amino acid
and releases it
5. The amino acid bonds to a growing chain
forming a poly peptide.
6. tRNA free to pick up next amino acid
7. mRNA moves on to next codon. This
keeps happening until a stop codon is
reached.
Mutations
A mutation is a change in DNA base sequence. It
could be a deletion of a base, swapping a base or
addition of a base in the sequence.
Each type of protein is made by a different amino
acid sequence. If sequence changed this can alter
how the protein works.
Types of mutation:
No effect: Although base sequence alters this
does not change the order of amino acids.
Beneficial: Change in base sequence means a
different amino acid is coded for, the change is
beneficial to the protein formed e.g. resistance.
Harmful: change in base sequence means a
different amino acid is coded for, the change is
harmful to the organism e.g. sickle cell anaemia.
B2 Topic 1: The building blocks of cells
Enzymes
Enzymes are proteins
Enzymes are biological catalysts
Enzymes catalyse the following reactions:
o Digestion
o DNA replication
o Protein synthesis
o + many others
Enzyme action
Enzymes work by binding to molecules called
‘substrates’ – once bound, enzymes catalyse the
change of substrate molecules into product
molecules.
o The enzyme is not changed during the
reaction
o Substrates bind to the enzyme at the
active site. This is where the reaction
will occur and be catalysed.
o Enzymes only work with a specific
substrate, they are highly specific.
Explaining using ‘lock and key’ hypothesis:
o Substrate binds to enzyme at active site
o Active site has specific shape which is
complementary to the substrate shape
o Each enzyme type has a different active
site shape.
o Enzyme active site is the lock
o Substrate is the key.
Factors affecting enzyme action:
Temperature
Most enzymes optimum temperature is 37°C
Below optimum enzymes work slowly as it is too
cold for them to catalyse efficiently.
Above optimum the active site shape begins to
change, this means substrate cannot bind and
enzyme is denatured.
pH
Most enzymes optimum pH is 7.
Stomach enzymes (pepsin) optimum pH is 1.
Above or below optimum the active site shape
begins to change, the substrate cannot bind, this
means the enzyme is denatured.
Concentration
As substrate concentration increases, more
molecules can bind to active site so rate of
reaction increases.
At very high concentrations of substrate, all
active sites are occupied so enzymes cannot work
any quicker, rate of reaction stabilises.
B2 Topic 1: The building blocks of cells