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Revision Notes
Topic 3
Prokaryotic organisms are prokaryotic cells
cells(single-celled like bacteria)- no nucleus
Eukaryotic organisms are eukaryotic cells
cells(complex, including animal and plant cells) – nucleus
-

Both types contain organelles which are parts of cells with a specific function
...

Animal cells- eukaryotic

Diagram

Description
-

-

Function

Large organelle surrounded by a
nuclear envelope (
(double
membrane)
Pores
Chromatin is made from DNA
and proteins
Nucleolus

-

Round organelle surrounded by
a single membrane
No clear internal structure
Acrosome= specialised lysosome

-

Membranes enclosing a fluid
fluidfilled space
Covered with ribosomes

-

Folds and processes proteins made by the
ribosomes

Small organelles that floats free
in the cytoplasm or is attached
to the RER
Made of proteins and RNA
No membrane

-

Site for protein synthesis

Fluid-filled, membrane-bound,
flattened sacs
Vesicles seen at the edge of the
sacs
Formed by fusion of vesicles

-

Modifies proteins
Processes and packages new lipids and proteins
in vesicles for transport
It also makes lysosomes

-

-

-

Controls the cells activities
DNA contains instructions for making proteins
Pores allow substances (e
...
RNA) to move
between the nucleus and cytoplasm
Nucleolus makes ribosomes

Contains digestive enzymes kept separate from
the cytoplasm by the surrounding membrane
Used to digest invading cells
Break down worn out components of the cells

-

-

Synthesises and processes lipids

-

Usually oval shaped
Double membrane
Inner one is folded- cristae
Matrix contains enzymes involved in
respiration

-

Site of aerobic respiration where ATP is
produced
Found in cells that are very active and
require a lot of energy

-

Small, hollow cylinders made of
microtubules (tiny, protein cylinders)
Found in animal cells
Only some plant cells

-

-

Cell surface membrane/
plasma membrane

Similar to RER but no ribosomes

-

Phospholipid bilayer
Partially permeable barrier

-

-

Protein production and transport
1
...


mRNA leaves the nucleus to ribosomes

3
...


Protein moves through the ER, creating a 3D shape

5
...


The vesiclesfuses to the Golgi apparatus

7
...
g
...


A vesicle containing the modified protein is pinched off
the Golgi apparatus

9
...
g
...

-

No nuclei
No membrane-bound cell organelles
Very small

-

DNA lies free in the cytoplasm
Cell wall is always present

-

Separation of chromosomes during
cell division
Formation of spindles
Transport within cell cytoplasm
Control what substances leave and
enter the cell

Generalised
bacteria cell

Plasma membrane
-

Cytoplasm
-

Made of lipids and
proteins
Controls movement
of substances in and
out of the cell

Cell wall
-

Supports the cell
Prevents change of shape
Made of murein which is aglycoprotein
Contains peptidoglycan (polysaccharide and
polypeptide combined)
Pili

No membrane bound organelles
Ribosomes(smaller than
eukaryotic ribosomes 70s vs 80s)

-

Flagellum (plagella)
-

Rotates to make the cell move
Not all prokaryotes have a flagellum
Some havemore than one

Slime capsule
Plasmid

Circular DNA
-

Free floating
No nucleus
Long coiled strand
Not attached to any histone proteins

Short hair like structures
Helps prokaryotes stick to
other cells
Used in the transfer of
genetic material between
cells

-

-

Small loops of DNA
Genes for antibiotic
resistancewhich can
be passed between
prokaryotes
Not always present

Mesosome
-

Inward folds of
the membrane

-

Site of
respiration

-

-

Protect bacteria from
attack by cells of the
immune system
Prevent dehydration
Not always present

Electron microscope images

Magnification, miscroscopes, micrometers
...
During fertilisation, they join to form a zygote
which develops into a new organisms
...

1
...


Sperm reaches the ovum
Chemicals released by the cells zona pellucida trigger the acrosome
reaction once the sperm makes contact

3
...

5
...


Digestive enzymes are released to digest the follicle cells and zona pellucida
The sperm fuses with the ovum membrane which triggers the cortical reaction
No other sperm can enter so it’s fertilised by only one
Sperm nucleus enters the egg cell and they fuse

Cortical reaction:
Lysosomes release cortical
granules which thicken the
thezona pellucida
...

Formation of gametes (meiosis)
Meiosis happens in the reproductive organs starting with somatic cells that have the full number of chromosomes
...


Crossing over

-

Before the first division, homologous pairs of
chromosomes pair up
...
loci) is the position of a gene on a chromosome
...
Only crossing over can split them up
Genes with loci closer together on a chromosome are more closelylinked
because crossing over is less likelyto split them up
...
Longer chromosomes have more genes
...


Females: XX
Males: XY

Males have one X chromosome so they often have only one allele for sex-linked
genes
...


The Y chromosome is
smaller and carries fewer
genes
...


Males are more likely to show recessive phenotypes for genes that are sexlinked

Genetic disorders caused by faulty alleles on sex chromosomes include colour blindnessand haemophilia
...


Cell cycle and mitosis
Mitosis is cell division that produces genetically identical cells
...


Interphase: cell carries out normal function but prepares
to divide
...
Organelles
are replicated so it has spare ones and ATP content is
increased
...


Prophase

-

Chromosomes condense(shorter and fatter)
...

Centrioles move to opposite ends of the cell
Nuclear envelope (membrane) breaks down and chromosomes lie
free in the cytoplasm- marks end of prophase
...


Metaphase

-

Chromosomes line up along the middle of the cell and become
attached to the spindle by the centromere
...


Anaphase

-

Centromeres
chromatids
...


-

When separated chromatids reach the poles, the spindle breaks
down
...


Telophase

-

The chromosomes unravel (long and thin)

divide

which

separates

the

pair

of

sister

Nuclear envelope forms around each group of chromosomes
forming two nuclei

Cytokinesis (cytoplasmic division after mitosis)
-

Protein filaments(actin and myosin) bond to the inside surface of the cell membrane and contracts until cell
divides
Cytoplasm divides to make two daughter cells that are genetically identical to the original cell and each other
Each daughter cell prepares to get ready for the next round of mitosis

Plant cells
Instead of undergoing constriction, plant cells synthesise a new cell plate between the two new cells
...
g
...

Number of cells with visible chromosomes
Total number of cells observed

What is mitosis used for?
Mitosis ensures genetic consistency as the daughter cells are genetically identical to each other and the parent cell
...

They use binary fission where one cell splits into
two
...
g
...

STEM CELLS
Embryonic cells:
At the 8 cell stage the cell are unspecialised and can divide by mitosis to become any type of cell (totipotent)
...
These are pluripotent embryonic stem cells as they can
form most cell types but NOT placenta or umbilical cord
...
Many differentiated cells can
de-differentiate and develop into a new plant Gardeners plant clones from root,
plant
...

Explants (small pieces of plant) are surface
surface-sterilised and placed in a solid agar
medium with nutrients and growth regulators
...

-

Altering the growth medium will encourage differentiation and forms a small group of calls similar to an
embryo
The embryos develop into complete plants that are genetically identical clones
Plant tissue culture allows commercial growers to produce large numbers of identical plants rapidly

Tissue culture can be used in plant research, plant breeding, genetic modification and in conservation of
endangered plants
...

Fertility clinicsget pluripotent stem cells from spare embryos and use in vitro
fertilisation where the ovum is fertilised outside the body
...

Embryos are placed into the woman’s womb and additional embryos are
used for stem cells
...
Stem cells are then cultured for research
...

-

Produces a diploid cell rather like a zygote

The cell divides by mitosis to form a blastocyst and then stem
cells are encouraged to develop into tissues (can lead to
organs)

Problem:
The tissue formed from
the stem cells may be
rejected by the immune
system of the person
receiving the transplant
...

Using tissue-typing
...
Used in degenerative conditions using the
patients own stem cells, for example:
-

Stroke patients have stem cells injected into the brain to replace damaged neurons
Stem cells injected into the joints of patients with arthritis to repair damaged cartilage
Patients with burns will have stem cells from the unburned area cultured and
transplanted to burned area
Blood vesselsor heart muscle

Isolation and
culture of stem
cells is
difficult and
costly
...
g
...
g
...

The epigenome influences which
genes are transcribed in a cell
...

Gene is switched off
...

The attachment of methyl groups
(usually cytosine) prevents
transcription to mRNA by stopping
the RNA polymerase binding
...

In cells of a particular type of tissue, e
...
pancreas, all the genes for the pancreas are active but genes specific to
other tissue, e
...
liver/kidneys, are switched off
...

Switching on an individual gene- LAC OPERON MODEL
Prokaryotes
Scientists studied control of genes in prokaryote E
...

When lactose is not present, a lactose repressor binds to the DNA to prevent transcription of beta-galactosidase
gene
...

When lactose is present, it binds to the repressor to prevent the repressor binding to the DNA and so the enzyme
gene is expressed
...

The operator and genes associated with it are known as operon (lac operon)
...




The structural genes code for
useful proteins such as enzymes



The control elements include a
promoter(a DNA sequence located
before the structural genes that
RNA polymerase binds to) and an
operator (a DNA sequence that
transcription factors bind to)



The regulatory gene codes for an
activator (increases rate of
transcription) or repressor
(decreases the rate of transcription)
Switching genes on/off in eukaryotes
Genes in uncoiled, accessible regions of DNA can be transcribed into mRNA
...
The gene remains switched
off until the enzyme attaches successfully to the promoter region
...

Transcription can be preventedby a protein repressor molecule attaching to the DNA of the promoter region
which blocks attachment site
...

How are cells organised into tissues
Cells have adhesion molecules (recognition proteins) on
their surface membranes which helps them recognise other
same cells so they can group into clusters
...
The
particular recognition proteins synthesised by a cell
determines which cells it can and can’t attach to
...

In an embryo, cells begin to form tissue as the start their specialised functions- the genes coding for the
recognition proteins are switched on
...

Some tissues can interact with the extracellular matrix, a network of molecules secreted by cells
...
g
...


Gene expression and development
The precise sequence of transcription and translation of genes determines the sequences of changes during
development
...

-

signals from inside or outside of the cell result in changes in the epigenome that alters genes
transcribed at specific times and locations
...
g
...

-

Master genes control development of each section
They were discovered by looking at mutations that cause the development of the wrong appendagefor
that segment

The master genes produce mRNA that is translated into signal proteins which switch on the genes needed to
produce the proteins needed forspecialisation of cells in each segment
...

Hermaphrodite flower: has both male and female
parts
Most hermaphrodite flowers contain four sets of floral organs:
1
...

3
...


Sepals
Petals
Male stamen
Female carpel

There are arranged in concentric whorls
...

-

Only A = sepals
Only B = carpels
A + B = petals
B + C = stamen

These genes produce mRNA
that codes for signal proteins
that switch on appropriate
genes which code for proteins,
creating a specialised organ
Genes and the environment
Continuous variation (in phenotype)
-

When individuals within a population vary within a
range with no distinct categories
Affected by both genotype and environment
Controlled by genes at many loci(polygenetic
inheritance)
Example:
1
...

-

Mass
Any mass within a range

3
...

-

Blood type
Group A
Group B
Group AB
Group O
No other option
...


Multifactorial

Two people who inherit the same susceptibility will depend on the
environmental factors to determine whether they get the disease, e
...
diet,
stress and exposure to toxins, which trigger the symptoms
...

e
...
Arctic animals can have dark hair in the summer and white hair in the winter
...


How polygenetic inheritance works
Alleles for eye colour are found at several loci
...
Pigment absorbs light so brown eyes appear dark whereas blue eyes have little pigment and so reflects
light
...
a genotype of bb bbbb would add no pigmentto the
iris which creates a pale blue
...

The greater number of loci, the greater number of
possible shades
...

-

Diet affects height as malnutrition prevents growth
Genotype predicts potential height
Environment determines actual height
Changes in the environment cause changes in gene expression

In eukaryotes, epigenetic control determines whether certain genes
are expressed, altering phenotype
...
g
...


Increased methylation of DNA represses a gene

Egg and sperm are specialised cells
...


Methyl group always attaches at a CpGsite where there is a cytosine and guanine base are next to each other
...

2
...
How condensed the chromatin is
determines the accessibility of the DNA
...

-

Acetylated histones make chromatin less condensed so proteins can
bind to the DNA and it can be transcribed

-

When acetyl groups are removed, the chromatin becomes highly
condensed and genes can’t be transcribed as proteins can’t bind,
repressing the genes
...


When a cell divides the epigenetic changes to gene expression are passed on to the daughter cells
...

Cancer
Cancer occurs when the rate of cell multiplication is faster than the rate of cell
death
...

Cancer is a result of damaged DNA from physical factors, such as UV light and
asbestos which comes from the environment or carcinogens which can be
environmental or made by cell metabolism
...
Proteins are produced that stimulate the next stageand also
proteins that can stop the cell cycle
...



Cancer cells don’t respond to the control mechanisms

Oncogenes:
-

Code for the proteins that stimulate the transition from one stage in the cell cycle to the next
...

This may cause excessive cell division, resulting in a tumour
...

DNA mutations or epigenetic changes means there’s no brake in
the cell cycle
...
For cancer to occur there needs to be damage to
more than one part of the system
...

It stops the cell cycle by inhibiting
enzymes at G1/S transition which
prevents the cell copying its DNA
...
These cells have lost
control of the cell cycle
...
Inheriting the allele doesn’t make
cancer inevitable but increases susceptibility through environmental DNA damage
...


Some cancers are triggered by virus infection
...




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