Biology Unit 2 Revision

Topic 3 Voice of the Genome

Prokaryotes: The cell in which the genetic material is not enclosed by a nuclear envelope.
They also contain no membrane bound organelles. Most prokaryotes possess a cell wall. An
example is bacteria which have a rigid protective cell wall made on peptidoglycan. Next
there is a cell membrane that holds the cells content. Bacteria have a circular piece of DNA
in a region of cytoplasm called a nucleoid and other smaller rings of DNA known as plasmids.
Bacteria feed by extracellular digestion. Protein synthesis occurs on ribosomes and cell
respiration occurs on mesosomes (inner extensions of the cell surface membrane). Some
bacteria also have flagella.

Eukaryotes: Contains organelles that are membrane bound that separate the contents of
the organelles from the cytoplasm of the cell. They are generally larger than prokaryotes and
can carry out chemical reactions very efficiently. Animals, plants, fungi and protoctists are
made up of eukaryotic cells.

Nucleus: Contains the cells DNA. Surrounded by a nuclear envelope (a double membrane
structured by a phospholipid by layer with a gap called perinuclear space). On the surface of
the nuclear envelope there are nuclear pores in which diffusion occurs. The nucleus contains
chromatin (DNA & protein) and a nucleolus (site of ribosome assembly).
Ribosomes: Small dense organelles usually round on the surface of RER but can also be
found independently. They consist of 65% rRNA and 35% protein. They are the site of
translation in protein synthesis.
Endoplasmic Reticulum: Connected to the nucleus. ER has a membrane that is continuous
with the outer membrane of the nuclear envelope. ER often has attachment sites of
ribosomes creating RER (rough endoplasmic reticulum). RER is folded over itself several
times forming flattened stacks called cisternae. Its function is to keep together and transport
the proteins made on the ribosomes. Newly made proteins are threaded through pores in
the membrane and accumulate in the RER lumen where they are free to fold into the 3-d
shape. RER is also a storage unit for enzymes and other proteins. Small vesicles contain
newly synthesised proteins bud off the end of ER and either fuse with the Golgi complex or
pass directly on to the cell surface membrane. SER are not continuous with the nuclear
envelope and is the site of steroid production. It also contains enzymes that detoxify or
make harmless a wide variety of organic molecules and it acts as a storage site for calcium in
skeletal muscle cells.
Mitochondria: Large, individual organelles, usually spherical or elongated that occur in large
numbers in most cells. The function is to create ATP via aerobic respiration. Mitochondria
have a double membrane. The outer membrane is smooth whereas the inner membrane is
folded. This gives a large internal surface area on which complex reactions of AR take place.
The inner folds are called cristae and also contain a matrix fluid.
Centrioles: Short bundles of micro-filaments, at right angles to each other, which are found
in the clear area of cytoplasm. Centrioles are the centre of cytoskeleton formation. For this
reason they are known as microtubule organising centres.
Lysosomes: Small vesicles that contain a mixture of powerful lytic enzymes. They are needed
to destroy old or surplus organelles, digest material taken into the cell e.g. bacteria that
have just been ingested by white blood cells are destroyed by lysosomes (Phagocytosis).
They also destroy whole tissues or cells as part of the cell cycle.
Golgi body: tightly packed group of flattened cavities or vesicles. The whole organelle is a
shifting, flexible structure; vesicles are constantly being added and lost. Vesicles or proteins
from RER join the Golgi body and are then altered and then bud off to undergo exocytosis.
Mitosis (PMAT): Prophase: The DNA has already replicated during Interphase and the
chromosomes now condense, each chromosome is a double structure made from two
genetically identical pairs.
Metaphase: The nuclear membrane has gone and the chromosomes arrange themselves on
the equator of the spindle.
Anaphase: The chromatids are pulled apart by spindle fibres and move to opposite poles.
Telophase: Cytokinesis is beginning. The chromosomes become no longer individually
visible.
Meiosis Prophase 1: Early P1 starts when the chromosomes condense and the nucleolus
disappears. Mid P1, the homologous chromosomes, one from each parent pairs up. Each
pair forms a bivalent. In Late P1, recombination (cross over) takes place. One or both of the
chromatids of the two homologous chromosomes breaks off at certain points and fuses with
the chromatid of the other chromosome in the bivalent forming joints called chiasmata.
Prophase 1 ends with
the chromatids of each
bivalent entwined and
joined by chiasmata.
Metaphase 1: Nuclear
membrane disappears
and spindle is fully
developed. The
bivalents move to the
equator of the spindle
in the same way as
mitosis.
Anaphase 1: The
chromatid pairs of a
bivalent are pulled
apart by spindle fibres. At the end of A1, the chromatid pair from one of the original
homologous chromosomes is positioned at one pole of the cell and the chromatid pair from
the other homologous chromosome is at the other end of the pole.
Telophase 1: Spindles disappear and the nuclear envelope re-forms around the two sets of
chromosomes. At the same time cytokinesis separates the cytoplasm, forming two daughter
cells preparing for the second meiotic division.
Prophase 2: For each chromosome, the chromatid pair attaches itself to the new spindle,
which forms at right angles to the first.
Metaphase 2: Each chromatid pair lines up on the equator of the spindle.
Anaphase 2: The chromatids are pulled apart and move to opposite poles.
Telophase 2: The spindle disappears, the nuclear membrane re-forms, chromosomes expand
and cytokinesis forms 2 separate cells forming 4 haploid cells known as a tetrad.
Genetic variation is achieved by: The homologous chromosome pairs originate in different
organisms, one maternal and one paternal and hence are genetically different. Blocks of
genes are swapped between the chromatids of homologous chromosomes as the chiasmata
form during prophase 1. Each daughter cell can receive a copy of either chromosome from a
pair, and each copy may have undergone cross-over and have different genes from the other
three. This is called independent assortment.
Sperm cell: Have a head composed of an acrosome (special type of lysosome) and a nucleus.
The acrosome contains digestive enzymes that digest through the zona pellucida in the egg
cell. The sperm has a middle piece that holds large amounts of mitochondria to provide
energy for the sperm to move. The rest of the cell is a flagellum made of microtubules.
Egg cell:








The sperm travel through the cervix and the uterus and into one of the oviducts. When the
sperm are in the oviduct, fertilisation may occur. The sperm swim towards the ovum in the oviduct.
Once the sperm cell comes into contact with the granulosa cells of the egg cell, an acrosome
reaction occurs. The enzymes digest the zona pellucida so that the sperm can move towards the
plasma membrane of the egg cell. The sperm head fuses with the cell membrane of the cell. This
triggers a cortical reaction. The egg cell releases the contents of the vesicles called cortical granules
in the space between the cell membrane and zona pellucida. This makes the zona pellucida thicken
and hence making it impermeable to other sperm. This ensures the fertilisation of only one of each
gamete. Only the sperm nucleus enters the egg cell (the tail is discarded). The 2 nuclei fuse. This
creates a zygote.
Plant fertilisation: A pollen grain land on the stigma of a flower
where the pollen grain absorbs water and splits open. A pollen tube
grows out of the grain down into the style. 3 nuclei are found the
pollen tube (one tube nucleus and 2 male gamete nuclei behind it).
The tube nucleus produces enzymes that digest surrounding cells
and hence making a way through for the pollen tube. When the
tube reaches the ovary, it grows through the micropyle (a tiny hole
in the ovule wall) and into the embryo sac within the ovule. In the
embryo sac, the tube nucleus disintegrates and the tip of the pollen
tube bursts, releasing the 2 male nuclei. One male nuclei fuses with
the egg nucleus to make a zygote and then divides by mitosis to
become the embryo of the seed. The 2
nd
male nucleus fuses with 2
other nuclei called polar nuclei at the centre of the embryo sac. This
produces a cell with a large nucleus which divides to become a food
store (endosperm) for the seed. This process is called a double
fertilisation.
Stem cell: An undifferentiated cell whose daughter may differentiate into many other cell
types. Somatic stem cells exist in small numbers in grown adults. They are hard to extract
and differentiate into a very limited number of cells hence being multipotent (less variation
than pluripotent). They can be found in bone marrow and are hard to culture outside of the
body.
Stem cell cloning: adult stem cell is removed from the body and the nucleus is placed inside
an empty ovum creating a new pre-embryo cell containing patient DNA. A mild electric
shock is administered to stimulate mitosis and create a collection of embryonic stem cells
with the patient DNA. Stem cells are then cultured into desired organ etc. The advantages
are: reduced need for donors, reduced rejection, and cure diseases. Disadvantages are: we
currently are not sure how to properly perform the procedure, concerns that the treatment
causes late onset cancer. Immunosuppressant.
Totipotent cell: Totipotent cells have the ability to keep differentiating into an entire
organism. For a cell to be totipotent it must have the capacity, present in the cells of an
embryo, to develop into any type of cell with none of its genes switched of permanently.
Plant cells are totipotent during their entire lifetime. Embryonic stem cells are usually
totipotent until the blastocyst state, when the embryo would implant in its mothers uterus,
where the inner cells become pluripotent.
Pluripotent cell: Refers to a stem cell that has the potential to differentiate into any of the
three germ layers: endoderm (interior stomach lining, gastrointestinal tract, the lungs),
mesoderm (muscle, bone, blood, urogenital), or ectoderm (epidermal tissues and nervous
system. Pluripotent stem cells can give rise to any foetal or adult cell type. However, alone
they cannot develop into a foetal or adult animal as they lack the potential to give rise to
extra embryonic tissue, such as the placenta. These can usually be found in adult stem cells,
which are located in the bone marrow. The blood that drains from the placenta and
umbilical cord after birth is a rich source of pluripotent stem cells. If this blood is frozen and
stored it can later be used for the treatment of the child - or their family - should they need
them for any specific reason.
However there are several problems, firstly it would take a lot of storage space to do this for
everyone which means huge financial funding. Secondly there is evidence that suggests that
the precursor cells of conditions such as leukaemia are already present in the blood at birth,
therefore because the benefits are as yet largely unproven, the only way parents can do this
in the UK is to pay around 500 pounds to do it privately.
Parkinson's Disease - A brain disorder that stops nerve cells from working as they don't
produce dopamine neurones, scientists hope that stem cell treatment will allow them to
replace the lost brain cells and restore dopamine production, letting people return to a
normal life.
Diabetes - Type 1 diabetes usually develops when people are young; the insulin secreting
cells in the pancreas are destroyed or stop making insulin, so the blood glucose
concentration is uncontrolled. Although insulin injections work well enough, people affected
by diabetes have to monitor their food intake. Stem cell therapy could give them working
pancreas cells again, restoring insulin production and blood glucose control.
Cellular differentiation: the process by which a less specialized cell becomes a more
specialized cell type. Differentiation occurs numerous times during the development of a
multicellular organism as the organism changes from a simple zygote to a complex system of
tissues and cell types. Differentiation is a common process in adults as well; adult stem cells
divide and create fully-differentiated daughter cells during tissue repair and during normal
cell replacement. Differentiation dramatically changes a cell's size, shape, membrane
potential, metabolic activity, and responsiveness to signals. These changes are largely due to
highly-controlled modifications in gene expression, as selected mRNA produces certain
proteins, which changes the cells complexion - therefore differentiating it. With a few
exceptions, cellular differentiation almost never involves a change in the DNA sequence
itself. Thus, different cells can have very different physical characteristics despite having the
same genome.
A cell that is able to differentiate into all cell types of the adult organism is known as
pluripotent. Such cells are called embryonic stem cells in animals and meristematic cells in
higher plants. A cell that is able to differentiate into all cell types, including the placental
tissue, is known as totipotent. In mammals, only the zygote and subsequent blastomeres are
totipotent, while in plants many differentiated cells can become totipotent with simple
laboratory techniques. In cytopathology, the level of cellular differentiation is used as a
measure of cancer progression.
Genetic Terms: Allele- this is a different version of a gene. Most plants, all humans and
animals have two copies of each gene, one from each parent - this is called diploid. The two
copies can be the same or different. When they are the same the organism is called a
homozygote and when they are different it is called a heterozygote. Different alleles have
different base sequences, which code for different versions of the same characteristics.
Phenotype- the characteristics the alleles produce.
Genotype- the alleles a person has.
Variation in phenotype- The variation in phenotype is largely influenced by the genotypes. A
variation in genotype results in a variation in phenotype. There are some characteristics that
are only controlled by one gene. These are called monogenic characteristics. However, most
characteristics are controlled by a number of genes; they are stated to be polygenic.
The Environment- There are some characteristics that are only influenced by genotype, e.g.
blood group; however the majority of characteristics are influence by both the genotype and
the environment.

1. Height- this is polygenic and affected by environmental factors. It is mostly determined by
a persons genotype, for example tall parents usually have tall children. However,
malnourishment can cause children to not reach their maximum height because protein is
required for growth.
2. Monoamine Oxidase A(MAOA)-this is an enzyme used to hydrolyse monoamines in
humans. The production of this enzyme is caused by a single gene, but environmental
factors such as smoking tobacco can reduce the amount produced, causing low levels of
MAOA. Low levels of MAOA are linked with mental health problems.
3. Cancer- this is when cells dont stop dividing leading to tumours (lumps of cells) forming.
The risk of developing cancer is affected by both genes and environmental factors such as
diet.
4. Animal hair colour- this is polygenic. However, the environment affects the hair colour for
some animals. For example, some arctic animals have adapted to have dark hair in summer
and white hair in winter. This is triggered by environmental factors such as temperature
change; however, this is not possible if the animal does not have the genes for it.