3.2.2 All cells arise from other cells

Eukaryotic organisms
In multicellular organisms not all cells retain the ability to divide. Eukaryotic cells that to retain the ability to divide show a cell cycle. Cell division can occur by mitosis or meiosis. Mitosis produces two daughter cells that have the same number of chromosomes as the parent. We will cover mitosis in this section. Meiosis produces four daughter cells each with half the number of chromosomes of the parent cell. Meiosis is covered in section 3.4.3.

Mitosis can be split into 5 stages:

Interphase
Mitosis is proceeded by a period during which the cell is not dividing (this is interphase). Here, DNA replication occurs. The two copies of DNA remain joined at the centromere.
Prophase

• Chromosomes first become visible by shortening and condensing
• (in animals) centrioles move to opposite ends/the poles of the cell
• spindle fibres develop from each centriole. These span the cell from pole to pole and are collectively called the spindle apparatus.
• the nucleolus disappears and the nuclear envelope breaks down leaving the chromosomes free in the cytoplasm.
• the chromosomes are drawn towards the equator by spindle fibres attached to the equator

Metaphase

• the chromosomes can now be seen to be made up of two chromatids (each an identical copy of the DNA from the parent cell unless mutation occurs)
• chromosomes arrange themselves along the equator of the cell

Anaphase

• centromeres divide in two and spindle fibres pull the individual chromatids apart
• chromatids more to their respective poles and are now referred to as chromosomes (energy provided by mitochondria situated around the centrioles)

Telophase (+ cytokinesis)

• chromosomes reach their respective poles and become longer and thinner and disappear altogether leaving widely spread chromatin
• spindle fibres disintegrate

NOTE: Mitosis is a controlled process. Uncontrolled cell division can lead to the formation of tumours and of cancers. Many cancer treatments are directed at controlling the rate of cell division.

We need to be able to calculate mitotic index:

mitotic index = number of cells in mitosis x 100 / total number of cells

Prokaryotic cells
These divide by binary fission:

• The circular DNA molecule replicates and both copies attach to the cell membrane
• the plasmids replicate (a variable number)
• the cell membrane begins to grow between the two DNA molecules and begins to pinch inward, dividing the cytoplasm in two
• a new cell wall forms between the two molecules of DNA dividing the original cell into two identical daughter cells (each with a single copy of the circular DNA and a variable number of copies of the plasmids).

Viruses

Viruses do not undergo cell division. They inject their nucleic acid into a host cell infecting it. The host cell replicates the viral particles.

3.2.1.2 Structure of prokaryotic cells and of viruses

Prokaryotic cells
Prokaryotic cells are much smaller than eukaryotic cells. They also differ from eukaryotic cells as they:

• lack membrane-bound organelles
• have smaller (70S) ribosomes
• have no nucleus (instead they have a single circular DNA molecule that is free in the cytoplasm and not associated with proteins)
• a cell wall containing murein (a glycoprotein)

Some prokaryotic cells might additionally have:

• one or more plasmids
• a capsule surrounding the cell
• one or more flagella

Viruses
These are acellular non living particles. They are smaller than bacteria (20-300nm) and contain nucleic acids (DNA or RNA) as genetic material. They can only multiply inside a host cell. The nucleic acid is enclosed in a protein coat (a capsid). Some viruses are further surrounded by a lipid envelope (e.g HIV). The lipid envelope/capsid has attachment proteins which allow the virus to identify and attach to a host cell.

3.2.1.3 Methods of studying cells Content

This section is all about microscopes. The material we put under the microscope is the object, the appearance of this material when viewed under the microscope is the image. The magnification of an object is how many times bigger the image is when compared to the object:

magnification = size of image / size of object

NOTE: remember to keep the units of measurement the same!

Resolution is different to magnification. The resolution of a microscope is the minimum distance apart that two objects can be in order for them to appear as separate items. The resolving power depends on the wavelength/form of radiation emitted from the microscope. Increasing the magnification will increase the size of an object but not necessarily the resolution (every microscope has a limit resolution).

Okay so there are three types of microscope we use to study cells: the optical/light microscope, the transmission electron microscope, and the scanning electron microscope. We need to know the principles and limitations of using each one:

• The light microscope
• can only distinguish between objects more than 2μm apart due to the long wavelength of light.
• the transmission electron microscope
• can be focused by electromagnets as electrons are negatively charged
• can resolve objects that are just 0.1nm apart
• beams pass through a thin section of the specimen. Parts of this specimen absorb electrons and appear darker (other parts allow the electrons to pass through and so appear bright)
• an image is produced on a screen which can be photographed to produce a photomicrograph
• the resolving power (0.1nm) cannot always be achieved due to difficulties in preparing the specimen/the high energy electron beam may destroy the specimen
• the main limitations are as follows:
• whole system must be in a vacuum (living specimens cannot be observed)
• image produced is black and white
• a complex staining process is required
• specimen must be extremely thin
• image may contain artefacts
• 2D image produced
• the scanning electron microscope
• can be focused by electromagnets as electrons are negatively charged
• can resolve objects that are 20nm apart
• directs a beam of electrons on to the surface of the specimen from above (rather than penetrating from below). The beam is passed back and forth across a portion of the specimen in a regular pattern - the electrons are scattered depending on the contours of the specimen surface.
• A 3D image is produced by computer analysis of the pattern of scattered electrons and secondary electrons produced.
• the main limitations are as follows:
• whole system must be in a vacuum (living specimens cannot be observed)
• image produced is black and white
• a complex staining process is required
• image may contain artefacts

Cell fractionation
This is used to obtain large numbers of isolated organelles. It is the process whereby cells are broken up and the different organelles are separated out. Before cell fractionation can occur the tissue is placed in a cold buffered solution of the same water potential. this is because:

• cold to reduce enzyme activity that might break down the organelles
• is of the same water potential to prevent organelles bursting/shrinking as a result of osmotic gain/loss of water
• buffered so that the pH does not fluctuate.

The two stages of cell fractionation are homogenation and ultracentrifugation:

• homogenation
• cells are broken up by a homogeniser which releases the organelles from the cell. The resultant fluid is known as a homogenate and is filtered to remove any complete cells/large pieces of debris
• ultracentrifugation
• this is the process by which the fragments in the filtered homogenate are separated in a machine (a centrifuge). this spins the tubes of homogenate at very high speeds which creates a centrifugal force:
• the tube of filtrate is placed in the centrifuge and spun at slow speeds
• the heaviest organelles are forced to the bottom and form a pellet
• the supernatant is removed
• the supernatant is transferred to another tube and spun in the centrifuge at a faster speed than before
• the next heaviest organelles are forced to the bottom
• etc

3.2.1.1 Structure of eukaryotic cells

Each type of cell has a special internal structure suited to its job. This is known as the cell ultrastructure. Eukaryotic cells have a distinct nucleus and membrane bound organelles. The most important stuffs in the cell are as follows:

• cell-surface membrane (covered in 3.2.3)
• nucleus (containing chromosomes, consisting of protein-bound, linear DNA, and one or more nucleoli) 
• this contains the organisms hereditary material, manufactures rRNA and ribosomes, and acts as the control centre of the cell through the production of mRNA and tRNA (hence protein synthesis)
• has a nuclear envelope (double membraned) whose outer surface is continuous with the endoplasmic reticulum. This controls the entry/exit of materials in and out of the nucleus
• Nuclear pores allow the passage of large molecules
• The nucleoplasm makes up the bulk of the nucleus (granular jelly-like material)
• Chromosomes consist of protein (histone) bound linear DNA
• mitochondria
• These are the sites of the aerobic stages of respiration so are responsible for the production of ATP.
• These are double membraned. This controls entry and exit of material. The inner membrane is folded to form cristae which provide a large surface area for attachment of enzymes involved in respiration (respiration occurs on the mitochondrial membrane)
• The matrix contains proteins, lipids, ribosomes, and DNA which allows the mitochondria to control the production of some of their own proteins.
• chloroplasts (in plants and algae)
• These are organelles that carry out photosynthesis
• the chloroplast envelope is a double plasma membrane that surrounds the organelle. Being highly selective, it controls what enters and exits the chloroplast
• the grana are stacks of up to 100 disc like structures (thylakoids). Within thylakoids there is chlorophyll (the photosynthetic pigment). The granal membranes provide a large surface area for the attachment of chlorophyll/electron carriers/enzymes that carry out the light-dependant reactions
• the stroma is a fluid-filled matrix where the second stage of photosynthesis occurs. The stroma contains other structures such as starch grains. The stroma possesses all the enzymes needed to make sugars in the second stage of photosynthesis.
• They contain DNA and ribosomes to quickly and easily manufacture some of the proteins needed for photosynthesis
• Golgi apparatus and Golgi vesicles
• This is similar to the SER but is more compact. It consists of a stack of membranes that make up flattened sacs (cisternae) will vesicles (small rounded hollow structures called vesicles).
• The proteins and lipids produced by the ER are passed through the Golgi apparatus which modifies the proteins (e.g adding prosthetic components) and labels them allowing them to be accurately sorted and sent to their correct destinations. Once sorted the proteins and lipids are transported in Golgi vesicles.
• The Golgi apparatus can:
• add carbohydrate to proteins to form glycoproteins
• produce secretory enzymes
• secrete carbohydrates 
• transport, modify, and store lipids
• form lysosomes
• lysosomes (a type of Golgi vesicle that releases lysozymes) 
• lysosomes are formed when the vesicles produced by the Golgi apparatus contain enzymes (e.g proteases and lipases).
• They hydrolyse material ingested by phagocytic cells
• they release enzymes to the outside of the cell (exocytosis) to destroy external material
• they digest worn out organelles
• they completely break down cells (autolysis)
• ribosomes 
• small cytoplasmic granules with two subunits - one small and one large. Each subunit contains rRNA and protein. They are the site of protein synthesis.
• may be associated with the RER
• 80S are larger and occur in eukaryotic cells
• 70S are smaller and occur in prokaryotic cells, mitochondria, and chloroplasts
• rough endoplasmic reticulum and smooth endoplasmic reticulum
• This is a system of membranes that spread through the cytoplasm of the cells. It is continuous with the outer membrane and encloses a network of tubules and flattened sacs called cisternae
• The rough endoplasmic reticulum has ribosomes present on the outer surface of the membranes. It provides a large surface area for the synthesis of proteins and glycoproteins and provides a pathway for the transport of materials through the cell (especially proteins)
• The smooth endoplasmic reticulum lacks ribosomes on its surface and might be more tubular in appearance. It synthesises, stores, and transports lipids and carbohydrates.
• cell wall (in plants, algae and fungi)
• consists of microfibrils (cellulose) embedded in a matrix. microfibrils have considerable strength and so contribute to the overall strength of the cell wall
• The cell wall consists of a number of polysaccharides (such as cellulose) and there is a thin layer (the middle lamella) which marks the boundary between adjacent cell walls and cements adjacent cells together
• The cell wall provides mechanical strength in order to prevent the cell bursting under changing osmotic pressure, gives mechanical strength as a whole to the plant, allows water through to contribute to the movement of water through the plant
• cell vacuole (in plants)
• a fluid-filled sac bounded by a single membrane (tonoplast)
• contains a solution of mineral salts, sugars, amino acids, wastes, and sometimes pigments such as anhthocyanins
• they support herbaceous plants by making cells turgid
• the sugars and amino acids can act as a temporary food store
• the pigments may colour petals which might attract pollinating insects

In complex multicellular organisms, eukaryotic cells become specialised for specific functions. Specialised cells are organised into:

• tissues
• for working efficiency cells are often aggregated together. A collection of similar cells that perform a specific function is known as a tissue. Examples include:
• Epithelial tissue
• xylem
• organs
• tissues are aggregated into organs. It therefore follows that an organ is a combination of tissues that are coordinated to perform a variety of functions (although they often have one predominant major function). Examples include:
• the stomach
• a leaf
• systems
• organs work together as a single unit known as an organ system. Examples include:
• the digestive system
• the respiratory system
• the circulatory system

3.8.3 Using genome projects

The Human Genome Project (HGP) was an international scientific research project with the goal of determining the sequence of nucleotide base pairs that make up human DNA, and of identifying and mapping all of the genes of the human genome.Projects to determine the entire DNA nucleotide base sequence of organisms work by mapping the DNA base sequences that make up the genes of the organism and then map these genes on the individual chromosome of that organism. In this way a complete map of all the genetic material in an organism (the genome) is obtained.

The HGP would have been impossible without the use of bioinformatics (the science of collecting and analysing complex biological data, e.g genetic codes). It uses computers to read, store, and organise biological data at a very fast rate and utilises algorithms to analyse and interpret biological data.

Determining the complete DNA base sequence of an organism uses the technique of whole-genome shotgun (WGS) sequencing. This involves researchers cutting the DNA into many small easily sequenced sections and then using computer algorithms to align overlapping segments to assemble the entire genome. Sequencing methods are continuously updated. This and increased automation of the processes involved have lead to extremely rapid sequencing of whole genomes.

One outcome of the HGP is that lots of medical advances have been made. E.g over 1.4 million SNPs (single nucleotide polymorphisms) have been found in the human genome. SNPs are single-base variations in the genome that are associated with disease/disorders. Medical screening of individuals has allowed quick identification of potential medical problems (this is good for early intervention).

The proteins a genome codes for are known as the proteome (all the proteins produced by the genome). A protein is only produced when a gene is switched on. The cellular proteome is all the proteins produced in a given type of cell at a given time under specific conditions. The complete proteome is all the proteins produced in a given organism at a given time under specific conditions.

Determining the genome and proteome of simpler organisms
We sequence the genomes of prokaryotic/single-celled eukaryotic cells to gain information to help cure disease and provide knowledge of genes that can be usefully exploited. E.g ones from organisms that can withstand extreme/toxic environmental conditions and so have potential uses in cleaning up pollutants or in manufacturing biofuels. Determining the proteome of prokaryotes is relatively easy because:

• the vast majority of prokaryotes have just one circular piece of DNA that is not histone associated
• there are none of the non-coding portions (introns) of DNA which are typical of eukaryotic cells

Knowledge of the proteome of prokaryotes has a number of applications. One is the identification of the proteins that act as antigens on the surfaces of human pathogens. These antigens can be used in vaccines against diseases caused by these pathogens. The antigens can be manufactured and then administered to people in appropriate doses. In response, memory cells are formed etc.

Determining the genome and proteome of complex organisms

The problem in complex organisms is translating knowledge of the genome into the proteome. This is because the genome of complex organisms contains many introns (non-coding genes) as well as others that have a role in regulating other genes.

3.8.2.3 Gene expression and cancer

Cancer is a group of diseases caused by damage to the genes that regulate mitosis and the cell cycle. This leads to unrestrained growth of cells and as a consequence an abnormal group of cells (a tumour) develops and constantly expands in size.

It is important to realise that not all types of tumour are cancerous. Cancerous tumours are known as malignant tumours, non-cancerous tumours are known as benign tumours). We need to know the main characteristics of them:

Benign (non-cancerous)	Malignant (cancerous)
Grow to a large size very slowly	Can also grow to a large size, but very rapidly
The cell nucleus has a normal appearance	The cell nucleus is often larger and darker due to an abundance of DNA
Cells are often specialised (differentiated)	Cells become unspecialised (de-differentiated)
Cells produce adhesion molecules that make them stuck together so they remain within the tissue from which they arise (primary tumours)	Cells do not produce adhesion molecules and so they tend to spread to other regions of the body (metastasis) (secondary tumours)
Tumours are surrounded by a capsule of dense tissue so remain as a compact structure	Tumours are not surrounded by a capsule and can therefore grow finger-like projections into surrounding tissue
Much less likely to be life threatening but can potentially disrupt the functioning of a vital organ	More likely to be life threatening as abnormal tumour tissue replaces normal tissue
Tend to have localised effects on the body	Often have systemic effects such as weight loss and fatigue
Can usually be removed by surgery	Removal usually involves radio/chemotherapy and also surgery
Rarely reoccur after treatment	More frequently reoccur after treatment

DNA analysis of tumours has shown that cancer cells are derived from a single mutant cell. Further mutation in one of the descendent cells leads to other changes that cause subsequent cells to be different from normal cells in growth and appearance. The two main types of genes that play a role in cancer are tumour suppressor genes and oncogenes. An importance difference between oncogenes and tumour suppressor genes is that while oncogenes cause cancer as a result of the activation of proto-oncogenes, tumour suppressor genes cause cancer when they are inactivated.

Oncogenes
Most oncogenes are mutations from proto-oncogenes. Proto-oncogenes stimulate a cell to divide when growth factors attach to a protein receptor on its cell-surface membrane. This activates the genes that cause the DNA to replicate and the cell to divide. If a proto-oncogene mutates into an oncogene it can become permanently activated (switched on) for two reasons:

• The receptor protein on the cell-surface membrane can be permanently activated so that cell division is switched on even in the absence of growth factors
• The oncogene may code for a growth factor that is then produced in excessive amounts stimulating excessive cell division

The result is that cells divide too rapidly and out of control resulting in either a tumour or a cancer. A few types of cancer are caused by inherited mutations of proto-oncogenes that cause the oncogene to be activated. Most cancer-causing mutations involving oncogenes are acquired not inherited.

Tumour supressor genes
Tumour suppressor genes slow down cell division (they suppress tumours/cell division), repair mistakes in DNA, and tell cells when to die (apoptosis). They have the opposite role to proto-oncogenes. A normal tumour suppressor gene maintains normal rates of cell division which prevents tumours. If a tumour suppressor gene mutates it becomes inactive/switched off. As a result the inhibition of cell division stops and cells begin to grow out of control. The mutated cells are structurally and functionally different from normal cells. Many die but those that survive can make clones of themselves and form tumours. Some cancers are caused by inherited mutations of tumour suppressor genes but most are acquired. 

Abnormal methylation of tumour suppressor genes
Abnormal DNA methylation is common in the development of a variety of tumours. The most common abnormality is hypermethylation/increased methylation. The process whereby this leads to cancer is as follows:

• Hypermethylation occurs in a specific region (a promoter region) of tumour suppressor genes
• This leads to the tumour suppressor gene being inactivated
• As a result transcription of the promoter regions of tumour suppressor genes is inhibited
• As the tumour suppressor gene normally slows the rate of cell division it's inactivation leads to increased cell division and the formation of a tumour.

Furthermore, hypomethylation/decreased methylation can occur in oncogenes where is leads it leads to their activation hence formation of tumours.

Oestrogen and breast cancer

Oestrogen plays an important role in regulating the menstrual cycle in women. The fat cells of breasts tend to produce more oestrogen after menopause. These locally [produced oestrogens appear to trigger breast cancer in postmenopausal women. Once a tumour has developed it further increases oestrogen concentration which therefore leads to increased tumour development. White blood cells that are drawn to the tumour also increase oestrogen production. Basically how this works is, if oestrogen binds to a gene that controls cell division and growth then the gene will be activated and its continued division could produce a tumour.  It is also known that oestrogen causes proto-oncogenes to develop into oncogenes which leads to the development of a tumour.

3.8.2.2 Regulation of transcription and translation

In eukaryotes, transcription of target genes can be stimulated or inhibited when specific transcriptional factors move from the cytoplasm into the nucleus. The general principles involved in controlling gene expression by controlling transcription are as follows:

• For transcription to begin the gene must be switched on by specific molecules (transcriptional factors) that move from the cytoplasm into the nucleus
• Each transcriptional factor has a site that binds to a specific bas sequence of the DNA in the nucleus
• When it binds it causes this region of DNA to begin the process of transcription
• mRNA is produced and the information it carries is translated into a polypeptide (this is translation)
• When a gene is not being expressed (it is switched off), the site on the transcriptional factor that binds to DNA is not active
• As the site is inactive it cannot cause transcription and polypeptide synthesis.

So we need to know a bit about the role of the steroid hormone oestrogen in initiating transcription. Well, hormones like oestrogen can switch on a gene and start transcription by combining with a receptor site on the transcriptional factor. This then activates the DNA binding site on the transcriptional factor (by causing it to change shape):

• Oestrogen, a lipid-soluble molecule, diffuses easily through the phospholipid bilayer of cell-surface membranes
• Once inside the cytoplasm oestrogen binds with a site on a receptor molecule of the transcriptional factor. They are complimentary to one another
• By binding with the site the oestrogen changes the shape of the DNA binding site on the transcriptional factor, activating it
• It can now bind to DNA
• The transcriptional factor enters the nucleus (through a nuclear pore) and binds to a specific base sequence of DNA
• This stimulates transcription of the gene that makes up that portion of DNA

Whilst genes determine the features of an organism, the environment can influence the expression of these genes. It is now believed that environmental factors can cause heritable changes in gene function without changing the base sequence of DNA. This process is known as epigenetics. This provides explanations as to how environmental influences such as diets, stress, toxins (etc), can alter the genetic inheritance of an organism's offspring. So, now about how it works. Basically, we already know that DNA is wrapped around proteins called histones. We now know that both the DNA and histones are covered in chemicals (tags). These chemicals/tags form the epigenome. The epigenome determines the shape of the DNA-histone complex. E.g it keeps genes that are inactive tightly packed in arrangement ensuring they cannot be read (epigenetic silencing). It can also unwrap genes so the DNA is exposed and can be easily transcribed (switching on these particular genes). Unlike DNA, the epigenome is not fixed (it is flexible). It is flexible because its chemical tags respond to environmental changes, factors such as stress and diet can cause the chemical tags to adjust the wrapping/unwrapping, switching genes on/off.

The epigenome of a cell is an accumulation of the signals it has received during its lifetime. It acts a bit like a cellular memory. In early development the signals come from within the cells of the foetus. The nutrition provided by the mother is important in shaping the epigenome at this stage (this is why it is imperative that pregnant woman keep a good diet and don't smoke etc).After birth environmental factors affect the epigenome (although signals, such as hormones, from within the body can still influence it). These factors cause the epigenome to activate/inhibit a specific set of genes. The environmental signal stimulates proteins to carry its message inside the cell from where it is passed by a series of other proteins into the nucleus. Here the message passes to a specific protein which can be attached to a specific sequence of bases on the DNA. Once attached the protein can change:

• acetylation of histones, leading to the activation/inhibition of a gene
• methylation of DNA by attractive enzymes that can add/remove methyl groups.

Okaaay so what does any of that actually mean. Well, when the association of histones with DNA is weak the DNA-histone complex is less condensed meaning that the DNA is accessible by transcriptional factors which can initiate transcription (basically, the gene is switched on). When the association is stronger, the reverse occurs and the gene is switched off. Condensation of the DNA histone complex inhibits transcription. It can be brought about by decreased acetylation of the histones or by methylation of DNA. So how do these processes work?

Decreased acetylation of associated histones
Acetylation is the process whereby an acetyl group is transferred to a molecule. In this case, acetylcoenzyme A donates an acetyl group. Deacetylation is the removal of an acetyl group from a molecule. Decreased acetylation increase the positive charges on histones as acetyl groups are negatively charged. This increases the attraction of the histones to the phosphate groups of DNA. The association between DNA and histones becomes stronger and the genes are switched off.

Increased methylation of DNA
Methylation is the addition of a methyl group to a molecule. In this case a methyl group is added to the cytosine bases of the DNA. It inhibits transcription in the following ways:

• prevents the binding of transcriptional factors to DNA
• attracts proteins that condense the DNA-histone complex by inducing the deacetylation of histones.

Epigenetic changes can be responsible for certain diseases. Altering the epigenetic process can cause abnormal activation/silencing of a gene. In some cases the activation of a normally inactive gene can cause cancer. In other cases, the inactivation (silencing) of a usually active gene causes a disease.

In specific sections of DNA (near promoter regions) that have no methylation in normal cells. In cancer cells these regions become highly methylated causing genes that should be active to switch off.

Whilst epigenetics do not alter the sequence of bases in a DNA molecule they can increase the incidence of mutations. For example, some active genes help to repair DNA (preventing cancers). Individuals with various types of inherited cancer have increased methylation of these genes causing the gene to be switched off. As a result, the damage to base sequences in DNA are not repaired. This can lead to the development of cancer.

It's not all bad news though, we can also use epigenetic treatments to counteract the epigenetic changes that cause certain genes to be activated/silenced. The treatments use drugs to inhibit certain enzymes involved in either histone acetylation or DNA methylation. E.g. drugs that inhibit enzymes that cause DNA methylation can reactivate silenced genes. 

Epigenetics have also been used in diagnostics tests to detect the early stages of diseases such as cancer/brain disorders/arthritis. The tests can identify the level of DNA methylation and histone acetylation at an early stage of disease. This allows patients to seek treatment asap.

RNA interference
This is the last little bit in this section. Basically, in eukaryotes (and some prokaryotes!) the translation of mRNA produced by a gene can be inhibited by breaking mRNA down before its coded information can be translated into a polypeptide.One type of RNA molecule that may be involved is small interfering RNA (siRNA). This mechanism involved small double-stranded sections of siRNA and operates as follows:

• An enzyme cuts large double stranded RNA molecules into smaller sections known as siRNA
• One of the two siRNA strands combines with an enzyme
• The siRNA molecule guides the enzyme to a mRNA molecule by pairing up its bases with the complimentary ones on a section of the mRNA molecule
• Once in position the enzyme cuts the mRNA into smaller sections
• the mRNA is no longer capable of being translated into a polypeptide meaning that the gene has not beed expressed (so it has been silenced/blocked)