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Genetics
DNA and RNA
Genes​ sections of DNA
are
They’re found on chromosomes
Genes code for proteins
They contain the instructions to make them
Proteins are made from amino acids
Different proteins have a different number and order of amino acids
The order of nucleotide bases in a gene determines the order of amino acids in a particular
protein
Each amino acid is coded for by a sequence of three bases in a gene
Codon​ the sequence of three bases in a gene, which codes for an amino acid
is
Different sequences of bases code for different amino acids
This is the genetic code
The sequence of bases in a section of DNA is the template that is used make a protein in protein
synthesis
DNA molecules are found in the nucleus of the cell
The organelles for protein synthesis are found in the cytoplasm
DNA is too large to move out of the nucleus
Instead, a section is copied into RNA
The RNA leaves the nucleus, and joins with a ribosome in the cytoplasm
Here it is used to synthesise a protein
RNA is made up of nucleotides, that contain one of four different bases
The nucleotides form a nucleotide strand with a sugar-phosphate backbone
RNA differs from DNA in three ways:
The sugar in the nucleotide is a ribose sugar, not deoxyribose

The nucleotides form a single polynucleotide strand, not a double one
Uracil replaces thymine as a base
It pairs with adenine during protein synthesis

Messenger RNA
mRNA is a single polynucleotide strand
Groups of three adjacent bases are usually called codes
mRNA is made in the nucleus during transcription
It carries the genetic code from the DNA in the nucleus to the cytoplasm

Transfer RNA
tRNA is a single polynucleotide strand
It is folded into a clover
Hydrogen bonds between specific base pairs hold the molecule in shape
Every tRNA molecule has a specific sequence of three bases at one end
An ​
anticodon​ the sequence of three bases in a gene, at the opposite end to the codon
is
They have an amino acid binding site at the other end
It is found in the cytoplasm
It carrier amino acids, that are used to make proteins to the ribosomes
DNA

mRNA

tRNA

Shape

double-stranded
twisted into a double helix
held together by hydrogen
bonds

single-stranded

single-stranded
folded into a clover shape
held together by hydrogen
bonds

Sugar

deoxyribose sugar

ribose sugar

ribose sugar

Bases

A, T, C, G

A, U, C, G

A, U, C, G

Other
features

codons

codons

anticodons
an amino acid binding site

Protein Synthesis
Transcription
During transcription, an mRNA copy of a gene is made in the nucleus
Transcription starts when RNA polymerase attaches to the DNA double-helix at the beginning
of a gene
The hydrogen bonds between the two DNA strands in the gene brake
This separates the strands
The DNA molecule uncoils at that point
One of the strands is then used as a template to make an mRNA copy
The RNA polymerase lines up free RNA nucleotides alongside the template strand
Specific base pairing means that the mRNA strands ends up being a complementary copy of
the DNA template strand
Once the RNA nucleotides have paired up with their specific bases, they’re joined together
This forms an mRNA molecule
The RNA polymerase moves along the DNA
This separates the strands, and assembles the mRNA strand
The hydrogen bonds between the uncoiled strands of DNA reform once the RNA polymerase
has passed by
The strands coil back into a double-helix
There are sequences of DNA called stop signals
When RNA polymerase reaches this, it stops making mRNA, and detaches from the DNA
The mRNA moves out of the nucleus through a nuclear pore
It attaches to a ribosome in the cytoplasm

mRNA in Eukaryotic Cells
Genes in eukaryotic DNA contain section that do not code for amino acids
These section are called introns

Sections that do code for amino acids are called exons
During transcription, the introns and exons are copied into mRNA
Pre-mRNA are mRNA strands containing introns and exons
Introns are removed from pre-mRNA, by splicing
Introns are removed, and exons are joined, forming mRNA strands
This takes place in the nucleus
The mRNA then leaves the nucleus for the next stage of protein synthesis

Translation
Translation occurs at the ribosomes in the cytoplasm
Amino acids are joined together to make a polypeptide chain, following the sequence of codons
The mRNA attaches itself to a ribosome
tRNA molecules carry amino acids to the ribosome on the mRNA
A tRNA molecule has an anticodon that’s complementary to the first codon on the mRNA
It attaches itself to the mRNA by specific base pairing
A second tRNA molecules attaches itself to the next codon on the mRNA in the same way
The two amino acids attached to the tRNA molecules are joined by a peptide bond
The first tRNA molecules moves away
It leaves its amino acid behind
A third tRNA molecules binds to the next codon on the mRNA
Its amino acid binds to the first two
The second tRNA molecules moves away
This process continues
A chain of linked amino acids or a polypeptide chain is produced
This continues until a stop signal on the mRNA molecule is found
The polypeptide chain moves away from the ribosome

Translation is complete

Genetic Code & Nucleic Acids
The genetic code is non-overlapping, degenerate and universal
The ​
genetic code​ the sequence of codons in mRNA
is
They code for specific amino acids
Each codon is read in sequence
They are separate from the triplet before and after it has been read
They do not share their bases
The code is non-overlapping
The genetic code is degenerate
There are more possible combinations of triplets than there are amino acids
Some amino acids are coded for by more than one codon
Some codons are used to tell the cell when to start and stop production of the protein
They’re found at the beginning and end of the mRNA
The genetic code is universal
The same specific codons code for the same amino acids in all living things

Regulation of Transcription & Translation
Transcription factors control the transcription of target genes
All the cells in an organism carry the same genes
the structure and function of different cells vary
This is because not all the genes in a cell are expressed
Different genes are expressed
This results in different proteins being made
These proteins then modify the cell, as they determine the cell structure and control cell
processes

The transcription of genes is controlled by protein molecules called transcription factors:
Transcription factors move from the cytoplasm to the nucleus
In the nucleus they bind to specific DNA sites near the start of their target genes
They control the expression by controlling the rate of transcription
Activators are transcription factors
They increase the rate of transcription
They help RNA polymerase bind to the start of the target gene
Repressors are also transcription factors
They decrease the rate of transcription
They bind to the start of the target gene preventing RNA polymerase from binding, stopping
transcription
Oestrogen affects the transcription of target genes
The expression of genes can also be affected by other molecules in the cell
Oestrogen is a hormone that can affect transcription by binding to a transcription factor
called an oestrogen receptor
This forms an oestrogen-oestrogen receptor complex
The complex moves from the cytoplasm into the nucleus
Here, it binds to specific DNA sites near the start of the target gene
The complex can either act as an activator, and help RNA polymerase
It can also act as a repressor, and block RNA polymerase
They way the complex acts depends on the type of cell, and the target gene
The level of oestrogen in a cell affects the rate of transcription of target genes

Gene expression is affected by small interfering RNA, known as siRNA
siRNA molecules are short, double-stranded RNA molecules
They can interfere with eh expression of specific genes

Their bases are complementary to specific sections of a target genes, and the mRNA that it’s
formed from
siRNA can interfere with both transcription and translation of genes
It affects translation through RNA interference
in the cytoplasm, siRNA and associated proteins bind to the target mRNA
The proteins cut up the mRNA into sections
This stops it from being translated
It prevents the expression of the specific genes, as its protein can no longer be produced

Mutations, Genetic Disorders & Cancer
Mutations​ changes to the base sequence of DNA
are
They can be caused by error during DNA replication
They can be caused by mutagenic agents
The order of DNA bases in a gene determines the order of amino acids in a protein
If a mutation occurs, the sequence of amino acids it codes for, and the protein formed could be
altered
Not all mutations affect the order of amino acids
The degenerate nature of the genetic codes means some amino acids are coded for by more
than one codon
A​
silent mutation ​
occurs when substitution of a base still codes for the same amino acid as
the original base
The mutation has no effect on the production of the final polypeptide
This is because the R group remains the same
Therefore the tertiary structure would not change
A​
nonsense mutation​
occurs when a substitution of a base occurs, leading to a premature
stop codon being coded for
This leads to the premature end to the synthesis of a polypeptide
Successful synthesis of the final protein would be unlikely

It would not be able to function normally
A ​issense mutation​
m
occurs when a change in a base leads to a different amino acid being
coded for
The polypeptide will have a single amino acid that is different
This mutation is determined by the role of the amino acid in the final polypeptide
Deletion​ when one base is deleted
is
Deleting a base can lead to the codons no longer being read properly
The bases are read in triplets
When one is deleted, the subsequent bases are shifted forward by one base
The amino acid sequence of the ‘new’ code will be different
Substitution​ when one base is substituted with another
is
Substituting a base can lead to the codons no longer being read properly
The bases are read in triplets
When one is added, the subsequent bases are shifted backward by one base
The amino acid sequence of the ‘new’ code will be different
Mutations occur spontaneously
Mutagenic agents​
increase the rate of mutation
Ultraviolet radiation, ionising radiation, some chemicals and some viruses are mutagenic
agents
They increase the rate of mutations by:
Acting as a base - chemicals called base analogs can substitute for a base during DNA
replication
This changes the base sequence in the new DNA
Altering bases - some chemicals can delete or alter bases
Changing the structure of DNA - some types of radiation can change the structure of DNA
This causes problems during DNA replication

Genetic disorders and cancer are caused by mutations
Hereditary mutations cause genetic disorders and some cancers
Genetic disorders​ inherited disorders caused by abnormal genes or chromosomes
are
Some mutations can increase the likelihood of developing certain cancers
If a gamete containing the mutation for a genetic disorder is fertilised, the mutation will be
present in the new fetus formed
Acquired mutations​ mutations that occur in individual cells after fertilisation
are
If these mutations occur in the genes that control the rate of cell division, it can cause
uncontrolled cell division
A​
tumor​ a mass of abnormal cells
is
They are caused by uncontrolled cell division
Cancers​ tumors that invade and destroy surrounding tissue
are
There are two types of gene that control cell division
Tumour-suppressor genes can be inactivated in a mutation occurs in the DNA sequence
When functioning normally, these genes slow cell division
They produce proteins that stop cells dividing
They can cause them to self-destruct
If a mutation occurs in this gene, the protein is not produced
This stimulates the cells to divide uncontrollably
This results in a tumour
Proto-oncogenes can be increased if a mutation occurs in the DNA sequence
A oncogene is a proto-oncogene which is mutated
When functioning normally, these genes stimulate cell division
They produce proteins that make cells divide
If a mutation occurs in this gene, it may become overactive
This stimulates the cells to divide uncontrollably

This results in a tumour

Diagnosing & Treating Cancer and Genetic Disorders
Cancer and most genetic disorders are caused by mutations
Knowing whether a disorder is caused by an acquired or inherited mutation affects the
prevention and diagnosis of the disorder
Identifying the specific mutation that causes a disorder in an individuals affects the
prevention, diagnosis and treatment

Acquired Mutations
Prevention
Cancer can be caused by acquired mutations
Acquired mutations can occur spontaneously
They can be caused by exposure to mutagenic agents
If you know that acquired mutations are caused by mutagenic agents, you can try to prevent
cancer be avoiding them
Protective clothing - people who work with mutagenic agents should wear protective clothing
Sunscreen - this should be worn when the skin is exposed to the sun
Vaccination - some acquired cancers are caused by viruses
A vaccine available should protect women, reducing the risk of developing certain types of
cancer

Diagnosis
Normally, cancer is diagnosed after symptoms appear
High-risk individuals can be screened for cancers that the general population are not screened
for
They can be screened earlier and more frequently
This can lead to earlier diagnosis of cancer
This increases the chances of recovery
Some types of cancer are caused by a particular mutation

Knowing which mutation a type of cancer is caused by can affect diagnosis
If the specific mutation is known, more sensitive tests can be developed
This leads to earlier and more accurate diagnosis
This increases the chances of recovery
Individuals diagnosed with cancer can have the DNA from the cancerous cells analysed
This helps find out which mutation has caused it

Treatment
Knowing which mutation a type of cancer is caused by can affect treatment
Treatment is different for different mutations
The aggressiveness of the treatment can differ
This depends on the mutation
Different mutations produce different types of cancer, which affects the treatment
If the specific mutation is known, then gene therapy may be used to treat it

Prevention
Cancer caused by hereditary mutation usually result in a family history of a certain type of
cancer
If an individual has a family history of cancer, measures can be take to prevent it developing
Most cancers are caused by mutations in multiple genes
People with a family history should avoid gaining extra acquired mutations, by avoiding
mutagenic agents
Individuals with a family history of cancer can have their DNA analysed to see if they carry
the specific mutation
If a mutation causes a very high risk of cancer, preventative surgery may be carried out
This involves removing the organ the cancer is likely to affect before the cancer develops

Diagnosis
Screening, or increased and earlier screening can be done if there’s a family history

This can lead to early detection
It also increases the chances of recovery
Screening, or increased and earlier screening can be done if there’s a hereditary mutation
This can lead to early detection
It also increases the chances of recovery

Treatment
Treatment is similar to treating cancer caused by acquired mutations
The treatment depends on the mutation
However cancer caused by hereditary mutations are often diagnosed earlier
This can change the treatment used

Hereditary Disorders
Diagnosis
If a person has family history of a genetic disorder, they can have their DNA analysed
This allows them to see if they have the mutation that causes it - or if they’re a carrier
If they’re tested and diagnosed before symptoms develop, available treatment can begin
earlier
Knowing if they have the disorder or if they’re a carrier can help find out if any children they
have or might have are at risk

Treatment
Gene therapy can be used to treat some genetic disorders
the treatment can be different for different mutations
Early diagnosis can affect treatment options

Prevention
Carriers or sufferers of genetic disorders can undergo preimplantation genetic diagnosis,
during IVF
This prevents any offspring having the disease

Embryos are produced by IVF, and screened for the mutation
Only embryos without the mutation are implanted into the womb

Stem Cells
Stem cells are able to mature into any type of body cell
Multicellular organisms are made from different cell types
They are specialised for their function
All these cells originally came from stem cells
Stem cells ​ unspecialised cells that can develop into other types of specialised cells
are
They divide to become new cells
They then become specialised
All multicellular organisms have ome form of stem cell
They are found in the embryo
Here, they all become specialised cells to form a fetus
They can be found in some adult tissues
Here, they become specialized cells that need to be replaced
Stem cells can mature into any type of body cell
Totipotent​ are stem cells that can mature into any type of body cell in an organism
cells
These are only present in the early life of an embryo
They lose their ability to specialise into all types of cells after this
Pluripotent​ are stem cells that have been chemically marked to become a certain type of
cells
cell
Only a few stem cells remain in mature organisms
Multipotent​ are stem cells that can only develop into a specified cell
cells
Mature plants also have stem cells
They’re found in areas where the plant is growing

All stem cells in plants are totipotent
They can mature into any cell type
They can be used to grow plant organs
They can be used to grow whole new plants in vitro
Stem cells become specialised because different genes are expressed
Stem cell all contain the same genes
During development not all of them are transcribed and translated
Under the right conditions some genes are expressed
Others are switched off
mRNA is only transcribed from specific genes
The mRNA from these genes is then translated into proteins
These proteins modify the cell
They determine the cell structure and control cell processes
Changes to the cell cause the cell to become specialised
These changes are difficult to reverse
Therefore when a cell becomes specialised, it remains specialised

Tissue Culture
Tissue culture​ growing plant tissue artificially
is
A single totipotent stem cell is taken from a growing area on a plant
The cell is placed in a growth medium
This contains nutrients and growth factors
It is sterile
This stops microorganisms from growing and competing with the plant cells
The plant stem cell will grow and divide into a mass of unspecialised cells
If the conditions are suitable, the cells will mature into specialised cells
The cells grow and specialise to form a plant organ or an entire plant
This depends on the growth factors used

Stem Cells in Medicine
Some stem cell therapies already exist
Stem cells can divide into other cell types
They can be used to replace cells damaged by illness or injury
Some stem cell therapies exist for diseases affecting the blood and immune system
Bone marrow contains stem cells
These are pluripotent
They can become specialised to form any type of blood cells
Bone marrow transplants can be used to replace the faulty bone marrow in patients that
produce abnormal blood cells
The stem cells in the transplanted bone marrow divide and specialise to produce healthy blood
cells
This technique has been used to treat lejeune and lymphoma
It has been used to treat some genetic disorders
Stem cells could be used to treat other diseases
For example:
Spinal cord injuries - stem cells could be used to replace damaged nerve tissue
Heart disease - stem cells could be used to replace damaged heart tissue
Bladder conditions - stem cells could be used to grow whole bladders, which are then
implanted into patients to replace damaged ones
Respiartory diseases - donated windpipes can be struipped down to their simple collagen
structure, and then covered with tissue generated by stem cells, which can then be translated
into patients
Organ transplants - organ could be grown from stem cells to provide new organs for people in
donor waiting lists
Stem cells could be genetically identical to a patient's own cells
This could then be used to grow new tissue or organs that the body would not reject

There are huge benefits to using stem cells in medicine
People who make decisions about the use of stem cells to treat human disorders have to
consider the potential benefits of stem cell therapies
They could save many lives
They could improve the quality of life for many people
Adult Stem Cells
These are obtained from the body tissues of an adult
They are obtained in a simple operation
There is very little risk involved
It can be uncomfortable
They are not as flexible as embryonic stem cells
They’re multipotent
They only specialise into a limited range of cells
Embryonic Stem Cells
These are obtained from embryos at an early stage of development
Embryos are created in a laboratory using in vitro fertilisation
The egg cells are fertilised outside the womb
Once the embryos are four to five days old, stem cells are removed
The rest of the embryo is destroyed
They’re totipotent
They can develop into all types of specialised cells
There are ethical issue surrounding the use of stem cells
Obtaining stem cells from an embryo raises ethical issues
The procedure results in the destruction of an embrou
It could become a fetus if placed in a womb

Some people believe an individual is formed at the moment of conception
Therefore it has the right to life
They believe it is wrong to destroy embryos
It is possible to obtain stem cells from unfertilised embryos
These are embryos made from egg cells that have not been fertilised by sperm
This causes fewer objections
This is because the embryos couldn’t survive a few days
It would not produce a fetus if placed in a womb
Some people think scientists should only use adult stem cells
Their production does not destroy an embryo
Adult stem cells cannot develop into all the specialised cell types that embryonic stem cells can
The decision makers in society have to take everyone’s views into account



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