Download 2.5.2 Heredity and Gene Expression

2.5 GENETICS
2.5.2 Heredity and Gene Expression
Definition of “heredity”.
Heredity is the passing on of features from parents to offspring by means of genes.
Heredity is also called genetic inheritance.
The features are passed from the parents to the
offspring by means of genes in the sex cells or
gametes.
Genes are the units of heredity.
Example of “heredity”.
Humans inherit genes that control features like
eye colour and ear shape.
Plants inherit genes that control features like
flower colour and seed shape.
Definition and example of “gene expression”.
Genes are located along the length of a chromosome.
Each gene is a section of DNA and contains the code for
making a particular protein.
Each gene or group of genes controls the development of a
particular characteristic or trait of an individual.
Gene expression is the way in which the genetic code in a gene is used to make a protein
Basically gene expression refers to the way in which genes work.
Gene expression depends on the environment which means that characteristics result from
heredity and environment.
A child might inherit the gene for tallness but if it doesn’t get enough food the genes will not
work and the child will not grow tall i.e. the genes will not be expressed
A plant inherits the genes to make chlorophyll but if it doesn’t get enough light the genes will
not work and no chlorophyll will be make i.e. the genes will not be expressed.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.5 GENETICS
2.5.3 Genetic Code
Definition of a “gene”.
Genes are located along the length of a chromosome.
Each gene is a section of DNA and contains the code for
making a particular protein.
Each gene or group of genes controls the development of a
particular characteristic or trait of an individual.
A gene is inherited information for making a protein
A gene is a section of DNA that causes the production of a specific protein
Role of a “gene”.
Many of the proteins made by genes are enzymes. Genes control a cell because the enzymes
they make control cell activities. Therefore, the role of a gene is to control a cell.
Chromosome structure.
Chromosomes are made of DNA (40%) and protein (60%).
The DNA is a very long molecule. It is very coiled and folded.
The proteins called histones are responsible for holding the DNA
in its folded state so that it can fit into the nucleus of a cell.
Genes are found on the DNA. Some genes are close together and others are far apart. There
are many parts of the DNA that contains no genes. This DNA is said to be non-coding and is
“junk DNA”.
2.5.4 DNA Structure, Replication and Profiling
Simple structure of DNA
DNA is a very long molecule. It consists of two strands. The two strands are linked together
by paired bases.
There are four different bases:
Adenine (A), Thymine (T), Guanine (G) and Cytosine (C)
Each base can only link with one other type, A with T and C with G.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.5 GENETICS
Coding and non-coding structures.
Genes are arranged along the DNA. Sometimes the genes are close together and sometimes
they are widely separated. The gene sections of DNA are known as coding structures. The
rest of the DNA is non-coding, as it doesn’t carry the code for the formation of a protein. This
is often called junk DNA.
There are 2 types of junk DNA
a. Non-coding DNA ( found between genes)
This DNA consists of repeating sequences of bases dispersed at random between
genes. The function of these repeating stretches of bases is unknown.
b.Non-coding DNA (found within genes)
The sections of DNA that code for a protein a gene are called exons (because they are
expressed). The non-coding sections of DNA in a gene are called introns (because
they intervene between the working sections).
Triplet base code.
DNA codes for a protein by using a sequence of three consecutive bases. Each group of three
bases is called a triplet or codon. Each triplet codes for an amino acid. A sequence of triplets
codes for a sequence of amino acids and hence a protein will be formed.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.5 GENETICS
H.2.5.14 NUCLEIC ACID STRUCTURE AND FUNCTION
(EXTENDED STUDY)
Structure of DNA – a nucleic acid
1. DNA is a very long double-stranded molecule.
2. It is composed of NUCLEOTIDES joined together (it is a polymer).
A nucleotide consists of ONE DEOXYRIBOSE SUGAR, ONE PHOSPHATE and
ONE OF 4 NITROGENOUS BASES – GCAT
3. The deoxyribose sugar and phosphate form the sides of the ladder. The bases form the
rungs.
4. The bases form specific pairs or SPECIFIC PURINE AND PYRIMIDINE COUPLES
The COMPLEMENTARY BASE PAIRS are:
Guanine (purine) and Cytosine (pyrimidine)
Adenine (purine) and Thymine (pyrimidine)
5. The bases are held together by HYDROGEN BONDS.
A and T form 2 hydrogen bonds while G and C form 3 hydrogen bonds.
6. The two strands run opposite to each other or are anti-parallel
7. The two strands are coiled in a regular manner forming a DOUBLE HELIX
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.5 GENETICS
DNA Replication
DNA is the only molecule able to make a copy of itself. The process of duplication is called
replication. Replication means to make a perfect identical copy.
As we learnt in mitosis, when a cell divides the DNA must produce an exact copy of itself.
This replication takes place during Interphase of mitosis.
Mechanism of DNA replication
1. The double helix unwinds (or uncoils).
2. An enzyme (DNA helicase) unzips the two complimentary strands of DNA by breaking the
bonds between the base pairs
3. Free complimentary nucleotides from the cytoplasm attach to the exposed bases on the
separated strands.
4. A new complimentary strand is built along each ‘old’ strand – the old strand acts as a
mould or template for the new DNA strand
The new strands are assembled by the anabolic enzyme DNA polymerase.
5. Two new DNA strands are made. Each new DNA is
a. half new DNA and half old DNA
b. identical to the original DNA and to each other (Replication is semi-conservative)
6. Each new DNA rewinds to form a double helix
Significance of DNA replication
The fact that DNA is able to replicate or make an exact copy of itself means that the exact
same DNA is passed on to each new generation of cells. Replication is the reason why both
cells produced as a result of mitosis are genetically identical to each other and to the original
parent. It is also the reason why every somatic cell of our body is genetically identical.
Human beings grew from a zygote with 46 chromosomes by the process of mitosis and
because of replication the new chromosomes passed on to the new cells are identical to that
zygote and all the new cells produced are identical to each other and all contain 46
chromosomes.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.5 GENETICS
DNA profiling
DNA profiling is a method of making a unique pattern of bands from the DNA of a person,
which can then be used to distinguish that DNA from other DNA
DNA profiling is also called genetic or DNA fingerprinting.
Stages involved in DNA profiling
1. DNA isolation
Cells are broken down to release DNA
2. DNA is cut into fragments
The DNA is cut into fragments using special restriction enzymes e.g. one restriction
enzyme always cuts the DNA at the base sequence GAATTC.
The cut sections of DNA are called restriction fragments and are different lengths
because the base sequences being cut may be close or far apart on the DNA strands
3. The DNA fragments are separated on the basis of size
The fragments are separated by gel electrophoresis. An electric current drags the
DNA fragments through a gel. The smaller fragments move further and faster
through the gel and so the fragments are sorted by size into distinct bands in the gel.
A photographic copy of the final pattern of DNA bands is obtained.
4. Patterns are compared
The DNA profile is analysed and compared to another DNA profile to see if there are
any similarities or differences.
Two applications of DNA profiling
1. Medical
DNA profiles can be used to solve paternity and maternity cases by comparing the child’s
DNA profile with that of the disputed parent. Paternity cases are important in
immigration, inheritance and rape cases
All the childs bands match the father or
the mother so the man is the father of the child
The child’s bands only match the mother
so the man is not the father.
2. Crime
If biological material like hair, blood, semen or saliva is left at the scene of a crime, it
can be used to prepare a DNA profile. This is then compared with the DNA profile of a
suspect. If the patterns match then the suspect is associated with the crime.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.5 GENETICS
Genetic screening: screening
Mistakes can happen in DNA replication. If a mistake happens in a gene, then the gene will
not work properly and the resulting cell will not be able to make protein properly. If this
happens in a somatic cell it might not be a big problem but if it happens in a gamete then
every cell in the offspring will have the mutated gene and will be affected. This is how a
genetic disease occurs.
Genetic Screening is a diagnostic test to identify the presence or absence of changed genes
Uses of genetic screening
a. Adults screening - To see if a person is carrying an altered gene which
may give rise to a genetic disorder
To assess the risks of a genetic disorder being passed on to offspring
b. Foetal screening – amniotic fluid around the baby is tested for the
presence of genetic disorder
Ethical issues
a. If an embryo is tested and shown to have a genetic disorder it might encourage abortion
b. Insurance companies and employers might discriminate against people who carry altered
genes or have genetic disorders
c. Should a person be told that they have a genetic disorder which would lead to death later in
life?
RNA (Ribonucleic Acid)
RNA is a nucleic acid. It is mostly found in the cytoplasm in the form of ribosomes (rRNA).
Messenger RNA (mRNA) is made in the nucleus using the code in DNA and it travels into the
cytoplasm where it is involved in making protein.
Structure
1. RNA is single stranded (half a ladder)
2. It has the sugar ribose
3. It has the bases guanine (G), cytosine (C), adenine (A) and
uracil (U). It does not have thymine (T)
4. It is a polynucleotide (nucleotide polymer)
RNA is a complementary structure to DNA because it is made
using the code in DNA. If DNA has base sequence TAGCAT then
the complementary RNA sequence will be AUCGUA.
Differences between DNA and RNA
DNA
RNA
Double stranded
Single stranded
Deoxyribose sugar
Ribose sugar
Thymine
Uracil
Self replicates
Cannot self replicate
Only in nucleus
In cytoplasm, nucleolus and nucleus
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.5 GENETICS
2.5.5 Protein Synthesis
Protein synthesis happens as follows:
• DNA contains the code for making protein
It uses a sequence of three consecutive
bases.
Each group of three bases is called a triplet
or codon.
• This code is transcribed to
mRNA
Messenger RNA (mRNA) is
made in the nucleus using
the code in DNA. If DNA
has the base sequence
TAGCATGAG then the
complementary mRNA will
have the sequence
AUCGUACUC. (The code is
transcribed).
• The transcribed code goes
to a ribosome
The mRNA leaves the DNA
and moves out of the nucleus
into the cytoplasm to a ribosome
• The code is translated and the
amino acids are assembled in the
correct sequence to synthesise a protein
As the mRNA passes through the ribosome
each triplet of bases causes the correct
sequence of amino acids to link together to
make a protein. (The code is translated).
• The protein folds into its functional shape.
The protein is folded as it leaves the ribosome so it can carry out its function.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.5 GENETICS
H.2.5.15 PROTEIN SYNTHESIS (EXTENDED STUDY)
1. Transcription
Enzymes unwind
the DNA double
helix at the site of
a gene.
The DNA code is
transcribed on to
a complementary
mRNA strand.
The nucleotides
are assembled by
the enzyme RNA
polymerase
Nucleus
2.
The mRNA
moves out of
the nucleus and
into the
cytoplasm. It
then moves into
a ribosome.
Codon
3. Translation
Free-floating tRNAs with
attached amino acids, in
the cytoplasm, are
attracted by their
‘binding site’ to the
complementary mRNA
attached to the
ribosome.
Amino acids are aligned
in a sequence
determined by the
codons of the mRNA.
The amino acids bond to
form a new protein
molecule
The protein is released
when the mRNA code
sequence is complete
Anticodon
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.5 GENETICS
Location of protein synthesis
Protein synthesis takes place on the surface of ribosomes.
Process of protein synthesis
Protein synthesis involves 3 stages (a) Transcription (b) Translation (c) Protein Folding
1. Transcription
Transcription is the making of mRNA using a DNA template
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Enzymes unwind the DNA double helix.
RNA nucleotides bond with one strand of the exposed DNA. The enzyme RNA
polymerase assembles these nucleotides to form mRNA. The mRNA, therefore, has a
series of bases that are complementary to those in DNA.
The mRNA moves into the cytoplasm. Each three base sequence of mRNA carries a
genetic code or codon that specifies a starting codon, a particular amino acid or a
stop codon.
The mRNA attaches to ribosome subunits (rRNAs). These subunits form the
ribosome which is the site of protein synthesis.
2. Translation
Translation is the making of protein using the code in mRNA.
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Free-floating tRNAs with their attached amino acids, in the cytoplasm, are attracted
by their ‘binding site’ or anti-codon to the complementary mRNA which is already
attached to the ribosome. This ensures the amino acids are aligned in a sequence
determined by the codons of the mRNA.
The aligned amino acids bond to form the peptide links of the new protein molecule.
The tRNAs continue to move to the ribosome, until a stop codon on the mRNA is
reached. The protein is released when the mRNA code sequence is complete and the
protein is synthesised.
3. Protein Folding
Protein folding is the folding of protein into its functional shape.
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The protein is folded into its characteristic shape as it leaves the ribosome so it can
carry out its function.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.5 GENETICS
2.5.8 Evolution
Definition of "evolution"
Evolution is the way living things change genetically to produce new forms of life over long
periods of time in response to changes in the environment
Evolution is a gradual change in the characteristics of a species. It takes a long time and is
related to changes in the environment. Eventually a new species is formed.
Broad outline of Darwin and Wallace Theory of Natural Selection.
Darwin suggested that evolution happens as follows:
Observation 1
a. When organisms reproduce there are more offspring than parents.
Observation 2
b. However, population numbers stay the same.
Conclusion 1
c. This means that there must be competition for resources and a struggle to survive.
Observation 3
d. The members of a population show genetic or inherited variations (differences).
(These variations come from sexual reproduction and mutations).
Conclusion 2
e. The organisms that have variations which allow them to adapt better to their
environment will survive and reproduce. They will pass their genes to the next
generation. Organisms with unfavourable variations will not survive and will not
pass their genes on to the next generation.
f. GRADUALLY the population changes and becomes BETTER ADAPTED to the
ENVIRONMENT.
g. Over a long time, so many changes happen that a NEW SPECIES is formed. This
new species cannot breed with the original species.
This theory of Natural Selection is called “survival of the fittest” and is basically about how
well adapted organisms survive to reproduce and pass on their genes to the next generation.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.5 GENETICS
Evidence from any one source: fossil studies of any one anatomical characteristic
Palaeontology is the study of fossils.
Fossils are the dead remains of plants and animals that lived long ago and are preserved in
rocks, peat or resin.
Fossils are evidence for evolution because they can be placed in a series that show a gradual
change over a long time and they can also show a change in response to a change in the
environment.
When the fossilised bones and teeth of horses are arranged in series we get a picture of the
evolution of the horse. These fossils show changes in anatomical (body) characteristics like,
increase in size, reduction in the number of toes, and the development of molar teeth (back
teeth). These anatomical changes can also be related to changes in the environment
The fossil records of horses are very well documented and really support the theory of
evolution because
1.
2.
3.
4.
They cover a long time span of 60 million years
They shows changes in the height of the horse
They can be related to changes in the environment
They involve many species most of which are now extinct
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.5 GENETICS
2.5.9 Genetic Engineering
Genetic engineering is the artificial manipulation and alteration of genes
The process of genetic engineering involves some or all of the following:
Isolation
cutting (restriction)
transformation (ligation)
introduction of base sequence changes
expression.
Isolation
This involves removing the
chromosome containing the
target gene from a human
cell and removing a plasmid
from a bacterial cell.
Cutting (Restriction)
This involves cutting the
chromosome and the
plasmid with the same
restriction enzyme. This
means the target gene will
easily fit into the plasmid as
the cut ends will be
complementary.
Transformation (ligation)
This is joining the exposed
cut ends of the human DNA
(target gene) and the
plasmid DNA. They are
joined using the enzyme
DNA ligase.
Introduction of base sequence changes
This involves inserting the plasmid with the new DNA into a new bacterial host and
replicating the host (cloning the cells).
Expression
This is the process where the bacterium with the new gene makes the required protein
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork
2.5 GENETICS
Three applications of genetic engineering: one plant, one animal, one microorganism.
Plant Application
Some crops have had bacterial genes added to them. This
makes the plants resistant to weedkiller (herbicides). This
means that when weedkiller is sprayed on the crop the weeds
will be killed but the crop is not affected
Animal Application
A human gene has been put into the DNA of sheep. This means that
the sheep will produce a clotting chemical in their milk. This clotting
factor is needed by haemophiliacs to clot their blood.
Microoganism Application
A human gene for making insulin has been inserted into the DNA of a
bacterial cell. This means that the bacterial cell will make insulin. This is a
protein needed by diabetics to regulate their sugar levels.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co.Cork