HEREDITY (ordinary level)
Gene
Expression DNA Chromosomes Non-coding Genes The Structure of DNA
The Double
Helix The Genetic Code DNA Replication DNA Profiling
Applications
of DNA Profiling Genetic
Screening RNA Protein Synthesis
Heredity is the passing
on of features from parents to offspring by means of genes. Organisms inherit
genes from their parents.
GENES
Genes are carried on DNA. A gene is a section of DNA that causes
the production of protein which is the production of the animal or plant part.
(Living material is built up by proteins.)
Many of the genes produced are enzymes.
Enzymes were discussed in the enzymes web page which can be found on the homepage.
Gene expression is the process by which inheritable
information from a gene is made into protein or RNA.
In plain language gene
expression is how a gene works. Gene expression functions in the environment
of the living organism. If the environmental
factors are not correct then the
characteristic produced by the gene may
not be expressed. The way an organism physically looks is called its phenotype. The phenotype is determined
by genes but is influenced by toe organisms’ environment.
DNA, GENES, AND CHROMOSOMES
DNA (deoxyribose nucleic acid)
molecules are large and complex. They carry the genetic code that determines
the characteristics of a living thing.
Except
for identical twins, each person’s DNA is unique. This is why people can be
identified using DNA fingerprinting. DNA can be cut up and separated, forming a
sort of “bar code” that is different from one person to the next.
Genes
A gene is a short section of DNA.
Each one codes for a specific protein by specifying the order in which amino
acids must be joined together.
Chromosomes
The cell’s nucleus contains chromosomes. Chromosomes are made up of protein and DNA. The protein holds the DNA in a tightly packed shape
place in the nucleus. The DNA is
very long and would not fit into the nucleus if this didn’t occur. Genes are located on the DNA. They are
placed in line. Some genes are close together and others are far apart from
other genes. There are many parts of chromosome that contain no genes. In fact,
about 97% of a chromosome of a human has no genes. These parts of the
chromosomes between the 2 genes are called non-coding
protein sequences (sometimes called “junk DNA”).
The diagram shows the relationship between the
cell, its nucleus, chromosomes in the nucleus and genes.
The regions with no genes are the non-coding
sequences
PERCENTAGE OF NON-CODING SEQUENCES IN VARIOUS SPECIES
DNA is composed of 4 bases. These
bases go together in pairs. The 4
bases are known by the first letter of their name:
A = Adenine
T
= Thymine
G = Guanine
C = Cytosine
Each base can only join with one other
base.
A
can only join with T
G can only join
with C
The way
that one type of base links to another type of base is known as complementary
base pairing.
It is the precise
number and arrangement of these base pairs along the DNA that forms the
organism's genetic code.
As can be seen above, each pair of bases are held on the
side strands (some times called the “backbones”) of the DNA. These side strands are made up of a pentose sugar (a 5-carbon sugar) and a phosphate
group, The entire unit is then made of three molecules: a pentose sugar (a 5-carbon sugar) and a phosphate
group a nitrogenous base This smallest unit of DNA is called a nucleotide.
DOUBLE
DNA IS FOUND IN
CELLS IN A TWISTED FORM CALLED A DOUBLE HELIX.
Proteins are made up of amino acids. Amino acids
are made up of a set of 3 nucleotides called
triplets or codons. There are 20 amino acids that are used to form a variety of
proteins. There are many combinations of amino acids that make up the proteins
that are an organism’s body.
1 amino acid Here we
see the formation of 3 amino acids
As we
learned in the mitosis web page when
a cell divides the DNA must produce an
exact copy of itself. This replication takes place during interphase of mitosis. The process can
be listed as follows:
1.
Nucleotides
are made in huge quantity in the cytoplasm.
2.
An
enzyme unzips the two complementary strands of DNA.
3.
New
complementary nucleotides link to
the exposed bases on the separated strands.
REMEMBER
A
can only join with T
G can only join
with C
4.
A
new complementary strand is built along each ‘old’ strand.
5.
Two
DNAs, identical to the original and each other, are now present.
As a result of DNA replication the 2 cells formed
by mitosis are exact duplicates.
DNA
profiling is popularly
known as DNA fingerprinting. It produces a unique pattern from an individual’s
DNA which can then be used to distinguish that individual form another.
Extremely variable regions of non-coding DNA are used. Genetically different
individuals produce different profiles. The closer the
genetic relationship between individuals the more similar their profiles.
METHOD OF DNA PROFILING
1.
The DNA is released from a cell: The cells of an
uncontaminated biological sample (blood, semen, hair root, cheek cell) are
broken open and the DNA is released and separated. If the amount of DNA is
small the DNA can be amplified (increased) by a process called polymerase chain reaction (PCA).
2.
Digestion: Special enzymes are used to cut the DNA at specific points and produce a
set of fragments of varying lengths. These enzymes are called restriction enzymes. The DNA sections
that are cut are called restriction
fragments. Certain enzymes will cut the DNA at specific bases (T, G, A, and C). Each restrictive
fragment will be specific for that particular person. The bases as well as the
distance between the bases (non-coding
DNA) are characteristics of that individual organism (person).
3.
Separation: The fragment mixture is placed in a
block of gel and separated by gel
electrophoresis on the basis of size. The fragments are exposed to an
electrical charge. This causes the fragments to move. The smaller the fragment
the further it travels. The bands of small fragments are separated from the
bands of the large fragments. After other treatments, a photographic copy of
the pattern of DNA is compiled. Using certain methods the positions of the DNA
fragments on X-ray film appear as dark bands. The positions of these bands are
the genetic profile of the individual and the more numerous the bands the more
reliable the ‘fingerprint’. The bands are produced and vary in thickness
depending on how many DNA fragments are present in a certain length of DNA. Remember that there are many parts of
the chromosomes that don’t contain coding DNA.
4.
Pattern comparison: The DNA fingerprint can be used to
be compared to other DNA fingerprints. The pattern is unique to an individual.
In this way comparisons of known DNA to unknown DNA can be made to determine if
the unknown DNA can be identified.
- Crime: Forensic medicine uses medical
evidence in legal disputes. DNA profiling is used in forensic
medicine. Material such as hair, semen, blood, and saliva which is left at
a crime scene can be compared with possible suspects to find out if the
suspect was at the crime scene.
- Medical: DNA profiles can be used to determine
the parents of a child.
Genetic Screening is used to find
out if the parents of a child or the child itself (through removal of cells
from the foetus) carry defective genes that could develop into health problems.
Although the parent may not have the disorder he/she could be a carrier for the condition. With genetic
screening, prospective parents can determine if there is a possibility that their future children could have the health
problem.
RNA, like DNA, is
made up of 2 of 4 bases. The bases of RNA are as follows:
As
you can see, Uracil takes the place
of Thymine in RNA. So, the bases in
RNA are:
A = Adenine
U
= Uracil
G = Guanine
C = Cytosine
A
= Adenine is
complementary (combines with) U = Uracil
G
= Guanine is complementary
with C = Cytosine
Unlike DNA, RNA is single
stranded. The RNA produced is complementary to the DNA which produced it.
Also, RNA can move out into the cytoplasm while DNA remains in the nucleus.
DNA
RNA
G C
G C
A U
(There is no T in RNA so U is the complement of A)
A U
T A
C G
As stated previously, genes are responsible for the formation of
protein. The protein could be new
body cells or enzymes.
The base sequence of the
DNA determines the properties of the new protein produced.
Each 3 segment group (triplet/codon) produces an amino acid.
The amino acids form protein.
STAGES OF
PROTEIN SYNTHESIS
1. Initiation: Just like in DNA
Replication the DNA strands separate into 2 chains. (occurs
in the nucleus)
2. Transcription:
RNA complementary bases attach to the bases on one side of the strand.
When this happens the code has been transcribed.
This RNA strand is called messenger RNA
(mRNA).
3. The mRNA detaches from the DNA strand and moves
out of the nucleus and into the cytoplasm.
4. The mRNA enters
and passes through a ribosome. The amino
acids (codons/triplets) form proteins. We say that the mRNA has been
translated at the ribosome to form protein.
Each
codon/triplet forms a particular amino acid. As seen above, CUU forms AA6 and
GUA forms AA12.
Many
of these amino acids (up to 20 depending on the protein formed) join to form
the new protein.
Click here for an
interactive animation of DNA and Protein Synthesis