Chapter 14: DNA and
RNA
Leaving Certificate Biology
Higher Level
Heredity and Gene
Expression
• Heredity is the passing on of
characteristics/traits from one generation
to the next
• A gene is a short region of a chromosome
that contains a code for the production of a
protein
• Gene expression is the process by which
the code in DNA is used to make a protein
Genetic Code
• The code for a particular protein can be
thousands of bases long
• Only approx 3% of DNA is thought to
actually code for proteins (coding DNA)
• The rest (97%) is called non-coding DNA
– does not code for any proteins
Chromosome Structure
• The genes are contained within a much longer piece of DNA
called a chromosome
• The genes are spread out along the length of the
chromosome
• There is coding DNA (DNA that codes for a specific protein)
and non-coding DNA (DNA whose function is generally
unknown)
• The non-coding regions of the chromosomes used to be
called ‘junk DNA’
Gene (coding
DNA)
Non-coding
DNA
Chromosome Structure
• Chromosomes are composed of 40%
DNA: 60% protein
• The protein (histones) makes the DNA
very stable and enable it to be supercoiled
into a very small space (i.e. the nucleus)
Chromosome Structure
• DNA wraps around
proteins called
histones, which then
supercoil into a
chromosome
• NOTE: chromosome
only exist during
mitosis
• At all other times the
DNA is in the form of
chromatin
DNA Structure
• DNA – deoxyribonucleic acid – is a polymer
• DNA consists of two strands – made up of
alternating sugar (deoxyribose) and phosphate
molecules
• The two strands are attached to each other by
nitrogenous bases
• DNA contains 4 bases:
–
–
–
–
]
]
Adenine (A)
Guanine (G)
Thymine (T)
Cytosine (C)
Purines
Pyrimidines
DNA Structure (continued)
• The strands are twisted on themselves creating
a spiral ladder
• The spiral ladder shape is called a double helix
• The bases attach the two strands together in
pairs (complementary base pairing)
• The bases always attach to the sugar molecules
Complementary base pairing
• Complementary base pairing occurs between
the bases in DNA:
–
–
–
–
–
Adenine can only pair with thymine (A = T)
Cytosine can only pair with guanine (C ≡ G)
A = T: double hydrogen bond
C ≡ G: triple hydrogen bond
Individual hydrogen bonds are very weak but as there
are so many hydrogen bonds they are collectively
very strong – holding the two strands of DNA together
and making DNA very stable
Complementary base pairing
DNA
Nucleotide
• A nucleotide is a 3 molecule unit
composed of a phosphate, sugar
(deoxyribose), and base (A, T, C or G)
• It is the basic unit of the structure of
DNA
DNA replication
• DNA replication occurs towards the end of
interphase
– An enzyme unwinds and unzips the DNA
– Free nucleotides diffuse in from the cytosol
and are placed into their complementary
position by the enzyme DNA polymerase
– Once the DNA has been replicated the DNA
coils and supercoils into chromosomes in
preparation for mitosis/meiosis.
DNA replication
DNA replication
DNA Profiling
• DNA profiling is a method of making a
unique pattern of bands from a sample
of DNA for identification purposes
– Note: for the leaving cert you have to be able
to give two applications of DNA profiling
Applications of DNA Profiling
• Species identification
• Criminology: placing suspect at a crime
scene
• Medical: used often in paternity testing
DNA Profiling Method
1. DNA isolation: extraction/release of DNA
from cells
•
•
DNA is released from cells by using a type
of detergent that splits open cell membranes
Even if the sample to be tested is very small
(such as a hair follicle/blood smear) the
amount of DNA can be increased (by DNA
replication) several million-fold in a few
hours!
DNA Profiling Method (cont.)
2. Cutting: DNA is cut into fragments
•
•
•
‘Restriction enzymes’ cut DNA at specific
base sequences
Products of this process are fragments of
DNA (restriction fragments) that are different
sizes
Everyone’s DNA is different which means
that restriction enzymes will cut everyone’s
DNA in slightly different places
2. Cutting of DNA
DNA Profiling Method (cont.)
DNA sample 1
Restriction
enzymes
Isolated
DNA
DNA sample 2
Restriction
fragments
Restriction
enzymes
Restriction
fragments
DNA Profiling Method (cont.)
3. Separation: restriction fragments have to
be separated
•
•
•
Because everyone has their own unique
DNA they also have their own unique set of
restriction fragments after the cutting stage
The mixture of restriction fragments can be
separated out into a unique pattern of bands
The process of separating out the different
fragments is carried out by gel
electrophoresis
DNA Profiling Method (cont.)
3. Separation (cont.): Gel Electrophoresis
•
•
•
•
Agarose gel is poured into specialised
shallow tray and allowed to set
The mixture of DNA is loaded into ‘wells’ at
the top end (negative end) of the gel and an
electric current is passed through the gel
DNA is a negatively charged molecule and
will be attracted towards the positive end
The large restriction fragments will move
more slowly than the short fragments – this
creates a unique pattern of bands of
fragments
DNA Profiling Method (cont.)
–
Sample 1
–
+
Battery
+
Sample 2
Crime scene
sample
Restriction fragments (of
the same size grouped
together)
Agarose gel
3. Separation by Gel
Electrophoresis
Gel
Electrophoresis
Gel Electrophoresis
Gel
Electrophoresis
DNA Profiling Method (cont.)
4. Pattern analysis
•
•
•
An invisible pattern has been produced by
the gel electrophoresis
To make the pattern visible the whole gel is
stained (e.g. ethidium bromide) and then
viewed under UV light
The patterns produced can then be
compared and analysed for identification
purposes
UV illumination of a gel
RNA
• RNA – ribonucleic acid
• RNA is single stranded
• RNA contains nitrogenous bases:
– Adenine (A)
– Uracil (U)
– Cytosine (C)
– Guanine (G)
• RNA contains the sugar ribose
• Nucleotides in RNA are composed of a
phosphate, sugar (ribose) and a base (A,
U, C, or G)
RNA bases
Experiment: to isolate DNA from onion
• Add finely chopped onion to 3 g salt and 100
ml distilled water
• Heat (60˚C) and stir mixture gently for 15
minutes
• Cool in ice bath for 5 minutes
• Blend mixture for 3 seconds
• Filter blended mixture through coffee filter
paper
• Take 3 ml of filtrate in test tube and add 3
drops of freshly squeezed kiwi fruit juice –
slowly swirl the test tube to mix
• Slowly add 10 ml ice cold ethanol down the
side of the test tube
• DNA becomes visible at the junction
between the filtrate and ethanol