DNA and RNA
Chapter 16
Structure of DNA
• DNA (deoxyribonucleic acid) is a very long
coiled molecule.
• The structure of DNA can be considered
under 3 headings:
– nucleotides
– base pairs
– the double helix
Nucleotides
• A nucleotide consists of a phosphate group,
a sugar and a nitrogen-containing base.
• The sugar used in DNA is called
deoxyribose.
• There are 4 different bases:
– Adenine (A), Thymine (T)
– Guanine (G), Cytosine (C)
• Adenine and Guanine are called purines.
• Thymine and cytosine are called
pyrimidines.
There are four DNA nucleotides:
P-D-A
P-D-G
P-D-T
P-D-C
(P = phosphate, D = deoxybibose)
Base pairs
• Adenine and Thymine both form 2 weak
hydrogen bonds.
• This allows them to join together, they are
complementary base pairs.
• Guanine and Cytosine form 3 hydrogen
bonds and are another complementary base
pair.
• In each base pair there is a purinepyrimidine link.
• DNA resembles a ladder.
The Double Helix
• Scientists Watson and Crick proposed that
DNA was a double helix.
• The complementary strands have the
phosphate on the outside; the deoxyriboses
are next; with the bases on the inside.
DNA replication
• The sequences of bases on one strand of a
double helix determines the sequence of
bases on the complementary strand.
• E.g. one side of DNA has the base sequence
AGGCCTTA, then the other side must be
TCCGGAAT.
DNA Replication
• DNA replication takes
place in the nucleus of
the cell during
interphase.
• It results in the singlestranded chromosome
forming two identical
strands that are held
together at the
centromere.
gene
Chromosome
(single-stranded)
Chromosome
(double-stranded)
DNA replication occurs as follows...
1. The double helix unwinds.
2. Enzymes break the hydrogen bonds.
3. The complementary sands move apart.
4. DNA nucleotides move from the
cytoplasm into the nucleus and attach to
their complementary bases on the
exposed strands.
5. The new strands contain exactly the
same sequence of bases. Each new
stretch of DNA re-forms into a double
helix.
Significance of DNA replication
• DNA is able to produce exact copies of
itself.
• This allows exactly the same DNA to be
passed to each daughter cell during mitosis.
The Genetic Code
• Genes are composed of long sequences of
DNA bases.
• Genes cause a sequence of amino acids to
be assembled to form a protein.
• To allow the correct protein to be assembled
DNA carries a genetic code.
• This code operates as a sequence of three
DNA bases called a triplet or codon.
• It takes many triplets to from a gene.
A
T
C
C
C
G
This triplet codes
for amino acid 2
This triplet codes
for amino acid 14
Coding and non-coding DNA
• About 97% of the DNA in a human nucleus
does not cause the production of proteins.
• This junk DNA has no known function.
• Some of this non-coding DNA is located
between the genes, with much of it at the
centromeres and at the ends of the
chromosomes.
• Much of it is found within the genes.
DNA profiles
• A DNA profile is a unique pattern of DNA
from a person.
• It is compared with the DNA profile of
another person.
• DNA profiling is also called genetic or
DNA fingerprinting.
Preparing DNA profiles.
1. Release DNA from cells.
DNA is released from the cell. The cells can
be obtained from saliva, hair roots or semen.
2. Cut the DNA into fragments.
The DNA is cut into pieces using special enzymes
(called restriction enzymes). These enzymes cut
DNA when they encounter specific base
sequences. The DNA sections obtained in this way
will vary in length from very small sequences of
bases to very long sequences.
3. Separate the fragments.
The DNA fragments are separated according to
their length. This involves placing the fragments
in a gel and pass an electric current through the
gel. Small fragments move faster through the gel
than large ones. A photograph of the final result is
obtained.
Each DNA profile looks like a bar code. No two
people have the same DNA profile.
Uses of DNA profiles
• Crime Investigations
• Medical (Paternity tests)
Genetic Screening
• Genetic screening means testing a person’s
genes for the presence of abnormal or
altered genes.
• This can be very valuable for couples who
know particular genetic disorders run in
their families.
• They can be advised of the probability of
their children having a disorder.
Ethical problems
• If an embryo is tested and shown to have a
disorder, it may encourage the couple to
have an abortion.
• Should a person be told they have a disorder
that will develop later in life and lead to
death?
• Should insurance companies be informed of
genetic screening results?
RNA
RNA (ribonucleic acid) differs from DNA as
follows
DNA
RNA
C, A, T, G (has C, A, U, G (has
urcil)
thymine)
Single-stranded
Double helix
Mainly found in
the nucleus
Only one type
Found in the
nucleus and
cytosol
Three types
mRNA, tRNA,
rRNA
Remember…
CATGUT for the difference in bases
between DNA and RNA, where CATG are
the bases in DNA, and U replaces T in RNA!
•RNA bases are complementary to DNA bases.
•Example, if DNA has the base sequence
TAGGC, the RNA complementary
base sequence will be AUCCG.
Protein Synthesis
• Genes control cells by producing enzymes.
• Enzymes are proteins.
• To make the correct proteins it is important
that amino acids are assembled in the
correct order in ribosomes.
• Genes work by forming the correct proteins.
• This involves the genetic code in DNA
being transcribed to mRNA (messenger
RNA).
• This code must then be translated into the
correct sequence of amino acids.
• Transcription takes place in the nucleus and
translation takes place in the ribosomes.
Remember...
Translation
Transcription
DNA
RNA
Protein
Main steps in protein synthesis...
1.The double helix unwinds at the site of a
gene that is to from a protein.
2.The sequence of bases on the DNA (gene)
is used for a complementary strand of
mRNA. This process is called transcription.
3.The mRNA moves out of the nucleus into
the cytoplasm.
4.Ribosomes are made mainly of rRNA
(ribosomal DNA).
5.The mRNA move into the ribosomes.
6.There are large numbers of tRNA (transfer
RNA) molecules in the cytoplasm. Each
tRNA carries a specific amino acid.
7.Triplets or codons on the mRNA strand
attract complementary triplets (called anticodons) in tRNA molecule.
8.Each tRNA carries its own amino acid.
9.The amino acids are detached from their
tRNA molecule and bonded together to
form new proteins.
10. The protein then folds into the correct 3D
shape to allow it to function properly.