2. 3.5.1 Compare the structure of RNA and DNA.
Limit this to the names of sugars, bases and the number of
strands.
3.5.2 Outline DNA transcription in terms of the formation of an
RNA strand complementary to the DNA strand by RNA
polymerase
TRANSCRIPTION
3. 3.5.3 Describe the genetic code in terms of codons composed of
triplets of bases.
3.5.4 Explain the process of translation, leading to polypeptide
formation. (Include the roles of messenger RNA (mRNA), transfer
RNA (tRNA), codons, anticodons, ribosomes and amino acids).
3.5.5 Discuss the relationship between one gene and one
polypeptide. (Originally, it was assumed that one gene would
invariably code for one polypeptide, but many exceptions have
been discovered).
TRANSLATION
4. DNA & RNA
There are TWO types of Nucleic acids:
– DNA
– RNA
Both DNA and RNA consist of chains of nucleotides, each
composed of a sugar, a base and a phosphate.
However, there are THREE differences between DNA and RNA:
Feature DNA RNA
No. of Strands Two Strands One Strand
Sugar Deoxyribose Ribose
Bases A-T, C-G A-U, C-G U = Uracil
5. The Genetic Code
The functional unit of information on the chromosome is the gene
A gene consists of a unique sequence of bases that code for a
polypeptide or an RNA molecule.
For the DNA molecule to influence the activities of a cell, it has to
first be translated into proteins.
One gene codes for a polypeptide or an RNA molecule.
So how can 4 bases be translated into a sequence of amino
acids when there are 20 possible amino acids to code for?
6. The Triplet Code
If each base coded for one amino acid only 4 amino acids could be
sequenced.
If we had pairs of bases coding for amino acids, how many
combinations would then be possible?
– 16 still not enough
The answer is to use 3 bases to code for each amino acid.
– With three bases 64 combinations are possible.
– More than enough for the 20 amino acids and start and stop signals.
This is called the Triplet Code.
7. The Triplet Code
Experiments have verified that the genetic code is made up of
base triplets.
These base triplets are called codons (on mRNA).
Each triplet codes for an amino acid.
Out of the 64 possible codons:
– 61 code for amino acids.
– 3 are stop codons (they do not code for an amino acid).
The genetic code is degenerate. This means that it is possible
for two or more codons to code for the same amino acid.
The genetic code is universal. This means that all living
organisms use the same code. Viruses also use this code.
9. Protein Synthesis
The central dogma of molecular genetics is that the flow of
information for organisms is unidirectional.
– It only flows in one direction (some viruses can reverse this).
DNA RNA Protein
There are two main processes involved in Protein Synthesis:
– Transcription.
• Where a molecule of mRNA is formed from the DNA template strand.
– Translation
• Where the mRNA sequence is converted into a polypeptide chain.
10. Transcription
DNA RNA.
Basically it is a process in which the message written in DNA
code is transcribed into a working copy of mRNA (messenger
RNA).
The process is:
– The RNA polymerase enzymes separate the two strands of DNA.
– One strand of the DNA is used as a template for mRNA synthesis.
– RNA polymerase forms the mRNA molecule using Uracil instead of
Thymine.
– When the mRNA molecule is complete, it breaks away from the DNA and
travels through the nuclear pores into the cytoplasm.
– The DNA strands rejoin.
12. Transcription
In Transcription, complementary base pairing occurs as in DNA
replication, except that uracil replaces thymine:
– ie. Cytosine – guanine
– Adenine – uracil
DNA base sequence: T A G G C T T G A T C G
mRNA base sequence: A U C C G A A C U A G C
13. Translation
RNA Protein
Basically it is a process in which a polypeptide chain is built from
a codon sequence on the mRNA molecule.
The process is:
– The mRNA molecule attaches to a Ribosome.
– tRNA molecules bring specific amino acids to the ribosome according to
the codon on the mRNA.
– There is a different tRNA molecule for each of the 61 codons.
– Each tRNA molecule is about 80 nucleotides long and is folded into a
clover shape.
– At one end there is an exposed triplet of bases called an anticodon and
at the other a specific amino acid.
– The anticodon on the tRNA matches the codon on the mRNA.
14. Translation
The ribosomes provide the platform where the tRNA-amino acid and mRNA are
brought together.
As the amino acids are bought alongside one another they are joined together by
enzymes to form a polypeptide.
Translation begins with a signal code, the start codon AUG.
– AUG codes for the amino acid Methionine.
The ribosome moves along the mRNA strand one codon at a time.
As the tRNA molecule deposits its amino acid, it is released back to the cytoplasm
to link with another amino acid.
When a stop codon is reached, translation ceases and the polypeptide chain is
completed and released from the ribosome.
The polypeptide folds into its final protein shape.
If the protein is more than one polypeptide, the chains link and form their tertiary structure.
16. One Gene - One Polypeptide
1.DNA codes for a specific sequence of polypeptides.
2. The DNA code for one polypeptide is a gene.
3. DNA is transcribed into mRNA.
4. mRNA is translated into a polypeptide.
5. Originally it was thought that one gene always codes for one polypeptide.
6. Some genes do not code for a polypeptide.
7. Some genes code for tRNA and rRNA.
8. Some sections of DNA code for regulators which are not polypeptides.
9. Antibody production does not follow the transcription and translation pattern.
10. Change in gene due to mutation will affect the primary structure of the
polypeptide.
17. 3.5.1 Compare the structure of RNA and DNA.
Limit this to the names of sugars, bases and the number of
strands.
3.5.2 Outline DNA transcription in terms of the formation of an
RNA strand complementary to the DNA strand by RNA
polymerase
TRANSCRIPTION, IBO guide:
18. 3.5.3 Describe the genetic code in terms of codons composed of
triplets of bases.
3.5.4 Explain the process of translation, leading to polypeptide
formation. (Include the roles of messenger RNA (mRNA), transfer
RNA (tRNA), codons, anticodons, ribosomes and amino acids).
3.5.5 Discuss the relationship between one gene and one
polypeptide. (Originally, it was assumed that one gene would
invariably code for one polypeptide, but many exceptions have
been discovered).
TRANSLATION, IBO guide:
