University of Johannesburg Compiled by Janice Williamson

1. UNIT 2:
GENETICS –
NUCLEIC ACID
RNA
CAMPBELL & REECE,
2010. CHAPTERS 17,

2. STRUCTURE OF AN
RNA MOLECULE
• RNA is made up of:
• Ribose sugar (a pentose sugar with 5
carbons),
• Phosphate and
• A nitrogenous base; e.g. Purines (Adenine
and Guanine) and Pyrimidines (Cytosine
and Uracil).
• RNA exists largely as single nucleotide chains
in living cells.

3. STRUCTURE OF AN
RNA MOLECULE
• The RNA strand is made up of alternating
molecules of ribose sugar and phosphate.
• The nitrogen bases are attached to the sugar
molecules in the strand and ’stick out’
laterally as in DNA.
• A sugar, a nitrogenous base and a phosphate
together form a ribonucleotide.
• An RNA molecule is a polymer of
ribonucleotides.

4. STRUCTURE OF AN
RNA MOLECULE
• Although RNA exists generally as single
polynucleotide chain, some segments of RNA
molecules may pair temporarily in double-
helical form or may fold back on themselves
to set up extensive double-helical regions.
• These fold-back double helices and their
arrangement are important to RNA functions.

5. BASIC STRUCTURE OF AN RNA
MOLECULE

6. RNA STRUCTURAL ELEMENTS

7. DIFFERENCES BETWEEN DNA & RNA
Characters DNA RNA
1
Molecule Double
stranded, helical
Single stranded, straight or
variously folded and twisted.
2
Pentose sugar Deoxyribose Ribose
3
Pyrimidine
base
Thymine Uracil
4
Complementa
ry base pairing
Always present
and exists
between A = T
and G = C
Normally absent, but may be
present in twisted segments of
a molecule.
If present, pairing is between A
= U and G = C
5
Ratio of
Purines:
Pyrimidines
Always 1:1 Not necessarily 1:1

8. TYPES AND
LOCATION OF RNA
1. mRNA (MESSENGER RNA) –
2. tRNA (TRANSFER RNA)
3. rRNA (RIBOSOMAL RNA)
4. cRNA (catalyticRNAs)
5. snRNA (Small Nuclear RNA)
6. snoRNA (Small Nucleolar RNA)
The blue RNA’s are the most important

9. mRNA
• Single strand RNA nucleotides.
• A polynucleotide strand synthesized according
to the code of the DNA.
• It carries the code in base triplet (codon) form,
from the DNA to the ribosomes.
• Found in the nucleus of the cell.

10. tRNA
• A single RNA strand folded in
the shape of a clover leaf.
• It carries a specific amino acid
on one end and transfers it to
the ribosomes.
• Has an anticodon on the other
end; the anticodon base-pairs
with a complementary codon
on mRNA.
• Found in the cytoplasm of the
cell.

11. tRNA

12. rRNA
• The two ribosomal subunits (large and small)
are made of proteins and ribosomal RNA
(rRNA)
• The single-stranded molecule of rRNA is
variously folded and twisted upon itself in
certain regions forming a secondary structure.
• Constitutes 50% of a ribosome.
• Help to bond mRNA to protein of the
ribosome.
• Found in the ribosomes in the cytoplasm of
the cell.

13. rRNA

14. PROTEIN SYNTHESIS
• PROTEIN SYNTHESIS CONSIST OF
2 DISTINCT STAGES:
• TRANSCRIPTION
• TRANSLATION

15. TRANSCRIPTION
• The 3 stages of transcription:
(1) Initiation
(2) Elongation
(3) Termination

16. TRANSCRIPTION: INITIATION
1. RNA-polymerase attaches to the
beginning of the DNA code called the
promotor
2. It unwinds the DNA molecule and
breaks the weak hydrogen bonds
between the complementary strands –
a “bubble forms”
3. The one strand now acts as a template
for the formation of the mRNA strand.

17. TRANSCRIPTION: INITIATION

18. TRANSCRIPTION : ELONGATION
4. Free nucleotides in the nucleus bonds
to the complementary bases of the
DNA template strand. (Uracil replaces
Thymine in mRNA)
5. More free nucleotides bond to their
complementary bases, to elongate the
mRNA strand, until the entire code has
been transcribed.
The DNA parts already transcribed rewound.

19. TRANSCRIPTION : ELONGATION

20. TRANSCRIPTION : TERMINATION
6. The mRNA will detach from the DNA
template.
7. The RNA polymerase detaches and
starts all over again at a different
location where needed.
8. Now the pre-mRNA strand has to
undergo a modification and RNA
splicing before it can leave the nucleus.

21. TRANSCRIPTION : TERMINATION

23. mRNA modification and splicing
Each end of a pre-mRNA molecule is
modified in a particular way:
 5’ end receives a modified nucleotide
5’ cap
 3’ end gets a poly-A tail

24. mRNA modification and splicing
RNA splicing removes introns (non-coding
RNA) & joins exons (coding RNA), creating
an mRNA molecule with a continuous
coding sequence.
RNA splicing is carried out by spliceosomes.

25. Why are these modifications
necessary?
–Seems to facilitate the export of mRNA
–Protect mRNA from hydrolytic enzymes
–Help ribosomes attach to 5 end

26. TRANSLATION
oA cell translates an mRNA message into protein
oWHAT IS NEEDED TO DO THIS?
 mRNA (Carries the code)
 tRNA (pick up amino acid and
takes it to the mRNA
 Amino acid (connect to form
protein)
 Ribosome (Location for protein
synthesis)

27. THE 3 STAGES OF TRANSLATION
Initiation
Elongation
Termination

28. TRANSLATION: INITIATION
1. Small ribosomal subunit binds with mRNA
2. Small subunit moves along mRNA until it
reach the start codon (AUG)
3. The matching anti-codon of the tRNA
(with amino acid Methionine)bonds with
the start codon.
3. Add the large subunit which completes the
translation initiation complex.

29. TRANSLATION: INITIATION

30. TRANSLATION: ELONGATION
1. More tRNA anticodons attach to mRNA
codons.
2. The amino acids attached to the tRNA’s
attach to one another by means of
peptide bonds.
3. Amino acids form a long polypeptide
chain.
4. tRNA releases amino acid to pick up
more amino acids

31. TRANSLATION: ELONGATION

32. TRANSLATION: TERMINATION
Termination occurs when a stop codon in
the mRNA reaches the A site of the
ribosome
The A site accepts a protein called a
release factor.
The release factor causes the addition of a
water molecule instead of an amino acid.
This reaction releases the polypeptide, &
the translation assembly then comes apart

33. TRANSLATION: TERMINATION

34. TRANSLATION: MODIFICATIONS
Often translation is not sufficient to make
a functional protein
Polypeptide chains are modified after
translation
Completed proteins are targeted to
specific sites in the cell

35. CRACKING THE GENETIC CODE
 64 codons have been deciphered.
Of the 64 triplets, 61 code for amino
acids; 3 triplets are “stop” signals to end
translation.
No codon specifies more than one amino
acid
Genes can be transcribed and translated
after being transplanted from one species to
another.

36. CRACKING THE GENETIC CODE
(codon)