GNE2601 SUMMARY | Fransché Beukes
Learning Unit 3: The structure and function of DNA and RNA
THE MOLECULAR STRUCTURE OF DNA AND RNA
NUCLEIC ACID STRUCTURE
Four levels of complexity in nucleic acids:
1. form the structural unit of nucleic acids.
2. in a linear manner to form a strand of DNA or RNA
3. Two strands interact to form a .
4. The double helix folds and bends to form a 3 .
Nucleotides are building blocks of nucleic acids:
Nucleotide has 3 components
o 1–3 groups
o Pentose
DNA – deoxyribose sugar
RNA – ribose sugar
o Nitrogenous
Purines (Adenine and Guanine) – 2-ringed structure
Pyrimidines (Cytosine, Uracil and Thymine) – 1-ring structure.
RNA: Uracil replace Thymine
Nucleotides are linked together to form a strand by phosphodiester bond
Phosphodiester bond links to – 3’ carbon of one nucleotide with the 5’ phosphate of the next.
Alternating sugars and phosphates make up the back bone of the molecules.
When a sequence of bases are written down it is important to write down 5’ or 3’ as well. Eg. 5’-
TACG-3’. The sequence of bases in a DNA strand remains the same over time, except in rare
cases when mutations occur.
When going from top to bottom
then it is in 5’ to 3’ direction and
vice versa.
Watson and Crick:
o Two strands are joined
together to form a right-
handed double helix.
o Bases in opposite
strands hydrogen-bond
according to the AT/GC
rule.
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, GNE2601 SUMMARY | Fransché Beukes
o Adenine binds with Thymine using two H-bonds;
o Guanine binds with Cytosine using three H-bonds.
o Two strands are antiparallel
Three forms of double helix DNA’s:
o B DNA – predominant form of DNA in living cells
o A DNA
o Z DNA
Nucleosides: A base and sugar complex. Adenine – adenosine, guanine – guanosine, thymine,
thymidine, cytosine – cytidine and uracil – uridine.
Chargaff’s rule: The idea that the amount of A in DNA equals the amount of T,
and the amount of G equals C.
DNA REPLICATION
Replication occurs using the semiconservative method – the old (parental) strands serve as
templates for the synthesis of two new molecules.
Two daughter molecules that are produced consist of one parental strand and one new strand.
Eukaryotes use RNA polymerase I, II, and III to
Many bacterial promoters have sequence
transcribe different cateogries of genes.
elements at the -35 and -10 regions. The
Prokaryotic and eukaryotic RNA polymerases
transcriptional start site is at +1.
have similar structure.
During the initiation phase of transcription in E.
coli, sigma (σ) factor, which is bound to RNA
polymerase, binds into the major groove of DNA Eukaryotic promoters consist of a core promoter
and recognizes sequence elements at the and regulatory elements such as enhancers and
promoter. This process forms a closed complex. silencers.
Following the formation of an open complex, σ
factor is released.
During the elongation phase of transcription, RNA Transcription of structural gens in eukaryotes
polymerase slides along the DNA and maintains requires RNA polymerase II, five general
an open complex as it goes. RNA is made in the transcription factors, and mediator. The five
5’ to 3’ direction according to complementary general transcription factors and RNA polymerase
base pairing. assemble together to form an open complex.
Transcriptional termination of RNA polymerase II
In a given chromosome, the use of the template
occurs while the 3’ end of the transcript is being
strand varies from gene to gene.
processed.
Transcriptional termination in E. coli occurs by a
rho (ρ)-dependent or ρ-independent mechanism.
8|Page
Learning Unit 3: The structure and function of DNA and RNA
THE MOLECULAR STRUCTURE OF DNA AND RNA
NUCLEIC ACID STRUCTURE
Four levels of complexity in nucleic acids:
1. form the structural unit of nucleic acids.
2. in a linear manner to form a strand of DNA or RNA
3. Two strands interact to form a .
4. The double helix folds and bends to form a 3 .
Nucleotides are building blocks of nucleic acids:
Nucleotide has 3 components
o 1–3 groups
o Pentose
DNA – deoxyribose sugar
RNA – ribose sugar
o Nitrogenous
Purines (Adenine and Guanine) – 2-ringed structure
Pyrimidines (Cytosine, Uracil and Thymine) – 1-ring structure.
RNA: Uracil replace Thymine
Nucleotides are linked together to form a strand by phosphodiester bond
Phosphodiester bond links to – 3’ carbon of one nucleotide with the 5’ phosphate of the next.
Alternating sugars and phosphates make up the back bone of the molecules.
When a sequence of bases are written down it is important to write down 5’ or 3’ as well. Eg. 5’-
TACG-3’. The sequence of bases in a DNA strand remains the same over time, except in rare
cases when mutations occur.
When going from top to bottom
then it is in 5’ to 3’ direction and
vice versa.
Watson and Crick:
o Two strands are joined
together to form a right-
handed double helix.
o Bases in opposite
strands hydrogen-bond
according to the AT/GC
rule.
7|Page
, GNE2601 SUMMARY | Fransché Beukes
o Adenine binds with Thymine using two H-bonds;
o Guanine binds with Cytosine using three H-bonds.
o Two strands are antiparallel
Three forms of double helix DNA’s:
o B DNA – predominant form of DNA in living cells
o A DNA
o Z DNA
Nucleosides: A base and sugar complex. Adenine – adenosine, guanine – guanosine, thymine,
thymidine, cytosine – cytidine and uracil – uridine.
Chargaff’s rule: The idea that the amount of A in DNA equals the amount of T,
and the amount of G equals C.
DNA REPLICATION
Replication occurs using the semiconservative method – the old (parental) strands serve as
templates for the synthesis of two new molecules.
Two daughter molecules that are produced consist of one parental strand and one new strand.
Eukaryotes use RNA polymerase I, II, and III to
Many bacterial promoters have sequence
transcribe different cateogries of genes.
elements at the -35 and -10 regions. The
Prokaryotic and eukaryotic RNA polymerases
transcriptional start site is at +1.
have similar structure.
During the initiation phase of transcription in E.
coli, sigma (σ) factor, which is bound to RNA
polymerase, binds into the major groove of DNA Eukaryotic promoters consist of a core promoter
and recognizes sequence elements at the and regulatory elements such as enhancers and
promoter. This process forms a closed complex. silencers.
Following the formation of an open complex, σ
factor is released.
During the elongation phase of transcription, RNA Transcription of structural gens in eukaryotes
polymerase slides along the DNA and maintains requires RNA polymerase II, five general
an open complex as it goes. RNA is made in the transcription factors, and mediator. The five
5’ to 3’ direction according to complementary general transcription factors and RNA polymerase
base pairing. assemble together to form an open complex.
Transcriptional termination of RNA polymerase II
In a given chromosome, the use of the template
occurs while the 3’ end of the transcript is being
strand varies from gene to gene.
processed.
Transcriptional termination in E. coli occurs by a
rho (ρ)-dependent or ρ-independent mechanism.
8|Page
