Prof Z Mbita (ZM)
Announcement
Class representatives:
Female Class Rep: Andile Maseko
Male Class Rep: Osofekun Solomon Ilerioluwa
Quiz:
At the end of each section,
First one ~ 19 July 2024.
1
Learning outcomes (LOs)
Two (2) LOs
Understand (Study) the chemistry and structure of DNA and RNA, and to
understand their functional importance in biochemical reactions
Understand the complexity of DNA organisation (genes and genomes),
replication, mutation, recombination, damage and repair
1
Different DNA conformations
2
Different DNA conformations
3
Assignment
Discuss the meaning of the characteristics that differentiate the various
conformations of DNA double helices.
Due date : 31 July 2024
Time : 16H00
Format and instructions: A rubric on blackboard
3
Organisation of DNA in Prokaryotes
Organisation of DNA in eukaryotes
Eukaryotic chromatin
Two forms of chromatin conformations:
Euchromatin:
A lesser coiled transcriptionally active region which can be
easily accessed by the RNA polymerase.
Heterochromatin:
Highly condensed → transcriptionally inactive region.
The genes in this region cannot be accessed by the RNA
polymerases for active transcription.
Posttranslational modification of nucleosomes
Genes XII ~ Chapter 8
Posttranslational modification of nucleosomes
Genes XII ~ Chapter 8
Effect of chromatin structure on gene expression
Histone modifications → either activates/inactivates gene
expression at the transcriptional level
Acetylation (How?) ↓ DNA condensation ↑ Transcription of a gene
Ubiquitination (How?)
Ubiquitination of H2A → Transcriptional inactivation
Ubiquitination of H2B → Transcriptional activation
Methylation of DNA
Target site are the cytidine residues in CpG islands.
↑ methylated Cytidine ↓ transcriptional activity.
Importance of DNA
This chemical substance is present in the nucleus of all
cells in all living organisms.
DNA controls all the chemical changes which take place in
cells.
The kind of cell which is formed (Skin, muscle, blood,
nerve etc.) is dependent on DNA ~ Cell differentiation.
The kind of organism that develops is controlled by its
DNA.
Our eyes, hair, growth, etc.
DNA Function
There are three main functions:
1. Storage of genetic material: generation to the next.
Preservation through complex DNA organisation.
Ability to replicate during cell division.
2. DNA transforms cells.
3. Defines cellular and functional proteins.
DNA is the macromolecule that ultimately controls every
aspect of cellular function, primarily through protein
synthesis ~ Central dogma.
DNA Function
Each strand serves as the template during DNA replication.
Ability to replicate during cell division:
Polymerization of nucleotides
When nucleotides are joined together ~ nucleic acids.
The hydroxyl groups to which esterification occurs are those
bonded at 3’ and 5’carbons,
The linkage of the nucleotides is a 3’, 5’-phosphodiester bond.
The nucleotides residues of nucleic acids are numbered from
the 5’end, which carries the phosphate group, to the 3’end,
which usually has a free hydroxyl group. NB: Replication
Polymerization of nucleotides
DNA or RNA?
Polymerization of nucleotides
DNA or RNA?
DNA Function
2. DNA transforms cells.
DNA Function
3. Defines cellular and functional proteins.
DNA is the macromolecule that ultimately controls every
aspect of cellular function, primarily through protein
synthesis. How?
DNA specifies proteins
Properties of DNA
Watson and Crick ~ 1953,
Through X-Ray diffraction patterns,
A=T and G=C (Chargaff’s rule),
Both evidences = DNA contains two polynucleotide chains
wrapped around each other to form a helix.
Hydrogen bonds between bases on opposite chains ~ align
the helix.
The sugar-phosphate backbone ~ outer part of the helix,
The two chains run antiparallel ~ 3’-5’ and 5’ to 3’.
The Double Helix-Base pairing
Properties of DNA
DNA stability
DNA is generally more stable than RNA. Why?
Several chemical and physical factors affect the nucleic acids.
The chemical factors include hydrolysis by acids, alkali, enzymes,
and mutagenic factors of the DNA bases.
The physical factors: heat, pH, salt concentration, and base
composition.
DNA denaturation
During processes such as replication ~ 2 strands separate.
In laboratories, DNA strands can be disrupted. (How?)
Heat, Acids and alkali.
Properties of DNA
DNA melting
Dissociation of the DNA double helix (separation of strands).
Due to heat ~ Melting temperature (Tm), a temperature at which
half of the helical structure is lost.
Due to cellular enzymes (during replication ~ helicases in an ATP
dependent manner).
Hypochromism
Stacked bases ~ absorb less ultraviolet light than unstacked bases.
Where would you find these?
DNA and RNA.
Melting of nucleic acids ~ monitored by measuring absorption of
light @ 260nm (where it’s maximal).
Properties of DNA
DNA melting
Properties of DNA
DNA melting
The melting temperature depends:
The length of the DNA, and
The nucleotide sequence composition,
Higher GC content, higher Tm.
The triple hydrogen bonds between G and C need more
energy to disrupt than the double bonds between A and T.
Properties of DNA
DNA melting
Properties of DNA
Annealing
Separated strands ~ reassociate to form a double helix (when the
temperature has been reduced to below the Tm).
Application ~ hybridization (Northern and Southern blotting
experiments).
I Thank you
