14. DNA replication
Copying of DNA – DNA from DNA
Only once
Semi conservative
Each strand acts as template
Polarity is maintained.
Reading 3’ to 5’
Synthesis 5’ to 3’
15.
16. Steps of replication
Identification of Origin
Unwinding of DNA
Replication fork and RNA primer synthesis
Synthesis and elongation of DNA - Polymerization
Replication bubbles and ligation of gaps
Chromatin assembly formation
17. Origin of replication
E.coli – Dna A protein finds
Ori C
Yeast- Origin Recognition
complex (ORC)
Eukaryotes - DNA
unwinding element (DUE)
18.
19. Replication fork
As DNA synthesis continues, there is a replication
fork with two prongs.
Eukaryotes have replication at multiple origins
DNA helicase unwinds DNA
Helicases - separate strands of a DNA double helix
Also remove nucleic acid-associated proteins.
20.
21. Single-strand binding proteins bind to
the DNA (SSBPs)
(Gyrase)Topisomerases I and II –
removes supercoils by nicking and
sealing.
RNA primer
11- 60 nts length
Dna G synthesize – RNA polymerase
activity
Leading strand – one
Lagging strand – many
22. DNA polymerase
Reading 3’ to 5’
Synthesis 5’ to 3’
Leading strand (forward) –
continuous
The lagging strand is synthesized
in short segments as Okazaki
fragments.
24. Termination
In Eukaryotes DNA replication is unable to reach the
very end of the chromosomes.
The telomere of the daughter DNA strand shortens.
So somatic cells can only divide a certain number of
times.
In germ cell line, telomerase enzyme extends the
repetitive sequences of the telomere region to prevent
degradation.
Telomerase if active in somatic cells, leads to cancer.
25.
26. DNA ligase
DNA ligases close nicks in the
phosphodiester backbone of DNA.
DNA ligases are essential for
joining Okazaki fragments .
Involved in DNA repair process.
27. Reverse transcriptase
In retroviruses, the RNA is the template.
Reverse transcriptase – DNA from RNA
DNA-RNA hybrid is synthesized
RNAase H degrades the RNA
Then dsDNA is synthesized from DNA
template.
Retroviruses (e.g., HIV), and RT inhibitors
are widely used as antiretroviral drugs.
30. Clinical applications
A disregulation of the cell cycle lead to tumor
formation.
Cell cycle inhibitors, RB, p53 etc. on mutation, form a
tumor.
G1 is the most variable phase of the cell cycle.
31. DNA repair
DNA damage needs repair.
Ineffective repair lead to three possible states:
1. an irreversible state of dormancy, known as senescence
2. cell suicide, also known as apoptosis or programmed
cell death
3. unregulated cell division, which can lead to the
formation of a tumor that is cancerous
32. Types of DNA damage
1. Endogenous damage - attack by reactive oxygen
species produced from metabolism.
2. Exogenous damage by external agents
ultraviolet , X-rays and gamma rays
Thermal disruption, toxins, mutagenic chemicals,
aromatic compounds that intercalate DNA
viruses.
33.
34. 1.Single base alteration
2.Two base alteration
a.UV light induced thymine- thymine
dimer
b.Alkylating agent cross linkage
3. Chain breaks
It is due to ionizing radiation,
radioactivity, oxidative free radicals.
4. Cross linkage
35. The mechanisms of DNA repair
1. Nucleotide excision repair (NER)
2. Mismatch repair (MMR)
3. Base excision repair (BER)
4. Homologous recombination repair
(HRR)
5. Non homologous end joining (NHEJ)
38. Transcription
Synthesis of RNA from DNA
Reading of DNA from 3’ to 5’
Synthesis of RNA from 5’ to 3’
RNA polymerase
Promoter region regulates transcription
Steps- Initiation , elongation, termination
41. Translation – RNA to proteins
Synthesis of proteins
mRNA – contains the message in the
form of codons
tRNA carries amino acids and
recognize codons with its anticodon
rRNA is involved in translation