DNA SYNTHESIS
DNA replication
``````````````````````````````````````````````````````````````````
DNA replication is a process by which a double stranded DNA molecule is copied to produce 2
identical DNA molecules. This process occurs at the S phase
1. 2 DNA strands are unzipped by DNA Helicase forming a replication fork
2. DNA Primase forms a primer which is a new starting point for DNA construction
3. DNA polymerase starts adding nucleotide bases to the leading strand (5’to 3’)
4. The leading strand is made continuously
5. The other strands is a lagging strand meaning it can only be made discontinuously using
Okazaki fragments because it was synthesized in a 3’ to 5’ direction
6. DNA primase forms a primer fragment down the strand
7. DNA polymerase adds the nucleotide bases and the process is repeated until the strand in
complete
8. Exonuclease enzyme removes all the primers then DNA polymerase comes back and lls them
in
9. DNA ligase seals both strands forming a continuous double strand
DNA transcription
DNA transcription is the process by which DNA gene is converted to mRNA
1. RNA polymerase 2 binds to the promoter site to initiate transcription as it unwinds the double
helix of the DNA
2. RNA polymerase moves along the DNA template
3. RNA polymerase continuously unwinds DNA double helix thus exposing the nucleotides
4. As it moves along the DNA template the DNA reforms into the double helix con guration
5. Transcription continues until it recognizes a stop codon (UAA.UAG,UGA)
6. The newly formed RNA strand is released
In prokaryotes this is the end of transcription, but in eukaryotes theres post-transcriptional events
- Capping
- Polyadenylation
- RNA splicing
CAPPING
- 7-methyl guanosine is added to the ends of pre-mRNA at 5’ to protect them from degradation
and for it to signal initiation of translation
POLYADENYLATION
- poly(A) tail is added to the 3’ end of pre-mRNA where it catalyzes poly(A) to polymerase. This
enhances eukaryotic mRNAs stability and plays an imp role in the transport of mRNA from the
nucleus to the cytoplasm
RNA SPLICING
- turns pre-mRNA into mature mRNA by removing introns and keeping exons
fi fi
DNA replication
``````````````````````````````````````````````````````````````````
DNA replication is a process by which a double stranded DNA molecule is copied to produce 2
identical DNA molecules. This process occurs at the S phase
1. 2 DNA strands are unzipped by DNA Helicase forming a replication fork
2. DNA Primase forms a primer which is a new starting point for DNA construction
3. DNA polymerase starts adding nucleotide bases to the leading strand (5’to 3’)
4. The leading strand is made continuously
5. The other strands is a lagging strand meaning it can only be made discontinuously using
Okazaki fragments because it was synthesized in a 3’ to 5’ direction
6. DNA primase forms a primer fragment down the strand
7. DNA polymerase adds the nucleotide bases and the process is repeated until the strand in
complete
8. Exonuclease enzyme removes all the primers then DNA polymerase comes back and lls them
in
9. DNA ligase seals both strands forming a continuous double strand
DNA transcription
DNA transcription is the process by which DNA gene is converted to mRNA
1. RNA polymerase 2 binds to the promoter site to initiate transcription as it unwinds the double
helix of the DNA
2. RNA polymerase moves along the DNA template
3. RNA polymerase continuously unwinds DNA double helix thus exposing the nucleotides
4. As it moves along the DNA template the DNA reforms into the double helix con guration
5. Transcription continues until it recognizes a stop codon (UAA.UAG,UGA)
6. The newly formed RNA strand is released
In prokaryotes this is the end of transcription, but in eukaryotes theres post-transcriptional events
- Capping
- Polyadenylation
- RNA splicing
CAPPING
- 7-methyl guanosine is added to the ends of pre-mRNA at 5’ to protect them from degradation
and for it to signal initiation of translation
POLYADENYLATION
- poly(A) tail is added to the 3’ end of pre-mRNA where it catalyzes poly(A) to polymerase. This
enhances eukaryotic mRNAs stability and plays an imp role in the transport of mRNA from the
nucleus to the cytoplasm
RNA SPLICING
- turns pre-mRNA into mature mRNA by removing introns and keeping exons
fi fi
