Introduction
In the field of molecular biology, understanding the processes of DNA replication, transcription, and translation is crucial. These processes are fundamental to the flow of genetic information within a cell, often summarized by the central dogma of molecular biology: DNA → RNA → Protein. This study note document will delve into each of these processes in detail, breaking down complex concepts into digestible sections.
DNA Replication
DNA replication is the process by which a cell duplicates its DNA, ensuring that each daughter cell receives an exact copy of the genetic material. This process is semi-conservative, meaning that each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.
Key Steps in DNA Replication
- Initiation
- Replication begins at specific locations called origins of replication.
- Enzymes like helicase unwind the DNA double helix, creating a replication fork.
- Single-strand binding proteins (SSBs) stabilize the unwound DNA.
- Elongation
- Primase synthesizes a short RNA primer to provide a starting point for DNA polymerase.
- DNA polymerase III adds nucleotides to the 3' end of the RNA primer, synthesizing the new DNA strand.
- The leading strand is synthesized continuously, while the lagging strand is synthesized in short fragments called Okazaki fragments.
- Termination
- DNA polymerase I removes RNA primers and replaces them with DNA.
- DNA ligase joins the Okazaki fragments on the lagging strand, forming a continuous DNA molecule.
Enzymes Involved
- Helicase: Unwinds the DNA double helix.
- Primase: Synthesizes RNA primers.
- DNA Polymerase III: Adds nucleotides to the growing DNA strand.
- DNA Polymerase I: Removes RNA primers and replaces them with DNA.
- DNA Ligase: Joins Okazaki fragments.
Ensure you understand the role of each enzyme in DNA replication, as this is a common topic in exams.
Transcription
Transcription is the process by which the genetic information in DNA is copied into messenger RNA (mRNA). This process occurs in the nucleus of eukaryotic cells.
Key Steps in Transcription
- Initiation
- RNA polymerase binds to the promoter region of the gene.
- The DNA double helix unwinds, exposing the template strand.
- Elongation
- RNA polymerase moves along the template strand, synthesizing a complementary RNA strand.
- RNA nucleotides pair with their complementary DNA bases (A with U, T with A, C with G, and G with C).
- Termination
- Transcription continues until RNA polymerase reaches a termination sequence.
- The newly synthesized mRNA strand is released.
Post-Transcriptional Modifications
- 5' Capping: Addition of a methylated guanine cap to the 5' end of the mRNA.
- Polyadenylation: Addition of a poly-A tail to the 3' end of the mRNA.
- Splicing: Removal of introns (non-coding regions) and joining of exons (coding regions).
Post-transcriptional modifications are crucial for the stability and export of mRNA from the nucleus to the cytoplasm.
