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DNA replication and transcription are two fundamental processes that occur in cells to ensure the accurate transmission of genetic information and the synthesis of proteins. Let's delve into each process separately:
DNA Replication:
DNA replication is the process by which a cell duplicates its DNA before cell division. It occurs during the S phase of the cell cycle. The steps involved in DNA replication are as follows:
1. Initiation: The replication process begins at specific sites on the DNA molecule called origins of replication. Enzymes called helicases unwind and separate the DNA strands, forming a replication fork.
2. Elongation: DNA polymerase, along with other enzymes and proteins, adds complementary nucleotides to each separated DNA strand. The leading strand is synthesized continuously in the 5' to 3' direction, while the lagging strand is synthesized discontinuously in small fragments called Okazaki fragments.
3. Priming: Primase synthesizes short RNA primers on the lagging strand, providing a starting point for DNA polymerase to begin synthesis.
4. DNA synthesis: DNA polymerase adds nucleotides to the growing DNA strands, using the existing strands as templates. Adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G), following the base-pairing rules.
5. Termination: The replication process continues until the entire DNA molecule is replicated. At the termination site, specific proteins halt the replication process and ensure the completion of replication.
Transcription:
Transcription is the process by which genetic information encoded in DNA is copied into a complementary RNA molecule. It occurs in the nucleus of eukaryotic cells and the cytoplasm of prokaryotic cells. The steps involved in transcription are as follows:
1. Initiation: RNA polymerase binds to a specific region on the DNA molecule called the promoter. The DNA strands unwind, forming a transcription bubble.
2. Elongation: RNA polymerase synthesizes a complementary RNA molecule using one of the DNA strands as a template. Adenine (A) in DNA pairs with uracil (U) in RNA, while cytosine (C) pairs with guanine (G).
3. Termination: Transcription continues until a termination signal is reached on the DNA molecule. At this point, RNA polymerase detaches from the DNA, and the newly synthesized RNA molecule is released.
4. Post-transcriptional modifications: In eukaryotic cells, the newly synthesized RNA molecule undergoes various modifications before it can be used to synthesize proteins. These modifications include the addition of a 5' cap and a poly-A tail, as well as the removal of introns through a process called splicing.
In summary, DNA replication ensures the faithful duplication of the entire DNA molecule, while transcription copies specific segments of DNA into RNA molecules. These processes are vital for the accurate transmission of genetic information and the synthesis of proteins, which are essential for the functioning of cells.