DNA Replication, Transcription, and Translation

Posted on Oct 26, 2024 in Biology

DNA Replication

DNA replication is semiconservative, creating two new DNA molecules from a double helix. One strand acts as a template, and a new complementary strand is synthesized. This results in a double-stranded helix.

Stages of DNA Replication

  1. Initiation: Replication begins at specific DNA sequences called origins of replication.
  2. Unwinding and Opening: Enzymes and proteins, collectively known as the replisome, facilitate this process.
    • Helicases unwind the double helix.
    • Gyrases and topoisomerases relieve torsional stress.
    • SSBPs (single-strand binding proteins) prevent the template strands from reannealing.
  3. Synthesis: DNA polymerases synthesize new strands in the 5′-3′ direction, reading the template in the 3′-5′ direction.
    • DNA polymerase III performs most of the synthesis and error correction.
    • DNA polymerase I also contributes to error correction.
    • DNA polymerase II repairs damage caused by physical agents.
    • The leading strand is synthesized continuously.
    • The lagging strand is synthesized in short fragments called Okazaki fragments.
    • RNA primers are required for initiation of synthesis.
  4. Error Correction: DNA polymerase III corrects errors during replication. Other enzymes involved include:
    • Endonucleases, which cut out incorrect segments.
    • DNA polymerases, which fill the gaps.
    • DNA ligases, which join the corrected segments.

RNA Types

mRNA (Messenger RNA)

mRNA carries genetic information from DNA to ribosomes for protein synthesis. In eukaryotes, pre-mRNA undergoes processing, including splicing to remove introns.

tRNA (Transfer RNA)

tRNA transports specific amino acids to ribosomes during protein synthesis.

rRNA (Ribosomal RNA)

rRNA is a component of ribosomes and plays a structural and functional role in protein synthesis.

snRNA (Small Nuclear RNA)

snRNA forms complexes with proteins and is involved in processes such as splicing.

Transcription in Eukaryotes

Transcription is the process of transferring genetic information from DNA to RNA. Three types of RNA polymerase (I, II, and III) are involved. Eukaryotic genes are often fragmented into exons (coding regions) and introns (non-coding regions). Introns are removed before translation.

Stages of Transcription

  1. Initiation: RNA polymerase II binds to the promoter region of DNA.
  2. Elongation: RNA synthesis proceeds in the 5′-3′ direction. A methylguanosine cap is added to the 5′ end.
  3. Termination: Transcription terminates, often signaled by a specific DNA sequence. A poly-A tail is added to the 3′ end of the pre-mRNA.
  4. Maturation: Introns are removed by splicing, and the mature mRNA is formed.

Translation

Translation occurs in ribosomes and involves the synthesis of proteins based on the mRNA sequence.

Stages of Translation

  1. Initiation: The ribosome binds to the mRNA and the initiator tRNA carrying methionine.
  2. Elongation: Amino acids are added to the growing polypeptide chain.
  3. Termination: Translation stops when a stop codon is encountered.

Genetic Code

  • The genetic code is based on triplets of nucleotides called codons.
  • The reading frame is 5′-3′.
  • The genetic code is non-overlapping.
  • Three codons are stop codons.
  • The genetic code is degenerate, meaning multiple codons can code for the same amino acid.
  • The genetic code is nearly universal.