Biology Exam Preparation: Transcription and Translation

Central Dogma of Molecular Biology

• The central dogma refers to the flow of genetic information from DNA to mRNA to protein.

• This process includes two main stages: transcription and translation.

Transcription

• Definition: Transcription is the process where the genetic information in DNA is transferred to mRNA.

• Location: Transcription occurs in the nucleus of the cell.

• Key Points:

  • Unlike DNA replication, which copies the entire DNA molecule, transcription only copies one gene at a time.

  • RNA polymerase (an enzyme) is responsible for synthesizing RNA from the DNA template.

Steps of Transcription:

1. Initiation:

   • RNA polymerase binds to a specific region of the DNA called the promoter sequence (marked in green).

   • The promoter indicates where transcription should begin.

2. Strand Separation:

   • The DNA strands are unwound (hydrogen bonds between bases are broken) to expose the gene.

3. Elongation:

   • RNA nucleotides align along the DNA template strand following base pairing rules:

     • Adenine (A) pairs with Uracil (U) in RNA.

     • Cytosine (C) pairs with Guanine (G).

   • RNA polymerase links the RNA nucleotides to form a new strand of mRNA as it moves along the DNA.

4. Termination:

   • Transcription ends when RNA polymerase reaches a terminator sequence, a specific sequence that signals the end of the gene.

   • At this point, the RNA polymerase disassociates from the DNA and releases the newly formed mRNA.

5. Processing of mRNA:

   • The mRNA undergoes several modifications:

     • Addition of a 5' cap and a 3' poly-A tail for protection and stability.

     • Removal of non-coding regions called introns, while keeping the coding regions called exons together.

   • Result: Mature mRNA is now ready to move from the nucleus to the cytoplasm.

Translation

• Definition: Translation is the process by which the sequence of the mature mRNA is used to synthesize proteins.

Key Concepts in Translation:

• Codons: Groups of three nucleotides on the mRNA that encode for specific amino acids.

  • Each codon corresponds to one amino acid in a protein sequence.

• Understanding the genetic code is crucial, as each codon specifies which amino acids should be added during protein synthesis.

Important Notes:

• Be able to transcribe a given DNA sequence to produce the correct mRNA sequence by using the correct pairing rules (e.g., for a DNA sequence AGT, the corresponding mRNA would be UCA).

• Distinguish clearly between requests for mRNA sequence versus the complementary DNA strand, as they are different processes.

To distinguish clearly between requests for mRNA sequence versus the complementary DNA strand:

• mRNA Sequence Request:

  • Involves transcribing the DNA sequence into mRNA.

  • mRNA is synthesized based on the DNA template, replacing (A)adenine with uracil (U).

  • Example: For the DNA sequence AGT, the corresponding mRNA sequence would be UCA.

• Complementary DNA Strand Request:

  • Involves determining the complementary DNA sequence.

  • Pairs adenine (A) with thymine (T) and cytosine (C) with guanine (G).

  • Example: For the same DNA sequence AGT, the complementary DNA strand would be TCA.